Nonpolar solvents, ordered surfactant

The anion series in the order of their influence on the surface activity of cationic surfactants and on the properties of insoluble monolayers coincide with the lyotropic series. The anion nature is manifested in the specific interaction with cationic surfactants with the formation of ionic pairs or in the anion penetration into the adsorption monolayers. Apparently, charge transfer complexes are frequently formed at the water-air and water-oil interfaces between the adsorbed cations and anions. Mukerjee and Ray estabhshed that these complexes exist in ionic pairs, between Br or J anions and dodecylpyridine ions in chloroform, and also on the micelle surface formed by the same cations in water [84-86]. The data on the ionic pair structure at the interface can be obtained by spectral study of monolayers and solutions of ionogenic surfactants in nonpolar solvents. According to Goddard, the investigation on the mixed monolayers formed by the surface-active anions and quaternary ammonium compounds will enable us to better understand the specific properties of the interaction between the adsorbed ions [72]. 

The surfactant aggregation in the non-aqueous phase usually begins at 10" — 10 M and is several orders lower than the CCM in the aqueous phase [126]. Therefore, the study of the aggregation and the determination of activity coefficients at such concentrations turn out to be a comphcated experimental task. The surface pressure isotherms at the boundary between water and surfactant solutions in nonpolar solvents and other methods can be employed for this purpose. (Surfactants dissolved in the organic phase significantly affect the water-air interfacial tension [129]). 

Microheterogeneous environments, such as those found in reverse micelles (RMs) and microemulsions, have tremendous promise because of the nonstandard environments they present. Often, chemistries that occur in these solutions do not occur in homogeneous liquid solutions [1-4]. Essentially, RMs are spatially ordered macromolecular assemblies of surfactants formed in nonpolar solvents, in which the polar head groups of the surfactants point inward toward a polar core and the hydrocarbon chains point outward toward the nonpolar medium [5, 6] (see schematic representation in Fig. 14.1). 

When surfactants dissolve in a solvent forming a solution, the surface energy of the solution wiU decrease rapidly and linearly with increasing concentration. This decrease is due to the preferential enrichment and ordered arrangement of the surface of surfactant molecules on the solution surface, i.e., hydrophilic heads inside the aqueous solution and/or away from the nonpolar solution or air. 

---