A microemulsion can be defined as a thermodynamically stable, transparent dispersion of two immiscible liquids, stabilized by an interfacial film of surfactants. Microemulsions have many applications in such diverse areas as pharmaceutical preparations, cosmetics, enhanced oil recovery, etc. An important aspect of microemulsion design is the ability to solubilize the maximum amount of one liquid (dispersed phase) into another (continuous phase). This can also be considered as the ability to solubilize a fixed amount of the dispersed phase with a minimum amount of surfactant.

There have been extensive studies done on microemulsions in the last few decades. Many works focus on microemulsions made using ionic surfactants, and most involve one surfactant. As it has been demonstrated that single surfactants will not necessarily produce the best microemulsions, many studies involving addition of a cosurfactant have also been done. This cosurfactant is usually a short chain alcohol, ranging from two carbons (ethanol) to four carbon atoms (butanol), though alcohols up to decanol have been studied by some investigators. Also, the effect of salt in the water phase has also been of great interest, so many studies exist on the effect of pH or electrolyte concentration on microemulsion properties.

The use of mixtures of nonionic surfactants is essential because the benefit of a cosurfactant such as a short chain alcohol is not available. These alcohols are not considered edible and may be irritating to the skin. Since it is known that a single surfactant alone is generally not optimum, mixtures are used to get an improvement in microemulsion properties similar to those obtained by the addition of a cosurfactant.

This chapter of the dissertation will demonstrate the value of mixtures of surfactants in the formulation of microemulsion systems that solubilize large amounts of water in oil. This research will be performed using nonionic surfactants and W/O microemulsions. In order to be more generally useful, we try to develop general rules for the formation of highly solubilizing microemulsions, that will hopefully be more widely applicable than just to the systems studied here. Current design of microemulsion systems involves trial and error methods to arrive at an optimum formulation. The intention of this research is to improve the understanding of the synergism between surfactants in a microemulsion environment, so that highly solubilizing microemulsion systems can be formulated in a more rational, scientific fashion.

Nonionic Surfactants

Nonionic surfactants are studied in the present work because of their unique nature: compared to ionic surfactants they are much less harmful to humans and thus can be considered for food, pharmaceutical, and cosmetic applications. Nonionics are compatible with all other types of surfactants, and are resistant to polyvalent cations (hard water) and high ionic strength solutions.

A good surfactant should have a low solubility in both the bulk and dispersed phases. Nonionic surfactants generally have lower solubility and CMC than ionics. This can be seen by the critical micellization concentration (CMC), which is 8 mM for the anionic surfactant sodium dodecylsulfate, and only 0.06 mM for the nonionic C₁₂E₄ (lauryl tetraethoxylate). These values demonstrate the low solubility of the nonionic surfactant monomer in water.

The molecular structures of nonionic surfactants are usually one of the following: polyoxyethylenated alkylphenols (mostly p-octyl-, p-nonyl-, p-dodecyl-, dinonylphenols), polyoxyethylenated straight chain alcohols, polyoxyethylenated polyoxypropylene glycols, polyoxyethylenated mercaptans, long chain carboxylic acid esters (glyceryl and polyglyceryl esters of natural fatty acids, propylene glycol, sorbitol, and polyoxyethylenated sorbitol esters, polyoxyethylene glycol esters) and alkanolamines (diethanolamine-, isopropanolamine-fatty acid condensates). The edible surfactants are mainly the esters based on glycerol, sorbitol, and propylene glycol [Rosen, 1989].

Surfactants used in foods are usually called emulsifiers [El-Nokaly et al., 1991]. Many of these have been assigned an Acceptable Daily Intake (ADI) value by the FDA. Many food surfactants are fatty acid esters. Raw materials in the synthesis of these surfactants are usually naturally occurring fats and oils. Most are nonionic, the exceptions being succinic, citric, and diacetyl tartaric acid esters. Lecithin is the only approved surfactant containing a positive charge. The following table summarizes some ADI values for emulsifiers.

For cosmetic applications, the preferred nonionic surfactants include polymeric ethers, alkoxylated amines, and ethoxylated sorbitan esters. Specific examples include blends of glyceryl stearate and POE (100) stearate, isostearic acid, cetearyl alcohol, choleth-24, ceteath-24, and stearyl triethanolamine [Zecchino et al., 1991].

Rosen [1992] suggests that commercially available surfactants should be studied, because questions of toxicity, biodegradability, and general environmental impact must now be addressed whenever FDA approval is sought for a new compound, a costly and time consuming procedure. So, in the search for improved surfactant performance and properties, a less costly approach is to investigate combinations of existing acceptable surfactants that show synergy, rather than designing new chemical structures.

HLB: Definition and application. HLB, the hydrophilic-lipophilic balance, has proven to be a useful tool for the design of emulsions. The HLB number is a parameter that has been assigned to each surfactant molecule, and has a range of 0 to 20 for nonionic surfactants [Griffin, 1949; 1954]. A higher number represents a more hydrophilic molecule. This system was designed to score nonionic surfactants, but has been applied to ionic surfactants also. It has been suggested that the HLB number of nonionic surfactants should be a function of temperature, as nonionic surfactants become increasingly hydrophobic with increasing temperature [Shinoda and Friberg, 1986, p. 56].

The HLB temperature, or PIT (phase inversion temperature) is an important parameter for microemulsions containing nonionic surfactants. As temperature is increased through this value, the microemulsion will go through a change from O/W to W/O. There is some correlation between HLB number and PIT, and empirical estimates of the former can be made from the former [Shinoda and Friberg, p. 63]. Shinoda found nonlinear, but monotonic relationship between PIT and HLB for emulsions, using heptane as the oil phase, and various surfactants; POE nonylphenyl ethers, dodecylphenyl ethers, Tweens, and some binary mixtures of the three classes [Shinoda and Friberg, p. 113].

HLB number is used in the design of emulsions by applying the concept of required HLB (RHLB). There are two RHLB values assigned to each oil, one where W/O emulsions should be formed and one for O/W emulsions. These numbers are approximate, to within ± 1 HLB number. It is possible to make a mixture of surfactants, one surfactant with HLB below the RHLB and one surfactant with HLB above, so that a mixture of the surfactants can be found that will have the RHLB value. The HLB of a surfactant mixture is simply the weighted average of the HLB of the two individual components:

Ideally, the RHLB should be independent of the surfactants selected. RHLB is expected to be in the range 3-6 for W/O emulsions and 8-18 for O/W emulsions [ICI, 1984].

HLB number can be estimated in several different ways [Shinoda and Friberg, 1986, p. 66]. One method for polyhydric alcohols with polyoxyethylene is to multiply the ratio of the weight of the hydrophilic part of the molecule with the total weight of the molecule, as in [Griffin, 1949]

Another method for polyhydric alcohol fatty acid esters determines the HLB number as a function of saponification number of the ester and acid number of the acid [Shinoda and Friberg, 1986, p. 69]. A third method assigns a number to each functional group in a molecule, which are combined to compute the HLB number [Shinoda and Friberg, p. 84].

Shinoda et al. [1984] attempt to clarify confusion about HLB number and HLB. The HLB number of a surfactant is a parameter defined by Griffin [1949], and is fixed. HLB, on the other hand, is the hydrophile-lipophile balance of a surfactant, and will change in response to such environmental variables as surfactant composition, type of oil, temperature, valence of counterions, salt concentration, etc. The point is made that solubilization should be studied as a function of the HLB.

Microemulsions

The solubilization of one phase into another in a microemulsion system is affected by a balance of attractive and repulsive forces. As microemulsions are thermodynamically stable, the droplets will not coalesce and precipitate over time. The nature and environment of emulsion and microemulsion droplets must be considered to be completely different. Emulsion droplets are much larger, generally greater than a micron, while microemulsion droplets are in the 10-100 nanometer range. It is clear from this size difference that the curvature at the interface is much more significant for microemulsion droplets. Droplets of both types are affected by thermal motion of the solution and will collide with each other. The interface of emulsion droplets can be considered as a monolayer of surfactant. Colliding emulsion droplets will either repel each other or coalesce. This coalescence is a monotonic process, it will continue to occur over time, resulting in an increase in the average droplet size over time. Microemulsion droplets, on the other hand, behave quite differently. One may consider the interface to be less perfect, with the dispersed phase coming in contact with the continuous phase. Collisions between droplets occur, and while most will repel, some will coalesce and then spontaneously break apart again. This results in a mixing of the contents of the microemulsion droplets. There must be a balance of forces present to drive the coalesced microemulsion droplets into splitting again. Over time, the average droplet size of a microemulsion will remain constant.

A microemulsion can be characterized by the amount of the dispersed phase solubilized in the continuous phase, at a given temperature. This can be done by titration with the dispersed phase while stirring, until a final clear to cloudy transition is observed. A solubilization parameter can be defined as the ratio of the volume of dispersed phase to the volume of the continuous phase present in the microemulsion. Another characterization technique [Abe et al., 1986] is by production of a middle phase (Winsor III) microemulsion, where the microemulsion is in equilibrium with an excess oil phase and an excess water phase. The middle phase contains an equal amount of water and oil, and the solubilization parameter can be defined as the volume ratio of oil (or water) to surfactant.

Maximum solubilization has been reported for many different systems. As an example, Bansal et al. [1979] reports a water/oil ratio of 0.48 in a W/O microemulsion system consisting of sodium stearate, hexadecane, water, and one of three cosurfactants, either n-pentanol, n-hexanol, or n-heptanol. The initial system was composed of 1 g sodium stearate, 4 ml alcohol, and 10 ml hexadecane. A linear relationship is shown between maximum solubilization of water and the surfactant/oil ratio. Reported w/o ratios of 0.16 and 1.12 for 5% and 20% w/v sodium stearate/hexadecane, respectively, were reported.

The theoretical maximum ratio of dispersed phase to total volume is 0.74. This is the volume fraction assuming equal sized spheres packed in the face centered cubic (FCC) or hexagonal close packing (HCP) configurations. The solubilization parameter presented here is the ratio of volume of dispersed phase to volume of continuous phase, so the theoretical maximum of this parameter is 2.85. Several factors work to reduce this value. First, the thickness of the interface is not zero, but rather finite, and gains more relative importance as the radius of the particles diminishes. For an interface 1% of the radius, the theoretical maximum parameters fall to 0.72 and 2.76, and for an interface 10% of the radius these parameters fall to 0.56 and 2.14. One cannot also assume a monodisperse size distribution of particles. Second, a distribution in the particle size will also result in lower maximum parameters, as the ideal packing structure will be disrupted.

Nature of microemulsion droplet. What does a microemulsion droplet look like, and what is its dynamic behavior? Perhaps it is helpful to think of a microemulsion droplet as an intermediate between a micelle and an emulsion droplet. Some researchers even consider a W/O microemulsion droplet to be a hydrated reverse micelle. One can consider an emulsion droplet to be a rather static structure, a large droplet of water suspended in a continuous phase of oil, with the oil/water interface stabilized by a monolayer of surfactant. On the other hand, a micelle is a very dynamic structure. It is a bit deceiving to see drawings of micelles in journals, because this representation is by necessity static, and micelles are actually dynamically changing and loosely bound aggregates. There is an equilibrium and a constant exchange between surfactant monomers and surfactant in the micellar aggregation. Micelles may have lifetimes as short as milliseconds, while emulsion droplets have long lifetimes, without the disintegration we see in micelles, but potentially with the occurrence of some coalescence over time. Given these two extremes of micelles and emulsion droplets, what will be our model for microemulsion droplets? It is known that the contents of mixed microemulsions do mix, this allows reactions to occur using microemulsion droplets as tiny microreactors [Kumar et al., 1993]. Because the water droplet is so small, we know through NMR studies that there exist two significant states of water, the "free" water in the interior of the droplet, and the water at the interface, associated with the surfactant molecules. Since we know that some fraction of microemulsion droplet collisions result in coalescence, yet the mean droplet size in the microemulsion remains constant over time, there must be a driving force to break up these aggregates again. Positive interfacial tension is the driving force for decreasing interfacial area of a given droplet, so there must be some "negative" interfacial tension present to force an increase in interfacial area and the resulting breakup of microemulsion droplets that are too large. At the optimum droplet size, the forces are balanced and the interfacial tension must approach zero. We can envision a microemulsion as a dynamic entity, a tiny droplet surrounded by a monolayer or aggregate of surfactant, sometimes combining with its neighbors, and subsequently breaking apart, sometimes exposing the water phase to the oil phase.

Microemulsions using nonionic surfactants. For oil-in-water (O/W) microemulsions made with ionic surfactants, the primary force preventing coalescence is electrostatic charge repulsion. These microemulsions display the greatest ability to solubilize one phase within another. For W/O microemulsions, and microemulsions made with nonionic surfactants, there is only steric hindrance preventing coalescence of the droplets. This is a weaker force than electrostatic repulsion, so these microemulsions display less solubilization ability. Steric hindrance is affected only by the part of the surfactant molecule exposed to the continuous phase, for example, the hydrophobic region in a W/O microemulsion. Repulsion is enhanced by the length of this region, as well as disorder introduced by such features as double bonds, which introduce bends and interfere with orderly packing of the surfactant molecules at the interface.

Importance of cosurfactant free microemulsions. Microemulsions have traditionally been formed using a single surfactant and a cosurfactant, usually a small molecule. Typical cosurfactants are short chain alcohols, ethanol to butanol, glycols such as propylene glycol, or medium chain alcohols, amines, or acids.

The suitability of cosurfactant depends on the application. For cosmetics [Zecchino et al., 1991], long chain fatty alcohols may be used, from 12-18 carbon atoms. Abe et al. [1986] concludes that the role of the cosurfactant is to destroy liquid crystalline or gel structures that form in place of a microemulsion phase. He also concludes that cosurfactant free microemulsions in most systems (they studied sulfonates) cannot be made except at high temperatures. To achieve a microemulsion phase in the 25-40^oC range, cosurfactant must be used. Kahlweit et al. [1991] and Strey and Jonstroemer [1992] have investigated the effect of medium chain length alcohols in water/octane/POE octylether microemulsions. They characterize the role of the alcohol, and quantify its partitioning behavior between the oil, water, and interface phases. El-Nokaly et al. [1991] summarizes the role of a cosurfactant in the following table:

Reference: [El-Nokaly et al., 1991]

Effect of oil phase. The nature of the oil phase is of great interest in the formulation of W/O microemulsions. Microemulsions are formed well using nonpolar oils such as the normal alkanes, but oils such as triglycerides are important because they are edible. The triglycerides are much more polar than the alkanes, and generally form poor microemulsions. For cosmetic applications, silicone oils such as cyclomethicone may be used, along with a triglyceride and/or squalane as moisturizing components (squalene is 2,6,10,15,19,23-hexamethyltetraeicos-2,6,10,14,18,22-hexene). El-Nokaly et al. [1991] believes that a surfactant with a higher HLB must be used to compensate for this partial polar nature of the oil.

The choice of oil phase depends on the application. For cosmetic formulations, skin irritation is the most important factor. Zecchino et al. [1991] summarizes some oils that are generally acceptable. Hydrocarbons may be 12-18 carbon straight chain or 12-30 carbon branched alkyl compounds. Acceptable animal oils are cod liver oil, lanolin oil, mink oil, orange roughy oil, and shark liver oil. Acceptable vegetable oils include almond, apricot kernel, avocado, castor, coconut, corn, evening primrose, jojoba, olive, safflower, sesame, soybean, and wheat germ oils. Some silicone oils are acceptable, such as certain linear polysiloxanes and cyclic dimethyl polysiloxanes such as cyclomethicone. The solubility of lipophilic surfactants in the oil phase will influence the concentration of surfactant at the interface. Too high a solubility or too high a CMC in the oil phase will result in poor microemulsion solubilization. The CMC of a surfactant will decrease geometrically with molecular weight [Shinoda and Friberg, 1986, p.87].

The type of oil also has a significant effect on the PIT. Table 3-3 summarizes PIT for 10% POE (9.6) nonylphenyl ether in a volume ratio 1.0 water/oil emulsion system. Note that W/O is the state above the PIT and O/W is the state below the PIT [Shinoda and Friberg, p.97]. A lower PIT correlates well with the ability of the oil to dissolve the surfactant. For this case, benzene is the best solvent and hexadecane is the worst. The effect of mixing oils displays no nonlinear effects with respect to volume ratio, from observation of the change in PIT for mixtures of heptane with paraffin, cyclohexane, xylene, and benzene [Shinoda and Friberg, p. 109].

Reference: [Shinoda and Friberg, 1986, p.97]

Experimental techniques for studying microemulsions. Droplet size measurements can be performed using QELS (quasi-elastic light scattering). QELS is a general technique for characterizing solutions of macromolecules or particles by studying the time dependent differences in laser light scattered from a sample solution. Droplet radius cannot be determined directly, but is derived from the diffusion coefficient, given certain assumptions, such as a monodisperse particle size distribution of spherical particles at infinite dilution (noninteracting particles). The Stokes-Einstein relationship is

where k is the Boltzmann’s constant, h is the viscosity, T is the absolute temperature, and D₀ is the diffusion coefficient. The measured radius (R_H) is the hydrodynamic radius, not the precise radius of the particle, as it includes such things as the bound water or oil that moves with the particle.

Several researchers have investigated microemulsion droplet size using light scattering. Hou et al. [1987] reviewed the literature to date on QELS investigations of microemulsions, provided an overview of the theory and limitations of the technique, and investigated water-in-oil microemulsions using AOT as a surfactant. The effect of additives such as NaCl, propanol, heptanol, Arlacel 20, as well as the effect of different alkanes as the oil phase were reviewed. Beckman and Smith [1990] investigated droplet size and diffusion coefficient of w/o microemulsions formed from a mixture of ethoxylated alcohols and acrylamide in a mixture of ethane and propane near the critical point. Aoudia et al. [1991] studied the effect of butanol as a cosurfactant on droplet size and interfacial fluidity for a O/W system of octane, NaCl, water, sodium dodecylbenzenesulfonate and butanol.

Design of Water-in-Oil Microemulsions

Surfactant structure studies. There have been several studies of the effect of surfactant molecular structure on microemulsion formation. It is well known that AOT (bis-2-ethylhexyl sodium sulfosuccinate) forms cosurfactant free W/O microemulsions. This is an anionic surfactant molecule with double branched hydrophobic tails. Because of its unusual shape, with its large two tailed hydrophobic domain, it forms reverse micelles, and thus W/O microemulsions.

Israelachvili [1992] described a packing parameter (R), which predicts the aggregate structure of a micellar solution. R is itself dependent on structural parameters of the surfactant. R is defined as V_H/l_ca₀, where V_H is the volume occupied by the hydrophobic groups in the micellar core, l_c is the length of the hydrophobic group in the core, and a₀ is the cross-sectional area occupied by the hydrophilic group at the micelle-solution interface. The following table shows the expected structure for different R values.

Reference: [Israelachvili, 1992]

The importance of double tailed surfactants was studied by Abe et al. [1986], using polyoxyethylenated alkanesulfonates. Their goal was to characterize the effect of different hydrophobic structures, both double tailed and branched, to produce cosurfactant free microemulsions at higher salinity and lower temperature than previously reported. They produced middle phase (Winsor III) microemulsions, which contain an equal amount of oil and water in a surfactant phase at equilibrium with an excess oil phase and an excess water phase. The phases used were hexadecane and aqueous NaCl solutions, along with polyoxyethylene sulfonates where the hydrophilic group was attached near the middle of a 12-20 carbon atom linear alkane. Their choice of a surfactant series allows for the alteration of the hydrophobic group, by changing the length of the group and the point of attachment of the hydrophilic group, as well as the alteration of the hydrophilic group by changing the length of the polyoxyethylene chain between the hydrophobic structure and the sulfonate. They concluded that alkane sulfonates with chain length of 14-18, and hydrophilic attachment at or near the middle carbon, can form microemulsions without cosurfactant. Microemulsions can be formed with an alkane chain length of 12, when three ethylene oxide units are added to the hydrophilic group. Lowering the surfactant molecular weight improves salt tolerance, but also lowers the solubilization parameter. An increase in NaCl concentration acts to decrease solubilization, as does an increase in temperature. Adding two ethylene oxide units to the hydrophilic group increased tolerance to NaCl concentration as well as temperature. Note that ionic and nonionic surfactants have different partitioning behavior with temperature, ionic surfactants tend to be more hydrophilic with an increase in temperature, while nonionic surfactants tend to be more hydrophobic. It is estimated that CaCl₂ has 4-5 times the effect of NaCl in the aqueous phase.

Abe et al. [1986] concluded that there are three conflicting concepts that must be considered simultaneously when designing a cosolvent free microemulsion system. First, the surfactant must partition equally between the oil and water phases, through optimization of temperature, surfactant hydrophilic group structure, surfactant hydrophobic group structure, oil phase composition, and electrolyte composition in the aqueous phase. Choice of cosolvent and cosolvent concentration also effect partitioning when present. Second, the surfactant molecules must reach maximum linear extension, to produce high solubilization and low interfacial tension. Surfactant concentration must be above the critical micelle concentration. Third, net surfactant lateral interactions must be weak. The system must be above the melting point of all extended structures, such as liquid crystals and gels, so that the more disordered microemulsion state will be thermodynamically most stable. This can be accomplished by shortening the hydrophobe length, increasing hydrophobe branching, adding ethylene oxide units, increasing temperature, and decreasing electrolyte concentration. Adding cosolvent can also help when present. So, through optimization of the hydrophobe branching, surfactant molecular weight, and hydrophilic group, Abe was able to form cosurfactant free microemulsions, at lower temperatures than previously reported, with good solubilization, and at NaCl concentrations from 0.1 M to 3.4 M without affecting microemulsion quality. Note that all surfactants used were ionic, with the polar group attached to the linear alkyl chain in middle carbon positions. Also, all surfactants studied had a limited molecular weight range, from 350 to 410. There was not much difference either in the fully extended length of the surfactant molecules, including the major tail, the ethylene oxide units and the sulfonate group. The best surfactants had three ethylene oxide units. These molecules are not available commercially, as they had all been synthesized for the study.

Reference: [Abe et al., 1986]

Shinoda and Kunieda [1977] discussed formation of microemulsions using nonionic surfactant, with the goal of formulating microemulsions with less surfactant (equivalent to obtaining maximum solubilization with a given amount of surfactant). Hydrophilic nonionic surfactants generally have a long polyoxyethylene chain attached to the hydrophobic part of the molecule. This chain is attached by a polymerization reaction, and the number of monomer units per molecule cannot be controlled exactly. This results in surfactant which contains a distribution of POE chain lengths. The effect of the polyoxyethylene chain length distribution on the phase diagram of a microemulsion system was investigated.

Shinoda and Friberg [1986, p. 36] presents a phase diagram of temperature dependence vs. solvent oil fraction for the water/cyclohexane/5 wt% POE (7.4) nonyl-phenylether system. A similar shaped phase diagram is given for POE chain length vs. solvent oil fraction, where the POE chain length variation is achieved by mixing n=7.4 and n=9.7 polyoxyethylene nonylphenylethers. Both of these graphs show W/O and O/W microemulsion regions, as well as a three phase region. The similarity between these two phase diagrams are an excellent example of the analogy between the change in HLB and a change in temperature for nonionic surfactant microemulsions. Shinoda also presents phase diagrams for other nonylphenylether surfactants, as well as for mixtures of these and ionic surfactants.

Surfactant mixture studies. Characterizing the effect of surfactant mixtures on the solubilization of water in oil is a more difficult problem than the solubilization using just one surfactant as described in the previous section. Since one is now working with at least a four phase system, triangular phase diagrams can no longer be used to represent the data. One way around this is to use a pseudophase diagram, where the vertices now represent a fixed ratio of two components. One can make one vertex water, another oil and one surfactant, and the last oil and the second surfactant. There has been no mention in the literature of enhanced solubilization by mixing surfactants, though several investigators have studied different proportions of cosurfactant to surfactant, where the cosurfactant is a small molecule, usually an alcohol.

It is important to point out that many commercially available surfactants, and especially nonionics, are mixtures. This is a direct result of their synthesis. The hydrophilic group of a nonionic surfactant is usually polyoxyethylene. This is formed by polymerization of ethylene oxide. The product of this reaction is not an exact number of monomers attached to each surfactant molecule, but rather a distribution of chain lengths. The distribution has been analyzed for some commercial surfactant products, and an example is presented by Shinoda and Friberg [1986]. The distribution is usually a Poisson distribution. For block copolymers, there may be a distribution in the hydrophobic domain also. This often involves polymerization of propylene oxide. Some other surfactants may be a blend which has proven to be commercially successful. Neodol 23 (Shell) has a mixture of 12 and 13 carbon chains in its hydrophobic moiety. Arlacel 186 (ICI) is a mixture of glycerol oleate and propylene glycol. Arlacel 83 and Arlacel C (ICI) are mixtures of sorbitan mono- and dioleates. Atlas G1086 and G1096 (ICI) are POE versions of Arlacel 83.

Shinoda et al. [1984] stress the importance of selection of the size of the hydrophilic group of the surfactant, or selection of temperature, in order to adjust HLB and achieve high solubilization. Of course, another way to adjust HLB is to mix surfactants with different HLB numbers. Through the ability to continuously adjust HLB over a range of values, maximum solubilization for the system can be realized. As examples, he presents solubilization data for C₁₂E₅ in tetradecane, and also data for mixtures of water/ decane/ calcium dodecyl ethoxysulfate/ isooctyl monoglyceride, 3% NaCl aq./ decane/ SDS/C₈E₂, 3% NaCl aq./ decane/ SDS/ isooctyl monoglyceride, and 1% NaCl aq./ liquid paraffin/ sodium hexadecyl sulfate/ isooctyl monoglyceride. He found the mixtures of nonionic and anionic surfactants had high solubilization, both on the W/O and the O/W side of the phase diagram.

Shinoda et al. [1971] investigated the effect of the size and distribution of the oxyethylene chain on stability of emulsions using nonionic surfactants. He used POE nonylphenylethers and POE dodecylethers, concluding that the stability of an emulsion increased when the hydrophilic groups are broadly distributed. It was also observed that the PIT became ambiguous and changed sensitively with concentration for surfactant mixtures containing of widely different HLB numbers. It is thus expected that mixing long and short oxyethylene chain homologs will increase the stability of emulsions.

Very few microemulsion studies exist in the literature involving cosurfactant-free formulations of nonionic surfactant mixtures. Only two were found, by Lissant [1974] and Shinoda and Friberg [1986], and these did not provide any insight into optimum mixing of surfactants. The following table summarizes relevant surfactant mixture studies found in the literature.

The goal of this effort is to discover a means to solubilize a maximum amount of water into an oil phase, in microemulsion form. Mixtures of surfactants will be used to achieve better results than possible with any single surfactant, taking advantage of synergism between the surfactants. By solubilization experiments the microemulsion region of the phase diagram can be mapped out for different surfactant mixtures. Confirmation of the existence of microemulsion droplets can be made with quasi-elastic light scattering, to estimate microemulsion droplet sizes, and using polarized filters, which allow detection of liquid crystal phases.

Solubilization experiments were performed by titrating water into mixtures of surfactant and hexadecane. Given a fixed surfactant to oil ratio and a specific oil phase, the only freedom remaining is the choice of surfactant. Single surfactants have been tested, along with binary mixtures of oil soluble and water soluble surfactants. For these binary mixtures, the oil soluble surfactant was a member of the Span or Arlacel family (sorbitan esters), and the water soluble surfactant was a Tween (polyoxyethylenated sorbitan esters).

Design rules for the choice of surfactants, as well as the optimum surfactant ratio, for maximum solubilization are sought. Of all previous work on microemulsions described in the introduction of this chapter, little exists on nonionic surfactant mixtures, and no design guidelines are apparent. Rules analogous to the guidelines for the use of surfactant HLB to select surfactants and mixture ratios for emulsification (macroemulsions) [Griffin, 1949; 1954], and the packing parameter of Israelachvili [1976; 1992] for determination of surfactant aggregation structure, would be very useful in developing microemulsions for a variety of applications.

In this thesis clear examples of synergism in mixtures of nonionic surfactants are presented, and several design rules for formulating microemulsions are proposed.

Experimental Procedure

W/O solubilization. Solubilization is a very simple experimental technique. Surfactants are mixed with the oil phase and agitated. In most cases, the surfactants will dissolve in the oil. In some cases the mixture will separate, and continuous stirring may be necessary to achieve homogeneity before titration. Water or aqueous solution is added dropwise into the mixture until a permanent clear to cloudy phase transition is observed, which represents the maximum water solubilization for that particular oil and surfactant mixture. For the cases where the surfactant does not dissolve in the oil phase, the initial mixture will be cloudy and addition of water may result in a transition of cloudy to clear solution, representing the minimum solubilization for the surfactant mixture. The kinetics of solubilization vary, initially water is solubilized very fast, limited by the stirring speed. Near the end point, though, solubilization can be very slow, often taking hours for the cloudy (or hazy) solution to clear up. Solubilization was performed by titrating water dropwise into a prepared mixture of oil and surfactant, while stirring. 3.0 g of total surfactant were mixed 7.0 ml of hexadecane. This amounts to 42.9% w/v or 35.7 wt% initial surfactant. Water used was deionized and distilled. Experiments were performed at 25^oC.

Liquid crystals. One complication is the potential formation of a liquid crystalline phase. Under certain conditions a combination of oil, water and surfactant will result in a phase where there are orderly planes of oil and water separated by monomolecular layers of surfactant. Formation of these liquid crystals must be prevented if a microemulsion phase is desired. Several authors have addressed the factors necessary for the formation of liquid crystals. For the surfactant combinations studied in this dissertation, it is quite common for liquid crystals to form spontaneously for mixtures with an HLB of greater than 11. Israelachvili [1992] reports that liquid crystals (lamellar structures) will form for a certain range of R values (see Introduction of this Chapter). Liquid crystal formation can be detected by two methods; 1) a large increase in viscosity should be observed, and 2) checking with polarizing filters, one may see that the solution is birefringent, and thus has undergone a transition from an isotropic microemulsion solution to an ordered liquid phase.

Materials. Arlacel, Brij, Span and Tween families of surfactants were supplied by ICI Americas Inc. (Wilmington, DE). Several different oils were used in the investigations. The alkane series of decane, dodecane, tetradecane and hexadecane were purchased from Fisher Scientific (Fair Lawn, NJ). Water used was distilled and deionized.

Binary Mixture Results and Discussion

The intent of this series of experiments is to obtain high solubilization levels by mixing a more hydrophobic with a more hydrophilic surfactant. For the following examples, Tween 80 or Tween 85 are used as the more hydrophilic surfactant, and Span or Arlacel types are used as the more hydrophobic surfactant.

Synergistic mixtures of surfactants show increased solubilization capacity for many of the mixtures studied. A maximum value of solubilization is seen for most Span/Tween mixtures at a certain ratio. These solubilizations are much higher than any microemulsion made with a single surfactant. Several mixtures can achieve solubilization of greater than 0.5 (v/v) water/oil, with the Arlacel 83/Tween 85 system achieving a maximum of 0.78 water/oil (see Table 3-7).

Cases with synergism. The binary surfactant mixture results are graphed with the Tween surfactant being the common factor, and the lines being defined by different Span surfactants. Figure 3-1 shows mixtures of Tween 85 with Arlacel 83, Span 20, 80 and 85. Figure 3-2 shows Tween 80 with the same Spans. Figure 3-3 shows Tween 20 with Span 20 and 80. Figure 3-4 shows Tween 21 with Span 20 and 80.

The Tween 85 systems show the greatest solubilization. The buildup to maximum solubilization with increasing HLB number is much more gradual than the sharp falloff after the maximum is achieved. This appears to be analogous to solubilization curves presented by Kon-No et al. [1971] who studied the effect of temperature on solubilization in W/O microemulsions. Though he used ionic surfactants, his graphs show multiple phase changes for a given temperature near the region of maximum solubilization. This could explain the extremely sharp drop-off that is seen, as the second microemulsion region that occurs when more water is added beyond the initial maximum solubilization region is not mapped out. For nonionic surfactants, the hydrophilicity of the surfactant is increased with decreasing temperature, so the temperature studies of Kon-No et al. [1971] may be qualitatively analogous to the mixture N_HLB excursions presented here.

Tween 80 showed maximum synergism with Span 20. Solubilization was also reasonable with Span 80. No synergism was observed with Span 85. As the HLB of Tween 80 is 15, higher HLB mixtures could be made than with Tween 85. This resulted in many
 

(Figure 3-1. W/O solubilization, binary mixtures of Spans with Tween 85.)
 

(Figure 3-2. W/O solubilization, binary mixtures of Spans with Tween 80.)
 

(Figure 3-3. W/O solubilization, binary mixtures of Spans with Tween 20.)
 

(Figure 3-4. W/O solubilization, binary mixtures of Spans with Tween 21.)
 

mixtures which solubilized significant water, but in the form of liquid crystals rather than microemulsions. Generally, liquid crystalline states were observed for mixtures with N_HLB greater than or equal to 12.

Synergism was less obvious with the other Tweens. Tween 20 and Tween 21 systems showed no maximum solubilization behavior. Solubilization was generally low, and increased with N_HLB until the onset of liquid crystalline states in the 10-11 range. It seems that much higher solubilization would be possible with these Tweens if the liquid crystal region occurred at higher HLB.

Cases without synergism. One of the exceptions showing no synergism is mixtures using Tween 81. This surfactant by itself displays an unusually high solubilization, and any mixture of Tween 81 with other Spans shows no improvement. Since Tween 81 has the lowest HLB (10) for any of the Tweens, mixtures with other Tweens were attempted to see if there was any improvement. No enhancement was seen with any of the Tween 81/Tween mixtures. Figure 3-5 shows Tween 81 with Span 20 and 80, as well as Tween 20, 80 and 85.

Another exception showing no synergism are mixtures involving Span 85. No enhancement in solubility is seen by mixing Span 85 with any Tween. As this is the most hydrophobic surfactant studied, with an HLB number of 1.8, it is possible that the majority of the Span 85 partitions into the oil phase and is not present to any appreciable extent at the interface. As seen in Figure 3-6, solubilization for mixtures of Span 85 and Tween 85 over the range studied corresponds quite well with the level predicted for hydration. This implies that the mixture may exist as a molecular mixture, with no formation of structure, such as
 

(Figure 3-5. W/O solubilization, binary mixtures of Spans with Tween 81.)
 

(Figure 3-6. Solubilization assuming hydration, binary mixtures of Spans with Tween 85.)
 

microemulsion droplets. Also, the QELS experiments on Span 85/Tween 85 mixtures showed little scattering. This would confirm the idea that this surfactant combination does not form microemulsion droplets, and the solubilization is only due to the hydration of the surfactant.

Hydration of the surfactant. Figure 3-6 shows solubilization curves for Span 80 and Span 85 with Tween 85, along with the estimated solubilization that would occur due to hydration, where only one water molecule per oxygen in the surfactant is solubilized. The graphs suggest that microemulsions may be formed for mixtures where excess solubilization occurs above the hydration line.

It may be assumed that the initial water added to a surfactant/oil mixture will result in hydration of the surfactant, and will not form microemulsion droplets. The solubilization by the Span 80/Tween 85 mixtures are mapped out in Figure 3-6, along with the hydration estimate. There are several regions of interest. It can be seen that for HLB values over 9.5, the solubilization tracks the hydration calculation quite well. No microemulsion droplets are formed here, as confirmed by QELS. An HLB of 11 is the upper limit for this mixture, as it is the HLB of Tween 85. Between HLB 8 and 9.5, there is a large excess of solubilization over the hydration limit. This is the region where a synergism between the surfactants is quite constructive in stabilizing the water droplets in the hexadecane. At HLB values of less than 8, one can see that the solubilization drops off to values below that predicted by hydration. In this regime, something must be preventing the surfactant from hydrating, as it does in the region from HLB of 9.5 to 11.

Formulation rules. HLB design rules may be found for microemulsions, analogous to the emulsion rules of Griffin [1949]. The optimum W/O microemulsion solubilization occurs in the HLB range of 8.5 to 11. This differs from the HLB range (3 to 6) that one would expect for W/O emulsions [ICI,1984]. Table 3-7 summarizes the maximum solubilization of water in hexadecane observed for various surfactants and binary combinations. Another HLB based rule is the onset of liquid crystallinity in the solution. Above an HLB of 12, apparently all formulations are liquid crystals.

*No maximum observed, solubilization is for pure Tween surfactant.

Ternary Mixture Results and Discussion

Given the improvements seen with binary mixtures of surfactants, the possibility that ternary mixtures would lead to even greater solubilization was next to be explored. Having three components complicates the search for an optimum ratio of components, so initial attempts involved fixing the ratio of two of the surfactants, and mixing these two with the third.

The first ternary system studied was Span 20, Tween 80 and Tween 85. This system showed a sharp maximum in solubilization with respect to HLB, as can be seen in Figure 3-7. The peak is at a high HLB of 10.8. The surfactant ratio was x:1:1, so the relative amounts of the Tweens were equal, and only the ratio of Span 20 to the Tweens was changed to achieve a range of HLB values. Above N_HLB=11.5, liquid crystalline systems were formed.

The second system studied was Span 20, Span 80 and Tween 85. This system (Figure 3-7) had a peak and shape of the solubilization curve similar to that of the first. The surfactant ratio was 1:1:x, with equal amounts of Spans, and the ratio of Tween 85 to Spans changed to achieve different HLB values. The peak solubilization occurred at an HLB number which appears to be the average between the Span 20/Tween 85 peak and the Span 80/Tween 85 peak, suggesting that no synergism exists due to the Span/Span mixtures.

Effect of Surfactant/Oil Ratio on Solubilization

As all previous experiments were performed at a fixed surfactant/oil ratio (35.7 wt% surfactant), the dependence of solubilization on this parameter was not explored. Two systems were studied. The first was Tween 81, chosen because it had greatest water solubilization by itself, and the second was an N_HLB=10 Span 80/Tween 85 mixture.
 

(Figure 3-7. W/O solubilization in ternary Span/Tween mixtures.)
 

Figure 3-8 shows water solubilization using Tween 81, and Figure 3-9 shows solubilization with the Span 80/Tween 85 mixture, vs. wt% surfactant. Additionally, water/surfactant ratio vs. water/oil ratio (Figure 3-10) is presented for the Span 80/Tween 85 mixture. Maximum solubilization vs. wt% surfactant was approximately linear for the Tween 81/water/hexadecane system. The Span 80/Tween 85 mixture showed an interesting deviation from this linear behavior. Above 20 wt% initial surfactant, the relationship appears linear. Below 20%, there is a region of excess solubilization, peaking at 10%. The peak at 10% is so high that solubilization is the same as the mixture with 30% surfactant.
 
 

Solubilization experiments were extended to alkane solvents other than hexadecane. Shorter linear alkanes were considered, as hexadecane is the longest chain alkane that is still a liquid at room temperature (MP=18.2 ^oC). Possibilities exist for synergistic effects between the oil and the surfactant (chain-length compatibility effect), as discussed in Shaio et al. [1996].

Experimental Procedure

Solubilization was performed as described in the previous section (Water/Hexadecane/Sorbitan Ester Microemulsions). Concern must be given to the evaporation of the oil, as the shortest alkane used, octane, has a boiling point of 125.7 ^oC.

Results and Discussion

Tween 81. In hexadecane, Tween 81 has the highest solubilization capacity of any of the surfactants. There is a drop-off in solubilization with the shorter alkanes, but the
 

(Figure 3-8. W/O solubilization, weight % excursion for Tween 81.)
 

(Figure 3-9. W/O solubilization, weight % excursion for Span 80/Tween 85 mixtures.)
 

(Figure 3-10. W/S ratio vs. S/O ratio, for Span 80/Tween 85 N_HLB=10 mixture.)
 

decrease is not large. Figure 3-11 shows water solubilization vs. alkane carbon number for 20 wt% Tween 81 in the n-alkane series from decane to hexadecane. Solubilization of Tween 81 in corn oil was very poor. Several references mention the poor ability of triglycerides in forming W/O microemulsions.

Span/Tween mixtures. Figure 3-12 shows solubilization water in alkanes using Span 80/Tween 85, and Figure 3-13 shows solubilization water in alkanes using Span 20/Tween 85 mixtures, and Figure 3-14 shows solubilization water in alkanes different by four carbon atoms in length, using Span 20/Tween 85 mixtures. There is something unusual in the apparent solubilization maxima for C₈, C₁₂ and C₁₆, as they seem significantly higher and more regularly spaced than the maxima for the other alkanes studied. There must be some explanation for this optimal four carbon step in the alkanes, perhaps there is some chain length compatibility argument that can be applied to explain this result.

(Figure 3-11. W/O solubilization in C₁₀-C₁₆ n-alkane/Tween 81 microemulsions.)
 

(Figure 3-12. W/O solubilization in C₈-C₁₆ n-alkane/Span 80/Tween 85 microemulsions.)
 

(Figure 3-13. W/O solubilization in C₈-C₁₆ n-alkane/Span 20/Tween 85 microemulsions.)
 

(Figure 3-14. W/O solubilization in C₈, C₁₂, C₁₆ n-alkane/Span 20/Tween 85 microemulsions.)

Interfacial curvature model. A possible explanation for the solubilization results in different alkanes can be made when one considers a model for the extent of oil penetration into the surfactant layer. One must consider the surfactant monolayer as being composed of a head group layer and a tail layer. When comparing a Span molecule (sorbitan alkylate) with Tween 85 (ethoxylated sorbitan trioleate), one sees that the Span has the much smaller head group relative to the size of the tail (Figure 3-15). If a monolayer were to be made entirely of Span, it would tend to curve towards the head group side (head group inside micelle or droplet) while Tween 85 would tend to curve the other way. At an oil/water interface, an excess of Span would cause a high interfacial curvature and thus small water-in-oil droplets. Adding Tween would tend to lower the curvature of the interface. A point is reached where sufficient Tween is added and the interface would be flat, and beyond this additional Tween at the interface would drive the curvature to flip the other way (Figure 3-15). This flat interface scenario is a reasonable model for the point of maximum solubilization, while the inversion of the direction of curvature is a possible explanation for the sudden collapse of solubilization when Tween is added just beyond the point of maximum solubilization. This can also be an explanation for the liquid crystal formation, which occurs when the interfacial curvature is optimally zero. This model
 

(Figure 3-15. Curvature model for the solubilization behavior of Span/Tween W/O microemulsions.)
 

of the curvature is related to the packing parameter developed by Israelachvili et al. [1976] and applied to the determination of aggregation structure.

The effect of the oil phase can also be explained by this model. It is reasonable to assume that a shorter alkane, such as octane, will penetrate into the tail layer of the interfacial film to a greater extent. Oil penetration will be in the tail side only, causing curvature to increase, resulting in smaller droplets, in favor of the W/O microemulsion (Figure 3-15). This expansion of the tail side of the interfacial film will thus allow more Tween to be present before the microemulsion fails. This is supported by the data in Figure 3-12 and 3-13, and summarized in Table 3-9, which show greater Tween present at microemulsion solubilization failure in the order octane > dodecane > hexadecane. The octane penetrates more into the tail layer than the other oils, and thus the Span/Tween/octane microemulsions have the greatest percentage of Tween at the point of maximum water solubilization.

Given the success with the alkane/sorbitol ester mixtures, it was important to see if similar results would be found with oils and surfactants of different structure. Cyclohexane was studied as the oil phase, and POE nonylphenyl ethers as the surfactants (Igepal CO series). The different Igepals have varying amounts of polyoxyethylene. The POE number is an average, the distribution in POE numbers for a given average results from the lack of precision in the ethylene oxide polymerization reaction. The CO series has a nonylphenyl hydrophobic group. The nonyl- moiety is produced by polymerization of propylene, and thus is a branched alkane structure. This microemulsion system was used by Kumar et al. [1993] for nanoparticle synthesis.

Experimental Procedure

Solubilization was performed as described in the previous section (Water/Hexadecane/Sorbitan Ester Microemulsions), but with different proportions. The Igepal CO and DM series, Emulphor and Alkamuls series of surfactants were supplied by Rhone-Poulenc, Inc. (Cranbury, NJ). The initial oil phase consisted of 1.5 g total of surfactant in 7.76 ml of hexadecane. Alternately, 1.2 g of total surfactant was dissolved in 6.23 ml cyclohexane (density=0.7791). These proportions result in 20 wt% initial surfactant in the oil phase. Cyclohexane (BP=80.7^oC) is much more volatile than hexadecane (BP=287^oC), so care must be taken to cap the sample vials during titration, if the sample is to be left stirring for extended periods.

Results and Discussion

Solubilization in cyclohexane. Table 3-10 shows mixtures of Igepal CO-430/CO-610, and CO-210/CO-720 in hexadecane. Figure 3-16 shows mixtures of CO-210/CO-720, as well as some single surfactants, in cyclohexane. The surfactant mixtures in cyclohexane performed fairly well. Surfactants were initially soluble in oil, unlike the hexadecane system. This Igepal/cyclohexane system is interesting, because it is the one used by Kumar [1993] for superconducting ceramic nanoparticle synthesis. CO-430 was used at 28 wt%. The results in Figure 3-16 clearly show the superiority in solubilization ability for mixtures of surfactants, over single surfactant systems. The mixture chosen was the most
 

(Figure 3-16. W/O solubilization, binary mixtures with Igepal CO series surfactants in cyclohexane.)
 

hydrophobic Igepal CO, CO-210, with an N_HLB of 4.6, and the most hydrophilic that is still liquid at 25^oC, CO-720, with an N_HLB of 14.2.

Solubilization in hexadecane. The solubilization results for the Igepal mixtures are not as impressive as for the Span/Tween mixtures in hexadecane, but much better in cyclohexane. Maximum solubilization is summarized in Table 3-10. The Igepal CO surfactant mixtures were not initially soluble in hexadecane. Only after the addition of an initial volume of water was a microemulsion state observed in hexadecane. Solubilization was observed in the N_HLB range of 7.7 to 9. At 9.5, no microemulsion phase was observed for the mixtures studied.

Possible explanations for the poor solubility in hexadecane may be the relative shortness of the hydrophobic moiety. All surfactants have the same nonylphenyl structure, which is short and multiply branched as compared to the long, linear lauryl and oleyl structures in the Span and Tween surfactants. Poor steric hindrance of the nonylphenyl structures may cause poor microemulsion formation. The large difference in size between the hydrophobic moiety and the long POE chain of the hydrophilic moiety may result in poor packing of molecules at the interface, which may also disrupt microemulsion formation.

Electrolyte solubilization. One application of microemulsions is the preparation of nanoparticles, mixing w/o microemulsions prepared with different aqueous phases that will form an insoluble precipitate on contact. By mixing the microemulsions, collisions of microemulsion droplets cause the formation of nanoparticles of the precipitate material, much smaller in size than can be made by traditional methods. Figure 3-17 shows the solubilization of aqueous electrolyte solutions in Igepal CO-430 and cyclohexane. The applicability of microemulsion solubility studies with pure water to solubilization of electrolyte solutions is not obvious. The current effort examined solubilization of monovalent (NaCl) and divalent (CaCl₂) solutions in Igepal CO-430 in cyclohexane. This system has been used in several nanoparticle synthesis studies, with cations of up to tetravalent charge. The solubilization increases with ionic strength and valence for the solutions studied, up to 1 M concentration.

Conclusion. It appears that similar rules governing the formation of W/O microemulsions, based on HLB, are more general and can be applied to a variety of surfactant systems. These rules should help reduce the number of microemulsion systems screened in the development of new microemulsion applications.
 

(Figure 3-17. Solubilization of electrolyte solutions in Igepal/cyclohexane W/O microemulsions.)
 
 

Solubilization in hexadecane with several other pure surfactants was tried, as well as a few other systems of binary mixtures, with the most successful being the Span/EL-719 mixtures (Figure 3-18). Solubilization in other oil phases was tried, including corn oil (a triglyceride) and diesel oil. Tables 3-11 and 3-12 summarize the surfactants and oils used in both successful and unsuccessful attempts at solubilization.

Experimental. ICI (Wilmington, DE) supplied the Span, Brij, and Alkamuls EL-719 surfactants. Corn oil, peanut oil and olive oil were commercial products available in any supermarket. Diesel fuel was purchased directly from an Exxon service station.

Results. Several O/W microemulsion solubilization experiments (the inverse of the previous W/O studies) were performed using water and hexadecane. Single surfactants were tried over a wide HLB range. No successful solubilization was achieved. Over the HLB range of 10 to 13.5, opaque gels were formed, while outside that range, two liquid phases were observed.

(Figure 3-18. W/O solubilization in hexadecane/Span/ethoxylated castor oil microemulsions.)
 

Light Scattering Experiments

Quasi-elastic light scattering is a powerful technique for determination of macromolecular or particle size of these objects in a solution. Light scattering has been used in this research to determine the presence of microemulsion droplets, as well as to quantify the trend of droplet size with increasing surfactant/oil ratio.

Experimental. Dynamic light scattering measurements were performed using a Brookhaven Instruments Corp. (Holtsville, NY) model BI-2030 correlator, 200SM laser light scattering goniometer, and EMI-9865 photomultiplier cathode. Light scattering measurements were taken at a scattering angle of 90^o, using a Spectra Physics (Mountain View, CA) argon ion laser operating at a wavelength of 514.5 nm.

Samples were prepared at different volume fractions (f ), defined as:

where V is the total volume of the mixture, V_s is the volume of surfactant added and V_w the volume of water added. Extrapolation to zero volume fraction is done. Samples were filtered through a 0.2 micron PFT membrane and placed into a clean cylindrical cell. Samples were maintained at 25^oC during the measurements.

Data analysis involves an autocorrelation analysis of the time variation of the scattered signal. The Brookhaven Instruments software provides a particle size measurement and error, based on a monodisperse size distribution assumption required by the Stokes-Einstein relation. A cumulant analysis is also performed which assumes multiple exponential decay signals, and calculates the apparent mean particle size for a polydisperse mixture.

Results. Quasi-elastic light scattering measurements were taken on Span 85/Tween 85 samples, and on Tween 81 samples, in order to help elucidate the microstructure. A real analysis problem exists for these microemulsions, in that an absolute diffusion coefficient (and thus droplet hydrodynamic radius) cannot be absolutely measured, as the droplet size is found to be a relatively strong function of the light scattering sample time. This dependence suggests that one or more of the assumptions needed for QELS analysis is being violated (monodisperse, no interparticle interactions, sufficiently low particle density to avoid multiple light scatter).

Droplet size determination was done for Tween 81 surfactant in hexadecane, with initial surfactant/oil ratios of 20, 25 and 30%. Water is present at the maximum solubilization value, a water/oil ratio of 0.263, 0.327, and 0.389, respectively. This represents 2.0 ml water in 7.8 ml oil, 2.5 in 7.8, and 2.9 in 7.6 ml. Droplet size is shown to increase as surfactant/oil or water/oil increases. (see figure) Both the single exponential decay calculation (higher droplet radius value) and the polydisperse calculation (lower radius value) are presented.

The effect of sample time of the collection is also investigated. For the 30 wt% surfactant/oil sample, the sample time was varied from 1 to 50 microseconds. The change in measured droplet size is significant with the change in sample time. The droplet size is expected to increase with increasing water solubilized in the oil. The change in calculated particle diameter with a change in sample time is expected for a polydisperse sample. The error in the gamma value for the cumulant analysis is lower than the error in gamma for a single exponential, lending weight to the assumption that we have a polydisperse sample.

From the above experiments on W/O microemulsions, the following can be concluded: