Solubilization. Phase Diagrams of Three-Component Systems

8 Solubilization. Phase Diagrams of Three-Component Systems 

The solubilization phenomenon, which refers to the dissolution of normally insoluble or only slightly soluble compounds in water caused by the addition of surfactants, is one of the most striking effects encountered for surfactant systems. Solubilization is of considerable physico-chemical interst, such as in discussion of the structure and dynamics of micelles and of the mechanism of enzyme catalysis, and has numerous practical applications, such as in detergency, in pharmaceutical preparations and in micellar catalysis. In biology, solubilization phenomena are most significant, e.g., cholesterol solubilization in phospholipid bilayers and fat solubilization in fat digestion and transport. 

The influence of solubilization on solution properties may vary from no appreciable effects to very marked ones. We may take solutions of CTAB as an illustrative example of the manifold of possibilities. Here solubilization of cyclohexane has a very small influence on a variety of rheological and spectroscopic properties while addition of aromatic compounds and long-chain alcohols may cause extensive changes. 

Rheological properties are particularly sensitive and for some solubilizates, such as a-methylnaphthalene, the solutions may become viscoelastic the appearence of viscoelasticity often depends on subtle effects in chemical structure29). Certain spectroscopic features are strongly influenced. Thus the H and 13C NMR line widths show large increases, the 81 Br quadrupole relaxation may be strongly affected and there may be the appearance of linear dichroism, birefringence and conductance anisotropy for flowing systems. 

The very complex variation of the amount solubilized, as well as physico-chemical properties, with chemical structure of solubilizate and surfactant as well as with surfactant concentration cannot be adequately discussed solely in terms of the energetical conditions of the solubilizate in the micelles. Thus one should also consider the conditions in the phase which separates out at the solubilization limit this is in most cases a liquid crystalline phase. A fundamental basis for a proper understanding of solubilization in surfactant systems is, therefore, a detailed information on phase equilibria in three-component systems surfactant-solubilizate-water. Due in particular 

---