Emulsion Characterization - Advances in Chemistry (ACS Publications)

3 Emulsion Characterization Randy J. Mikula

Downloaded by CORNELL UNIV on December 7, 2012 | http://pubs.acs.org Publication Date: May 5, 1992 | doi: 10.1021/ba-1992-0231.ch003

Department of Energy, Mines, and Resources, C A N M E T , Fuel Processing Laboratory, P.O. Bag 1280, Devon, Alberta, Canada, TOC 1E0

This chapter outlines emulsion characterization techniques ranging from those commonly found in field environments to those in use in research laboratories. Techniques used in the determination of bulk emulsion properties, or simply the relative amount of oil, water, and solids present, are discussed, as well as those characterization methods that measure the size distribution of the dispersed phase, rheological behavior, and emulsion stability. A particular emphasis is placed on optical and scanning electron microscopy as methods of emulsion characterization. Most of the common and many of the less frequently used emulsion characterization techniques are outlined, along with their particular advantages and disadvantages.

AJM

E M U L S I O N IS U S U A L L Y D E F I N E D

as a system c o n s i s t i n g o f a l i q u i d

d i s p e r s e d w i t h o r w i t h o u t a n e m u l s i f i e r i n an i m m i s c i b l e l i q u i d , u s u a l l y i n droplets o f larger t h a n c o l l o i d a l sizes. I n p e t r o l e u m e m u l s i o n s , solids p l a y an extremely i m p o r t a n t role i n b o t h the f o r m a t i o n a n d stability o f e m u l s i o n s . T h e s e solids c a n b e oil-phase c o m p o n e n t s s u c h as wax crystals o r p r e c i p i tated asphaltenes, o r m i n e r a l c o m p o n e n t s that are p a r t i a l l y o l e o p h i l i c , a p r o p e r t y that allows t h e m to act as stabilizers b e t w e e n t h e o i l a n d w a t e r phases. C h a r a c t e r i z a t i o n o f s u c h emulsions therefore o f t e n involves t h r e e phases: the water phase, t h e o i l phase, a n d the solids. C o m p l e t e charac t e r i z a t i o n o f a n e m u l s i o n c o u l d therefore i n v o l v e d e t a i l e d c h e m i c a l a n d p h y s i c a l analysis o f a l l o f the e m u l s i o n c o m p o n e n t s , as w e l l as any b u l k p r o p e r t i e s that m i g h t b e o f interest (viscosity, density, etc.). T h i s l e v e l o f d e t a i l is clearly b e y o n d the scope o f this d i s c u s s i o n . F o r t h e purposes o f this chapter, e m u l s i o n c h a r a c t e r i z a t i o n w i l l b e d e f i n e d as the q u a n t i f i c a t i o n o f the phases present, the d e t e r m i n a t i o n o f the nature a n d size d i s t r i b u t i o n o f 0065-2393/92/0231-0079 $13.65/0 © 1992 American Chemical Society

In Emulsions; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

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the d i s p e r s e d phase, a n d the m e a s u r e m e n t o f t h e stability o f t h e d i s p e r s e d phase.


C h e m i c a l p r o p e r t i e s o f the i n d i v i d u a l phases w i l l not b e d i s c u s s e d i n this c h a p t e r because a n y n u m b e r o f c o n v e n t i o n a l analytical t e c h n i q u e s c a n b e a p p l i e d to t h e separated o i l , water, a n d solids phases that m i g h t m a k e u p a t y p i c a l e m u l s i o n . F r o m a p r o c e s s i n g p o i n t o f v i e w , w h e n t r e a t i n g (separat ing) e m u l s i o n s , t h e m a i n c o n c e r n s u s u a l l y are total o i l , water, a n d solids i n each o f t h e f e e d , p r o d u c t , a n d tailings streams. A s l o n g as w a t e r i n the p r o d u c t o i l a n d o i l i n t h e tailings w a t e r are l o w , t h e n t h e process is w o r k i n g , and d e t a i l e d analysis o f t h e c o m p o s i t i o n o f t h e e m u l s i o n c o m p o n e n t s is n o t r e q u i r e d . A f u n d a m e n t a l u n d e r s t a n d i n g o f t h e interactions b e t w e e n e m u l sion c o m p o n e n t s that d e t e r m i n e stability is o f t e n o n l y r e q u i r e d o r " r e s o r t e d t o " w h e n process upsets o c c u r . I n t e r f a c i a l p r o p e r t i e s , film r i g i d i t y o r strength, a n d surface t e n s i o n b e t w e e n t h e various e m u l s i o n phases are ex t r e m e l y i m p o r t a n t i n d e t e r m i n i n g stability o f t h e d i s p e r s e d phase, b u t they w i l l n o t b e d i s c u s s e d i n d e t a i l h e r e because these measurements f a l l u n d e r the category o f t e c h n i q u e s u s e d to characterize t h e i n d i v i d u a l e m u l s i o n c o m p o n e n t s . A n u m b e r o f r e v i e w articles a n d books discuss these t e c h n i q u e s a n d m a n y aspects o f e m u l s i o n science a n d e m u l s i o n c h a r a c t e r i z a t i o n (1-13). E m u l s i o n c h a r a c t e r i z a t i o n a n d t e c h n o l o g y d e v e l o p m e n t have b e e n d r i v e n b y t h e m e d i c a l , a g r i c u l t u r a l , f o o d , a n d cosmetics i n d u s t r i e s ; t h e p e t r o l e u m a n d o i l i n d u s t r i e s have b o r r o w e d these technologies a n d a d a p t e d t h e m to t h e i r p a r t i c u l a r a p p l i c a t i o n s . A n u m b e r o f books a n d r e v i e w articles discuss aspects o f e m u l s i o n technologies specifically r e l a t e d to o i l - f i e l d a n d p e t r o l e u m applications (14,15). T h e s e p e t r o l e u m applications have b e c o m e especially i m p o r t a n t since t h e advent o f surfactant flooding a n d o t h e r ter tiary o i l r e c o v e r y m e t h o d s i n w h i c h e m u l s i o n s are u s e d and/or f o r m e d . T h e c h a r a c t e r i z a t i o n t e c h n i q u e s that w i l l b e d i s c u s s e d h e r e are u s e d i n field situations, o n - l i n e , a n d i n t h e laboratory. I n o r d e r to characterize a n e m u l s i o n , it is necessary to d e t e r m i n e t h e a m o u n t o f each phase present, the nature o f t h e d i s p e r s e d a n d c o n t i n u o u s phases, a n d t h e size d i s t r i b u t i o n o f the d i s p e r s e d phase. T h e stability o f an e m u l s i o n is a n o t h e r i m p o r t a n t p r o p erty that c a n b e m o n i t o r e d i n a variety o f ways, b u t most o f t e n , f r o m a p r o c e s s i n g p o i n t o f v i e w , stability is m e a s u r e d i n terms o f t h e rate o f phase separation over t i m e . T h i s p h e n o m e n o l o g i c a l a p p r o a c h serves w e l l i n p r o cess situations i n w h i c h e m u l s i o n f o r m a t i o n a n d b r e a k i n g p r o b l e m s c a n b e v e r y site specific. H o w e v e r , e m u l s i o n stability is u l t i m a t e l y r e l a t e d to t h e d e t a i l e d c h e m i s t r y a n d physics o f the e m u l s i o n c o m p o n e n t s a n d t h e i r i n t e r actions, a n d these details cannot b e c o m p l e t e l y i g n o r e d . T h i s c h a p t e r is s t r u c t u r e d a c c o r d i n g to t h e types o f i n f o r m a t i o n p r o v i d e d b y the various c h a r a c t e r i z a t i o n t e c h n i q u e s . A p p l i c a t i o n s i n t h e field o r i n t h e laboratory are discussed, a l o n g w i t h advantages a n d disadvantages. A n e x c e p t i o n to this f o r m a t is m a d e f o r t h e m i c r o s c o p i c t e c h n i q u e s , w h i c h ,


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because o f t h e i r w i d e a p p l i c a b i l i t y (and because they h a p p e n to b e m y specialty), w i l l b e c o v e r e d separately. M i c r o s c o p y is seen b y m a n y as the u l t i m a t e c h a r a c t e r i z a t i o n t o o l , at least i n terms o f d r o p l e t size d i s t r i b u t i o n , because d i r e c t o b s e r v a t i o n o f a sample is s i m p l e to i n t e r p r e t . I n spite o f the p e r c e p t i o n o f m i c r o s c o p y as the u l t i m a t e c h a r a c t e r i z a t i o n m e t h o d , m a n y p r o b l e m s a n d pitfalls c a n be e n c o u n t e r e d i n i n t e r p r e t i n g m i c r o s c o p i c o b servations, a n d these w i l l be d i s c u s s e d at l e n g t h . T w o other t e c h n i q u e s that m i g h t also w a r r a n t separate d i s c u s s i o n , elec trokinetics a n d viscosity d e t e r m i n a t i o n s , are c o v e r e d i n d e t a i l i n C h a p t e r s 2 and 4 a n d w i l l o n l y b e b r i e f l y m e n t i o n e d h e r e . T h e figures are g i v e n w i t h d e t a i l e d captions so that they m a y be r e f e r r e d to i n d e p e n d e n t l y o f the m a i n text.

Bulk Properties A l t h o u g h surface p h e n o m e n a d e t e r m i n e the f u n d a m e n t a l p r o p e r t i e s o f e m u l s i o n s i n terms o f size d i s t r i b u t i o n s a n d stability, the b u l k p r o p e r t i e s o r b u l k c o m p o s i t i o n s are the yardsticks b y w h i c h p l a n t operators a n d process p e r s o n n e l measure process efficiency. A c c u r a t e d e t e r m i n a t i o n o f the o i l , water, a n d solids (if present) is therefore one o f the most i m p o r t a n t aspects of emulsion characterization.

Oil-Continuous or Water-Continuous Emulsions. I n most e m u l s i o n systems, the nature o f the d i s p e r s e d phase is q u i t e clear. T h e r e is usually little d o u b t that an o i l - w a t e r e m u l s i o n w i t h 5 % w a t e r is a w a t e r - i n - o i l e m u l s i o n , o i l b e i n g the c o n t i n u o u s phase. I n m a n y separation a n d treatment processes, i n a d d i t i o n to the o i l p r o d u c t ( w h i c h m i g h t c o n t a i n some e m u l s i fied water), a n d the w a t e r tailings ( w h i c h m i g h t c o n t a i n some e m u l s i f i e d o i l ) , there c a n be an interface e m u l s i o n , a s o - c a l l e d rag layer, whose c o n t i n u o u s and disperse phases are generally u n k n o w n . F i g u r e 1 shows an o p t i c a l m i c r o g r a p h o f s u c h an e m u l s i o n i n w h i c h the o i l a n d w a t e r are b o t h c o n t i n u o u s and d i s p e r s e d , d e p e n d i n g u p o n w h e r e i n the sample one looks. O f t e n these e m u l s i o n s b u i l d to a c e r t a i n l e v e l , c o n t i n u o u s l y r e - f o r m a n d break i n the separator, a n d n e v e r cause o p e r a t i o n a l p r o b l e m s . O c c a s i o n a l l y , h o w e v e r , they c a n b u i l d to s u c h an extent that they r e q u i r e r e m o v a l a n d separate treatment. K n o w l e d g e o f the nature o f the d i s p e r s e d phase is t h e r e f o r e c r i t i c a l i n d e t e r m i n i n g an effective treatment. T h e ratio o f the o i l to w a t e r alone is not sufficient to d e t e r m i n e w h i c h is the d i s p e r s e d phase because the p r e s e n c e o f emulsifiers o r solids c a n signifi cantly affect the a m o u n t o f d i s p e r s e d phase d i s t r i b u t e d i n a g i v e n a m o u n t o f c o n t i n u o u s phase. F i g u r e 2 shows an example o f a fire flood e m u l s i o n that is w a t e r - i n - o i l , a l t h o u g h the e m u l s i o n contains 6 3 % (by w e i g h t ) water. E x p l o sives are o f t e n w a t e r - i n - o i l e m u l s i o n s w i t h u p to 9 2 % w a t e r phase (16, 17).




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Figure 1. Optical micrograph of a rag-layer emulsion showing complex structure. In reflected mode with blue-violet light, the water component (W) is dark, and the oil component (O) fluoresces yellow (bright in this black-andwhite reproduction). On a very short scale both oil-in-water and water-in-oil emulsions can be seen.



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Figure 2. Scanning electron micrographs (at three magnifications) of afire flood emulsion illustrating a case in which, although the water-oil ratio is 2.5:1, water is the dispersed phase. The composition of this emulsion is 63% water, 11% solids, and 26% oil. The compositions of the dispersed and continu ous phases were determined from the X-ray signal excited in the electron microscope. The size of the dispersed water phase ranges from less than 0.1 μηι up to about 10 yum. The large features labeled Ο are regions of oil phase that can be described as oil emulsified in a continuous phase of a water-in-oil emulsion. These complex systems are difficult to characterize with anything but microscopic methods.

Several t e c h n i q u e s d e t e r m i n e w h e t h e r the c o n t i n u o u s phase is o i l o r water. T h e s i m p l e s t is the d i l u t i o n m e t h o d , i n w h i c h a d r o p o r t w o o f t h e e m u l s i o n is a d d e d to water. I f it is a n o i l - i n - w a t e r e m u l s i o n i t w i l l s p r e a d a n d disperse. I f i t is w a t e r - i n - o i l i t w i l l r e m a i n as a d r o p (18). T h e d i l u t i o n test c a n b e effective, b u t care m u s t b e taken that s a m p l i n g the e m u l s i o n does n o t i t s e l f d e t e r m i n e the c o n t i n u o u s phase. F o r instance, d r a w i n g a w a t e r - i n - o i l e m u l s i o n u p t h r o u g h t h e c a p i l l a r y o f a d r o p p e r c a n cause the e m u l s i o n to



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invert because of interactions of the water phase with the hydrophilic glass walls. This phenomenon is extremely important in the microscopic characterization of emulsions and will be discussed further in that section. Another option is to dye the continuous phase (J 9, 20). Dyeing is best done under a microscope where the coloring of the continuous phase can be observed with an appropriate water- or oil-soluble dye. Several water-soluble dyes, such as methylene orange or methylene blue, can be used. A common oil-soluble dye is fuchsin. If methylene blue is mixed with the emulsion and no color change is observed, then the emulsion is most likely water-in-oil. The opaque nature of oil-field emulsions limits the applicability of these color techniques. Electrical conductivity or capacitance of the emulsion might determine the nature of the continuous phase because water-in-oil would be much less conductive than a similar oil-in-water emulsion. This technique is useful in the laboratory to monitor emulsion inversion as a function of oil, water, or chemical addition and also is the basis of many level sensors infieldsituations. A significant change in the amount of solids in the oil or water phases in a process situation might give a conductivity reading that is ambiguous in terms of defining the continuous phase (21), and therefore the water-oil interface level. Oil and Water Content with Solids. Many methods determine the relative amounts of water and oil in emulsions. Because many emulsions of interest to the oil industry also contain solids, determination of the solids content is also important (22, 23). The Institute of Petroleum (IP), the American Petroleum Institute (API), and the American Society for Testing and Materials (ASTM) have developed standard methods for these determinations, as have most organizations or laboratories where these determinations are routinely performed (24-28). These methods are all some modification of the Dean-Stark procedure, in which the sample is placed in a porous thimble and refluxed with a suitable organic solvent. Modified Dean-Stark Procedure. A schematic of the apparatus for the modified Dean-Stark procedure is shown in Figure 3. The sample is held in a porous thimble suspended above the refluxing organic solvent. The water in the emulsion sample is codistilled with the solvent and is trapped in the side arm where water content can be determined directly (27, 29, 30). The organic component is dissolved in the solvent and carried to the bottom of the apparatus where it can later be quantified gravimetrically after the solvent is removed. The solids are retained in the porous thimble and are also determined gravimetrically. The size range of solids retained on the thimble is naturally related to its porosity; a common modification of the technique involves centrifuging and decanting the bitumen or oil component (in solvent) to separate thefinesolids. A typical report of results would


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CONDENSER

WATER TRAP

Δ POROUS THIMBLE FOR SAMPLE

REFLUXING SOLVENT

Figure 3. Modified Dean-Stark apparatus. The solvent (usually toluene) drips through the sample, dissolving the organic component and leaving the solids behind. The water, which codistills with the solvent, condenses and is trapped in the side arm and measured volumetrically. The solids and organic phases are determined gravimetrically after evaporating the solvent from the sample thimble and the solvent flask.


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t h e n i n c l u d e p e r c e n t water, p e r c e n t solids, p e r c e n t b i t u m e n - o i l w i t h fines, p e r c e n t b i t u m e n - o i l w i t h o u t fines, a n d p e r c e n t solids w i t h fines. O f t e n the fine solids that pass t h r o u g h the t h i m b l e are i g n o r e d a n d s i m p l y i n c l u d e d w i t h the b i t u m e n o r o i l content. O t h e r c o m m o n m o d i f i c a t i o n s to the stand a r d p r o c e d u r e s i n c l u d e the type o f solvent u s e d o r the r e f l u x i n g t i m e .


T h e major disadvantage o f this t e c h n i q u e is the t i m e r e q u i r e d f o r the r e f l u x i n g a n d for s a m p l e w o r k u p after the extraction to c o m p l e t e the g r a v i m e t r i c d e t e r m i n a t i o n s . B e c a u s e b i t u m e n a n d heavy o i l m a y sometimes c o n t a i n asphaltenes o r heavy o r g a n i c c o m p o n e n t s that m a y be i n s o l i d f o r m i n the o i l phase, i n t e r p r e t a t i o n o f the fine solids c o m p o n e n t requires o t h e r i n f o r m a t i o n about solids c o m p o s i t i o n . T h e advantage o f the t e c h n i q u e is that the s a m p l e c a n b e relatively large, an i m p o r t a n t c o n s i d e r a t i o n i n m a n y situations w h e r e s a m p l e streams are q u i t e heterogeneous. Centrifugation. A n o t h e r c o m m o n l y u s e d t e c h n i q u e for d e t e r m i n a t i o n o f o i l , w a t e r , a n d solids is a s i m p l e c e n t r i f u g e test. A s w i t h the D e a n Stark m e t h o d , a s t a n d a r d p r o c e d u r e has b e e n d e v e l o p e d b y several o r g a n i z a tions (27, 31, 32). B a s i c a l l y , the test consists o f d i l u t i n g the e m u l s i o n w i t h a k n o w n a m o u n t o f solvent a n d c e n t r i f u g i n g f o r a fixed t i m e . W i t h the spe c i a l l y d e s i g n e d c e n t r i f u g e t u b e s h o w n i n F i g u r e 4, the a m o u n t o f w a t e r a n d solids i n the sample can be r e a d v o l u m e t r i c a l l y . T h e w a t e r - a n d - s o l i d s is the d e n s e r phase, so the results are g e n e r a l l y r e p o r t e d as B S & W (basic s e d i m e n t a n d water) as a v o l u m e t r i c p e r c e n t . B e c a u s e it is fast a n d r e l i a b l e , the c e n t r i f u g e test is p r o b a b l y the most c o m m o n l y u s e d t e c h n i q u e f o r field evaluation o f w a t e r content i n o i l p r o d u c t streams. T h e most c o m m o n v a r i a t i o n to the A P I , A S T M , a n d I P m e t h o d s for field use is the a d d i t i o n o f d e m u l s i f i e r o r k n o c k o u t drops to facilitate the separa t i o n o f the phases. T h e d e m u l s i f i e r is g e n e r a l l y a d d e d at concentrations significantly above w h a t w o u l d be n o r m a l o p e r a t i o n a l levels. T h e d i s a d vantages o f this t e c h n i q u e are that it does not separate the w a t e r a n d solids, a n d it is not u s e f u l f o r v e r y h i g h - w a t e r - c o n t e n t streams. F i l l i n g the c e n t r i fuge t u b e w i t h a representative s a m p l e c a n also be d i f f i c u l t , especially w i t h viscous e m u l s i o n s .

Oil and Water Content without Solids. T h e p r e s e n c e o f solids i n an e m u l s i o n system reduces the c h a r a c t e r i z a t i o n options because t e c h n i q u e s that c a n q u a n t i f y a l l t h r e e phases are not r e a d i l y available. I n m a n y situations, h o w e v e r , o n l y q u a n t i f i c a t i o n o f w a t e r i n the o i l p r o d u c t o r o i l i n the tailings w a t e r is i m p o r t a n t . I n o t h e r cases, the solids c o n t e n t is i n s i g n i f i cant. I n these situations, the range o f t e c h n i q u e s available is m u c h m o r e extensive a n d , i n g e n e r a l , m o r e a p p l i c a b l e to field a n d o n - l i n e a p p l i c a t i o n s . O b v i o u s l y , the m e t h o d s d i s c u s s e d e a r l i e r also a p p l y to these systems, a l o n g w i t h a v a r i e t y o f s p e c t r o s c o p i c a n d c h e m i c a l analytical t e c h n i q u e s .



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Figure 4. Centrifuge tubes are tapered to allow for precise determination of emulsion samples that have a low water content. The photograph shows that in the absence of a clear oil-water interface, it can be difficult to accurately determine water content (usually basic sediment and water). In addition, centrifugation time can significantly affect the amount of unresolved emulsion. The two centrifuge tubes on the left were centrifugea for 10 min; the two on the right were spun for only 5 min. The first three are the same sample; the tube on the right has a higher solids content. The arrows mark the unresolved emulsion layer. The tube on the far right shows the water phase with some black organic solids at the bottom of the centrifuge tube. This sedimentation can occur as a result of inadequate mixing of the oil phase (in heavy oils) with the solvent; estimation of the percentage of the oil phase relative to water and solids will be inaccurate. In this case, the black solids are oil phase closely associated with clays; therefore, they report with the bottom solids and water fraction.

Karl Fischer Titration. T h e K a r l F i s c h e r t i t r a t i o n is a fast a n d a c c u rate m e t h o d f o r d e t e r m i n i n g w a t e r content. A l t h o u g h t h e A S T M , A P I , a n d I P standards quote a w a t e r range o f 0.02 to 2 . 0 % (33), t h e t e c h n i q u e c a n b e successfully u s e d at h i g h e r w a t e r contents (>10%), T h e t e c h n i q u e involves t i t r a t i n g t h e e m u l s i o n sample w i t h t h e K a r l F i s c h e r reagent c o n s i s t i n g o f a m i x t u r e o f I , S O , a n d p y r i d i n e d i s s o l v e d i n m e t h a n o l . T h e i o d i n e is r e d u c e d b y t h e s u l f u r d i o x i d e i n the p r e s e n c e o f w a t e r to f o r m H I a n d S 0 . T h e s e are immediately complexed by the pyridine and neutralized. Once all of the w a t e r is r e a c t e d , h i g h l y c o n d u c t i v e free i o d i n e appears, a n d t h e e n d p o i n t is 2

£

3


88



i d e n t i f i e d b y t h e increase i n c o n d u c t i v i t y . A l t h o u g h most substances are i n e r t to t h e K a r l F i s c h e r reagent, mercaptans a n d sulfide s u l f u r w i l l i n t e r f e r e a n d must b e b e l o w 500 p p m b y w e i g h t . H i g h w a t e r d e t e r m i n a t i o n s b y this m e t h o d relative to those d e t e r m i n e d b y D e a n - S t a r k distillations m i g h t i n d i cate interferences f r o m o t h e r c o m p o u n d s o r m i n e r a l s that c a n react w i t h t h e K a r l F i s c h e r reagent (34, 35). Process streams w i t h h i g h m i n e r a l o r solids c o n t e n t c a n t h e r e f o r e b e d i f f i c u l t to analyze accurately. Electrical Properties. T h e e l e c t r i c a l p r o p e r t i e s o f o i l a n d w a t e r are q u i t e d i f f e r e n t i n terms o f c o n d u c t i v i t y a n d d i e l e c t r i c constant (both o f w h i c h c a n b e related). T h e s e differences c a n b e m e a s u r e d accurately w i t h a capacitance p r o b e a n d c o r r e l a t e d to t h e a m o u n t o f w a t e r i n a n o i l stream. T h i s type o f p r o b e is c o m m o n l y u s e d i n o n - l i n e situations to m o n i t o r p e r c e n t w a t e r i n o i l p i p e l i n e s (36, 37). G e n e r a l l y , w a t e r a n d solids cannot b e dif f e r e n t i a t e d , so the signal is p r o p o r t i o n a l to t h e total solids a n d w a t e r content. T h e s e systems have seen t h e greatest applications i n m o n i t o r i n g relatively l o w w a t e r contents. I n p r i n c i p l e , t e c h n i q u e s based o n e l e c t r i c a l p r o p e r t i e s c a n b e c a l i b r a t e d f o r process streams w i t h significant w a t e r a n d solids c o n tents. H o w e v e r , t h e capacitance o f the fluid changes w i t h e i t h e r a n increase i n solids o r a n increase i n water, so t h e use o f e l e c t r i c a l p r o p e r t i e s i n these situations is l i m i t e d to streams w h e r e o n l y o n e o r t h e o t h e r is c h a n g i n g . Other Methods. G a m m a - r a y attenuation measures t h e density o f the s a m p l e , w h i c h is r e l a t e d to c h a n g i n g o i l o r water content. G a m m a - r a y d e n sity meters are q u i t e c o m m o n i n process m o n i t o r i n g , b u t they are u s e f u l f o r e m u l s i o n c h a r a c t e r i z a t i o n o n l y i n cases w h e r e t h e solids content is k n o w n to b e z e r o o r c o m p l e t e l y constant (38). O t h e r w i s e t h e density i n f o r m a t i o n o b t a i n e d cannot b e r e l i a b l y r e l a t e d to o i l o r w a t e r content. M i c r o w a v e - b a s e d meters have also b e e n u s e d to m o n i t o r w a t e r content i n e m u l s i o n s (39). M i c r o w a v e t e c h n i q u e s c a n b e u s e d i n t w o ways: E i t h e r the attenuation o f t h e m i c r o w a v e r a d i a t i o n d u e to a b s o r p t i o n b y t h e w a t e r phase is m e a s u r e d , o r capacitance o r resonance changes i n a m i c r o w a v e cavity are n o t e d . T h e capacitance-change m e t h o d is m u c h m o r e sensitive, a l t h o u g h b o t h , l i k e the g a m m a - r a y a b s o r p t i o n m e t h o d , are l i m i t e d i n that solids c o n t e n t must b e constant o r zero i n o r d e r to accurately i n t e r p r e t t h e i n f o r m a t i o n o b t a i n e d . B o t h o f these t e c h n i q u e s are a p p l i c a b l e to field situa tions a n d o n - l i n e m o n i t o r i n g . I n special cases w h e r e t h e c o n t i n u o u s phase is reasonably transparent, absorbance o f l i g h t o r s i m p l e t u r b i d i m e t r y c a n b e r e l a t e d to o i l o r w a t e r content i n an emulsion.

Rheology. V i s c o s i t y a n d other fluid-flow parameters o f e m u l s i o n s are i m p o r t a n t , not just f o r e s t a b l i s h i n g p u m p i n g a n d h a n d l i n g p r o t o c o l s , b u t because they relate to o t h e r e m u l s i o n p r o p e r t i e s , s u c h as size d i s t r i b u t i o n o f the d i s p e r s e d phase, t h e p r e s e n c e o f solids o r emulsifiers, a n d t h e nature o f


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Characterization

the c o n t i n u o u s phase. M a n y c o m m e r c i a l i n s t r u m e n t s p e r f o r m r h e o l o g i c a l d e t e r m i n a t i o n s , a n d m a n y t h o r o u g h reviews c o v e r various aspects o f viscos ity a n d o t h e r r h e o l o g i c a l parameters (40-43).

T h e operating principles o f

the various i n s t r u m e n t s a n d the c h a r a c t e r i z a t i o n t e c h n i q u e s available w i l l b e covered i n Chapter 4. E m u l s i o n i n s t a b i l i t y a n d phase separation d u r i n g viscosity m e a s u r e ments l i m i t the a p p l i c a b i l i t y o f m a n y o f the m e a s u r e m e n t t e c h n i q u e s . T h i s p h e n o m e n o n is i l l u s t r a t e d i n F i g u r e 5 . T h e r e p r o d u c i b l e peak i n shear stress w i t h i n c r e a s i n g shear rate is r e l a t e d t o b i t u m e n separating f r o m this e m u l


s i o n at t h e r o t o r - p l a t e interface o f a c o n v e n t i o n a l v i s c o m e t e r .

Dispersed-Phase Properties T h e c h e m i c a l a n d p h y s i c a l nature o f t h e d i s p e r s e d phase is generally t h e p r i m a r y c o n s i d e r a t i o n i n o r d e r to define o r t o characterize a n e m u l s i o n .

10

SHEAR

RATE

1/s

Figure 5A: Shear stress versus shear rate for an emulsion sample with a high solids content. The peak at low shear is reproducible and is due to oil separating from the emulsion onto the rotors. In rheometers that cannot measure at such low shear rates, this peak can be incorrectly attributed to stress overshoot or yield strength.


90



10

ο

»

1

0

2 0 0

:

1

I

I

4 0 0

6 0 0

8 0 0

SHEAR

RATE

Li 1000

1 /s

Figure 5B: Shear stress versus shear rate for the same sample with this easily separable bitumen removed. Now the peak at low shear rate does not appear. The area enclosed by the upper and lower curves represents thixotropic (or shear-thinning) behavior in this emulsion.

A f t e r a l l , the stability a n d size d i s t r i b u t i o n o f this phase d e t e r m i n e most b u l k e m u l s i o n p r o p e r t i e s . F i x e d p r o p o r t i o n s o f o i l , water, a n d solids c a n b e c o m b i n e d i n various ways to p r o d u c e e m u l s i o n s h a v i n g d i f f e r e n t size d i s t r i butions o f the d i s p e r s e d phase, g i v e n o n l y s m a l l differences i n e m u l s i f i e r o r i o n additions to t h e water o r o i l phases. T h e s e p h y s i c a l differences c a n l e a d to significantly d i f f e r e n t viscosity a n d stability i n e m u l s i o n s w i t h n o m i n a l l y identical bulk composition. T h e selection o f o p t i m u m t r e a t m e n t p r o t o c o l s m a y d e p e n d significantly o n d e t e r m i n a t i o n o f t h e size d i s t r i b u t i o n o f t h e d i s p e r s e d phase. F o r i n stance, c e n t r i f u g a t i o n m i g h t n o t b e effective i n a system w i t h h i g h viscosity a n d a v e r y s m a l l size d i s t r i b u t i o n o f d i s p e r s e d phase. Stokes' l a w c a n b e u s e d to p r e d i c t t h e residence t i m e n e e d e d i f size d i s t r i b u t i o n a n d viscosity are k n o w n . T h e s m a l l e r t h e average size o f t h e d i s p e r s e d phase, the larger the residence t i m e r e q u i r e d . I n fact, t h e residence t i m e increases as t h e inverse o f t h e square o f t h e d i a m e t e r o f the d i s p e r s e d phase.


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91


T h i s section w i l l focus m a i n l y o n c h a r a c t e r i z a t i o n o f the p h y s i c a l n a t u r e o f the d i s p e r s e d phase o r its size d i s t r i b u t i o n . E l e c t r o k i n e t i c c h a r a c t e r i z a t i o n t e c h n i q u e s , w h i c h d e t e r m i n e the e l e c t r i c d o u b l e - l a y e r p r o p e r t i e s o f the d i s p e r s e d phase, w i l l be o n l y b r i e f l y m e n t i o n e d . A g a i n , e l e c t r o k i n e t i c p r o p erties, t h e i r significance, a n d t h e i r m e a s u r e m e n t have b e e n c o v e r e d i n r e v i e w articles (44, 45). A s i d e f r o m m i c r o s c o p y , the t e c h n i q u e s f o r d e t e r m i n i n g the size d i s t r i b u t i o n o f the d i s p e r s e d phase i n e m u l s i o n systems c a n be b r o a d l y d i v i d e d i n t o t h r e e categories: t e c h n i q u e s that d e p e n d u p o n the d i f f e r e n c e s i n elec t r i c a l p r o p e r t i e s b e t w e e n the d i s p e r s e d a n d c o n t i n u o u s phases, those that effect a p h y s i c a l separation o f the d i s p e r s e d d r o p l e t sizes, a n d those that d e p e n d u p o n scattering p h e n o m e n a d u e to the p r e s e n c e o f the d i s p e r s e d phase. O v e r v i e w s o f these types o f t e c h n i q u e s are f o u n d e l s e w h e r e (1-4,13, 46-49).

Size Distribution Using Electrical Properties. A s m e n t i o n e d earlier, f o r w a t e r content d e t e r m i n a t i o n s , t e c h n i q u e s that d e p e n d u p o n differences i n electrical properties do not distinguish between water and solids. T h i s p r o p e r t y l i m i t s t h e i r a p p l i c a b i l i t y to systems i n w h i c h solids are n e g l i g i b l e o r the process s t r e a m is so w e l l d e f i n e d that the differences i n signal c a n be a t t r i b u t e d to d i f f e r e n c e s i n size d i s t r i b u t i o n a n d not to total w a t e r a n d solids (50, 51). C l e a r l y these m e a s u r e m e n t s r e q u i r e extensive c a l i b r a t i o n a n d are not g e n e r a l l y a p p l i c a b l e to o i l - f i e l d e m u l s i o n s . A t e c h n i q u e that is w i d e l y u s e d i n spite o f these d r a w b a c k s , h o w e v e r , is p e r f o r m e d w i t h the a u t o m a t e d ( C o u l t e r ) c o u n t e r (51-53). I n this i n s t r u m e n t , the e m u l s i o n droplets (or particles) are d i l u t e d i n a n electrolyte a n d passed t h r o u g h a fine c a p i l l a r y that connects t w o larger c h a m b e r s c o n t a i n i n g i m m e r s e d electrodes. A p o t e n t i a l d i f f e r e n c e is a p p l i e d b e t w e e n the elec trodes. T h e resistance change that occurs w h e n an o i l d r o p l e t passes t h r o u g h the orifice b e t w e e n the plates is p r o p o r t i o n a l to the a m o u n t o f e l e c t r o l y t e d i s p l a c e d a n d t h e r e f o r e to the size o f the p a r t i c l e . F i g u r e 6 shows a sche m a t i c o f a t y p i c a l e x p e r i m e n t a l setup. A w i d e range o f o r i f i c e sizes is avail able to cover size ranges f r o m 0.4 μπι to about 500 μπι. T h i s t e c h n i q u e , o r variations o f it that m i g h t measure voltage, c u r r e n t , or capacitance changes, is also k n o w n as a sensing-zone t e c h n i q u e . T h e s e m e t h o d s always r e q u i r e c a l i b r a t i o n a n d are l i m i t e d to o i l - i n - w a t e r e m u l s i o n s because the t e c h n i q u e d e p e n d s u p o n the d i s p l a c e m e n t o f electrolyte i n the sensing z o n e . D i l u t i o n o f the e m u l s i o n is o f t e n r e q u i r e d because the appear ance o f t w o particles i n the sensing zone at one t i m e w o u l d be m e a s u r e d as a single larger p a r t i c l e . T h e size range a n a l y z e d i n a single c a p i l l a r y is also l i m i t e d because particles o n the o r d e r o f 4 0 % o f the c a p i l l a r y d i a m e t e r l e a d to b l o c k a g e , a n d particles s m a l l e r t h a n about 2 % o f the c a p i l l a r y d i a m e t e r d o not p r o d u c e a signal above the noise (background) a n d are e f f e c t i v e l y i n v i s i ble.




92

Figure 6. Schematic of a sensing-zone technique (top). As the particle or emulsion droplet (suspended in an electrolyte) passes through the sensing zone, the capacitance or resistance changes in proportion to the size of the particle. These signals can be sorted and interpreted as a size distribution by using an equivalent spherical diameter. Multiple droplets can be mistakenly interpreted as a single larger particle, but several alternative designs minimize this problem. The signal is proportional to the amount of electrolyte displaced; consequently, solids and emulsion droplets cannot be distinguished. These types of techniques are applicable only to oil-in-water emulsions. Bottom: The most common instrument for this technique, the Coulter counter.


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93

Size Distribution Using Scattering Properties. T h e size range p r o b e d b y the various scattering t e c h n i q u e s is a f u n c t i o n o f the w a v e length; neutron-scattering, X-ray-scattering, and light-scattering techniques are r e l a t e d i n terms o f the p h y s i c a l i n t e r a c t i o n b e t w e e n the r a d i a t i o n a n d the particles a n d cover sizes f r o m 0.4 n m to h u n d r e d s o f m i c r o m e t e r s .


Light Scattering. T h e most c o m m o n c o m m e r c i a l l y available s i z i n g i n s t r u m e n t s d e p e n d u p o n l i g h t scattering to o b t a i n size i n f o r m a t i o n . T h e availability o f inexpensive, w e l l - d e f i n e d l i g h t f r o m laser sources has r e s u l t e d i n a w i d e variety o f scattering t e c h n i q u e s u s i n g l i g h t (16, 54-59). L i g h t scattering can be b r o a d l y d i v i d e d i n t o t i m e - a v e r a g e d scattering, w i t h w h i c h e i t h e r spatial d i s t r i b u t i o n or i n t e n s i t y is m e a s u r e d , a n d t i m e fluctuation scattering, w h i c h i n c l u d e s p h o t o n c o r r e l a t i o n s p e c t r o m e t r y , i n w h i c h scattering is c o r r e l a t e d to the m i c r o s c o p i c m o t i o n o f i n d i v i d u a l scat t e r i n g centers. T h e s e t e c h n i q u e s have b e e n d i s c u s s e d i n d e t a i l i n several reviews ( 5 9 - 6 3 ) . O n l y a b r i e f o v e r v i e w o f the most c o m m o n t i m e - a v e r a g e d methods w i l l be given here. These methods include F r a u n h o f e r diffraction a n d l i g h t scattering at larger angles, ( M i e scattering), w h i c h are the basis o f m a n y c o m m e r c i a l l y available s i z i n g i n s t r u m e n t s . Q u a s i - e l a s t i c l i g h t scattering or p h o t o n c o r r e l a t i o n s p e c t r o m e t r y , F r a u n h o f e r d i f f r a c t i o n , a n d o t h e r t e c h n i q u e s that d e p e n d u p o n l i g h t have the same d r a w b a c k ; n a m e l y , the o p a c i t y o f most o i l p r o d u c t i o n samples makes t h e m u n s u i t a b l e for use. T y p i c a l p r o b l e m s w i t h the t h e o r y a n d subse q u e n t data r e d u c t i o n o f the scattering i n f o r m a t i o n to a size d i s t r i b u t i o n i n c l u d e an a s s u m p t i o n o f the nature o f the size d i s t r i b u t i o n (typically l o g n o r m a l , a l t h o u g h software is available w i t h o t h e r options) a n d an i n a b i l i t y to d i s t i n g u i s h aggregates f r o m large single p a r t i c l e s . Solids a n d the d i s p e r s e d phase o f the e m u l s i o n cannot be d i s t i n g u i s h e d f r o m e a c h other, a d i s a d vantage s h a r e d w i t h the sensing-zone t e c h n i q u e s . I n a d d i t i o n , the s a m p l e m u s t be d i l u t e e n o u g h to m i n i m i z e m u l t i p l e scattering. F i g u r e s 7 a n d 8 illustrate the e x p e r i m e n t a l setup a n d examples o f the signal o b s e r v e d i n F r a u n h o f e r d i f f r a c t i o n f o r m o n o d i s p e r s e particles a n d f o r a p o l y d i s p e r s e sample, respectively. T h e d e t e c t i o n system i n most c o m m e r c i a l i n s t r u m e n t s is e i t h e r a n array o f i n t e n s i t y sensors o r a single d e t e c t o r w i t h a m o v i n g mask that measures i n t e n s i t y differences o f the o v e r l a p p i n g c o n c e n t r i c rings that are not d i s c e r n i b l e i n F i g u r e 8. A l t h o u g h no c a l i b r a t i o n is necessary f o r m o n o d i s p e r s e s p h e r i c a l systems, the data are o u t p u t as an e q u i v a l e n t s p h e r i c a l d i a m e t e r a n d the range o f a p p l i c a b i l i t y is g e n e r a l l y for sizes e x c e e d i n g 10 |xm. F o r m a n y e m u l s i o n systems, the refractive i n d e x is close to that o f water; t h e r e f o r e , the p r a c t i c a l l o w e r l i m i t is 10 μπι (13). F o r s o l i d particulate systems, the refractive i n d e x is g e n e r a l l y large; conse q u e n t l y , the a p p l i c a b i l i t y o f this t e c h n i q u e c a n be e x t e n d e d to s m a l l e r sizes. M o s t i n s t r u m e n t m a n u f a c t u r e r s use scattering at larger angles, as w e l l as d i f f r a c t i o n , to p r o b e the s m a l l e r sizes (smaller t h a n about 10 μ π ι ) . S c a t t e r i n g at larger angles involves a d i s t i n c t d e p e n d e n c e u p o n refractive i n d e x , a n d


94

EMULSIONS IN THE PETROLEUM INDUSTRY SAMPLE

CD)

•

DIFFRACTED LIGHT DETECTORS /(small angles)

ABSORBED LIGHT DETECTORS (turbidity)


LIGHT SOURCE

SCATTERED LIGHT DETECTORS (large angles)

Figure 7. Schematic of a light-scattering apparatus. Three techniques are illustrated. First, attenuation of the incident light or turbid imetry can indicate the amount of dispersed phase but offers no information about the size distribution. Second, diffraction of the incident beam (Fraunhofer diffraction) offers size information for relatively large sizes when the particles are on the order of or larger than, the wavelength of the incident light. Third, scattering through larger angles (Mie scattering) occurs with particles smaller than the wavelength of the incident light. This large-angle scattering can be affected by the refractive index of the scattering centers. Computer data handling reduces these signals to a size distribution. Variations of these basic techniques involve detection of scattered light as a function of angle, correlation of the scattered photons (photon correlation spectroscopy), and detection of scattering as a function of wavelength or polarization of the incident light.

various manufacturers use either the p o s i t i o n o r w a v e l e n g t h d e p e n d e n c e o f the scattered light at larger angles. A s s u m p t i o n s about an " a v e r a g e " refrac tive i n d e x o r the nature o f the size d i s t r i b u t i o n ( b i m o d a l o r l o g - n o r m a l , f o r instance) must b e m a d e to d e t e r m i n e a size d i s t r i b u t i o n f r o m the l i g h t scattering i n f o r m a t i o n at larger angles. X-ray and Neutron Scattering. S m a l l - a n g l e X - r a y a n d n e u t r o n scat t e r i n g also c a n b e u s e d to p r o b e e m u l s i o n size d i s t r i b u t i o n s , b u t at a m u c h s m a l l e r r e s o l u t i o n , d o w n to the m o l e c u l a r l e v e l (about 4 - A r e s o l u t i o n w i t h n e u t r o n scattering) (64-67). T h i s l e v e l o f d e t a i l i n d e t e r m i n i n g m o l e c u l a r aggregates is c l e a r l y n o t a p p l i c a b l e to e m u l s i o n s c o m m o n l y e n c o u n t e r e d i n o i l extraction a n d p r o c e s s i n g situations, a l t h o u g h the p r i n c i p l e s are the same as f o r l i g h t - s c a t t e r i n g p h e n o m e n a .

Size Distribution by Physical Separation. Size d i s t r i b u t i o n s o b t a i n e d b y p h y s i c a l separations generally i n v o l v e systems o f s o l i d particles


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3.

Figure 8. Three typical Fraunhofer diffraction patterns. In polydisperse systems, the interpretation of the relationship between these patterns and the size distribution can be difficult and requires sensitive photomultipliers. The transmitted beam is blocked out, and the detectors are arranged outward from the center. In some cases a single detector has a movable mask to measure diffracted light intensity as a function of position. Subtle differences in size distribution (i.e., log-normal vs bimodal, etc.) cannot be distinguished, and generally some assumption must be input to the data reduction programs.


95

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that are not affected b y the h a n d l i n g r e q u i r e d . H o w e v e r , some n e w d e v e l o p ments i n h y d r o d y n a m i c c h r o m a t o g r a p h y , size e x c l u s i o n c h r o m a t o g r a p h y , a n d field-flow f r a c t i o n a t i o n m a y have applications to m i c r o e m u l s i o n systems a n d p e r h a p s to the larger e m u l s i o n s m o r e c o m m o n l y e n c o u n t e r e d i n the o i l field. D e t a i l e d d e s c r i p t i o n s o f these t e c h n i q u e s c a n b e f o u n d i n the l i t e r a t u r e (68). Chromatographic Techniques. H y d r o d y n a m i c a n d size e x c l u s i o n c h r o m a t o g r a p h y are s i m i l a r i n that they d e p e n d u p o n c o n v e n t i o n a l c h r o m a t o g r a p h i c p r i n c i p l e s o f flow (of particles o r droplets i n this case) i n a c a r r i e r fluid. I n size e x c l u s i o n c h r o m a t o g r a p h y , the larger particles exit the system first because they are not s l o w e d b y interactions w i t h pores i n the p a c k i n g m a t e r i a l . I n h y d r o d y n a m i c c h r o m a t o g r a p h y , the larger particles exit first because t h e y are too b i g to stay i n the s l o w c a r r i e r - f l u i d v e l o c i t y zones near the p a c k i n g o r at the walls o f the c h r o m a t o g r a p h i c c o l u m n . F i g u r e 9 illustrates these t e c h n i q u e s . T h e s e t e c h n i q u e s are most a p p l i cable to the separation o f m i c r o e m u l s i o n s , m i c e l l a r systems, or large m o l e cules. T y p i c a l size ranges u p to 1 μπι, a l t h o u g h h y d r o d y n a m i c c h r o m a t o g r a p h y has b e e n a p p l i e d to larger systems w h e n u s e d w i t h o u t p a c k i n g (up to 60 μπι) (69-72). U n s t a b l e systems cannot b e c h a r a c t e r i z e d w i t h these t e c h n i q u e s because o f interactions w i t h the c o l u m n o r p a c k i n g m a t e r i a l . T h e major advantage o f the t e c h n i q u e s is the p h y s i c a l separation o f the size fractions for f u r t h e r c h a r a c t e r i z a t i o n . Sedimentation Techniques. O t h e r t e c h n i q u e s that effect a p h y s i c a l separation i n c l u d e gravitational o r c e n t r i f u g a l s e d i m e n t a t i o n , i n w h i c h p a r t i cles o r e m u l s i o n droplets are separated o n the basis o f size a n d density. T h e separation that occurs c a n b e q u a n t i f i e d b y m o n i t o r i n g X - r a y o r l i g h t absorb a n c e as a f u n c t i o n o f p o s i t i o n . Stokes' l a w t h e n can be u s e d to d e t e r m i n e the p a r t i c l e size d i s t r i b u t i o n f r o m the absorbance data as a f u n c t i o n o f the s e d i m e n t a t i o n t i m e (73, 74). Field-Flow Fractionation. F i e l d - f l o w f r a c t i o n a t i o n ( F i g u r e 9) is l i k e the o t h e r p h y s i c a l separation t e c h n i q u e s except that a field is a p p l i e d at r i g h t angles to the flow; particles o r droplets are t h e r e b y separated d e p e n d i n g u p o n t h e i r i n t e r a c t i o n w i t h the field. T h e field can b e e l e c t r i c , m a g n e t i c , gravitational, t h e r m a l , o r w h a t e v e r force m i g h t interact w i t h the p a r t i c l e s . F r a c t i o n a t i o n u s i n g e i t h e r gravity o r c e n t r i f u g a l force is the p r i n c i p l e b e h i n d some c o m m e r c i a l l y available i n s t r u m e n t s (75-81).

Emulsion Stability D e t e r m i n i n g e m u l s i o n stability is one o f the most i m p o r t a n t tests that c a n b e p e r f o r m e d o n an e m u l s i o n . T h e ease w i t h w h i c h the o i l a n d w a t e r phases



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ο

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ο ο ο ο ο ο ο ο ο ο ο

ο

° Λ

°o° o ° °

οο οθ ο ο ο

ο

C

Figure 9. Three chromatographic techniques are mainly applicable to microemulsions and very stable systems because they depend upon interactions at surfaces and in pore spaces. A: In size exclusion chromatography, the sample is injected and is segregated in the column by virtue of the fact that the smaller particles interact and get held up in the small pore spaces, while the larger particles elute through more quickly. B: Hydrodynamic chromatography works because smaller particles and droplets can approach the column or capillary wall to near the boundary where carrier flow is essentially zero. The larger droplets are more strongly affected by the flow and elute more quickly than the smaller ones. C: Field-flow fractionation depends upon separation of flowing emulsion droplets in an applied field. A sample is injected into a carrier with an applied field perpendicular to the flow. The field is often just gravity but might be electrical or magnetic depending upon the nature of the emulsion. Several commercial instruments use sedimentation or gravity as the applied field. Subsequent detection of the droplets is often via a light-scattering type of technique.


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separate establishes the t r e a t m e n t p r o t o c o l f o r the e m u l s i o n a n d u l t i m a t e l y d e t e r m i n e s the cost o f treatment. D e t e r m i n a t i o n o f the most effective d e m u l s i f i e r f o r a p a r t i c u l a r process is g e n e r a l l y d o n e o f f - l i n e , a n d the effec tiveness o f a d e m u l s i f i e r d e p e n d s u p o n the degree o f d e s t a b i l i z a t i o n . T h e d e s t a b i l i z a t i o n is most o f t e n m o n i t o r e d b y s i m p l y o b s e r v i n g the phase sepa r a t i o n as a f u n c t i o n o f t i m e . W h e n a clear interface is present b e t w e e n the o i l a n d w a t e r layers, this m e t h o d c a n b e v e r y effective, b u t w h e n the separation is not so distinct, o p e r a t o r bias c a n affect the results. T o o v e r c o m e this bias, c e n t r i f u g a t i o n c a n b e u s e d to c l a r i f y the interface {82, 83), l i g h t scattering to automate the d e t e r m i n a t i o n o f separation {84, 85), o r m i c r o s c o p i c t e c h n i q u e s to m o n i t o r d r o p l e t coalescence {86-88).

Bottle Tests. T h e most c o m m o n m e t h o d o f d e t e r m i n i n g relative e m u l s i o n stability is the s i m p l e bottle test. T h e r e are p r o b a b l y as m a n y d i f f e r e n t bottle test p r o c e d u r e s as there are p e o p l e w h o r o u t i n e l y use t h e m . I n g e n e r a l they i n v o l v e d i l u t i o n o f the e m u l s i o n w i t h a solvent (to r e d u c e viscosity), s h a k i n g to h o m o g e n i z e the e m u l s i o n o r to m i x i n the d e m u l s i f i e r to be evaluated, a n d a w a i t i n g a n d w a t c h i n g p e r i o d d u r i n g w h i c h the extent o f phase separation is m o n i t o r e d a l o n g w i t h the c l a r i t y o f the interface a n d the t u r b i d i t y o f the w a t e r phase. D e p e n d i n g u p o n the viscosity o f the o r i g i n a l e m u l s i o n , the test may b e d o n e at elevated t e m p e r a t u r e s o r w i t h v a r y i n g amounts o f d i l u e n t . S e p a r a t i o n m i g h t also be e n h a n c e d b y c e n t r i f u g a t i o n , a l t h o u g h u s u a l l y not w h e n d i l u e n t is also a d d e d . F i g u r e 10 shows a bottle test as a f u n c t i o n o f t i m e i n w h i c h the u p p e r o i l phase steadily increases i n v o l u m e . T h i s type o f test p r o v i d e s a significant a m o u n t o f i n f o r m a t i o n r e l a t i n g to b o t h the stability o f the e m u l s i o n phase a n d the c l a r i t y o f the separated water.

Centrifugation. C e n t r i f u g a t i o n is a m o d i f i c a t i o n o f the bottle test i n w h i c h the s e d i m e n t a t i o n force is artificially i n c r e a s e d to effect separation. D i l u e n t m a y o r may not b e a d d e d d e p e n d i n g u p o n the stability o f the e m u l s i o n . S p e c i a l l y d e s i g n e d stroboscopic centrifuges m o n i t o r phase sepa r a t i o n as a f u n c t i o n o f t i m e ; t h e y p r o v i d e i n f o r m a t i o n exactly analogous to the b o t t l e test p r e v i o u s l y d e s c r i b e d (at h i g h e r gravity, o r g, forces) w h i l e c o n t i n u o u s l y o b s e r v i n g the sample interface. I n b o t h the s i m p l e b o t t l e test a n d the c e n t r i f u g e test, settling a n d separation o f the o i l a n d water phases are d e p e n d e n t u p o n the size o f the d i s p e r s e d phase a n d the viscosity o f the c o n t i n u o u s phase (Stokes' law). T h e r e f o r e , the u p p e r part o f the o i l phase o f t e n contains significantly less w a t e r t h a n the layer near the o i l - w a t e r i n t e r f a c e . D e p e n d i n g u p o n the process c o n d i t i o n s , the w a t e r content as a f u n c t i o n o f d e p t h a n d t i m e i n the o i l layer m i g h t b e a v e r y i m p o r t a n t p a r a m e t e r . T h e w a t e r content as a f u n c t i o n o f d e p t h i n the o i l layer c a n t h e n be d e t e r m i n e d b y s p i n o r c e n t r i f u g e tests (at h i g h s p e e d f o r w a t e r content d e t e r m i n a t i o n ) o r b y D e a n - S t a r k tests.



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Figure 10. This series of photographs from a bottle test shows the emulsion separating over time as evidenced by the steady increase in the upper oil phase. The lower water phase contains most of the solids but does not change in volume significantly. The interface emulsion in between the oil and water steadily decreases (destabilizes) in volume and resolves into the oil and water phases. As shown in Figure 1, these interface emulsions can have a complex morphology or structure.

Electrokinetics. B o t t l e tests a n d c e n t r i f u g a t i o n m a y b e s o m e w h a t c r u d e , b u t they d o o f f e r a relative measure o f e m u l s i o n stability that c o m b i n e s , to some extent, a l l o f t h e factors that affect stability. E l e c t r o k i n e t i c m e a s u r e m e n t s are s o m e w h a t m o r e elegant because they a l l o w d i r e c t m e a s u r e m e n t o f the degree o f electrostatic stability i n a n e m u l s i o n system. T h e zeta p o t e n t i a l , o r relative m a g n i t u d e o f the e l e c t r i c charge o n t h e surface, is


100


r e l a t e d to e m u l s i o n stability, as is the thickness o f the d o u b l e layer (the d i f f u s e layer o f c o m p e n s a t i n g charge). F o r electrostatically s t a b i l i z e d sys tems, the d o u b l e - l a y e r thickness c a n be as i m p o r t a n t i n d e t e r m i n i n g relative stability as the surface p o t e n t i a l as m e a s u r e d b y the zeta p o t e n t i a l .


A w i d e variety o f e l e c t r o k i n e t i c m e a s u r e m e n t s o r i n s t r u m e n t s c a n be u s e d to q u a n t i f y the electrostatic stability o f the d i s p e r s e d phase. T h e s e measurements w i l l only be summarized here. A d i s p e r s e d e m u l s i o n d r o p l e t (electrostatically stabilized) c a n b e t h o u g h t o f as a c h a r g e d c e n t e r w i t h a p l i a b l e cover o f c o m p e n s a t i n g charge d u e to o r i e n t a t i o n o f the w a t e r m o l e c u l e s (or c o u n t e r i o n s ) a r o u n d the d r o p let. W h e n exposed to an e l e c t r i c field, the d r o p l e t w i l l m o v e a c c o r d i n g to the surface charge. T h e m o t i o n o f the d r o p l e t i n an a p p l i e d e l e c t r i c field distorts the r e l a t i o n s h i p b e t w e e n the c h a r g e d c e n t e r a n d the o u t e r layer to create a d i p o l e . W i t h this s i m p l i s t i c v i e w , it is possible to u n d e r s t a n d the p r i n c i p l e b e h i n d e l e c t r o p h o r e t i c m o b i l i t y , w h e r e b y the relative m o t i o n o f particles o r e m u l sion droplets is m e a s u r e d w i t h an a p p l i e d e l e c t r i c field. T h e s e measure ments o f t e n d e p e n d u p o n m i c r o s c o p i c o b s e r v a t i o n o f the d r o p l e t m o t i o n i n the a p p l i e d e l e c t r i c field a n d a c a l c u l a t i o n o f d r o p l e t velocities to d e t e r m i n e t h e i r e l e c t r o p h o r e t i c m o b i l i t y . F i g u r e 11 is a schematic o f a t y p i c a l e x p e r i m e n t a l setup. B y o b s e r v i n g m a n y d r o p l e t s , the average e l e c t r o p h o r e t i c m o b i l i t y o r charge o n the e m u l s i o n c a n b e d e t e r m i n e d . F o r h i g h l y c h a r g e d systems, it m a y be possible to destabilize the d i s p e r s i o n b y adjusting p H . C o m p r e s s i o n o f the d o u b l e layer b y c h a n g i n g the c o n c e n t r a t i o n o f c o u n t e r i o n s c a n also destabilize e m u l s i o n s o r dispersions that are electrostatically s t a b i l i z e d . A l ternatively, the a d d i t i o n o f surface-active agents c a n b r i n g the system to its z e r o p o i n t o f charge, o r electrostatically d e s t a b i l i z e d state. T h e m e t h o d s f o r m a k i n g these m e a s u r e m e n t s range f r o m d i r e c t o b s e r v a t i o n , w h i c h c a n be v e r y tedious, to some f a i r l y a u t o m a t e d systems that c o u n t p a r t i c l e b y p a r t i c l e to give a d i s t r i b u t i o n o f e l e c t r o p h o r e t i c m o b i l i t i e s {44, 45). T h e s e a u t o m a t e d i n s t r u m e n t s are i n v a l u a b l e i n cases w h e r e mixtures o f d i s p e r s e d droplets a n d solids m i g h t have d i f f e r i n g e l e c t r o p h o r e t i c m o b i l i t i e s . F i g u r e 12 shows the e l e c t r o p h o r e t i c m o b i l i t y o f a p o p u l a t i o n o f o i l d r o p l e t s that have a significant average negative charge. T h e p h o t o g r a p h i n the same figure shows the b e h a v i o r o f these electrostatically s t a b i l i z e d p a r t i cles. T h e e l e c t r o p h o r e t i c m o b i l i t y o f the o i l droplets c a n b e m o d i f i e d i n a v a r i e t y o f ways b y a d d i n g o t h e r ions o r p o l y m e r s to affect the surface charge o r to n e u t r a l i z e i t . F i g u r e 13 illustrates the effect o f a c a t i o n i c p o l y m e r that neutralizes the charge somewhat a n d brings the droplets close to the z e r o p o i n t o f change. T h e a c c o m p a n y i n g p h o t o m i c r o g r a p h shows the treated particles coagulat ing.



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Settling Sample Potential Measurement

Β Figure 11. Schematic representation of the electrophoretic mobility (A) mea surement showing the major components. In an applied electric field, emulsion droplets move according to their surface charge. These charges can electrostat ically stabilize an emulsion system by preventing the droplets from coming into contact and coalescing. The motion of the droplets is visually observed, and the electrophoretic mobilities of a number of particles are measured to determine zeta potential. The sedimentation potential (B) is also illustrated.



102


Figure 12. Electrophoretic mobility of emulsified oil droplets. An electrostatically stabilized emulsion is shown in the photograph. The charge on the particles prevent them from approaching closely and agglomerating or coalescing. The droplets are electronegative, and therefore adding protons to the system (changing pH) can often bring the system to the zero point of charge and thus destabilize (at least electrostatically) the emulsion. The population versus electrophoretic mobility curves (determined with a Pen Kem 3000 instrument) show that the original emulsion is electrostatically stabilized (A). Lowering the pH (B) made the emulsion droplets less negative but did not bring them to the zero point of charge (i.e., did not destabilize the emulsion electrostatically). I n spite o f t h e d r o p l e t s b e i n g d e s t a b i l i z e d electrostatically, n o e v i d e n c e o f d r o p l e t coalescence is seen. B y t h e same t o k e n , a n electrostatically stabi l i z e d e m u l s i o n m i g h t still coalesce a n d separate, s e d i m e n t , o r c r e a m i f o t h e r d e s t a b i l i z i n g forces overbalance t h e electrostatic c o m p o n e n t . C r e a m i n g r e fers to c o n c e n t r a t i o n o f the d i s p e r s e d phase w i t h o u t c o m p l e t e l y separating the o i l a n d w a t e r phases. F i l m stability a n d i n t e r f a c i a l forces are i m p o r t a n t i n d e t e r m i n i n g e m u l sion stability a n d the l i k e l i h o o d o f c r e a m i n g o r c o m p l e t e separation o f the phases. C h a r a c t e r i z a t i o n o f these i n t e r f a c i a l effects is an i m p o r t a n t factor i n d e t e r m i n i n g the f u n d a m e n t a l p r o p e r t i e s that m i g h t u l t i m a t e l y d e t e r m i n e coalescence k i n e t i c s . S o m e relevant papers a n d reviews have b e e n p u b l i s h e d elsewhere (54, 89-96). T h e p r e s e n c e o f t h e e l e c t r i c d o u b l e layer a n d its d i s t o r t i o n w h e n d r o p lets m o v e is the p r i n c i p l e b e h i n d several r e l a t e d methods s u c h as the s e d i -



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Figure 13. Electrophoretic mobility (Pen Kem 3000) of the emulsion from Figure 12 after cationic polymer addition (A). The cationic polymer has neutralized the oil droplet surface charge and electrostatically destabilized the emulsion. The photomicrograph (B) shows this destabilized emulsion that has begun to flocculate or agglomerate but that is not coalescing. This electrostatic destabilization is not the only factor affecting emulsion stability. Factors such as interfacial tension andfilm strength can prevent coalescence of the emulsion droplets, even though they can now closely approach each other and agglomerate.

m e n t a t i o n p o t e n t i a l t e c h n i q u e s (also r e p r e s e n t e d i n F i g u r e 11). O n e f a i r l y n e w t e c h n i q u e that deserves s p e c i a l m e n t i o n is the e l e c t r o s o n i c a m p l i f i e r , w h i c h c a n m e a s u r e e l e c t r i c a l c u r r e n t s i n a n e m u l s i o n that is s o n i c a l l y agi t a t e d (and t h e r e b y create d i p o l e s i n t h e e l e c t r i c d o u b l e layer) o r measure t h e s o u n d w a v e g e n e r a t e d w h e n t h e e m u l s i o n is e l e c t r i c a l l y s t i m u l a t e d . T h e advantage o f this t e c h n i q u e is that n e i t h e r d i l u t e solutions n o r transparent systems are r e q u i r e d (to a l l o w f o r d i r e c t o b s e r v a t i o n o f the d i s p e r s e d phase). U n f o r t u n a t e l y , t h e r e l a t i o n s h i p b e t w e e n t h e m e a s u r e d signal a n d e l e c t r o -


104


p h o r e t i c m o b i l i t y o f the e m u l s i o n is n o t as s t r a i g h t f o r w a r d as i n c o n v e n t i o n a l m i c r o e l e c t r o p h o r e s i s that e m p l o y s d i r e c t o b s e r v a t i o n (97).


Microscopy M i c r o s c o p y is o f t e n the last w o r d i n t h e d e t e r m i n a t i o n o f t h e size d i s t r i b u t i o n o f d i s p e r s e d systems (98-101). T h r o u g h o u t the l i t e r a t u r e , d i s t r i b u t i o n s o b t a i n e d b y various p a r t i c l e a n d e m u l s i o n s i z i n g t e c h n i q u e s are c o m p a r e d to the values d e t e r m i n e d b y m i c r o s c o p y (13, 102-107). E s t a b l i s h i n g a r e p r e sentative sample is a c o n c e r n f o r a l l o f t h e t e c h n i q u e s discussed a n d is not necessarily a p a r t i c u l a r p r o b l e m f o r m i c r o s c o p i c observation, a l t h o u g h this c r i t i c i s m is o f t e n g i v e n f o r the m i c r o s c o p i c m e t h o d s . I n d e e d , m a n y o f the sample h a n d l i n g c o n c e r n s discussed i n this section a p p l y e q u a l l y to samples prepared for other techniques. I n s o l i d p a r t i c u l a t e systems, d i r e c t observation is justifiably t h e last w o r d . I n e m u l s i o n s w h e r e c r e a m i n g , s e d i m e n t a t i o n , a n d coalescence c a n change the nature o f the s a m p l e , m i c r o s c o p i c o b s e r v a t i o n has u n i q u e sample h a n d l i n g p r o b l e m s . I f these s p e c i a l s a m p l i n g p r o b l e m s are addressed, t h e n m i c r o s c o p y c a n i n d e e d p r o v i d e t h e b e n c h m a r k for t h e p h y s i c a l c h a r a c t e r i z a t i o n o f the d i s p e r s e d phase i n e m u l s i o n systems. F i g u r e 14 shows a m u l t i p l e e m u l s i o n easily c h a r a c t e r i z e d w i t h o p t i c a l m i c r o s c o p y i n t h e fluorescent m o d e . O t h e r techniques are n o t capable o f d i s t i n g u i s h i n g this e m u l s i o n f r o m a s i m p l e o i l - i n - w a t e r e m u l s i o n w i t h a m u c h larger size d i s t r i b u t i o n . T h e c o m p l e x m a t h e m a t i c a l treatments f o r l i g h t - s c a t t e r i n g experiments and the experimental complexities of some o f the other characterization t e c h n i q u e s m e a n that, i n g e n e r a l , greater care is taken i n t h e i n t e r p r e t a t i o n o f the results a n d operators are aware o f p o t e n t i a l data r e d u c t i o n p r o b l e m s . I n m i c r o s c o p y , because " s e e i n g is b e l i e v i n g " , the t e n d e n c y is to i g n o r e s a m p l i n g p r o b l e m s a n d to reach c o n c l u s i o n s that are sometimes b a s e d o n s a m p l i n g artifacts o r p e c u l i a r i t i e s o f the m i c r o s c o p i c o b s e r v a t i o n t e c h n i q u e . A s l o n g as t h e possible p r o b l e m s are k n o w n , m i c r o s c o p y c a n b e r e g a r d e d as the single most i m p o r t a n t e m u l s i o n c h a r a c t e r i z a t i o n t o o l . I n the a p p r o p r i a t e c i r c u m s t a n c e s i t c a n give i n f o r m a t i o n about t h e relative amounts o f o i l , water, a n d solids i n a n e m u l s i o n system; t h e i r interactions o r associations; the size d i s t r i b u t i o n o f the d i s p e r s e d phase; a n d the rate o f coalescence o f the d i s p e r s e d d r o p l e t s . V a r i o u s m i c r o s c o p i c t e c h n i q u e s c a n be u s e d to define n o t o n l y t h e p h y s i c a l nature o f t h e sample, b u t also the c h e m i c a l c o m p o s i t i o n , b o t h m i n e r a l a n d organic.

Optical Microscopy. O p t i c a l m i c r o s c o p y involves t h e use o f trans m i t t e d l i g h t , r e f l e c t e d l i g h t , p o l a r i z e d l i g h t , fluorescence, a n d m o r e r e cently, t e c h n i q u e s s u c h as c o n f o c a l m i c r o s c o p y . E a c h o f these variations has p a r t i c u l a r strengths a n d a p p l i c a b i l i t y .



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Figure 14. Optical micrograph using reflected fluorescent light showing a multiple emulsion that is extremely difficult to characterize by conventional techniques. The continuous water phase (W, dark) shows a large dispersed oil droplet (O, bright) that contains a water droplet that also contains emulsified oil. The arrow points out an oil-in-water in oil-in-water emulsion droplet. Characterization of these multiple emulsions can be accurately carried out only with microscopic techniques. T r a n s m i t t e d - l i g h t m i c r o s c o p y requires a s a m p l e sufficiently t h i n to a l l o w l i g h t to pass t h r o u g h i t . T h i s r e q u i r e m e n t is o f t e n a c c o m p l i s h e d b y s i m p l y s m e a r i n g t h e e m u l s i o n sample o n a s l i d e . C a r e m u s t b e t a k e n to ensure that t h e slide is p r o p e r l y p r e p a r e d to accept the c o n t i n u o u s phase. A h y d r o p h i l i c glass surface, for instance, c a n i n v e r t an o i l - c o n t i n u o u s e m u l s i o n to a w a t e r - c o n t i n u o u s o n e . C o r r e c t d e t e r m i n a t i o n o f s o m e t h i n g as basic as the nature o f the c o n t i n u o u s phase c a n t h e r e f o r e b e d i f f i c u l t w i t h e m u l s i o n s that are unstable. C a r e f u l observation o f e m u l s i o n b e h a v i o r u s i n g b o t h h y d r o p h i l i c a n d o l e o p h i l i c s a m p l e h o l d e r s is sometimes r e q u i r e d t o deter m i n e t h e effect o f e m u l s i o n interactions w i t h t h e s a m p l e h o l d e r s . W h e n e m u l s i o n i n s t a b i l i t y makes s a m p l e c o l l e c t i o n a n d observation d i f f i c u l t , fast f r e e z i n g t h e e m u l s i o n a n d subsequent observation o f the f r o z e n s a m p l e c a n a v o i d e m u l s i o n changes d u e to sample p r e p a r a t i o n a n d h a n d l i n g . T h e t r a n s m i t t e d - l i g h t t e c h n i q u e is l i m i t e d b y the o p a q u e nature o f most o i l samples a n d , i n cases w h e r e t h e sample cannot b e m a d e t h i n e n o u g h , a n alternative t e c h n i q u e u s i n g r e f l e c t e d l i g h t is available. F i g u r e 15 is a s c h e m a t i c o f the e x p e r i m e n t a l setup f o r r e f l e c t e d - l i g h t m i c r o s c o p y . B y use o f r e f l e c t e d l i g h t , t h e sample c a n s i m p l y b e p u t i n a s m a l l



106

OBSERVATION BY EYE, CAMERA OR PHOTOMULTIPLIER

POLARIZER AND/OR FILTERS

POLARIZER AND/OR FILTERS


\

Or-

- BEREK PRISM

FINAL OBJECTIVE LENS

Figure 15. Schematic of the optical microscope in reflected-light mode. Air or oil immersion objectives may be used. Oil immersion objectives require a glass cover slip over the sample and an oil drop of appropriate refractive index to bridge the gap between the objective lens and the sample cover glass; this setup has the advantage of much higher resolution. The light source can be plain or polarized white light (tungsten lamp) for observation of solids, or appropriately filtered blue-violet light (high-pressure mercury lamp) to excite fluorescence of the oil phase. To investigate fluorescence behavior, the reflected blueviolet or ultraviolet light is filtered out, and only the fluorescent light (longer wavelength) is returned to the detector. Other techniques such as dark-field illumination allow particles to be counted and not sized. The droplets are seen only as points of light on a dark background. c o n t a i n e r o r w e l l s l i d e . O f t e n a c o v e r slip is p u t over t h e s a m p l e , a n d a n o i l i m m e r s i o n lens is u s e d . T h i s setup allows o n e to focus b e y o n d t h e c o v e r s l i d e and

observe t h e s a m p l e past t h e l e v e l o f t h e c o v e r - s l i p - s a m p l e i n t e r f a c e

w h e r e a i r b u b b l e s are c o m m o n l y e n t r a p p e d . F i g u r e 16 shows a i r b u b b l e s



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Figure 16. Photomicrograph (white light, reflected mode) illustrating trapped air bubbles at the cover-slip—sample interface. These can easily be mistaken for the dispersed phase, especially in transmitted-light mode.

t r a p p e d at the sample-glass interface that m i g h t b e m i s t a k e n f o r d i s p e r s e d phase d r o p l e t s , especially i f t r a n s m i t t e d l i g h t is u s e d . C l a y s a n d o t h e r solids are o f t e n transparent to w h i t e l i g h t , a n d i n these cases, p o l a r i z e d light can be u s e d to observe the clays. T o e n h a n c e observa t i o n o f the o i l , the fluorescence b e h a v i o r o f the organic phase is u s e d . T h i s a p p r o a c h involves i n c i d e n t l i g h t o f violet o r ultraviolet w a v e l e n g t h a n d o b servation o f the fluorescent l i g h t i n the v i s i b l e r e g i o n . T h e i n c i d e n t r e f l e c t e d b e a m is filtered out, a n d the r e t u r n i n g l i g h t is d u e to the fluorescent behav i o r o f the o i l phase. F i g u r e 17 shows the fluorescence o f b i t u m e n associated w i t h clays i n contrast to the b e h a v i o r o f b i t u m e n that is relatively clay free. T h e d i f f e r i n g fluorescent b e h a v i o r m i g h t b e i n d i c a t i v e o f a p a r t i c u l a r o i l c o m p o n e n t that p r e f e r e n t i a l l y associates w i t h the m i n e r a l phase. T h i s fluorescence t e c h n i q u e is m a i n l y a p p l i c a b l e to o i l - i n - w a t e r e m u l sions i n w h i c h the o i l phase appears as b r i g h t spots i n a dark b a c k g r o u n d (because the w a t e r does not fluoresce). T h e effect is i l l u s t r a t e d i n F i g u r e s 18 a n d 19. F i g u r e 18 shows an o i l - i n - w a t e r e m u l s i o n u n d e r w h i t e l i g h t . A l t h o u g h the clays a n d other solids are v i s i b l e , there is l i t t l e o r no e v i d e n c e o f the d i s p e r s e d o i l phase. F i g u r e 19 illustrates that the o i l phase appears b r i g h t a n d is m u c h easier to resolve w i t h fluorescent l i g h t . T h e o i l droplets can be seen to b e quite distinct f r o m the c o n t i n u o u s w a t e r phase (although the clays are n o w i n v i s i b l e ) . T h i s type o f i m a g e is p a r t i c u l a r l y suitable f o r i m a g e analysis a n d a u t o m a t e d d r o p l e t c o u n t i n g a n d size c h a r a c t e r i z a t i o n . A p o t e n t i a l p r o b l e m w i t h o p t i c a l m i c r o s c o p y , especially w i t h h i g h - i n t e n sity m e r c u r y v a p o r lamps (for b l u e - v i o l e t i n c i d e n t light) is l o c a l i z e d sample h e a t i n g . W i t h s o m e m a r g i n a l l y stable e m u l s i o n s , the h e a t i n g effect c o u l d be e n o u g h t o break the e m u l s i o n . T h i s effect is i l l u s t r a t e d i n the series o f p h o t o g r a p h s i n F i g u r e 20. T h e first image is o f a m u l t i p l e e m u l s i o n (water-



108


Figure 17. Fluorescence behavior of the organic phase can be an important indicator of the composition of the oil phase or of oil-solid interactions. These oil components (bitumen associated with clays versus bitumen that is clay-free) clearly differ in fluorescence behavior, a result indicating different organicphase compositions or different oil-solid interactions. In this black-and-white reproduction, the oil component that fluoresces blue (B) appears brighter than the component that is fluorescing yellow (Y). The free organic component exhibits the blue fluorescence; the organic material associated with clays fluoresces yellow.

Figure 18. White-light (polarized) photomicrograph in reflected mode of an oil-in-water emulsion with a significant solids content. With polarized light, the clays (C) appear bright, but the oil droplets cannot be seen at all.



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Figure 19. Reflected-light photomicrograph of the same field of view as Figure 18 in the fluorescence mode showing bright oil droplets in a dark watercontinuous phase. In this photograph the clays cannot be seen. This type of image with high contrast between the phases is ideal for automated analysis. However, droplets not exactly in focus (O) may be incorrectly sized.

i n - o i l - i n - w a t e r ) u n d e r w h i t e l i g h t that is b r o k e n after a short o b s e r v a t i o n p e r i o d u n d e r b l u e - v i o l e t l i g h t (fluorescence m o d e ) . T h e availability o f low-cost c o m p u t i n g a n d image analysis c o m p a t i b i l i t y has h e l p e d to r e d u c e t h e t i m e i n v o l v e d i n q u a n t i f i c a t i o n o f m i c r o s c o p i c analysis to d e t e r m i n e size d i s t r i b u t i o n . T h e c o m p a r i s o n o f images to r u l e r s p h o t o g r a p h e d u n d e r t h e same c o n d i t i o n s a n d t h e use o f s p l i t - i m a g e m i c r o scopes (105-107) have largely b e e n r e p l a c e d b y a u t o m a t e d image-analysis t e c h n i q u e s . M o s t suppliers o f image-analysis e q u i p m e n t o f f e r p r o g r a m s o r routines t o separate o r " d e a g g l o m e r a t e " t h e spheres w h e n o i l droplets are t o u c h i n g o r agglomerated. T h e s e p r o g r a m s a n d t h e a b i l i t y to a u t o m a t i c a l l y size e m u l s i o n d r o p l e t s greatly reduces t h e t e d i u m o f size analysis b y m i c r o scopic m e t h o d s . T o b e c o n f i d e n t o f an average size analysis w i t h i n 10%, a p p r o x i m a t e l y 150 particles s h o u l d b e s i z e d . T o increase t h e c o n f i d e n c e l e v e l to 5 % , a p p r o x i m a t e l y 7 4 0 particles s h o u l d b e s i z e d (104). O f course these n u m b e r s are o n l y a r o u g h g u i d e , a n d t h e actual c o n f i d e n c e levels w i l l d e p e n d u p o n the nature o f the size d i s t r i b u t i o n . F i g u r e 2 1 shows a p r o b l e m that c a n o c c u r w h e n t o o m u c h r e l i a n c e is p l a c e d o n a u t o m a t e d i m a g e analysis, n a m e l y , i n a c c u r a t e s i z i n g o f droplets that are slightly o u t o f focus. T h e fields o f v i e w to b e a n a l y z e d , e i t h e r m a n u a l l y o r w i t h a u t o m a t e d m e t h o d s , have to b e chosen carefully.




110

Figure 20. Heating of an emulsion sample under blue-violet light. The top photograph shows a water-in-oil-in-water emulsion under white light in the reflected mode. After a short observation period under blue-violet light (middle), the multiple emulsion is broken and only the oil-in-water component remains (bottom).



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Figure 21. White-light (top) and blue-light fluorescence mode (bottom) photomicrographs of a water-in-oil emulsion. With white light the water droplets have internal reflections that lead to a halo effect and an incorrect size estimate. With incident blue-violet light to excite oil-phase fluorescence, the emulsified water droplets appear as dark circles in a bright oil background and are significantly easier to size. However, droplets that are above or below the plane of focus will still be incorrectly sized.

B e c a u s e o f the interactive nature o f the m i c r o s c o p i c t e c h n i q u e , i n o t h e r w o r d s , t h e h u m a n factor, there c a n b e differences i n size analyses b y d i f f e r ent operators, a n d o p e r a t o r bias to e i t h e r s m a l l o r large particles. M i s s i n g large particles affects t h e mass d i s t r i b u t i o n , a n d n e g l e c t i n g s m a l l particles affects t h e n u m b e r d i s t r i b u t i o n . O f course these concerns are not l i m i t e d to m i c r o s c o p i c analysis, i f size d i s t r i b u t i o n s are b i a s e d .



112


Particles s m a l l e r than about 0.5 μπι b e g i n to a p p r o a c h the r e s o l u t i o n l i m i t o f the o p t i c a l m i c r o s c o p e , a n d o f t e n particles c a n be r e c o g n i z e d b u t not s i z e d p r o p e r l y because o f l i m i t a t i o n s i n b o t h the r e s o l u t i o n a n d the d e p t h o f field o r focus i n the o p t i c a l system. T h e c o n f o c a l m i c r o s c o p e solves some o f these p r o b l e m s a n d adds a n e w d i m e n s i o n to o p t i c a l m i c r o s c o p i c analysis. T h e c o n f o c a l m i c r o s c o p e digitizes the i n t e n s i t y i n f o r m a t i o n i n a field o f v i e w a n d , b y adjusting the focus, makes it possible to r e c o n s t r u c t an i m a g e that is i n focus over a significant d e p t h i n a s a m p l e . T h r o u g h this r e c o n s t r u c t i o n , the o p t i c a l i m a g e that c a n be p r o d u c e d p r o v i d e s m o r e i n f o r m a t i o n about associations b e t w e e n the water, o i l , a n d s o l i d phases. F i g u r e 22 shows a series o f c o n f o c a l m i c r o s c o p e p h o t o graphs o f a t y p i c a l interface e m u l s i o n . T h e d i a m e t e r o f the o i l d r o p l e t increases as the p l a n e o f focus passes i n sections t h r o u g h the d r o p l e t .

Electron Microscopy. B o t h s c a n n i n g a n d t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y have b e e n u s e d extensively to characterize e m u l s i o n sys tems (107-110). T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y is somewhat less c o m m o n a n d almost i n v a r i a b l y involves the observation o f replicas o r m e t a l r e p r o d u c t i o n s o f the e m u l s i o n s a m p l e . S c a n n i n g e l e c t r o n m i c r o s c o p y ( S E M ) is m u c h m o r e analogous to c o n v e n t i o n a l o p t i c a l m i c r o s c o p y . T h e s c a n n i n g e l e c t r o n m i c r o s c o p e offers significant advantages over the o p t i c a l m i c r o scope i n terms o f d e p t h o f field a n d r e s o l u t i o n . H o w e v e r , the v a c u u m e n v i r o n m e n t a n d energy d e p o s i t e d b y the e l e c t r o n b e a m means that sample h a n d l i n g a n d p r e p a r a t i o n are m u c h m o r e d i f f i c u l t . T h e t w o m a i n t e c h n i q u e s c o m m o n l y discussed i n the l i t e r a t u r e are k n o w n as d i r e c t observation (or f r o z e n h y d r a t e d observation) a n d the o b servation o f r e p l i c a s . B o t h t e c h n i q u e s i n v o l v e the fast f r e e z i n g o f the sample i n a c r y o g e n s u c h as l i q u i d n i t r o g e n , p r o p a n e , o r f r e o n . T h e f r o z e n s a m p l e is t h e n f r a c t u r e d to reveal the i n t e r i o r features. T h i s f r a c t u r e d surface c a n be c o a t e d w i t h a m e t a l film o r o b s e r v e d d i r e c t l y . O f t e n , the m e t a l film is r e m o v e d f r o m the sample a n d o b s e r v e d as a r e p l i c a . T h i s type o f p r o c e d u r e allows the c r e a t i o n o f a p e r m a n e n t archive o f the samples p r e p a r e d , a n d the o b s e r v a t i o n is the same as w i t h any o t h e r e l e c t r o n m i c r o s c o p e sample w i t h no c o n c e r n about c o n t a m i n a t i o n o f the m i c r o s c o p e o r b e a m damage to the sample. T r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , w i t h f e w exceptions, involves the c r e a t i o n o f replicas because it depends u p o n the e l e c t r o n b e a m passing t h r o u g h the s a m p l e , w i t h regions o f l o w o r h i g h density a p p e a r i n g as b r i g h t a n d d a r k areas. W h e n replicas are u s e d i n t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , the m e t a l o r c a r b o n r e p l i c a is s h a d o w e d w i t h a second m e t a l to accentuate the t o p o g r a p h y o f the e m u l s i o n o n the r e p l i c a . T h e s e m e t h o d s are o u t l i n e d i n F i g u r e 23. S c a n n i n g e l e c t r o n m i c r o s c o p y is relatively s i m p l e c o m p a r e d to t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , a n d the images o b t a i n e d are significantly easier to


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3.

Figure 22. This series of confocal micrographs are composites of both polarized white light (on the left) to show the clays and of fluorescent light (on the right) to show the oil component. The confocal microscopic technique allows digitization of the data from images focused at discrete depths in the sample. This feature gives the effect of observing slices of the sample at successive intervals of depth. The increase in apparent size of the large oil droplet is due to slices being taken progressively closer to the center of the droplet. This technique makes it easier to characterize the relationship between the solids and the dispersed phases. Computer reconstruction of the slices can give a threedimensional effect (greater depth of field) similar to that obtained with scanning electron microscopy. (Photographs taken by V. A. Munoz and W. W. Lam of CANMET, at the Ontario Laser and Light Wave Research Centre.)



114


Figure 23. The freeze-fracture sample preparation technique and the six main steps in preparing a sample for electron microscopy. The first step is freezing the sample rapidly enough to prevent large ice crystal formation and resulting sample distortion. After the sample is frozen, it is fractured to reveal the interiorfeatures. This fractured sample can then he put into an electron microscope with appropriate cryogenic capability (frozen hydrated or direct observation). More commonly a replica of the sample is created by coating the fractured surface with metal, shadowing with a second metal, dissolving away the original sample, and then observing the replica in a conventional electron microscope. For transmission electron microscopy, in which the image formation depends upon differences in sample density, the replica must be shadowed, or coated directionally with metal to provide a density contrast in the peaks and valleys of the emulsion replica. During the freezing and metal coating steps, the sample must be kept in a vacuum environment to keep it frozen and to prevent frost deposition. Light frost can sometimes be mistaken for the dispersed phase. With direct observation, the sample can be carefully warmed (to about 130-150 K), and any frost layer can be sublimed away. This step obviously cannot be done with a replica. Another advantage of direct observation is the X-ray information that can help identify the composition of the various emulsion components.


3.

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Emulsion

Characterization

115


i n t e r p r e t . I n a d d i t i o n , the d i r e c t o b s e r v a t i o n o p t i o n gives the p o t e n t i a l f o r m u c h m o r e i n f o r m a t i o n about the c h e m i c a l c o m p o s i t i o n o f the e m u l s i o n . F i g u r e s 24 a n d 25 s h o w t y p i c a l w a t e r - i n - o i l e m u l s i o n s s t u d i e d by f r o z e n h y d r a t e d observation i n a s c a n n i n g e l e c t r o n m i c r o s c o p e . W i t h d i r e c t o b s e r v a t i o n , the s a m p l e m u s t b e k e p t c o l d i n the e l e c t r o n m i c r o s c o p e , a n d care is r e q u i r e d to p r e v e n t s a m p l e damage i n the b e a m a n d to p r e v e n t m i c r o s c o p e c o n t a m i n a t i o n . I n a d d i t i o n , these f r o z e n samples are often d i f f i c u l t to i m a g e because o f c h a r g i n g effects that distort the i m a g e . T h e benefit o f this extra care i n s a m p l e h a n d l i n g , h o w e v e r , is that e l e c t r o n b e a m interactions w i t h the sample p r o d u c e characteristic X - r a y signals that a l l o w i d e n t i f i c a t i o n o f c o m p o n e n t s o f the e m u l s i o n b e i n g o b s e r v e d . T h i s t e c h n i q u e has b e e n r e f i n e d to the p o i n t w h e r e , i n s p e c i a l cases, c h e m i c a l c o m p o s i t i o n a l differences at the e m u l s i o n interface can b e i d e n t i f i e d , as w e l l as the c o m p o s i t i o n o f the d i s p e r s e d a n d c o n t i n u o u s phases (109, 110). F i g u r e 26 shows a n o i l - i n - w a t e r e m u l s i o n w i t h c o r r e s p o n d i n g X - r a y spectra o f the c o n t i n u o u s a n d d i s p e r s e d phases a n d o f the interface itself. F i g u r e 2 7 illustrates the r e s o l u t i o n i m p r o v e m e n t s o f S E M o b s e r v a t i o n r e l a tive to o p t i c a l m i c r o s c o p y . F i g u r e 28 shows c o r r e s p o n d i n g X - r a y spectra that i d e n t i f y the droplets as o i l a n d suggest that t h e y m a y b e s t a b i l i z e d b y fine clay p a r t i c l e s . H i g h - s p e e d c o m p u t e r s a n d the a b i l i t y to d i g i t i z e e l e c t r o n signals at v i d e o rates m e a n that, i n spite o f p o o r i n i t i a l i m a g e q u a l i t y i n d e a l i n g w i t h d i r e c t o b s e r v a t i o n o f f r o z e n h y d r a t e d samples, several r e l a t i v e l y noisy i m -

Figure 24. Electron micrograph showing the relatively featureless surface and the fractured interior of a water-in-oil emulsion. This image was prepared with a metal-coated frozen sample, a modification of direct observation in which the sample is coated sufficiently to prevent sample charging but not enough to produce a replica. This technique still requires an electron microscope with cryogenic capability.



Figure 25. Electron micrographs showing three typical views of a water-in-oil emulsion by direct observation. The resolution and depth of field are significantly better than can be achieved via optical microscopy.



3.

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Emulsion

Characterization

117

Figure 26A. Electron micrograph of an oil-in-water emulsion. X-ray spectra of the dispersed and continuous phases and the interface are shown in Figures 26B and 26C. The chemical composition of the interface itself can also be characterized. The resolution of the image is much greater than the resolution of the X-ray information. The spots marking the X-ray acquisition points approximately represent the area where the X-rays are produced.

ages ean be averaged to r e d u c e the noise l e v e l a n d p r o d u c e an i m a g e that is close i n q u a l i t y to that o b t a i n e d f r o m replicas. T h e o p t i c a l a n d e l e c t r o n m i c r o s c o p i c t e c h n i q u e s are q u i t e c o m p l i m e n tary i n terms o f the i n f o r m a t i o n that they c a n p r o v i d e . O p t i c a l m i c r o s c o p y , i n fluorescence m o d e o r w i t h p o l a r i z e d light, c a n p r o v i d e i n f o r m a t i o n about the organic phases i n the e m u l s i o n . E l e c t r o n m i c r o s c o p y , t h r o u g h the X - r a y s excited i n the sample, can p r o v i d e i n f o r m a t i o n about the i n o r g a n i c o r m i n e r a l phases present. T h e p r a c t i c a l l o w e r l i m i t o f e m u l s i o n s i z i n g w i t h o p t i c a l m i c r o s c o p y is o n the o r d e r o f 0.5 μπι. T h i s l i m i t is m u c h l o w e r w i t h e l e c t r o n m i c r o s c o p y , o n the o r d e r o f 0.1 μπι o r less w i t h d i r e c t observation o f f r o z e n samples i n a s c a n n i n g e l e c t r o n m i c r o s c o p e , a n d 0.01 μπι o r less w i t h replicas a n d trans m i s s i o n e l e c t r o n m i c r o s c o p y . Sizes s m a l l e r t h a n these l o w e r l i m i t s c a n b e r e c o g n i z e d w i t h each o f these t e c h n i q u e s , b u t q u a n t i f i c a t i o n o f the size d i s t r i b u t i o n b e c o m e s d i f f i c u l t . F u r t h e r m o r e , at levels o f about 0.01 μπι, it is e x t r e m e l y d i f f i c u l t to a v o i d artifacts a n d subsequent m i s i n t e r p r e t a t i o n s . A s m e n t i o n e d earlier, sample p r e p a r a t i o n is an e x t r e m e l y i m p o r t a n t c o n s i d e r ation i n b o t h o p t i c a l a n d e l e c t r o n m i c r o s c o p i c t e c h n i q u e s . W i t h o p t i c a l


118


Water


S

Oil

S|

10

0

X-Ray Energy (keV) Figure 26B. X-ray spectra of the dispersed and continuous phases and interface of the oil-in-water emulsion in Figure 26A doped with calcium. The top X-ray spectrum of the continuous phase shows no X-ray peaks, only background bremsstrahlung radiation, because this particular detector is not sensitive to the oxygen in the water phase. The bottom spectrum shows only a sulfur peak typical of many bitumens and heavy oils. The middle spectrum is of the interface and clearly shows chlorine and calcium (in this part) or iron (in Figure 26C), which are not present in either the dispersed or continuous phases. The chlorine is present in the emulsifier that was used to prepare this emulsion.



3.

MIKULA

Emulsion

Characterization

119

10

X-Ray Energy (keV) Figure 26C. X-ray spectra of the dispersed and continuous phases and interface of the oil-in-water emulsion in Figure 26A doped with iron. See also the caption to Figure 26B. m i c r o s c o p y , interactions o f t h e sample w i t h t h e sample h o l d e r c a n affect w h i c h phase is o b s e r v e d as c o n t i n u o u s a n d , w i t h e l e c t r o n m i c r o s c o p y , a r t i facts d u e to t h e f r e e z i n g process c a n affect i n t e r p r e t a t i o n o f t h e r e sults (108). M i c r o s c o p i c techniques offer t h e p o t e n t i a l f o r c o m p l e t e e m u l s i o n c h a r a c t e r i z a t i o n because they are capable o f q u a n t i f y i n g v o l u m e t r i c a l l y the relative amounts o f o i l , water, a n d solids present, d e t e r m i n i n g t h e size d i s t r i b u t i o n o f the d i s p e r s e d phase, a n d d e t e r m i n i n g some c h e m i c a l c o m p o sitional i n f o r m a t i o n about b o t h t h e organic a n d i n o r g a n i c c o m p o n e n t s .



120


Figure 27. Electron micrograph of discrete oil droplets on the interior surface of an air bubble. These oil droplets are less than 1 μπι in diameter and illustrate the high resolution possible using frozen hydrated observation. H o w e v e r , t h e researcher must b e aware o f the l i m i t a t i o n s o f this t e c h n i q u e i n terms o f sample h a n d l i n g a n d p r e p a r a t i o n , a n d o f the v e r y r e a l d a n g e r o f o v e r i n t e r p r e t i n g images once t h e y are a c q u i r e d .

New Developments N e w d e v e l o p m e n t s i n e m u l s i o n c h a r a c t e r i z a t i o n c a n s i m p l y m e a n recent applications o f w e l l - e s t a b l i s h e d technologies to e m u l s i o n systems o r t h e a p p l i c a t i o n o f u n c o n v e n t i o n a l m e t h o d s that, a l t h o u g h not i n w i d e s p r e a d use, m a y b e w e l l established i n p a r t i c u l a r operations. S e v e r a l o f t h e t e c h n i q u e s d i s c u s s e d p r e v i o u s l y c o u l d have b e e n assigned to this section; conversely, some o f those discussed h e r e m i g h t n o t b e r e g a r d e d as n e w b y those w h o m a y b e u s i n g these t e c h n i q u e s extensively. A d m i t t e d l y , t h e d i s t i n c t i o n is p a r t l y a r e f l e c t i o n o f m y o w n bias.

Nuclear Magnetic Resonance Spectroscopy. N u c l e a r m a g n e t i c resonance ( N M R ) spectroscopy offers several i n t r i g u i n g possibilities to i d e n t i f y w a t e r " s t r u c t u r e " , o r the o r d e r e d a r r a n g e m e n t o f w a t e r m o l e c u l e s at a n e m u l s i o n o r s o l i d surface. T h i s type o f i n f o r m a t i o n m i g h t h e l p i n u n d e r s t a n d i n g t h e differences b e t w e e n e m u l s i f i e d w a t e r a n d c o n t i n u o u s phase water, especially i n those e m u l s i o n s that c o n t a i n p o r t i o n s o f b o t h . I n f o r m a t i o n o n t h e range o f structure i n t h e w a t e r phase helps i n u n d e r s t a n d i n g t h e effective size o f some d i s p e r s e d a n d s o l i d phases i n e m u l s i o n systems a n d t h e r e f o r e some o f the factors a f f e c t i n g t h e i r stability.



3.

MIKULA

J2 &

Emulsion

Characterization

0

121

10

10

c φ

QC X

X-Ray Energy (keV) Figure 28. X-ray spectra of the small oil droplets in Figure 27. The upper spectrum was acquired with an incident beam energy of 10 keV. At this energy the electrons do not significantly penetrate the dispersed oil phase, and the Xray signal shows a high sulfur component typical of a heavy oil. At 15 keV the electrons penetrate the droplet, and the X-ray signal comes from behind the oil droplet. This spectrum shows significant Al and Si; hence, fine clays may play a role in stabilizing this emulsion.

N u c l e a r m a g n e t i c resonance spectroscopy c a n p r o b e b u l k p r o p e r t i e s o f e i t h e r t h e w a t e r (via p r o t o n Ν M R spectroscopy) o r t h e o r g a n i c phases (via c a r b o n - 1 3 Ν M R spectroscopy). O n - l i n e sensors have b e e n d e v e l o p e d to d e t e r m i n e o i l a n d w a t e r c o n t e n t i n c e r t a i n e m u l s i o n systems, a l t h o u g h the N M R t e c h n i q u e r e q u i r e s a m a g n e t i c field a n d r a d i o f r e q u e n c y generators to p r o d u c e t h e signal, w h i c h means that i t is n o t r u g g e d e n o u g h f o r m a n y o n l i n e applications (111). H o w e v e r , as a q u i c k l a b o r a t o r y test f o r o i l o r w a t e r


122


content, it has a w i d e r range o f a p p l i c a b i l i t y . T h e signal c a n b e affected b y m a g n e t i c o r p a r a m a g n e t i c species, a n d to a lesser extent, b y the s o l i d phase, i f f o r e x a m p l e , the organic species are a b s o r b e d o n a s o l i d surface.


Near-Infrared Spectroscopy. N e a r - i n f r a r e d ( N I R ) spectroscopy is a t e c h n i q u e that has b e e n a r o u n d f o r some t i m e b u t , l i k e Ν M R spectros copy, has o n l y r e c e n t l y b e e n i m p r o v e d a n d d e v e l o p e d f o r o n - l i n e a p p l i c a tions. N e a r - i n f r a r e d analysis ( N I R A ) is a n o n d e s t r u c t i v e t e c h n i q u e that is versatile i n t h e sense that i t allows m a n y constituents to b e a n a l y z e d s i m u l t a n e o u s l y {112, 113). T h e N I R s p e c t r u m o f a sample d e p e n d s u p o n t h e a n h a r m o n i c b o n d v i b r a t i o n s o f the constituent m o l e c u l e s . T h i s c o n d i t i o n means that the t e m p e r a t u r e , m o i s t u r e content, b o n d i n g changes, a n d c o n centrations o f various c o m p o n e n t s i n the sample c a n b e d e t e r m i n e d s i m u l t a neously. I n a d d i t i o n , scattering b y particles s u c h as s a n d a n d clay i n t h e s a m p l e also allows ( i n p r i n c i p l e ) t h e d e t e r m i n a t i o n o f p a r t i c l e size d i s t r i b u tions b y N I R A . S u c h analyses c a n b e u s e d to d e t e r m i n e the size o f d r o p l e t s in oil-water emulsions. T o d e t e r m i n e t h e o i l , w a t e r , a n d solids contents s i m u l t a n e o u s l y , sophis t i c a t e d statistical t e c h n i q u e s m u s t usually b e a p p l i e d , s u c h as p a r t i a l leastsquares analysis ( P L S ) a n d m u l t i v a r i a t e analysis ( M V A ) . T h i s a p p r o a c h r e quires a great d e a l o f p r e p a r a t i o n a n d analysis o f standards f o r c a l i b r a t i o n . N e a r - i n f r a r e d peaks c a n g e n e r a l l y b e q u a n t i f i e d b y u s i n g B e e r ' s law; conse q u e n t l y , N I R A is a n excellent analytical t o o l . I n a d d i t i o n , N I R A has a fast spectral a c q u i s i t i o n t i m e a n d c a n b e a d a p t e d t o fiber optics; this a d a p t a b i l i t y allows t h e i n s t r u m e n t to b e p l a c e d i n a c o n t r o l r o o m somewhat isolated f r o m the p l a n t e n v i r o n m e n t . I n F i g u r e 29, t h e s p e c t r u m o f an o i l - s a n d sample shows t h e f u n d a m e n t a l C - H peaks at 3.5 μπι. F r o m t h e t w o peaks i n this r e g i o n , o n e c o u l d deter m i n e the a r o m a t i c - a l i p h a t i c ratio o f the h y d r o c a r b o n s present i n t h e s a m p l e . T h e f u n d a m e n t a l w a t e r v i b r a t i o n is at a p p r o x i m a t e l y 3 μπι (this p e a k w o u l d be substantially larger i n a c o n v e n t i o n a l e m u l s i o n sample), a n d t h e f u n d a m e n t a l v i b r a t i o n s d u e to clays are at a p p r o x i m a t e l y 2.8 μπι. T h e shape o f t h e clay peaks indicates that k a o l i n i t e a n d a s m a l l a m o u n t o f s w e l l i n g clays s u c h as b e n t o n i t e are present i n this sample.

Differential Scanning Calorimetry. D i f f e r e n t i a l s c a n n i n g calor i m e t r y ( D S C ) is a t e c h n i q u e w i t h the p o t e n t i a l to d e t e r m i n e the relative a m o u n t s o f free a n d e m u l s i f i e d water. T h e f r e e z i n g , o r m o r e c o r r e c t l y , the s u p e r c o o l i n g b e h a v i o r o f e m u l s i f i e d w a t e r is v e r y d i f f e r e n t f r o m that o f free water, so t h e a m o u n t o f free versus e m u l s i f i e d w a t e r i n a sample c a n b e c h a r a c t e r i z e d . T h i s p a r a m e t e r is i m p o r t a n t i n the c h a r a c t e r i z a t i o n o f p r o d u c e d fluids a n d interface e m u l s i o n s i n w h i c h w a t e r m i g h t exist s i m u l t a n e o u s l y as b o t h c o n t i n u o u s a n d e m u l s i f i e d phases.


3.

MIKULA

Emulsion

123

Characterization

3.0

h

Λ ν, Ο

c

J

2.5

UJ

2.0

ϋ

-


First Combinations

Fundamentals

First Overtones

1.0 -

Vν

0.5

-.-

e

,ο

Near-SB 1.0

-.1 ..... 1.5

Carbonates

Mid-IR region

region -

1. 2.0

/

w /

1 2.5

1 3.0 MICROMETERS

Ι 3.5

1 4.0

1 4.5

5.0

Figure 29. NIR spectrum of an oil-sand sample illustrating peaks due to clay minerals (C), oil (O), and water (W) phases. These spectra can be obtained via fiber optics; therefore, this technique has the potential for on-line quantifica tion of oil, water, and mineral emulsion components. Either fundamentals, first combinations, or first overtones can be used to quantify particular emulsion components. The method requires calibration with standards that can be diffi cult, given complex field emulsions.

F i g u r e 30 shows a p l o t o f the a m o u n t o f heat a b s o r b e d b y the s a m p l e as a f u n c t i o n o f t i m e f o r a sample that has b o t h e m u l s i f i e d a n d free water. T h e e m u l s i f i e d water, o r dispersed-phase water, is i n s m a l l v o l u m e s , a c o n d i t i o n that lessens t h e l i k e l i h o o d o f a n u c l e a t i o n site f o r t h e b e g i n n i n g o f t h e f r e e z i n g process a n d results i n s u p e r c o o l i n g o f t h e d i s p e r s e d phase (114). T h e latent heat o f crystallization o r s o l i d i f i c a t i o n is t h e n t a k e n u p at a l o w e r t e m p e r a t u r e t h a n f o r the free water, w h i c h freezes at a h i g h e r t e m p e r a t u r e . T h e degree o f s u p e r c o o l i n g is greater f o r e m u l s i o n s o f s m a l l e r size d i s t r i b u tions, a n d , i n s p e c i a l cases, t h e t e c h n i q u e c a n also give a n i n d i c a t i o n o f t h e size d i s t r i b u t i o n o f t h e e m u l s i o n . T h e s e latent heats a n d f r e e z i n g t e m p e r a tures c a n also b e affected b y solutes that depress t h e f r e e z i n g p o i n t a n d b y fine solids that p r o v i d e n u c l e a t i o n sites. I n spite o f these p o t e n t i a l i n t e r f e r ences, t h e t e c h n i q u e is o n e o f the f e w that w i l l a l l o w a d e t e r m i n a t i o n o f free versus e m u l s i f i e d water.



124


1

L J

210

1

1 ι ι 230 250 TEMPERATURE (K)

_j

ι

i _ J

270

Figure 30. Freezing behavior of an emulsion characterized by differential scanning calorimetry. The free water will freeze at approximately 273 K. Emulsified water will supercool and freeze at lower temperatures, depending upon size distribution. The smallest droplets freeze last because of the smaller volume, and so fewer nucleation sites are available for ice crystal formation and water freezing. The different freezing behavior of free versus emulsified water gives this technique the potential to quantify the relative proportions of these two types of water. (Reproduced with permission from reference 114. Copyright 1984.)

Summary T o select a n d use t h e e m u l s i o n c h a r a c t e r i z a t i o n t e c h n i q u e best s u i t e d to t h e a p p l i c a t i o n at h a n d , i t is necessary to d e v e l o p a n u n d e r s t a n d i n g o f t h e u n i q u e capabilities a n d l i m i t a t i o n s o f each m e t h o d . O i l - f i e l d e m u l s i o n c h a r a c t e r i z a t i o n r e q u i r e m e n t s are generally f a i r l y s t r a i g h t f o r w a r d . O p e r a tions, p r o d u c t i o n , a n d research p e r s o n n e l are generally i n t e r e s t e d i n deter-


3.

MIKULA

Emulsion

Characterization

125

m i n i n g t h e extent o f change f r o m n o r m a l o p e r a t i o n i n terms o f some base l i n e data. I n most cases, this d e t e r m i n a t i o n involves s i m p l e analysis for total water, o i l , a n d solids. W h e n operations b e g i n to d r i f t , a w i d e range o f c o r r e c t i v e actions c a n b e t a k e n to r e m e d y t h e s i t u a t i o n ; the most i m p o r t a n t factor is t h e r e f o r e s p e e d o f analysis so that d r i f t i n p r o d u c t o r tailings q u a l i t y c a n b e q u i c k l y r e c o g n i z e d . T h i s r e q u i r e m e n t f o r s p e e d means reliance o n


o n - l i n e sensors o r , m o r e c o m m o n l y , o n a r e g u l a r s a m p l i n g p r o t o c o l a n d c e n t r i f u g e tests. S o m e o f the m o r e sophisticated t e c h n i q u e s o f f e r d e t a i l e d i n f o r m a t i o n o r levels o f accuracy that are n o t r e q u i r e d i n d a y - t o - d a y operations. H o w e v e r , w h e n o p e r a t i o n a l upsets cannot b e h a n d l e d b y n o r m a l m e t h o d s , details o f the e m u l s i o n p r o p e r t i e s have to b e u n d e r s t o o d . F o r example, subtle changes i n t h e size d i s t r i b u t i o n o f t h e d i s p e r s e d phase (while total o i l , water, a n d solids r e m a i n constant) c a n b e i m p o r t a n t i n d e t e r m i n i n g process p e r f o r mance. A n oil-in-water or water-in-oil emulsion can invert during processing as o n e o r t h e o t h e r phase is r e m o v e d , a n d t h e p o i n t i n t h e process w h e n this i n v e r s i o n occurs c a n have i m p l i c a t i o n s f o r t h e efficiency o f t h e o p e r a t i o n . T h e a d d i t i o n o f d i l u e n t to r e d u c e oil-phase viscosity, f o r instance, is m u c h m o r e efficient i f o i l is t h e c o n t i n u o u s phase. T h e efficiency o f any w a t e r - r e m o v a l steps d e p e n d s u p o n t h e size d i s t r i b u t i o n o f t h e d i s p e r s e d w a t e r a n d t h e stability o f t h e e m u l s i o n . E m u l s i o n f o r m a t i o n m a y b e exacerbated b y i n a p p r o p r i a t e p u m p i n g s p e e d o r o t h e r process variables. A n evaluation o f t h e c h e m i c a l a n d p h y s i c a l factors that d e t e r m i n e e m u l s i o n size d i s t r i b u t i o n o r e m u l s i o n b u l k p r o p e r t i e s is essential to o p t i m i z e e m u l s i o n b r e a k i n g efficiency. T h e d e t a i l e d e m u l s i o n c h a r a c t e r i z a t i o n m e t h o d s discussed h e r e i n c a n b e u s e d to h e l p resolve o p e r a t i o n a l upsets o n l y i f a base l i n e o f data exists f o r n o r m a l o p e r a t i o n . I n fact, w i t h o u t a t h o r o u g h c h a r a c t e r i z a t i o n o f the n o r m a l e m u l s i o n p r o p e r t i e s such as size d i s t r i b u t i o n a n d m i n e r a l a n d o r g a n i c c o m p o s i t i o n , t h e t e c h n i q u e s f o r d e t a i l e d c h a r a c t e r i z a t i o n m a y actually h i n d e r the u n d e r s t a n d i n g a n d u l t i m a t e s o l u t i o n o f a p a r t i c u l a r p r o c e s s i n g p r o b l e m b y i n t r o d u c i n g extraneous i n f o r m a t i o n . W h e n a base l i n e o f data exists, d e t a i l e d i n f o r m a t i o n o n the size d i s t r i b u t i o n a n d t h e r e l a t i o n s h i p b e t w e e n the d i s p e r s e d , c o n t i n u o u s , a n d s o l i d phases is i n v a l u a b l e .

Acknowledgments T h a n k s to K . C . M c A u l e y f o r p r e p a r a t i o n o f the figures a n d p h o t o g r a p h s a n d to V . A . M u n o z a n d W . W . L a m , w h o c o n t r i b u t e d m a n y o f t h e p h o t o g r a p h s r e l a t e d to t h e o p t i c a l a n d e l e c t r o n m i c r o s c o p y . T h a n k s are also d u e to C . K . Preston and J. C . D o n i n i for the contribution o n N I R A , C . A . A n g l e for the e l e c t r o k i n e t i c data, R . Z r o b o k f o r t h e r h e o l o g y scans, a n d H . A . H a m z a f o r u s e f u l c o m m e n t s a n d discussion o n c h a r a c t e r i z a t i o n technologies i n g e n e r a l .


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R E C E I V E D for review December 18, 1990. A C C E P T E D revised manuscript A p r i l 24, 1991.


Emulsion Characterization - Advances in Chemistry (ACS Publications)

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