Interfacial micellar solutions

C. A. Miller, R.-N. Hwan, W.J. Benton, and T.J. Fort Ultralow Interfacial Tensions and Their Relation to Phase Separation in Micellar Solutions. J. Colloid Interface Sci. 61,554(1977). 

Dungan et al. [186] have measured the interfacial mass transfer coefficients for the transfer of proteins (a-chymotrypsin and cytochrome C) between a bulk aqueous phase and a reverse micellar phase using a stirred diffusion cell and showed that charge interactions play a dominant role in the interfacial forward transport kinetics. The flux of protein across the bulk interface separating an aqueous buffered solution and a reverse micellar phase was measured for the purpose. Kinetic parameters for the transfer of proteins to or from a reverse micellar solution were determined at a given salt concentration, pH, and stirring... 

For small amounts of solubilized water, as a polar additive, the stability of the micelle is markedly increased, as shown by a decrease in the CMC. On the other hand, large amounts of water as a polar additive decrease the stability of the micelle. It is known that a solution of AOT in iso-octane solubilized up to 50 moles of water per mole of surfactant. As the concentration of water increases, the isotropic reverse micellar solution changes to a water-in-oil microemulsion. A clear understanding of the complex analyte-micelle-water pool interactions, especially analyte concentration and pH at the head group interfacial region, is under intensive study (Cline Love and al., 1984 Little and Singleterry, 1964 Luisi and Straub, 1984 Mclntire, 1990). 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

Interfacial Electron Transfer Processes In Micellar Solutions... 

D. Fennell Evans is the director of the Center for Interfacial Engineering and professor of chemical engineering and materials science at the University of Miimesota. He is the author of more than 180 publications on self-assembly processes in water and nonaqueous solvents, microemulsions, diffusion in liquids and micellar solutions, and characterization of surfaces using scanning probe techniques. He has published two textbooks. The Colloidal Domain and The Fundamentals of Interfacial Engineering. 

Ideally, the injected micellar solutions will be miscible with the fluids that they are in contact with in the reservoir and can thus miscibly displace those fluids. In turn, the micellar solutions may be miscibly displaced by water. Highest oil recovery will result if the injected micellar solution is miscible with the reservoir oil. If there are no interfaces, interfacial forces that trap oil will be absent. Injection of compositions lying above the multiphase boundary initially solubilizes both water and oil and displaces them in a misciblelike manner. However as injection of the micellar solution progresses, mixing occurs with the oil and brine at the flood front, and surfactant losses occur because of adsorption on the reservoir rock. These compositional changes move the system into the multiphase region. The ability of... 

Because of the energy crisis, there has been an increasing amount of work on applications of micellar solutions as injection fluids for enhanced oil recovery. The basic aim is to eliminate (reduce to less than 10 3 dyne cm ) the interfacial... 

The effect of alcohol concentration on the solubilization of brine has been studied in this laboratory (41). It was observed that there is an optimal alcohol concentration which can solubilize the maximum amount of brine and can also produce ultralow interfacial tension. The optimal alcohol concentration depends on the brine concentration of the system. The effect of different alcohols on the equilibrium properties and dynamics of micellar solutions has been studied by Zana (42). 

Chemical Shielding Study. 13C NMR spectra were recorded for samples lying within the phase and which originated from the 14.7 wt% aqueous SLS solution. The MMA content was varied up to a composition lying at the phase boundary. Spectra were also recorded for the microemulsions containing hexanol and the 14.7 wt% SLS solution. In addition, systems were studied which were models for the distinct aqueous, interfacial and oily environments found Ln microemulsions. These were aqueous solutions of MMA, aqueous micellar solutions of hexanol and SLS, and MMA solutions of hexanol, respectively. 

Miller, C.A. et ah, Ultralow interfacial tensions and their relation to phase separation in micellar solutions, J. Colloid Interface Sci., 68, 221, 1977. 

The effect of the curvation of the micelle on solubilization capacity has been pointed out by Mukerjee (1979, 1980). The convex surface produces a considerable Laplace pressure (equation 7.1) inside the micelle. This may explain the lower solubilizing power of aqueous micellar solutions of hydrocarbon-chain surfactants for hydrocarbons, compared to that of bulk phase hydrocarbons, and the decrease in solubilization capacity with increase in molar volume of the solubilizate. On the other hand, reduction of the tension or the curvature at the micellar-aqueous solution interface should increase solubilization capacity through reduction in Laplace pressure. This may in part account for the increased solubilization of hydrocarbons by aqueous solutions of ionic surfactants upon the addition of polar solubilizates or upon the addition of electrolyte. The increase in the solubilization of hydrocarbons with decrease in interfacial tension has been pointed out by Bourrel (1983). 

Thus, micellar solutions consist of three regions of distinctly different solvation properties, a continuous polar aqueous domain, non-polar cores and interfacial regions of intermediate polarity. They are all present in a single homogeneous, thermodynamically stable solution. The totality of the three regions can be treated as separate reaction regions... 

The appearance of ultra-low interfacial tension determines the use of such microemulsion systems for enhancing the degree of oil pool recovery. Microemulsion systems, sometimes also referred to as the micellar solutions, are pumped into the secondary satellite holes located at a certain distance from the production oil well. Water containing the required amount of electrolyte is pumped further into these satellite wells. While penetrating the oil pool, this microemulsion with substantial surfactant content, washes off the oil and forces it towards the production well [25,26]. 

The solubilisation of oil or water in a micellar solution of non-ionic surfactant, a) two-phase diagram (O - oil, W - water, 0, - oil in micellar solution, - water in inverse micellar solution, D - phase separation temperature region), b) interfacial tension as a function of T, according to Shinoda Friberg 1975... 

All the theories described above are based on the ideal solution thermodynamics, on the one hand, and on a rather heuristic molecular treatment of micelles as a phase particle, on the other hand. Despite of their obvious successes in predicting micellar solution properties, these theories have some essential drawbacks. The number of adjusting parameters at the evaluation of different contributions to the free energy is too high, as well as the number of oversimplifications, which have been used in order to estimate these parameters. For example, the micellar core is considered as a very small fluid phase droplet surrounded by a second fluid phase and the free energy of micelle surface is estimated as the interfacial tension between these two fluid phases. In order to elucidate this problem Eriksson et al, [24] attempted to... 

The equilibrium and dynamics of adsorption processes from micellar surfactant solutions are considered in Chapter 5. Different approaches (quasichemical and pseudophase) used to describe the micelle formation in equilibrium conditions are analysed. From this analysis relations are derived for the description of the micelle characteristics and equilibrium surface and interfacial tension of micellar solutions. Large attention is paid to the complicated problem, the micellation in surfactant mixtures. It is shown that in the transcritical concentration region the behaviour of surface tension can be quite diverse. The adsorption process in micellar systems is accompanied by the dissolution or formation of micelles. Therefore the kinetics of micelle formation and dissociation is analysed in detail. The considered models assume a fast process of monomer exchange and a slow variation of the micelle size. Examples of experimental dynamic surface tension and interface elasticity studies of micellar solutions are presented. It is shown that from these results one can conclude about the kinetics of dissociation of micelles. The problems and goals of capillary wave spectroscopy of micellar solutions are extensively discussed. This method is very efficient in the analysis of micellar systems, because the characteristic micellisation frequency is quite close to the frequency of capillary waves. 

Solubilization, Microemulsions and Emulsions. - Micellar solutions with both normal (Li) and reverse (L2) curvature, e.g. o/w and w/o type systems, can be swollen by oil and water to obtain water/oil/amphiphile ternary or pseudoternary systems. These systems have been widely used as solubilizing media for structural investigations of the immobilized solubilizate (a protein for instance), for drug delivery systems, and also for reaction media, (micellar catalysis). Ternary systems based on water, oil, and amphiphile mixtures can form a variety of Li and L2 monophasic regions. When these systems form isotropic solutions spontaneously, they are termed microemulsions. The formation of a microemulsion is related mainly to a substantial decrease of the interfacial tension (Yo/w) at the oil-water interface, due to the amphiphilic molecules located at the polar-apolar interface. This occurs in agreement with the typical equation ... 

Figure 17.4. Schematic description of the Ifee-energy profile of the interfacial water species. The species are in dynamic equihbrium with themselves and with water present in the bulk region of the micellar solution. The reaction coordinate is arbitrary and does not imply any distance. Barrier heights are also arbitrary. Figure adapted with permission from J. Phys. Chem. B, 107 (2003), 5194-5202. Copyright (2003) American Chemical Society.

S. Pal, S. Balasubramanian, and B. Bagchi, Identity, energy, and environment of interfacial water molecules in a micellar solution. J. Phys. Chem. B, 107 (2003), 5194-5202. 

The situation is more complicated for nonspontaneous bimolecular reactions involving a second reactant, whose distribution between the two pseudophases has to be considered. The simplest situation is that for reaction of a hydrophobic species whose solubility in water is sufficiently low that it is incorporated essentially quantitatively in the association colloid. For example, for reactions of nucleophilic amines in aqueous micelles, second-order rate constants in the micellar pseudophase calculated in terms of local concentrations are lower than in water [103,104], because these reactions are inhibited by a decrease in medium polarity and micelle/water interfaces are less polar than bulk water [59,60,99101]. Nonetheless, these bimolecular reactions are generally faster in micellar solutions than in water because the nucleophile is concentrated within the small volume of the micelles. Similar results were obtained for the reaction of 2,4-dinitrochlorobenzene (5) with the cosurfactant -hexylamine in O/W microemulsions with CTABr and w-octane [99], again consistent with the postulated similarities in the interfacial regions of aqueous micelles and O/W microemulsions. 

Interfacial tension studies are conducted with various concentrations of micellar solution components to determine optimal concentration ranges. Measurements are usually made with the spinning drop, pendent drop, or sessile drop techniques. 

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