B Factors Determining the Extent of Solubilization

Because of the importance of solubilization in the removal of oily soil by detergents and in the preparation of pharmaceutical, cosmetic, insecticide, and other types of formulations, a good deal of work has been done on elucidating the factors that determine the amount of solubilizate that can be solubilized by various types of surfactants. The situation is complicated by the existence of the different sites for the solubilization of different types of materials. 

The extent to which a substance can be solubilized into a particular micelle depends upon the portion of the micelle that is the locus of the solubilization. The volume of that portion depends upon the shape of the micelle. As we have seen (Chapter 3, Section IIA), the shape of the micelle is determined by the value of the parameter Vj/lcao. As that value increases, the micelle in aqueous medium becomes increasingly asymmetrical, with the result that the volume of the inner core increases relative to that of the outer portion. We can therefore expect that the solubilization of material in the core will increase relative to that in the outer region of the micelle with increase in asymmetry (increase in the value of Vf/lcad). The amount solubilized in any location will also increase with increase in the volume of the micelle, e.g., with increase in the diameter of a spherical micelle. 

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). 

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