Organized Surfactant Assemblies in Supercritical Fluids - ACS

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Chapter 5

Organized Surfactant Assemblies in Supercritical Fluids John L . Fulton and Richard D. Smith

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Pacific Northwest Laboratory, Battelle Boulevard, Richland, WA 99352

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has l i t t l e or no effect. Potential applications of this new class of solvents are discussed. Micelles and microemulsions are thermodynamically stable aggregates which are homogeneous on a macroscopic scale and form transparent solutions. Reverse (or inverse) micelles are small, dynamic aggregates of surfactant (amphophilic) molecules forming shells around core regions containing a polar phase. Reverse micelle phases, as well as the water "swollen" microemulsion phase, can be considered subsets of a wide range of possible organized molecular structures which include a multitude of liquid crystalline and bicontinuous phases. In addition to potential applications in enhanced oil recovery (1,2), there is increasing interest in utilizing reverse micelles and microemulsions for separation of proteins from aqueous solutions (3, 4), as reaction media for catalytic (5) or enzymatic (6) reactions, and as mobile phases in chromatographic separations (7,8). Recently, the first observation of reverse micelles in supercritical fluid (dense gas) solvents has been reported (3.) for the surfactant sodium bis(2-ethyhexyl) suifosuccinate (AOT) in fluids such as ethane and propane. The properties of these systems have several attributes which are relevant to secondary oil recovery. In the supercritical fluid region, where the fluid temperature and pressure are above those of the critical point, the properties of the fluid are uniquely different from either the gas or liquid states. 1

Correspondence should be addressed to this author. 0097-6156/88/0373-0091$06.00/0 ° 1988 American Chemical Society Smith; Surfactant-Based Mobility Control ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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SURFACTANT-BASED MOBILITY CONTROL

Carbon dioxide, ethane and propane are examples of pure components which can exist as s u p e r c r i t i c a l f l u i d s at t y p i c a l o i l well temperatures and pressures. In secondary o i l recovery, an inexpensive and abundant f l u i d , such as C02r may be i n j e c t e d into the w e l l to sweep out remaining o i l (1_Q_) . The a d d i t i o n of surfactant to t h i s f l u i d may improve sweep e f f i c i e n c i e s by further reducing c a p i l l a r y e f f e c t s and by increasing v i s c o s i t y to reduce sweep i n s t a b i l i t i e s . As the sweep front moves through the porous bed, the s u r f a c t a n t - f l u i d phase behavior may be a l t e r e d when intermixing with the o i l and changes i n pressure and temperature occur. A d d i t i o n a l l y , the o r i g i n a l o i l can contain s i g n i f i c a n t amounts of dissolved, low molecular weight alkanes such as ethane, propane or butane. The presence of these gases may a f f e c t the phase behavior of the s u r f a c t a n t s o l u t i o n i n the sweep f l u i d because intermixing of these two s o l u t i o n s occurs as the sweep f r o n t progresses. For these reasons, an understanding of the e f f e c t of pressure, temperature and concentration on the phase behavior of s u p e r c r i t i c a l f l u i d - s u r f a c t a n t solutions i s important i n developing more e f f i c i e n t recovery methods. The combination of the unique properties of s u p e r c r i t i c a l f l u i d s ( v i s c o s i t y , d i f f u s i o n rates and solvent properties) with those of a dispersed m i c e l l e phase creates a whole new c l a s s of solvents. The p h y s i c a l properties of a s u p e r c r i t i c a l f l u i d are v a r i a b l e between the l i m i t s of normal gas and n e a r - l i q u i d properties by c o n t r o l of pressure or temperature, and the solvent power of a s u p e r c r i t i c a l f l u i d i s d i f f e r e n t at each d e n s i t y . T y p i c a l l y , s u p e r c r i t i c a l f l u i d s have densities between 0.1 and 0.8 of t h e i r l i q u i d density. Under these conditions, t h e i r d i f f u s i o n c o e f f i c i e n t s are s u b s t a n t i a l l y greater than l i q u i d s . For example, the d i f f u s i v i t y of C O 2 above the c r i t i c a l temperature (31 *C) t y p i c a l l y v a r i e s between 10~ and 10~ cm /s, whereas l i q u i d s t y p i c a l l y have d i f f u s i v i t i e s of 98%, "purum") and was further p u r i f i e d according t o the method of Kotlarchyk (IB.) . In the f i n a l step, the p u r i f i e d AOT was d r i e d i n vacuo f o r eight hours. The molar water-to-AOT r a t i o , W = [H20]/[AOT], was taken t o be 1 i n the p u r i f i e d , d r i e d s o l i d (18.) . Solutions of 50 mM AOT i n iso-octane had an absorbancë of less than 0.02 A.U. at 280 nm, which compares favorably with AOT p u r i f i e d by HPLC (£). Potentiometric t i t r a t i o n indicated that a c i d impurities were l e s s than 0.2% mole percent (£.) . The p u r i f i e d AOT was analyzed by mass spectrometry using 70 eV e l e c t r o n i o n i z a t i o n of the sample from a d i r e c t probe introduction. Two trace impurities were i d e n t i f i e d : 2-ethyl-l-hexanol and maleic acid. The ethane and propane were both "CP" grade from Linde. The iso-octane (GC-MS grade) was used as received from Burdick and Jackson. Distilled, deionized water was used throughout. The phase behavior o f the A O T / w a t e r / s u p e r c r i t i c a l f l u i d systems was studied i n a high-pressure s t a i n l e s s s t e e l view c e l l having a 3/4-in. diameter by 3-in. c y l i n d r i c a l volume, capped on both ends with 1-in. diameter by 1/2-in. t h i c k sapphire windows. S i l v e r plated metal "C" r i n g seals (Helicoflex) formed the sapphire to metal s e a l . The f l u i d mixtures were a g i t a t e d with a 1/2-in. long Teflon-coated s t i r bar driven by a magnetic s t i r r e r (VWR, Model 200) . The i n s u l a t e d c e l l was heated electrically. Temperature was c o n t r o l l e d t o ±0.1 °C with a platinum r e s i s t a n c e probe and a three-mode c o n t r o l l e r (Omega, No. N2001) monitored with a platinum r e s i s t i v e thermometer (Fluka, No. 2180A, ±0.3 °C accuracy). The f l u i d pressure was measured with a bourdon-tube type pressure gauge (Heise, ± 0.3 bar accuracy). While s t i r r i n g , the f l u i d was allowed to e q u i l i b r a t e thermally f o r 10 min. before each new reading. Much longer observation periods (~ one day) were

Smith; Surfactant-Based Mobility Control ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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used to access e q u i l i b r i a were

the phase s t a b i l i t y of s e l e c t e d systems, although e s t a b l i s h e d r a p i d l y i n the systems r e p o r t e d .

The p r o c e d u r e f o r f i n d i n g a p o i n t on t h e t w o - p h a s e b o u n d a r y o f t h e n - a l k a n e / A O T / w a t e r s y s t e m s was a s f o l l o w s . A w e i g h e d amount o f s o l i d AOT was p l a c e d i n t h e v i e w c e l l , a n d t h e n a f t e r f l u s h i n g a i r from the cell w i t h low p r e s s u r e alkane, the cell was f i l l e d to w i t h i n 10 b a r o f t h e d e s i r e d p r e s s u r e w i t h a h i g h p r e s s u r e syringe pump ( V a r i a n 8 5 0 0 ) . T h i s A O T / a l k a n e s o l u t i o n was d i l u t e d w i t h p u r e w a t e r b y i n j e c t i n g s u c c e s s i v e 2 7 μΐ* i n c r e m e n t s o f w a t e r u n t i l the t w o - p h a s e b o u n d a r y was r e a c h e d . A h a n d o p e r a t e d s y r i n g e pump ( H i g h P r e s s u r e E q u i p m e n t , N o . 8 7 - 6 - 5 ) was u s e d t o s l o w l y i n j e c t t h e water through a metering valve into the supercritical-reverse micelle solution. B y k e e p i n g t h e w a t e r i n t h e s y r i n g e pump a t a constant pressure slightly above the view cell pressure, the amount of i n j e c t e d water c o u l d be d e t e r m i n e d from t h e v e r n i e r s c a l e on the screw o f t h e pump. T h e same p r o c e d u r e was u s e d t o study phase behavior i n the liquid iso-octane system. At each temperature, four different AOT c o n c e n t r a t i o n s (0.020, 0.050, 0.075, and 0.150 M) w e r e p r e p a r e d t o s t u d y p h a s e b e h a v i o r i n t h e r a n g e o f pressures f r o m 100 t o 350 b a r . The a c c u r a c y o f t h e l o c a t i o n o f t h e phase b o u n d a r y , d e t e r m i n e d by the above method, was verified using a slightly different technique. Before p r e s s u r i z i n g with the alkane f l u i d , the weighed AOT s a m p l e was placed i n the cell, along with a predetermined amount o f w a t e r . T h e v i e w c e l l was p r e s s u r i z e d t o w i t h i n 20 b a r o f the expected single p h a s e p r e s s u r e a n d t h e n s t i r r e d f o r 10 m i n . The fluid pressure was then i n c r e a s e d b y 10 b a r b y a d d i n g the a l k a n e f o l l o w e d b y s t i r r i n g f o r 10 m i n . u n t i l a s i n g l e p h a s e system was o b t a i n e d . The phase b o u n d a r i e s f o r f i v e s y s t e m s were f o u n d t o agree w i t h i n ± 5% o f the values from the previous measurement technique. T h e s o l u t i o n c o n d u c t i v i t y was m e a s u r e d u s i n g a Y e l l o w S p r i n g s Instrument c o n d u c t i v i t y meter ( Y S I M o d e l 34) with a high pressure conductivity cell (cell constant of 0.0044 c m ) . The high pressure cell consisted of ten stacked, stainless steel disc electrodes (10-mm d i a m e t e r d i s c s ) , insulated with Teflon washers. The meter i s particularly well s u i t e d f o r use w i t h t h i s type of c e l l because capacitance e r r o r s are minimized by the a c t i v e circuit and electrode over-potential is eliminated by measurement potentials of less than 1 v o l t . -

1

T h e s o l u b i l i t y o f A O T i n s u p e r c r i t i c a l e t h a n e a n d p r o p a n e was d e t e r m i n e d by s a m p l i n g an e q u i l i b r i u m c e l l u s i n g c h r o m a t o g r a p h i c techniques. A n e x c e s s o f s o l i d A O T w a s l o a d e d i n t o a 17 mL h i g h pressure vessel. T h e f l u i d was s a t u r a t e d w i t h AOT b y r e c i r c u l a t i o n t h r o u g h t h e s o l i d b e d o f A O T u s i n g a m a g n e t i c a l l y c o u p l e d g e a r pump (Micropump, No. 182-346). U s i n g a HPLC i n j e c t i o n v a l v e , ΙΟΟ-μΐ* samples of this solution were introduced into a UV a b s o r b a n c e detector (ISCO V ) . The m o b i l e phase, m a i n t a i n e d at c o n s t a n t flow r a t e , was p u r e l i q u i d e t h a n e o r p r o p a n e a t 300 b a r a n d 25 ° C . The amount o f AOT i n t h e 1 0 0 μΐι s a m p l e w a s d e t e r m i n e d b y integrating the absorbance peaks ( m o n i t o r e d a t 2 3 0 nm) f o l l o w i n g c a l i b r a t i o n u s i n g s o l u t i o n s o f known c o n c e n t r a t i o n . 4

A high pressure v i b r a t i n g tube densimeter ( M e t t l e r - P a a r DMA 512) was u s e d t o m e a s u r e t h e d e n s i t y o f t h e AOT/water/supercritical ethane s o l u t i o n s . By r e c i r c u l a t i n g water from a t h e r m o s t a t e d water bath through the water-jacketed measuring c e l l , the temperature of the c e l l c o u l d be c o n t r o l l e d t o ± 0.01 *C. The m i c e l l e solutions w e r e p r e p a r e d b y l o a d i n g m e a s u r e d a m o u n t s o f AOT a n d w a t e r i n t o a


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FULTON & SMITH

Organized Surfactant Assemblies in Supercritical Fluids

50 mL high pressure v e s s e l placed i n the water bath. A f t e r the vessel was f i l l e d with s u p e r c r i t i c a l ethane, the s o l u t i o n was mixed and r e c i r c u l a t e d through the v i b r a t i n g tube sensor by means of the magnetically coupled gear pump. The temperature and pressure were measured using the previously described instruments. The p a r t i a l molal volume of AOT, v^, i n s u p e r c r i t i c a l ethane was c a l c u l a t e d from the expression, v

2

= ν - y i (3v/dyi) , T

P

where ν i s the s p e c i f i c volume of the s o l u t i o n and y i i s the ethane mole f r a c t i o n . The measured AOT concentration was converted to AOT mole f r a c t i o n using an i t e r a t i v e procedure. I n i t i a l l y the value of V2 f o r pure AOT s o l i d was used to estimate y , allowing a new value of v~ to be c a l c u l a t e d from which a better estimate of y could be determined. 2

2

2

R e s u l t s and D i s c u s s i o n A simple v i s u a l experiment i n which polar dyes or proteins which are i n s o l u b l e i n the pure f l u i d are s o l u b i l i z e d by s u p e r c r i t i c a l f l u i d - s u r f a c t a n t solutions i s convincing evidence f o r the existence of a reverse m i c e l l e phase. A colored azo dye, malachite green [PfP "(p-phenylmethylidene)bis(N,N-dimethylaniline)] is very soluble i n a 0.075 M AOT/supercritical ethane s o l u t i o n at 37 *C and 250 bar when the water-to-AOT r a t i o , W, i s above 3. S u p e r c r i t i c a l propane reverse m i c e l l e s at 103 *C and 250 bar can s o l u b i l i z e s u b s t a n t i a l amounts of high molecular weight p r o t e i n s such as Cytochrome C. These p o l a r substances were determined to have n e g l i g i b l e s o l u b i l i t y i n the pure f l u i d and i n the water saturated fluid. In the binary solvent of AOT and f l u i d (where we assume W ~ 1 due to the d i f f i c u l t y of completely drying the AOT) these polar substances are only sparingly soluble. However, by increasing W to 3 or above, the s o l u b i l i t y of polar compounds i s greatly increased. S o l u b i l i z a t i o n of Cytochrome C i n propane/AOT/water solutions i s p a r t i c u l a r l y convincing evidence f o r m i c e l l e formation in s u p e r c r i t i c a l f l u i d s because i t excludes the p o s s i b i l i t y of a simple ion-pair mechanism of s o l u b i l i z a t i o n . It seems l i k e l y that t h i s l a r g e , water s o l u b l e enzyme i s s o l v a t e d by the h i g h l y hydrophobic f l u i d s only i f the p o l a r f u n c t i o n a l groups on the surface of the p r o t e i n are shielded from the f l u i d by surfactant molecules. AOT r e a d i l y forms reverse m i c e l l e s i n nonpolar solvents without the a d d i t i o n of cosurfactants i n part because the twin alkane t a i l groups of the s u r f a c t a n t molecule provide a very favorable geometric packing f o r i n v e r t e d s t r u c t u r e s . The surfactant's r e l a t i v e l y high molecular weight (444.5) and anionic head group r e s u l t i n very low s o l u b i l i t y of the monomeric form i n low molecular weight, nonpolar f l u i d s . The d i s s o l u t i o n of the m i c e l l a r form i s much more favorable because the polar head groups are s h i e l d e d from the nonpolar f l u i d phase. A measure of s o l u b i l i t y i n the s u p e r c r i t i c a l f l u i d phase can be obtained from the s o l u b i l i t y of the surfactant i n the near c r i t i c a l l i q u i d s at 25 'C where, although the temperatures are much lower, the f l u i d d e n s i t i e s are near the upper l i m i t of those normally obtained for the s u p e r c r i t i c a l f l u i d phase. The s o l u b i l i t y of AOT i n various f l u i d s at 25 *C i s shown i n Table I I . AOT i s i n s o l u b l e i n pure l i q u i d and supercritical C0 at temperatures from 25 to 100 C 1

e

2


97

98


and a t p r e s s u r e s up t o e t h a n e above 200 b a r and Table I I . AOT Fluid

C0

AOT

400 b a r . AOT i s very soluble i n l i q u i d i s likewise very soluble i n s u p e r c r i t i c a l Solubility

Solubility @ 25 *C

MW

Insoluble, surfactant remains s o l i d up t o 200 b a r

i n F l u i d s a t 25 T CO

P (Bar)

c

*C Polarizability 10~ cm

c

24

44

31.3

72.9

2.9

131

16.6

57.6

4.2

30

32.3

48.1

4.5

Propane V e r y s o l u b l e a t p r e s s u r e s above 10 b a r

44

96.7

41.9

6.3

CO -10% V e r y s o l u b l e IPA

45

60.8

2

Xenon

a

M e l t s a t 100 b a r f l u i d density =1.76 g/cm

at a

3

3

Ethane

V e r y s o l u b l e above 200 b a r . Surfactant m e l t s a t 100 bar.

2

SF

Insoluble, surfactant remains s o l i d up t o 200 b a r

6

a

b

S u p e r c r i t i c a l a t 25 *C. E s t i m a t e d by method g i v e n

146

b

45.5

i n Reference

90.0

37.1

b

3.2

6.5

19.

e t h a n e (at Τ = 37 'C and a t a s l i g h t l y h i g h e r p r e s s u r e o f 250 b a r ) . I n l i q u i d p r o p a n e a t 25 *C, AOT solubility i s very high at p r e s s u r e s above t h e p r o p a n e v a p o r p r e s s u r e o f 9 b a r and r e m a i n s v e r y s o l u b l e i n s u p e r c r i t i c a l p r o p a n e , Τ = 103 *C, above 100 bar. At a p r e s s u r e o f 100 b a r , AOT begins to melt i n supercritical xenon, a phenomena w h i c h i s a l s o o b s e r v e d i n e t h a n e a t about t h e same p r e s s u r e . A l t h o u g h s o l u b i l i t y a t h i g h e r xenon p r e s s u r e s has not been i n v e s t i g a t e d , t h e b e h a v i o r m i m i c s AOT/ethane b e h a v i o r up t o 100 b a r . The o r d e r o f i n c r e a s i n g s o l u b i l i t y a p p e a r s t o be C0 , xenon, e t h a n e , and propane, which f o l l o w s t h e o r d e r o f i n c r e a s i n g polarizability. The more p o l a r i z a b l e f l u i d s m i g h t be e x p e c t e d t o b e t t e r s o l v a t e t h e s u r f a c t a n t monomer (which has a l a r g e permanent d i p o l e moment) as w e l l as t h e l a r g e m i c e l l a r s t r u c t u r e s which have l o c a l l y high p o l a r i z a b i l i t i e s . However, AOT i s i n s o l u b l e i n s u l f u r hexafluoride w h i c h has a relatively large p o l a r i z a b i l i t y , and s i m i l a r b e h a v i o r i s o b s e r v e d f o r f r e o n 13, i n d i c a t i n g t h a t s u c h an explanation i s overly s i m p l i s t i c . By a d d i n g 10% i s o - p r o p y l a l c o h o l (IPA) t o t h e l i q u i d C0 (by v o l u m e ) , AOT becomes v e r y s o l u b l e and i s a l s o s o l u b l e i n t h i s b i n a r y f l u i d a t 80 *C and 200 b a r . We do 2

2


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FULTON & SMITH

Organized Surfactant Assemblies in Supercritical Fluids

not yet know whether the s o l u b i l i z e d AOT i s i n the m i c e l l a r form i n the IPA-CO2 s o l u t i o n but the IPA may act as a cosurfactant being l o c a l l y concentrated at the surfactant i n t e r f a c e and thus further s h i e l d i n g the i o n i c head groups from the nonpolar, predominately CO2, continuous phase. The pressure dependence of the phase behavior of s u p e r c r i t i c a l f l u i d solutions containing a reverse m i c e l l e phase i s s t r i k i n g and can be i l l u s t r a t e d by a d e s c r i p t i o n of the s o l v a t i o n process from view c e l l s t u d i e s . The d i s s o l u t i o n of 1 g of AOT s o l i d (W n 1) i n t o 25 mL of s u p e r c r i t i c a l ethane or propane proceeds i n four d i s t i n c t stages. At low pressures the AOT s o l i d i s i n e q u i l i b r i u m with a low density f l u i d containing a small or n e g l i g i b l e amount of dissolved s o l i d . At somewhat higher pressures (80 to 100 bar) the AOT begins to "melt", forming a system with three phases: s o l i d AOT, a viscous AOT l i q u i d with a small amount of d i s s o l v e d f l u i d , and a f l u i d phase containing d i s s o l v e d s u r f a c t a n t . At moderate pressures a two-phase system e x i s t s c o n s i s t i n g of a v i s c o u s , predominantly AOT l i q u i d i n e q u i l i b r i u m with a f l u i d c o n t a i n i n g appreciable amounts of s u r f a c t a n t . F i n a l l y , at higher pressures ( t y p i c a l l y >120 bar) a single, reverse m i c e l l e - c o n t a i n i n g phase i s created with the AOT completely solvated by the f l u i d . The ternary phase diagrams f o r s u p e r c r i t i c a l ethane, propane and l i q u i d iso-octane surfactant solutions are shown i n Figures 2, 3, and 4, r e s p e c t i v e l y . The region of i n t e r e s t i n t h i s study i s the alkane r i c h corner of the phase diagram represented from 80 to 100% alkane and less than 10% water by weight. Each phase diagram shows the l o c a t i o n of the phase boundaries separating the s i n g l e and two-phase regions at several d i f f e r e n t pressures i n the range of 100 to 350 bar. The areas to the r i g h t of these boundaries are regions where a s i n g l e , reverse m i c e l l e phase e x i s t s ; to the l e f t of these l i n e s , a two-phase system exists containing a l i q u i d and a gas phase. This l i q u i d phase i s predominantly water c o n t a i n i n g d i s s o l v e d surfactant most l i k e l y i n the form of monomer or normal m i c e l l e aggregates. The phase boundary l i n e s a l s o define the maximum water-to-surfactant r a t i o , W . At a given pressure W appears to be nearly constant over the range of AOT concentrations studied. The s u p e r c r i t i c a l ethane data shown i n Figure 2 (at 37 *C) are 5 *C above the ethane c r i t i c a l temperature; the s u p e r c r i t i c a l propane data are at the same reduced temperature ( T / T ) as the ethane (6 * C above the c r i t i c a l temperature of propane). To compare the phase behavior of an alkane l i q u i d with that of s u p e r c r i t i c a l propane, the phase diagram f o r l i q u i d i s o octane at 103 *C and various pressures i s shown i n Figure 4. The phase boundary l i n e s f o r s u p e r c r i t i c a l ethane at 250 and 350 bar are shown i n Figure 2. The surfactant was found to be only s l i g h t l y s o l u b l e i n ethane below 200 bar at 37 *C, so that the ternary phase behavior was studied at higher pressures where the AOT/ethane b i n a r y system i s a s i n g l e phase. As pressure i s increased, more water i s s o l u b i l i z e d i n the m i c e l l e core and larger m i c e l l e s can e x i s t i n the s u p e r c r i t i c a l f l u i d continuous phase. The maximum amount of water s o l u b i l i z e d i n the s u p e r c r i t i c a l ethane-reverse m i c e l l e phase i s r e l a t i v e l y low, reaching a W value of 4 at 350 bar. In contrast to ethane, the maximum amount of s o l u b i l i z e d water i n the s u p e r c r i t i c a l propane-reverse m i c e l l e system i s much higher, having a W value of 12 at 300 bar and 103 *C. Again, the W values increase as pressure increases from a W value of 4 at 100 bar to 12 at 300 bar, as shown i n Figure 3. The phase behavior i n Q

Q

c

Q

Q

0


99

100


100%

0%

A

•ψ

Ethane 9 0 %

ilJliUlJillliil

mnnmwnl illllliliilUill 10% Water

F i g u r e 2. E t h a n e - r i c h corner of the ethane/AOT/water p h a s e d i a g r a m ( w e i g h t %) a t 37 * C a n d a t t w o p r e s s u r e s , 350 b a r .

100%

a

ternary 250 and

0%

Propane 9 0 %

l/jiiimiiuju uuwnwrii

lUiiiiiiimul 80% 10% Water

Figure phase 200

3.

Propane-rich

diagram

and

300

(weight

corner

%)

at

103

of

the

propane/AOT/water

*C a n d a t

three

ternary

pressures,

bar.


100,

5. FULTON & SMITH

Organized Surfactant Assemblies in Supercritical Fluids 101

the s u p e r c r i t i c a l f l u i d surfactant systems i s markedly d i f f e r e n t than that of the l i q u i d iso-octane reverse m i c e l l e system. In the l i q u i d iso-octane system there i s no substantial e f f e c t of pressure on the phase behavior at the temperature studied. The c r i t i c a l m i c e l l e concentration (CMC) defines the minimum amount of surfactant monomer required t o form the reverse m i c e l l e phase and may be considered t o represent the s o l u b i l i t y of the surfactant monomer (although the CMC i s much l e s s c l e a r l y defined than i n normal m i c e l l e systems). In a reverse m i c e l l e s o l u t i o n t h i s small amount of monomeric surfactant e x i s t s i n e q u i l i b r i u m with the bulk of the surfactant i n the form of m i c e l l a r aggregates. For example, the CMC of AOT i n l i q u i d iso-octane i s -6 χ 10~ M. The s o l u b i l i t y of surfactant monomer i n a p a r t i c u l a r solvent i s dependent on s p e c i f i c s o l v e n t - s o l u t e f o r c e s . The dominant intermolecular i n t e r a c t i o n s between a polar surfactant molecule and alkane solvent molecules are the dipole-induced d i p o l e and the induced dipole-induced dipole forces. In s u p e r c r i t i c a l f l u i d s , the magnitudes of these forces are strongly dependent on the pressure and temperature c o n d i t i o n s o f t h e f l u i d which determine the intermolecular distances (19) . At s i m i l a r molecular d e n s i t i e s , hexane and iso-octane are expected to be better solvents f o r polar surfactant molecules because t h e i r p o l a r i z a b i l i t i e s (12xl0~ and 17xl0~ cm^, respectively) and, hence, t h e i r induced dipoles are greater than those f o r ethane and propane (4.4xl0~ and 6.3xl0~ cm^, r e s p e c t i v e l y ) . Even so, AOT e x h i b i t s very high s o l u b i l i t y i n s u p e r c r i t i c a l ethane and propane at moderate d e n s i t i e s , as shown i n Figures 5 and 6. For ethane, the s o l u b i l i t y i s much higher than one would expect f o r a high molecular weight, polar molecule i n a low molecular weight f l u i d . This high s o l u b i l i t y i s r e a d i l y explained i n terms of formation of AOT aggregates; i . e . , a reverse m i c e l l e phase dispersed i n the f l u i d . I t seems apparent from the s o l u b i l i t y data that at moderate pressures the surfactant monomer i s soluble above the CMC i n ethane and propane, although these data show no evidence of changes i n s o l u b i l i t y due to the CMC. As i n d i c a t e d i n Figures 5 and 6, there i s a nearly l i n e a r r e l a t i o n s h i p between the log[AOT] s o l u b i l i t y and the f l u i d density over several order of magnitude of AOT concentration. This type of behavior would be expected f o r the s o l u b i l i t y of a non-aggregate forming, s o l i d substance i n a s u p e r c r i t i c a l f l u i d (JLL). The s o l u b i l i t y and phase behavior of s o l i d - s u p e r c r i t i c a l f l u i d systems has been described by Schneider (2H) and others, and such behavior can be predicted from a simple Van der Waal's equation of s t a t e . C l e a r l y , t h i s approach i s not appropriate f o r p r e d i c t i n g surfactant s o l u b i l i t i e s i n f l u i d s , because i t does not account f o r the formation of aggregates or t h e i r s o l u b i l i z a t i o n i n a s u p e r c r i t i c a l f l u i d phase. In Figures 5 and 6, one might expect t o see two d i f f e r e n t s o l u b i l i t y regions. At low f l u i d d e n s i t i e s where intermolecular forces are reduced and the surfactant concentration i s below the CMC, t h e s o l u b i l i t y should increase g r a d u a l l y as the d e n s i t y increases. At higher d e n s i t i e s , above the CMC, the s o l u b i l i t y should increase r a p i d l y because the t o t a l surfactant s o l u b i l i t y i s dominated by the saturation concentration of micelles i n the f l u i d . This type of behavior i s not apparent i n Figures 5 and 6, perhaps because the CMC i s below 10~ M. An a l t e r n a t i v e explanation i s that the CMC f o r AOT i n s u p e r c r i t i c a l f l u i d s i s density dependent. This might be expected 4

24

24

24

4


24

102


Water

Figure 4. Iso-octane r i c h corner of the iso-octane/AOT/water t e r n a r y phase diagram (weight %) at 103 *C and at three pressures, 100, 200 and 300 bar.

10-

1

r

ίο- 1 5

0.0

ι

I

0.1

ι

I

0.2

«

1

1

0.3

1

1

0.4

1 0.5

3

Ethane Density (g/cm )

Figure 5. S o l u b i l i t y of AOT i n s u p e r c r i t i c a l ethane at 37, 50 and 100 *C, W = 1.


5. FULTON & SMITH


because the CMC can be s e n s i t i v e t o temperature and the nature of the continuous phase. As we have noted, at high d i l u t i o n there i s t y p i c a l l y a nearly l i n e a r r e l a t i o n s h i p between log [ s o l u b i l i t y ] and f l u i d density f o r s o l i d solutes. I f the AOT monomer conforms to t h i s behavior, the CMC might be expected t o have a s i m i l a r r e l a t i o n s h i p with f l u i d d e n s i t y (p); i . e . , l o g (CMC) °* p. Clearly, further studies are required to resolve these points. The e f f e c t of temperature on AOT s o l u b i l i t y i n ethane i s also shown i n Figure 5. The range of f l u i d d e n s i t i e s studied was l i m i t e d at higher temperatures by the pressure constraints of our apparatus. In our i n i t i a l correspondence i t was shown that the minimum ethane density necessary to support reverse micelles (at W had a n e a r l y l i n e a r inverse r e l a t i o n s h i p with temperature extending from the n e a r - c r i t i c a l l i q u i d (at 23 *C) to well into the s u p e r c r i t i c a l region (>100 C ) . (The previous experiments u t i l i z e d an AOT concentration of ~ 2 χ 10~ m o l e s / l i t e r , and corresponds to a s o l u b i l i t y measurement i n which the f l u i d density necessary for s o l v a t i o n was estimated by using the density of the pure f l u i d . The r e s u l t s are i n good agreement with the present more extensive measurements.) The s o l u b i l i t y of AOT i s greater i n propane than i n ethane even, at s i m i l a r temperatures, although the greater slope of the log [AOT] vs. p data suggests that the differences are small at higher d e n s i t i e s . In Figure 7 the conductivities of solutions containing reverse micelles dispersed i n s u p e r c r i t i c a l propane at 103* C are compared with c o n d u c t i v i t i e s of solutions containing reverse m i c e l l e s i n l i q u i d iso-octane at 25 and 103 *C, and pressures ranging from 75 to 350 bar. The AOT concentrations were approximately 37 and 80 mM at W ~ 1. In a l l cases the c o n d u c t i v i t i e s of these solutions are very low, below 10"^ mhos/cm. This evidence i s e n t i r e l y consistent with a reverse micelle structure forming i n a nonpolar supercritical fluid. Reverse m i c e l l e s o l u t i o n s formed i n s u p e r c r i t i c a l propane are more conducting than those formed i n l i q u i d iso-octane at the same temperature, pressure, and AOT and water concentrations. Part of t h i s difference can be explained by the higher m o b i l i t y of ions i n the lower v i s c o s i t y propane. The v i s c o s i t y of propane at 103 *C v a r i e s from 0.07 cp t o 0.09 cp between 175 t o 350 bar, whereas the v i s c o s i t y of iso-octane i s 0.5 cp at these conditions. The d i f f e r e n c e i n measured conductivity between propane and iso-octane solutions at 103 *C i s not as large as would be expected based s o l e l y on the f a c t o r of s i x d i f f e r e n c e i n v i s c o s i t y of the two f l u i d s . This indicates that other factors, such as differences i n the concentration of surfactant monomer, may be important. For the s u p e r c r i t i c a l propane solutions, conductivity decreases a t higher p r e s s u r e s . The p r e s s u r e dependence of c o n d u c t i v i t y i n propane can be e n t i r e l y explained i n terms of reduced i o n i c mobility as the v i s c o s i t y of the f l u i d increases at higher pressures. As shown i n Figure 7, adding surfactant t o propane increases the c o n d u c t i v i t y by several orders of magnitude over the binary system of propane saturated with pure water. The predominant contribution to conductance i n these solutions i s a n t i c i p a t e d to be from d i s s o c i a t e d surfactant monomer i n the continuous phase or from micelles containing one or more ionized molecules. The degree of d i s s o c i a t i o n i s quite low, but should be s l i g h t l y higher i n the l i q u i d alkane s o l u t i o n s due t o the somewhat l a r g e r d i e l e c t r i c constant. e

2


104


0.00

0.10

0.20

0.40

0.30

0.50

Propane Density (g/ml)

Figure 6. S o l u b i l i t y 103 *C, W = 1.

of AOT i n s u p e r c r i t i c a l

10-

propane

at

Propane. Τ = 1 0 3 ° C [AOT) = 7 9 m M .

7

io- U F

Iso-Octane 1 0 3 ° C , [AOT] = 8 0 m M

10Propane 1 0 3 ° C ,

Iso-Octane 2 5 ° C . [AOT] = 36 m M

10-

[AOT]

= 37 m M

9

Propane, Τ =

103°C

Saturated with H 0 2

10100

200

300

400

Pressure (bar)

Figure 7 . C o n d u c t i v i t y supercritical propane p r e s s u r e s , W = 1.

of reverse micelle and l i q u i d iso-octane

phases i n at various


5. FULTON & SMITH


Measurements of s u p e r c r i t i c a l ethane density versus the AOT concentration shown i n Figure 4 (T « 37 'C, Ρ = 250 bar) i n d i c a t e that the properties of the s u p e r c r i t i c a l continuous phase resemble those of the pure f l u i d . The dispersed m i c e l l e phase does not appear to increase the c r i t i c a l temperature or c r i t i c a l pressure of the binary s o l u t i o n to the point o f inducing a phase change i n the system. There i s a small increase i n density as surfactant i s added t o the system which confirms the v i s u a l observation that a second l i q u i d phase of much higher density i s not formed. From the data i n Figure 8/ the p a r t i a l molal volume o f an AOT molecule i n a m i c e l l a r aggregate dispersed i n s u p e r c r i t i c a l ethane at 37 *C and 250 bar i s estimated t o be -43.0 ± 30 cm /mole. A negative p a r t i a l molal volume for a solute i n a s u p e r c r i t i c a l f l u i d i s not s u r p r i s i n g since lower molecular weight solutes such as naphthalene i n ethylene near the c r i t i c a l point can have a p a r t i a l molal volume of -3000 cm /mole (21). This behavior i s due t o the l o c a l l y higher solvent density around the higher molecular weight, p o l a r i z a b l e solute molecule (22 21) . From the estimate of the p a r t i a l molal volume of AOT i n s u p e r c r i t i c a l ethane, the system may be seen t o be composed of a m i c e l l a r structure surrounded by an ethane s h e l l of density greater than the bulk f l u i d , and t h i s e n t i r e s t r u c t u r e i s dispersed i n the continuous, s u p e r c r i t i c a l ethane phase. 3

3

r

Conclusions The existence of a reverse m i c e l l e phase i n s u p e r c r i t i c a l f l u i d s has been confirmed from s o l u b i l i t y , c o n d u c t i v i t y and d e n s i t y measurements. The picture of the aggregate structure i n f l u i d s i s one of a t y p i c a l reverse micelle structure surrounded by a s h e l l of l i q u i d - l i k e ethane, with t h i s l a r g e r aggregate structure dispersed i n a s u p e r c r i t i c a l f l u i d continuous phase. The reverse micelle phase behavior i n s u p e r c r i t i c a l f l u i d s i s markedly d i f f e r e n t than i n l i q u i d s . By increasing f l u i d pressure, the maximum amount of s o l u b i l i z e d water increases, i n d i c a t i n g that these higher molecular weight structures are better solvated by the denser f l u i d phase. The phase behavior of these systems i s i n part due t o packing c o n s t r a i n t s of the surfactant molecules and the s o l u b i l i t y of large m i c e l l a r aggregates i n the s u p e r c r i t i c a l f l u i d phase. An understanding of the phase behavior of s u r f a c t a n t s u p e r c r i t i c a l f l u i d s o l u t i o n s may be r e l e v a n t t o developing e f f i c i e n t secondary o i l recovery methods because o i l d i s p l a c i n g f l u i d s , such as a C02/surfactant mixture, may be s u p e r c r i t i c a l at t y p i c a l well conditions. In addition, the o r i g i n a l o i l i n the well may contain d i s s o l v e d gases such as ethane, propane, o r butane, which may e f f e c t the phase behavior of the surfactant s o l u t i o n used to sweep out remaining o i l . A number of other important p o t e n t i a l a p p l i c a t i o n s of a m i c e l l a r phase i n s u p e r c r i t i c a l f l u i d s may u t i l i z e the unique properties of the s u p e r c r i t i c a l f l u i d phase. For instance, polar c a t a l y s t or enzymes could be molecularly dispersed i n a nonpolar gas phase v i a micelles, opening a new class of gas phase reactions. Because d i f f u s i v i t i e s of reactants or products are high i n the s u p e r c r i t i c a l f l u i d continuous phase, high transport rates t o and from a c t i v e s i t e s i n the c a t a l y s t - c o n t a i n i n g m i c e l l e may increase r e a c t i o n rates f o r those reactions which are d i f f u s i o n l i m i t e d .


106


0.49

0.44 Ο.000

τ

0.001

0.002

0.003

1

ι-

0.004

0.005

Figure 8. Density of A O T - s u p e r c r i t i c a l ethane 37 *C and 240 bar.

0.006

s o l u t i o n s at

The recovery of product or c a t a l y s t from the m i c e l l e core may be s i m p l i f i e d because the m i c e l l e s i z e , and even i t s existence, are dependent on f l u i d pressure, i n contrast t o l i q u i d systems where pressure has l i t t l e or no e f f e c t . Acknowledgment We thank the Department of Energy, O f f i c e of Basic Energy Sciences, f o r support of t h i s work through Contract DE-AC0 6-7 6RLO 1830.


5. FULTON & SMITH


Literature Cited 1.

Neogi, P. In Microemulsions: Structure and Dynamics: Friberg, S. E . ; Bothorel, P. Eds.; CRC Press: Boca Raton, 1987, pp. 197210. 2. Langevin, D. In Reverse Micelles: Luisi, P. L., Straub, Β. E . , Eds.; Plenum Press: New York, 1984; pp 287-303. 3. Luisi, P. L. Angew. Chem. Ind. Engl. 1985, 24, 439-450. 4. Goklen, K. E.; Hatton, T. A. In Separation Science and Technology; Bell, J. T . ; Watso, J. S., Eds.; Marcel Dekker: New York, 1987, pp. 831-841. 5. Leong, Y. S.; Candau, F. J. of Phys. Chem. 1982, 86, 22692271. 6. Luisi, P. L., Meier, P., Imre, V. E., Pande, A. In Reverse Micelles; L u i s , i , P. L.; Straab, Β. E., Eds.; Plenum Press: New York, 1984; pp. 323-337. 7. Hernandez-Torres, Μ. Α . ; Landy, J. S.; Dorsey, J. G. Anal. Chem. 1986, 58, 744-747. 8. Gale, R. W.; Smith, R. D.; Fulton, J. L. Anal. Chem. 1987, 59, 1977-1979. 9. Gale, R. W.; Fulton, J. L.; Smith, R. D. J. Am. Chem. Soc. 1987, 109, 920-921. 10. Orr, F. M.; Taber, J. J. Science 1984, 224, 563-569. 11. McHugh, Μ. Α.; Krukonis, V. J. Supercritical Fluid Extraction; Butterworths: Boston, 1986. 12. Smith, R. D.; Frye, S. L . ; Yonker, C. R.; Gale, R. W. J. Phys. Chem. 1987, 91, 3059-3062. 13. Streett, W. B. In: Chemical Engineering at Supercritical Fluid Conditions; Paulaitis, M. E.; Penninger, J.M.L.; Gray, R. D.; and Davidson, P., Eds.; Ann Arbor Science: Ann Arbor, 1983. 14. Peng, D.; Robinson, D. B. Ind. Eng. Chem., Fundam. 1976, 15, 59-64. 15. Eicke, H. F.; Kubik, R.; Hasse, R.; Zschokke, I. In Surfactants in Solution; Mittal, K. L.; Lindman, B.; Eds.; Plenum Press: New York, 1984, pp. 1533-1549. 16. Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979, 83, 480-486. 17. Kotlarchyk, M.; Huang, J. S.; Chen, S. H. J. Phys. Chem. 1985, 89, 4382-4386. 18. Kotlarchyk, M.; Chen, S.; Huang, J. S.; Kim, M. W. Physical Review A. 1984, 29, 2054-2069. 19. Prausnitz, J. M. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice-Hall: New Jersey, 1969. 20. Schneider, G. M. Angew. Chem. Int. Ed. Engl. 1978, 17, 716727. 21. Eckert, C. Α.; Ziger, D. H . ; Johnston, K. P.; Kim, S. J. Phys. Chem. 1986, 90, 2738-2746. 22. Yonker, C. R.; Frye, S. L.; Kalkwarf, D. R.; Smith, R. D. J. Phys. Chem. 1986, 90, 3022-3026. Received January 5, 1988


Organized Surfactant Assemblies in Supercritical Fluids - ACS

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