Micellization in Non-Aqueous Non-Polar Media: Reverse Micelles

by B. B. Bhuyan & R. Kalita

When a surfactant (e.g., sodium stearate) is dissolved in an aqueous solution (in a sufficient concentration), we know that micelles form, which are nearly spherical aggregates of the long surfactant-molecule chains, with their hydrophobic non-polar parts aggregating together in the core (inside) of the micelle and the hydrophillic polar heads lying on the surface. However when the same surfactant is dissolved (in a sufficient concentration, of course) in a  non-aqueous, practically non-polar solvent (e.g., liquid CO₂ or acetone), there also similar micellar aggregates form, but the location within the aggregates of the non-polar parts of the surfactant molecules and their polar heads would greatly differ. In these aggregates the core consists of the polar groups (say, carboxylate groups) including the ionogenic (i.e., ion-generating) parts, whereas the non-polar hydrocarbon chain-parts of the surfactant molecules lie towards the outer side of these nearly-spherical aggregates, in contact with the non-polar solvent molecules (figure). Naturally, these aggregates are termed as inverted micelles or reverse micelles.

The formation of these aggregates could be justified by the simple 'like prefers like' argument: as here the hydrocarbon chain parts of the surfactant and the solvent molecules are both non-polar, so the hydrocarbon-chain parts prefer to remain surrounded by the solvent molecules, with the polar and/or ionogenic heads preferring  to remain away from the solvent molecules, in the vicinity of one another. It is to be also noted here that because of the low permittivity of non-polar solvents, the ionogenic surfactants (e.g., sodium stearate) remains practically non-dissociated in them, in the form of  ion-pairs. It is suggested that the forces of dipole-dipole interactions among the ion-pairs as well as possible hydrogen-bonds have been responsible for the formation of a polar core. The presence of traces of water binding with the polar groups have been found to facilitate reverse-micelle formation in such non-polar non-aqueous media - stressing a positive role of hydrogen-bonds.

It can be easily visualized that a reverse micelle would encapsulate and dissolve some polar molecules (say, HF or H₂O molecules) within its core, just as a ordinary micelle takes up an oily (i.e., non-polar) dirt particle.

Microemulsions or Miceller Emulsions 

The term microemulsion was applied to systems prepared by emulsifying an hydrophobic liquid (termed as 'oil') in aqueous surfactants and then adding a fourth component called co-surfactant, which is generally an alcohol of intermediate chain length. Thus, benzene, water, potassium stearate and hexanol might be the components of a typical microemulsion formulation. What is observed experimentally is that formerly milky, i.e., turbid emulsions become transparent upon addition of the intermediate-chain-length alcohol. 

Light scattering and a group of other techniques reveal that the resulting microemulsion system consists of either an 'oil-in-water' or an 'water-in-oil'0 type of dispersion with particles having diameter in the 10-60 nm size-range. This particle dimension is at least an order of magnitude (i.e., 10 times) smaller than that of the coarse (usual) emulsions. Further, microemulsions are always isotropic, i.e., they have same directional properties in all directions, and are optically transparent while coarse emulsions are always cloudy (turbid). It is further found that microemulsions are thermodynamically stable, i.e., they are stable with respect to separation into their components. 

The thermodynamic stability of miceller emulsions is determined by the free energy of double layer formation, the entropy effect (for radius < 20 nm) and by the forces of electrical double layer, the van der Waals forces of attraction playing a secondary role. Miceller emulsions may be considered as swollen micelles. Another way of looking at microemulsions is to view them as complicated examples of miceller solubilisation. From this perspective, spontaneous formation or stability w.r.t. separation could be easily explained. In this viewpoint, ordinary and reverse micelles provide basis for the understanding of O/W ('oil-in-water') and W/O microemulsions. 

Microemulsions find application in many areas. Floor waxes, shaving lotions, beverage concentrates and pesticide preparations are common examples of microemulsion products. Microemulsion phenomenon is further used in recovery of mineral oils from natural reservoirs, after the easier primary and secondary recovery steps leave more than half of the oil reserve, trapped in the pore structures of surrounding stones and earth. 

Microemulsion is also used in emulsion polymerization technique for synthesis of polymers. In this technique, the hydrophobic monomer remains bound within the surfactant micelles dispersed within an aqueous medium. The polymerization initiator is water soluble, so that polymerization starts at the surface of the micelles where the monomer and the initiator comes in contact, and then proceeds inside the micelle where the monomer is remaining.