A Locus of Solubilization

FIGURE 4-2 Loci of solubilization of material in a surfactant micelle. Reprinted with permission from P. H. Elworthy, A. T. Florence, and C. B. MacFarlane, Solubilization by Surface-Active Agents, Chapman Hall, London, 1968, p. 68. 

Saturated aliphatic and alicyclic hydrocarbons and other types of molecules that are not polarized or not easily polarizable are solubilized in aqueous medium in the inner core of the micelle between the ends of the hydrophobic groups of the surfactant molecules. Their UV and NMR spectra indicate a completely nonpolar environment on solubilization. 

Polarizable hydrocarbons, such as short-chain arenes (benzene, isopropylbenzene), have been shown to be solubilized in quaternary ammonium solutions initially by absorption at the micelle-water interface, replacing water molecules that may have penetrated into the outer core of the micelle close to the polar heads, but solubilization of additional material is either deep in the palisade layer or located in the inner core of the micelle (Eriksson, 1965). The polarizability of the ji-electron cloud of the aromatic nucleus and its consequent ability to interact with the positively charged quaternary ammonium groups at the micelle-water interface may account for the initial adsorption of these hydrocarbons in that location. In POE nonionics, benzene may be solubilized between the polyoxyethylene chains of the hydrophilic groups (Nakagawa, 1967). 

Small polar molecules in aqueous medium are generally solubilized close to the surface in the palisade layer or by adsorption at the micelle-water interface. The spectra of these materials after solubilization indicate that they are in a completely, or almost completely, polar environment. Short-chain phenols, when solubilized in POE nonionics, appear to be located between the POE chains (Nakagawa, 1967). 

In concentrated aqueous surfactant solutions, although the shape of the micelles may be very different from that in dilute solution, the locus of solubilization for a particular type of solubilizate appears to be analogous to that in dilute solution that is, polar molecules are solubilized mainly in the outer regions of the micellar structures, whereas nonpolar solubilizates are contained in the inner portions. 

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