Introduction to Macro-and Microemulsions

1 Introduction to Macro- and Microemulsions

Macro- and Microemulsions Downloaded from pubs.acs.org by GEORGE MASON UNIV on 03/12/16. For personal use only.

M. K. SHARMA and D. O. SHAH Departments of Chemical Engineering and Anesthesiology, University of Florida, Gainesville, FL 32611

This paper reviews various aspects of macro- and microemulsions. The role of interfacial film of surfactants in the formation of these systems has been high-lighted. The formation of a surfactant film around droplets facilitates the emulsification process and also tends to minimize the coalescence of droplets. Macroemulsion stability i n terms of short and long range interactions has been discussed. For surfactant stabilized macroemulsions, the energy barrier obtained experimentally is very high, which prevents the occurrence of flocculation in primary minimum. Several mechanisms of microemulsion formation have been described. Based on thermodynamic approach to these systems, it has been shown that interfacial tension between oil and water of the order of 10 dynes/cm is needed for spontaneous formation of microemulsions. The distinction between the cosolubilized and microemulsion systems has been emphasized. -3

Macroemulsions have been known for thousands of years. The survey of ancient l i t e r a t u r e reveals that the emulsification of beeswax was f i r s t recorded i n the second century by the Greek physician, Galen (1). Macroemulsions are mixtures of two immiscible l i q u i d s , one of them being dispersed i n the form of fine droplets with diameter greater than 0.1 ym i n the other l i q u i d . Such systems are turbid, milky i n color and thermodynamically unstable ( i . e . the macroemulsion w i l l ultimately separate into two o r i g i n a l immiscible l i q u i d s with time). Since the early 1890s, extensive and careful studies have been carried out on macroemulsions and several excellent books have been written on various aspects of formation and s t a b i l i t y of these systems (2,10). In addition, several theories and methods of macroemulsion formation have been discussed i n the recent a r t i c l e s 17). In spite of this progress, we s t i l l do not have good predictive methods f o r the formation or breaking macroemulsions. For the f o r mation of a stable macroemulsion from two immiscible l i q u i d s , there i s no r e l i a b l e predictive method for selecting the emulsifier or

0097-6156/85/0272-0001$06.00/0 © 1985 American Chemical Society

MACRO- AND MICROEMULSIONS


2

technique of emulsification f o r obtaining the optimum r e s u l t s . One can use the concept of h y d r o p h i l i c - l i p o p h i l i c balance (HLB) f o r i n i t i a l screening (1,6) and most of the new macroemulsions are u l timately perfected by t r i a l and error approach. Macroemulsions are u t i l i z e d i n many applications and are very important from the technical point of view. Many technologies and processes involve production of stable emulsions, such as skin creams (cosmetics), metal cutting f l u i d s , f i b e r cleaning or removal of oil deposits (detergency), mayonnaise (food industry), bitumen emulsions (road construction), f u e l (energy), herbicides and pesticides ( a g r i c u l t u r a l sprays) and drug s o l u b i l i z a t i o n i n emulsions (pharmacy). In addition, it has been observed that some processes require emulsions of long-term s t a b i l i t y , whereas other require limited s t a b i l i t y of emulsions. There are processes such as formation of emulsions i n oil storage tanks and petroleum reservoirs where naturally occurring, unwanted stable emulsions have to be broken down. In view of the wide range of applications and technical importance of macroemulsions, it i s worth discussing various aspects of these systems. C l a s s i f i c a t i o n of Macroemulsions In this section, we w i l l b r i e f l y describe the c l a s s i f i c a t i o n of macroemulsions. Based on the dispersion of water or oil i n continuous phase and on the number of phases present i n the system, macroemulsions can be subdivided into two categories. Single Emulsions. These emulsions are formed by two immiscible phases (e.g. oil and water), which are separated by a surfactant f i l m . The addition of a surfactant (or emulsifier) i s necessary to s t a b i l i z e the drops. The emulsion containing oil as dispersed phase i n the form of f i n e droplets i n aqueous phase i s termed as oil-inwater (0/W) emulsion, whereas the emulsion formed by the dispersion of water droplets i n the oil phase i s termed as water-in-oil (W/0) emulsion. Figure 1 schematically i l l u s t r a t e s the 0/W and W/0 type emulsions. Milk i s an example of naturally occurring 0/W emulsion i n which f a t i s dispersed i n the form of fine droplets i n water. Double or Multiple Macroemulsions. These macroemulsions are formed by two or more than two immiscible phases which are separated by at least two emulsifier films. Multiple emulsions can also be subdivided as single emulsions i n two categories (0/W/O) and (W/O/W) emulsions (14). For a 0/W/O system, the immiscible water phase separates the two oil phases, whereas f o r a W/O/W system, the immisc i b l e oil phase separates the two aqueous phases. These emulsions are schematically shown i n Figure 2. The phase contained i n the subdrops i s often referred to as the encapsulated phase. These systems are very relevant to transport phenomena and separation processes (14), such as controlled release of drugs i n which the encapsulated phase can serve as a reservoir of the active ingredient. Mechanism of Macroemulsion Formation Macroemulsions can be produced i n d i f f e r e n t ways s t a r t i n g with two immiscible l i q u i d s and by applying mechanical energy, which deforms


1.

SHARMA AND SHAH

Oil-in-Water Figure 1.

3

Introduction

Schematic i l l u s t r a t i o n of oil (W/0) macroemulsions

Water-in-Oil oil-in-water

(0/W) and water-in-

Figure 2. Schematic i l l u s t r a t i o n of multiple W/O/W and 0/W/O macroemulsions

4

MACRO- AND

MICROEMULSIONS

the interface to such an extent that it generates droplets. The f o r mation of f i n a l emulsion droplets can be viewed as the stepwise pro cess. Therefore, the disruption of droplets i s a c r i t i c a l step i n the process of emulsification. During emulsion formation, the deformation i s opposed by the Laplace pressure. For spherical drop l e t of radius ( r ) , the difference i n pressure (Δρ) at the concave side of a curved interface with i n t e r f a c i a l tension (γ) i s 2 — . Further d i v i s i o n of droplets leads to an increase i n Δρ as r de creases. In order to disrupt such a small droplet, the pressure gra dient of the magnitude of must be applied externally. The v i s cous forces exerted by the continuous phase can also deform the emulsion droplets. The viscous stress (Gn.) should be of the same magnitude as the Laplace pressure to deform the droplets (9), where G i s the v e l o c i t y gradient and η i s the v i s c o s i t y of continuous phase. In any case, the pressure gradient or v e l o c i t y gradient required for emulsion formation are mostly supplied to the system by a g i t a t i o n . The various methods of a g i t a t i o n to produce emulsions have been de scribed recently (18). In addition, the emulsions of smaller droplets can be produced by applying more intense agitation to disrupt the larger droplets. Therefore, the l i q u i d motion during the process of emulsification i s generally turbulent (9) except for high v i s c o s i t y liquids. r


r

Energy Needed for Emulsion Formation. The t o t a l i n t e r f a c i a l area (A) generated due to emulsification process i s much larger because of the formation of the smaller droplets. Therefore, the increase i n surface free energy of the system i s given by γΔΑ. It can e a s i l y be calculated that the energy needed to produce the emulsion of average droplet diameter (2ym) from the two immiscible l i q u i d s (1 ml of each) with i n t e r f a c i a l tension γ = 10 dyne/cm would be about 3000 times higher than that of the surface free energy of the system. For the formation of some emulsions, the pressure^gradient of 2 χ 10 dynes/cm and v e l o c i t y gradient of 2 χ 10 /sec (assuming η = 1.0 cp) are needed (9). The large excess of energy required to produce emulsions can only be supplied by very intense agitation, which needs much energy. In order to reduce the agitation energy needed to produce a certain droplet s i z e , a suitable surfactant can be added to the system. The addition of surfactant reduces i n t e r f a c i a l tension, which i n turn decreases the surface free energy of the system. The formation of a surfactant f i l m around the droplets f a c i l i t a t e s the process of emuls i f i c a t i o n , and a reduction i n agitation energy by a factor of 10 or more can be achieved (9). The nature and concentration of surfactant also affects the droplet s i z e and energy requirement to form the emulsion. Besides lowering the i n t e r f a c i a l tension, the surfactant f i l m also tends to prevent the coalescence of droplets. I n t e r f a c i a l Film of Surfactants. The droplets are surrounded by an i n t e r f a c i a l f i l m of surfactant i n emulsion systems. The s t a b i l i t y of such films can be increased by adding appropriate surfactants. The rate of change i n i n t e r f a c i a l tension with surface area from i t s equilibrium value i s termed as the Gibbs e l a s t i c i t y Ε = 2dy/d(ln A) (9). The factors which control Ε are the rate of transport toward or from the interface and the structure of surfactant as well as the


1.

S H A R M A AND

SHAH

5

Introduction

rate of compression and expansion of the i n t e r f a c e . The f i l m elas t i c i t y also plays an important role to s t a b i l i z e emulsion droplets. As the f i l m i s stretched, the l o c a l concentration of surfactant i n the f i l m decreases. This causes a transient increase i n i n t e r f a c i a l tension. The highest e l a s t i c i t y i n the presence of s u f f i cient amount of surfactant, provides the greatest resistance against stretching (9). Moreover, Prins (19) has shown that a stretched thin f i l m can also break i f the i n t e r f a c i a l tension exceeds a c r i t i c a l value which depends on the system. For low surfactant con centration and rapid stretching, the c r i t i c a l value of i n t e r f a c i a l tension i s attained rapidly. Based on the above mentioned factors, it can be suggested that the surfactant i s of primary importance for the s t a b i l i t y , or f l o c c u l a t i o n of the emulsions. In conclu sion, the i n t e r f a c i a l tension gradients are e s s e n t i a l i n emulsion formation as suggested previously by Tadros and Vincent (20). S t a b i l i t y of Macroemulsions The emulsions are complex systems which present major challenges to the s c i e n t i s t s working i n this f i e l d . Previous investigators applied various t h e o r e t i c a l approaches at the droplet l e v e l and also at the molecular l e v e l to explain the behavior of these systems. The forces such as e l e c t r i c a l double layer, forces between emulsion droplets, hydrodynamic i n e r t i a l forces, entropie (Diffusional) forces and the dispersion forces which act on the droplets or be tween the droplets separated at tens or hundreds of nanometers. Sedimentation and f l o c c u l a t i o n processes involve the forces such as the c e n t r i f u g a l force, applied e l e c t r o s t a t i c force and gravi t a t i o n a l force. Before discussing the emulsion s t a b i l i t y i n terms of these forces, we would l i k e to explain the thermodynamics of emulsion s t a b i l i z a t i o n . Thermodynamic Approach to Emulsion S t a b i l i t y . In this section, we would l i k e to discuss thermodynamic approach to emulsion s t a b i l i t y . Let us assume that the t o t a l free energy of the emulsion can be separated into several independent contributions. Considering hypothetically the formation or coalescence of emulsion of two immis c i b l e l i q u i d s (e.g. oil and water), such that external f i e l d forces are absent. The t o t a l free energy (Gg) of the system j u s t before emulsif i c a t i o n process can be expressed i n the form (10) G

G

+

B - I

G

E

+

G

IE

+

G

(

S

1

)

where Gj, the free energy of the i n t e r n a l phase; G , the free energy of the external phase; Gjg, the free energy of the interface between two l i q u i d s and Gg, the free energy of the interface between the l i q u i d s and the surface of the container. In general, the s o l i d / l i q u i d i n t e r f a c i a l area w i l l be small and therefore, Gg can be ne glected. The free energies, Gj and G , w i l l remain almost the same before and after emulsification, whereas, G w i l l be at a minimum before emulsification. The i n t e r f a c i a l free energy, G j , can be expressed i n the form E

E

I E

E

= ΥΙΕ A

(2)

6


where ΎΙΕ> i n t e r f a c i a l tension and A, the i n t e r f a c i a l area. After emulsification, the i n t e r f a c i a l area greatly increases, therefore, G i s larger than that before emulsification. The free energy of emulsion formation can be written i n the form (10) t n e

I E

AG

Emul.= ^ I E

M

"

T A S

( 3 )

where AS i s the change i n entropy due to the process of emulsification. In general, the free energy of interface (γ^ ΔΑ) term i s much larger than the TAS term. Therefore, the change i n free energy of emulsion formation ( ^ G ) w i l l be p o s i t i v e . This indicates that most of the macroemulsions are thermodynamically unstable or metastable. More over, the free energy of emulsification i s p o s i t i v e ; this means that the free energy of demulsification ( A G ) i s negative. This im p l i e s that an extenral supply of free energy i s needed for the forma t i o n of macroemulsions, and once formed, they are unstable. Tadros and Vincent (20) have shown that the v a r i a t i o n i n free energy change as a function of the demulsification processes (e.g. f l o c c u l a t i o n , coalescence) i s continuous, and there i s no free energy b a r r i e r s to the processes u n t i l the drops are close enough for short range repul sive and a t t r a c t i v e forces. Ε


E m u l

D e m u l e

Short Range Interactions and Emulsion S t a b i l i t y . The s t a b i l i t y of ma croemulsions i n terms of short range (e.g. inter-droplet) interactions w i l l be discussed i n this section. The dispersion (London) forces a r i s e from charge fluctuations within a molecule associated with the e l e c t r o n i c motion (21). Therefore, these forces can operate even be tween nonpolar molecules. London (21) reported an equation for mu tual a t t r a c t i v e energy between two molecules i n vacuum i n the form V

-2- = ^ i (4) ο ,6 ,6 α α where h i s the Planck's constant; V , the c h a r a c t e r i s t i c frequency of the molecule; a, the p o l a r i z a b i l i t y of the molecule and d, the d i s tance between the molecules. The London forces between two molecules are short range as the i s inversely proportional to the s i x t h power of t h e i r separation. Assuming these forces between molecules could be summed f o r a l l the molecules i n a p a r t i c l e of radius ( r ) , the Equation (4) can be expressed (22) i n the form A 4

2

h

v

Q

Α

π

2 120H

l

3 1045H

v a l i d f o r Η > 150 A°, ν

Ar A * ~ Ϊ2Η

λ+3.5πΗ

44 4 ' 5.62x10 H T1

J

K

'

and (6)

v a l i d for Η < 150 A° where A i s the Van der Waals constant; λ , the wavelength of the i n t r i n s i c e l e c t r o n i c o s c i l l a t i o n s of the atoms (~10~5 cm); H, the i n t e r p a r t i c l e distance between droplets and r i s the average radius of droplets. The so c a l l e d Hamaker/Van der Waals constant, A, required to evaluate energy of a t t r a c t i o n between two droplets (Equations 5 and 6) i n vacuum i s defined by

1.

SHARMA AND SHAH

1

Introduction

A = ttVL.

(7)

where L. i s the London constant, B^, the number of atoms (molecules with itft kind contained i n cm of ^"the substance). I f the droplets are suspended i n a continuous medium, then the net i n t e r a c t i o n i s reduced and the Equation 7 can be rewritten i n the form 3

1 / 2

1 / 2

A « (A - A ) o-o w-w

2

(8)


1 3

1 4

and generally l i e s i n the range of 1 0 " to 1 0 " ergs (15, 17). The repulsive (e.g. e l e c t r i c a l double layer) forces have been discussed i n d e t a i l i n l i t e r a t u r e (23,25). The repulsive energy derived by Derjaguin and Kassakov (26) i s given by ,2 V

K H

R

= -y-^ l n ( l + e

)

(9)

where ε i s the d i e l e c t r i c constant of the medium; ψ , the surface potential and κ i s the r e c i p r o c a l "thickness" of the e l e c t r i c a l double layer, given by _ K

2 2>l/2 8πηζ e

(10)

" i ekT

where ζ i s the valency of counter ions; e, the e l e c t r o n i c charge; n, the number of ions per cm i n the s o l u t i o n ; k, the Boltzmann's constant and T, the absolute temperature. The t o t a l i n t e r a c t i o n energy i s obtained by summing the and V contributions. A schematic representation of these energies (_3) i s given i n Figure 3. This curve shows a primary minimum at very short distances between the droplets (e.g. close contact), a maxi mum at intermediate interdroplet distances and a secondary minimum at large interdroplet distnaces. For i r r e v e r s i b l e f l o c c u l a t i o n into the primary minimum to occur, the energy b a r r i e r has to be surmounted. The height of the b a r r i e r i s primarily controlled by the e l e c t r o l y t e concentration (15, 17). The r e v e r s i b l e f l o c c u l a t i o n may occur i n the secondary minimum as reported previously by several i n v e s t i g a tors (15,17,27). For surfactant s t a b i l i z e d emulsions, it has been reported that the energy b a r r i e r s obtained experimentally are very high, which prevents the occurrence of f l o c c u l a t i o n i n primary mini mum (15,27). 3

R

Long Range Interactions and Emulsion S t a b i l i t y . The processes such as sedimentation, creaming and f l o c c u l a t i o n can be c o n t r o l l e d by external forces, e.g. c e n t r i f u g a l , g r a v i t a t i o n a l or applied e l e c t r o s t a t i c forces. These forces are considered to be e s s e n t i a l l y long range. In t h i s section we would l i k e to discuss processes controlled by long range forces. Sedimentation and Creaming. The creaming and sedimentation processes occur i n emulsion systems mainly due to the density difference be tween the dispersed and continuous phases. Assuming a steady state,


8

MACRO- A N D MICROEMULSIONS

Figure 3.

Schematic i l l u s t r a t i o n of interaction energies as a function of i n t e r p a r t i c l e distance between two droplets.

1.

SHARMA AND SHAH

Introduction

9

the sedimentation or creaming rate (V) of non-interacting spherical droplets of radius (r) can be determined by equating two oppositely acting g r a v i t a t i o n a l and hydrodynamic forces as given by Stockes Law (28) i n the form 1

j π r

3

Apg = oTrn^rV

and


V=

(11)

In various emulsion systems, the main objective i s to decrease the rate of sedimentation or creaming rather than to increase it. Equation 11 indicates that this can be achieved by increasing n or decreasing Δρ. The former may be achieved by the addition of a structuring or g e l l i n g agent such as polymers, s i l i c a , etc., whereas, the l a t t e r by adding a suitable solvent. Other factors, such as deformation, polydispersity, droplet s i z e , f l o c c u l a t i o n and coales cence also affect the processes of sedimentation and creaming. Both these processes can be studied employing ultracentrifuge (29,30). Q

Flocculation and Coalescence. Flocculation being the primary process, the droplets of the dispersed phase come together to form aggregates. In this process, the droplets have not e n t i r e l y l o s t their i d e n t i t y and the process can be reversible. Since the droplets are surrounded by the double layer, they experience the repulsive effect of the dou ble layer. K i n e t i c a l l y , f l o c c u l a t i o n i s a second order reaction since it depends i n the f i r s t instance on the c o l l i s i o n of two drop l e t s and i s expressed i n the form (31) dn dt

2

1 ο

or 1/n. - 1/n = Κ t 1 ο 1

(12)

where η and n^ are the number of droplets present i n i t i a l l y and after time t. The rate constant (K-) depends upon the frequency of the c o l l i s i o n between droplets and i s governed by the repulsive forces between them. This constant i s also known as Smoluchowski s constant. The values of K- reported i n the l i t e r a t u r e are of the order of 1 0 " (15). Coalescence being the secondary process, the number of d i s t i n c t droplets decreases leading to a stage of i r r e v e r s i b i l i t y and f i n a l l y complete demulsification takes place. Coalescence rate very l i k e l y depends primarily on the f i l m - f i l m repulsion, f i l m drainage and on the degree of k i n e t i c s of desorption. K i n e t i c a l l y , coales cence i s a unimolecular process and the p r o b a b i l i t y of merging of two droplets i n an aggregate i s assumed not to affect the s t a b i l i t y at other point of contact (32). 1

1 3

dn,

10


or - In η = K t (13) Ο L L where n^ i s the number of i n d i v i d u a l droplets a f t e r time t. The rate of coalescence depends on the l a t e r a l adhesion properties of the surfactant f i l m at the interface. The coagulation process comprises the f l o c c u l a t i o n and coalescence of the system. These processes can be determined experimentally by l i g h t scattering, droplet counting and c e n t r i f u g a l methods. The t h e o r e t i c a l and experimental discussion of this topic i s given by the previous investigators (33-35). Macro- and Microemulsions Downloaded from pubs.acs.org by GEORGE MASON UNIV on 03/12/16. For personal use only.

In η

0

Microemulsions Microemulsions were f i r s t introduced by Schulman et. a l (36) i n 1943. The various properties of these systems were studied during the following years (37-40), and i n 1955 (39), the systems were c a l l e d both swollen m i c e l l a r solutions and transparent emulsions. This ambiguity i n the microemulsion terminology remains today (41). The microemulsions are defined as the clear thermodynamically stable dispersions of two immiscible l i q u i d s containing appropriate amounts of surfactants or surfactants and cosurfactants. The dispersed phase consists of small droplets with diameter i n the range of 100-1000A°. Because of these properties, such systems have several advantages over macroemulsions for i n d u s t r i a l applications. The small droplet s i z e i n microemulsions also leads to a large surface-to-volume r a t i o i n an oil-water system. This i s important for chemical reactions i n which the rate of reaction depends on the i n t e r f a c i a l area. The microemulsion can also be c l a s s i f i e d as W/0 or 0/W s i m i l a r to macroemulsion systems. Mechanism of Microemulsion Formation During the l a s t four decades, several investigators have proposed various mechanisms of microemulsion formation. The following i s a b r i e f description of these mechanisms. I n t e r f a c i a l Tension i n Microemulsions. Schulman and his collaborators (42) have postulated that the transient i n t e r f a c i a l tension has to be negative for the spontaneous uptake of water or oil i n microemul sions. During the process of microemulsion formation, one phase breaks up into the maximum number of droplets. The diameter of these droplets depends upon the i n t e r f a c i a l area produced by the surfactant molecules. The transient i n t e r f a c i a l tension (e.g. the spontaneous tendency of the interface to expand) produced by the mixing of the components became zero or a very small p o s i t i v e value at equilibrium. Schulman and h i s co-workers have s p e c i f i c a l l y mentioned that the negative i n t e r f a c i a l tension i s a transient phenomenon and that at equilibrium, the oil/water interface i n a mi croemulsion has either zero or a very small p o s i t i v e i n t e r f a c i a l tension. However, Schulman s explanation of transient i n t e r f a c i a l tension has been misquoted and misunderstood by various investiga tors. Schulman et a l . i n 1959 suggested various possible ways of producing transient negative i n t e r f a c i a l tension and, therefore, the formation of microemulsions. 1


1.

SHARMA AND SHAH

Introduction

11

The concept of transient i n t e r f a c i a l tension has been further extended by Davis and Haydon (43). They described an experiment by I l k o v i c (44) i n which a negative p o t e n t i a l was applied to a mercury drop i n an aqueous solution of a quaternary ammonium compound. At -8 v/cm applied p o t e n t i a l , the spontaneous emulsif i c a t i o n of mercury occurred. The spontaneous emulsification was observed for surfactant concentrations which exhibited negative values for i n t e r f a c i a l tensions upon extrapolation. These results indicate that for spontaneous emulsification, the dynamic i n t e r f a c i a l tension may approach transient negative values. Moreover, this does not mean that at equilibrium, the dispersed droplets w i l l have a negative i n t e r f a c i a l tension. Gerbacia and Rosano (46) have determined the i n t e r f a c i a l tension at oil-water interface a f t e r alcohol i n j e c t i o n into one of the phases. They observed that the i n t e r f a c i a l tension could be temporarily lowered to zero due to the d i f f u s i o n of alcohol through the i n t e r face. They concluded that the d i f f u s i o n of surfactant molecules across the interface i s an important requirement for reducing i n t e r f a c i a l tension temporarily to zero as well as for the formation of microemulsions. They further claimed that the formation of mi croemulsions depend on the order i n which components are added. It has also been shown from thermodynamic consideration (Equation 3), that i f the i n t e r f a c i a l tension i s very low, the thermodynamically stable emulsions can be formed. Previous investigators (20,45,47,48) have calculated that for a s i t u a t i o n l i k e l y to occur i n microemulsion formation, the i n t e r f a c i a l tensions would need to be i n the order of 10~ to 10~5 dynes/cm for thermodynamic s t a b i l i z a t i o n and for spontaneous formation of microemulsions. 4

Double Layer Interactions and I n t e r f a c i a l Charge. Schulman et a l (42) have proposed that the phase continuity can be controlled readily by i n t e r f a c i a l charge. I f the concentration of the counterions f o r the i o n i c surfactant i s higher and the d i f f u s e e l e c t r i c a l double layer at the interface i s compressed, water-in-oil microemulsions are formed. I f the concentration of the counterions i s s u f f i c i e n t l y decreased to produce a charge at the oil-water i n t e r f a c e , the system presumably inverts to an oil-in-water type microemulsion. I t was also proposed that for the droplets of spherical shape, the r e s u l t i n g microemulsions are i s o t r o p i c and exhibit Newtonian flow behavior with one diffused band i n X-ray d i f f r a c t i o n pattern. Moreover, f o r droplets of c y l i n d r i c a l shape, the r e s u l t i n g microemulsions are o p t i c a l l y anisotropic and non-Newtonian flow behavior with two d i fused bands i n X-ray d i f f r a c t i o n (9). The concept of molecular i n teractions at the oil-water interface for the formation of micro emulsions was further extended by Prince (49). Prince (50) also discussed the differences i n s o l u b i l i z a t i o n i n m i c e l l a r and micro emulsion systems. Scriven (78) proposed the r o l e of the e l e c t r i c a l double layer and molecular interactions i n the formation and s t a b i l i t y of microemulsions. According to them, the t o t a l i n t e r f a c i a l tension (γ ) can be expressed i n the form (14)


12


where γ i s the phase i n t e r f a c i a l tension which i s that part of the excess ^tangential stress which does not a r i s e i n the region of the d i f f u s e double layer and -γ^ i s the tension of the d i f f u s e double layer. This equation suggests that when exceeds Yp, the t o t a l i n t e r f a c i a l tension (γ^) becomes negative. For a plane interface, the d e s t a b i l i z i n g e f f e c t of a d i f f u s e layer i s primarily due to a negative contribution to the i n t e r f a c i a l tension. Adamson (51) proposed a model for W/0 microemulsion formation i n terms of a balance between Laplace pressure associated with the i n t e r f a c i a l tension at the oil/water interface and the Donnan Osmotic pressure due to the t o t a l higher ionic concentration i n the i n t e r i o r of aqueous droplets i n oil phase. The microemulsion phase can exist i n equilibrium with an e s s e n t i a l l y non-colloidal aqueous second phase provided there i s an added e l e c t r o l y t e d i s t r i b u t e d between droplet's aqueous i n t e r i o r and the external aqueous medium. Both aqueous media contain some alcohol and the t o t a l i o n i c concentration inside the aqueous droplet exceeds that i n the external aqueous phase. This model was further modified (52) for W/0 microemulsions to allow for the d i f f u s e double layer i n the i n t e r i o r of aqueous droplets. Levine and Robinson (52) proposed a r e l a t i o n governing the equilibrium of the droplet for 1-1 e l e c t r o l y t e , which was based on a balance between the surface tension of the f i l m at the boundary i n i t s charged state and the Maxwell e l e c t r o s t a t i c stress associated with the e l e c t r i c f i e l d i n the i n t e r n a l d i f f u s e double layer. In addition, Shinoda and Friberg (53) have summarized their extensive studies on the formation of microemulsions using nonionic surfactants. They proposed the following conditions to form mi croemulsions with minimum amount of surfactants: 1.

2. 3. 4.

Microemulsions should be formed near or at the phase inversion temperature (PIT) or HLB temperature for a given nonionic sur factant, since the s o l u b i l i z a t i o n of oil (or water) i n an aqueous (or nonaqueous) solution of nonionic surfactant shows a maximum at this temperature. The larger the s i z e of the nonionic surfactant, the greater i s the s o l u b i l i z a t i o n of oil i n water. The mixing r a t i o of surfactants should be such that it produces an optimum HLB value for the mixture. The closer the phase inversion temperature (PIT) of two surfac tants, the greater i s the s o l u b i l i z a t i o n and therefore, the minimum amount of the nonionic surfactants i s needed.

S t a b i l i t y and Structural Aspects of Microemulsions Several attempts have been made to explain the s t a b i l i t y and struc t u r a l aspects of various microemulsions (54-60). In this section, we would l i k e to describe some of the important aspects of micro emulsion s t a b i l i t y . S t a b i l i t y of Microemulsions. The f i r s t attempt to describe the mi croemulsion s t a b i l i t y i n terms of d i f f e r e n t free energy components was made by Ruckenstein and Chi (55) who evaluated the enthalpic (Van der Waals p o t e n t i a l , i n t e r f a c i a l free energy and the p o t e n t i a l due to the compression of the d i f f u s e double layer) and entropie

1.

SHARMA AND

SHAH

13

introduction

components. The q u a l i t a t i v e results of this model (55) for the given free energy charge (Ag) as a function of droplet s i z e (R) are shown i n Figure 4. This figure shows three curves depending on the values of uncharged surface free energy ( f ) . I f it i s very small, a stable dispersion of small droplets (microemulsion) can e x i s t . I f it i s too large, the dispersion cannot e x i s t . Moreover, for intermediate values, the metastable emulsions of large droplets can e x i s t . These three cases exist i n nature which indicates the v a l i d i t y of the model. This model also predicts the actual s i z e range of stable droplets. This treatment can also predict the occurrence of phase inversion. Phase inversion occurs at that volume f r a c t i o n for which the values of change i n free energy for both kinds of microemulsions are the same. In several recent papers, Ruckenstein and h i s co-workers (58, 61-63) have discussed the thermodynamic s t a b i l i t y of microemulsion systems. Eicke (64) has also explained the e f f e c t of cosurfactants on the thermodynamic s t a b i l i t y of microemulsion systems i n the presence of an additive. The presence of such an additive w i l l decrease or increase the m i s c i b i l i t y of the two-component system. An attempt has also been made by M i l l e r and Neogi (65) to explain the thermodynamic s t a b i l i t y i n terms of chemical p o t e n t i a l of the two phases, the i n t e r p a r t i c l e potentials, the entropie contribution and the i n t e r f a c i a l free energy. The k i n e t i c s t a b i l i t y of microemulsions has also been described by Eicke (66,67) using fluorescence technique to detect a rapid exchange of e l e c t r o l y t e solutions and water between inverse micelles i n iso-octane a r i s i n g from c o l l i s i o n s . Gerbacia and Rosano (46) experimentally observed that some W/0 microemulsions consist of surfactant-plus-cosurfactant concentration of 16% were not stable on a long term basis even i n the presence of a small amount of water (2.6%). A theory was presented (46) to explain the k i n e t i c s t a b i l i t y using microemulsion droplets as a model. The theory i s e s s e n t i a l l y limited to the energy changes occurring i n the layers of surfactant and cosurfactant during coalescence of water droplets of microemulsions. The evaluation of the free energy changes i n the i n t e r f a c i a l f i l m was based on the regular solution theory, both for entropie and enthalpic components.


s

Structural Aspects of Microemulsions. Several investigators have studied the structure of microemulsions using various techniques such as u l t r a c e n t r i f u g a t i o n , high resolution NMR, spin-spin relaxation time, ultrasonic absorption, p-jump, T-jump, stopped-flow, e l e c t r i c a l resistance and v i s c o s i t y measurements (56-58). The useful compilation of d i f f e r e n t studies on this subject i s found i n the books by Robb (68) and Shah and Schechter (69). Several s t r u c t u r a l models of microemulsions have been proposed and we w i l l discuss only a few important studies here. Based on various physical techniques, Shah et a l . (70) have proposed structures for the microemulsion and c o s o l u b i l i z e d systems (Figure 5). Two i s o t r o p i c clear systems with i d e n t i c a l compositions, except that one contains n-pentanol and the other n-hexanol, are s t r u c t u r a l l y quite d i s s i m i l a r systems. The proposed structure for the pentanol containing system i s a c o s o l u b i l i z e d system i n which one can v i s u a l i z e the surfactant and the cosurfactant forming a l i q u i d which can dissolve both water or oil as a molecular solution,


14

MACRO- AND

0 < f

Figure 4.

MICROEMULSIONS

SA SB SC < f

< f

Schematic i l l u s t r a t i o n of free energy change (Ag) as a function of droplet s i z e (R)

Pentanol

Hexanol

Cosolubilization (Molecular solution)

• Ο Ο

Microemulsion (Water-in-Oil)

Water I

Alkyl Alcohol ι Potassium Oleate

VSSSSSSAVA Hexadecane

Figure 5.

Schematic i l l u s t r a t i o n of the structure of c o s o l u b i l i z e d and microemulsion systems


1.

SHARMA AND SHAH

Introduction

15

whereas hexanol containing system i s a true water-in-oil microemulsion i n which water i s present as spherical droplets. The structures shown i n Figure 5 are schematic and should not be considered r i g i d l y . These authors have also mentioned that there may be small aggregates of water molecules or surfactant and cosurfactant molecules i n the cosolubilized system. However, the structure of cosolubilized system shown i n Figure 5 i s consistent with the change i n e l e c t r i c a l resistance upon addition of water to such systems. For microemulsion system (hexanol system) , since it contains water spheres i n a continuous oil medium, the addition of water forms more spherical droplets. The continuous medium i s s t i l l an oil phase and therefore, the e l e c t r i c a l resistance i s maintained at a high value i n the range of 10~-> ohms (70). However, for cosolub i l i z e d system (pentanol system), as the amount of water i s increased, the average distance between alcohol molecules as w e l l as between water molecules would change and this consequently would influence the hydrogen bonding a b i l i t y of water and alcohol molecules, which in turn would influence the chemical s h i f t of the resonance peak. Moreover, as one adds more and more water i n cosolubilized system, it becomes more e l e c t r i c a l l y conducting and, hence exhibits a continuous decrease i n the e l e c t r i c a l resistance (70). In summary, these authors (70) have proposed that the transparent, i s o t r o p i c , clear, stable systems prepared from oil/water/emuls i f i e r can be c l a s s i f i e d into one of three main categories: Normal or reversed micelles, water-in-oil or oil-in-water microemulsions or cosolubilized systems. One can distinguish these classes of structures by using a combination of physical techniques to study the properties of such systems. From the results of s e l f - d i f f u s i o n , Lindman et a l . (71) have proposed the structure of microemulsions as either the systems have a bicontinuous (e.g. both oil and water continuous) structure or the aggregates present have interfaces which are e a s i l y deformable and f l e x i b l e and open up on a very short time scale. This group has become more i n c l i n e d to believe that the l a t t e r proposed structure of microemulsion i s more r e a l i s t i c and close to the correct description. However, no doubt much more experimental and t h e o r e t i c a l investigations are needed to understand the dynamic structure of these systems. The ultrasonic absorption i n r e l a t i o n to the transitions and c r i t i c a l phenomena i n microemulsions has been studied by Lang et a l . (72). The ultrasonic absorption i s very sensitive to the concent r a t i o n fluctuations which occur near the c r i t i c a l temperature or composition i n binary l i q u i d s . Similar absorption maxima were also expected as the composition of the systems was varied i n the v i c i n i t y of composition where water-in-oil microemulsions convert into the oil-in-water microemulsions. However, the most puzzling feature of these data i s probably the very continuous change of the relaxation parameters with composition even i n the range where W/0 microemulsions turn into 0/W microemulsions. Friberg et a l . (73) have proposed a random structure of microemulsions with varying curvatures. Taupin and co-workers (74) have considered the presence of hard oil and water droplets with a r e l a t i v e l y sharp t r a n s i t i o n between these, while Shinoda (J3L 2ÎÙ proposed a lamellar structure with alternating water, amphiphilic N

9

A

S

16


and hydrocarbon layers. Talmon and Prager (77) have suggested a hard randomly arranged hydrophobic and hydrophilic polyhedra, whereas Scriven (78) has viewed the middle phase microemulsions as a complex periodic three dimensional networks with both hydrocarbon and water continuity. I t i s certain that future investigations on the structure of microemulsions w i l l reveal many interesting s t r u c t u r a l c h a r a c t e r i s t i c s and d i v e r s i t y of phenomena i n these systems.


Literature Cited 1. Becher, P. "Emulsions: Theory and Practice", Krieger Pub.: New York, 1977; p. 95. 2. Manegold, E. "Emulsionen, Chemie und Technic", Strassenbau: Heidelberg, 1952. 3. Kruyt, H.R. "Colloid Science", Vol. 1, Elsevier Pub.: New York, 1952. 4. Clayton, W. "Theory of Emulsion and Emulsification", Churchill: London, 1923. 5. Bancroft, W.D. "Applied Colloid Chemistry", McGraw-Hill: New York, 1932. 6. Sherman, P. "Emulsion Science", Academic Press: New York, 1963. 7. Beeker, P. "Emulsions: Theory and Practice", 2nd Ed., Van Nostrand Reinhold: New York, 1965. 8. Lissant, K.L., ed. "Emulsions and Emulsion Technology", Part I, Dekker: New York, 1975. 9. Becher, P., ed. "Encyclopedia of Emulsion Technology," Dekker: New York, 1983; Vol. 1 10. Lissant, K.L., ed. "Emulsions and Emulsion Technology", Parts II and III, Dekker: New York, 1976 & 1984. 11. Gouda, J.H.: Joos, P. Chem. Eng. Sci. 1978, 30, 521. 12. Sharma, M.K.; Sharma, M.; Jain, S.P.; Srivastava, S.N. J. Colloid & Int. Sci. 1978, 64, 179. 13. Torza, S.; Cox, R.G.; Mason, S.G. J. Colloid Int. Sci. 1972, 38, 395. 14. Straeve, P.; Varanasi, P.P. Separation & Purification Methods 1982, 11(1), 29. 15. Sharma, M.K.; Srivastava, S.N. Colloid & Poly. Sci. 1977, 255, 887, Agra Univ. J. Res. (Sci.) 1974, 23, 35. 16. Overbeek, J.G. J. Colloid Interface Sci. 1977, 58, 408. 17. Sharma, M.K.; Bahadur, P. and Srivastava, S.H., Indian J. Technol., 1075, 13, 419. 18. Walstra, P. in "Encyclopedia of Emulsion Technology", P. Becher, Ed.; Dekker: New York, 1983. 19. Prins, A. in "Foams"; R.J. Ackers, Ed.; Academic Press: London, 1972. 20. Tadros, T.F. and Vincent, B. in "Encyclopedia of Emulsion Technology"; P. Becher, Ed.; Dekker: New York, 1983. 21. London, F. Z. Phys. 1930, 63245. 22. Schenker, J.H.; Kitchener, J.A. Trans. Faraday Soc. 1960, 56, 161. 23. Verwey, E.W. and Overbeek, T.G. "Theory of Stability of Lyphobic Colloids", Amsterdam, 1948. 24. Bell, G.D.; Peterson, G. J. Colloid Interface Sci. 1972, 42, 542.

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25. 26. 27. 28. 29.


30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60.

Introduction

17

Devereux, O.F. and de Bruyn, P.L. Ph.D. Thesis, "Interaction of Plane Parallel Double Layers", MIT, Cambridge, Mass., 1963. Derjaguin, B.; Kussakov, M. Acta Phys. Chem. USSR, 1939, 10, 25 and 153. Sharma, M.K.; Srivastava, S.N. Indian J. Technol. 1977, 15, 82. Stokes, G.G. Phil. Mag. 1851, 1, 337. Levich, V.G. "Physicochemical Hydrodynamics", Prentice-Hall: New York, 1962. O'Brien, R.N.; Echer, A.I.; Leja, J. Zh. Fiz. Khim. 1947, 21, 1183. Smoluchowski, M.V. Z. Phys. Chem., 1917, 92, 129. Van den Tempel, M. Ind. Intern. Cong. Surf. Act. Vol. 1, 1957, p. 439. Muller, H. Kolloid-Z. 1926, 38, 1 Spielman, L.A. J. Colloid Interface Sci. 1970, 36, 562. Van den Tempel, M. Rec. Trav. Chem. 1953, 72, 433 and 442. Hoar, T.P.; Schulman, J.H. Nature 1963, 152, 102. Schulman, J.H.; McRoberts, T.J. Trans. Faraday Soc. 1946, 42B, 165. Schulman, J.H.; Riley, D.P. J. Colloid Sci. 1948, 3, 383. Bowcott, J.E.; Schulman, J.H. Z. Electrochem. 1955, 59, 283. Schulman, J.H.; Friend, J.A. J. Colloid Sci. 1949, 4, 497. Prince, L.M., Ed. "Microemulsions", Academic Press: New York, 1977. Schulman, J.H.; Staeckenius, W.; Prince, L.M. J. Phys. Chem. 1959, 63, 7716. Davis, J.T.; Haydon, D.A. Ind. Intern. Cong. Surf. Activity, Vol. 1, 1957, p. 417. Ilkovic, D. Coll. Trav. Chim. Tchecosl. 1932, 4, 480. Hsieh, W.C.; Manohar, C.; Shah, D.O. J. Colloid Interface Sci. (IN PRESS). Gerbacia, W.; Rosano, H.L. J. Colloid Interface Sci. 1973, 44, 242. Reiss, H. J. Colloid Interface Sci. 1975, 53, 61. Ruckenstein, E.; Chi, J.C. J. Chem.Soc.,Faraday Trans. II. 1975, 71, 1690. Prince, L.M. J. Colloid Interface Sci. 1969, 29, 216. Prince, L.M. J. Colloid Interface Sci. 1975, 52, 182. Adamson, A.W. J. Colloid Interface Sci. 1969, 25, 261. Levine, S.; Robinson, K. J. Phys. Chem. 1972, 76, 876. Shinoda, K.; Friberg, S. Advances in Colloid and Interface Sci. 1960, 4, 281. Lang, J.; Djavanbakht; Zana, R. J. Phys. Chem. 1980, 84, 1541. Ruckenstein, E.; Chi, J.C. J. Chem. Soc. Faraday Trans., II. 1975, 71, 1690. Shah, D.O.; Hamlin, R.M . Science 1971, 171, 483. Bansal, V.Κ.; Chinnaswamy, Κ.; Ramachandran, C.; Shah, D.O. J. Colloid Interface Sci. 1979, 72, 524. Lindman, B.; Kamenka, N.; Kathopoulis, T.M.; Bilsson, P.G. J. Phys. Chem. 1980, 84, 2485. Bansal, V.K.; Shah, D.O.; O'Connell, J.P. J. Colloid Interface Sci. 1980, 75, 462. Sjoblom, E.; Friberg, S.E. J. Colloid Interface Sci. 1978, 67, 16.

18

61. 62. 63.


64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78.


Ruckenstein, E.; Narayanan, R. J. Phys. Chem. 1980, 84, 1349. Ruckenstein, E. Chem. Phys. Letters 1980, 98, 573. Ruckenstein, E. in "Surfactant in Solution-Theoretical and Applied Aspects"; K.L. Mittal, Ed.; Plenum Press: New York, 1983; Vol. 3, p. 1551. Eicke, H.F. J. Colloid Interface Sci. 1979, 68, 440. Miller, C.A.; Neogi, P. J. Amer. Inst. Chem. Eng. 1980, 26, 212. Eicke, H.F. "Topics in Current Chemistry", Springer-Verlag: Berlin, 1980: Vol. 87. Eicke, H.F. J. Colloid Interface Sci. 1975, 52, 65. Robb, I.D., Ed. "Microemulsions", Plenum Press: New York, 1982. Shah, D.O. and Schechter, R.S., Eds. "Improved Oil Recovery by Surfactant and Polymer Flooding", Academic Press: New York, 1977. Shah, D.O.; Walker, R.D., Jr.; Hsieh, W.C.; Shah, N.J.; Dwivedi, S.; Nelander, J.; Pepinsky, R.; Deamer, D.W. SPE 5815 presented at Improved Oil Recovery Symposium, 1976. Lindman, B., Kamenka, N., Brun, Β., Nilsson, P.G. in "Micro emulsions"; I.D. Robb, Ed.; Plenum Press: New York, 1982; p. 115. Lang. J., Djavanbakht, Α., Zana, R. in "Microemulsions"; I.D. Robb, Ed.; Plenum Press: New York, 1982; p. 238. Friberg, S.; Lapczynska, I.; Gillberg, G. J. Colloid Interface Sci. 1976, 56, 19. Lagues, M.; Ober, R.; Taupin, C. J. de Physique Letters 1978, 39, 487. Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 70. Saito, H.; Shinoda, K.; J. Colloid Interface Sci., 1970, 32, 647. Talmon, Y.; Prager, S. J. Chem. Phys. 1978, 69, 517. Scriven, L.E. in "Micellization, Solubilization and Microemul sions"; K.L. Mittal, Ed.; Plenum Press: New York, 1977; Vol. 2, p. 877.

RECEIVED December 20, 1984

Introduction to Macro-and Microemulsions

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