Effect of urea on thermodynamics of micellization

Abstract

The thermodynamic parameters of micellization for two series of surface-active agents have been derived from surface tension and calorimetric measurements in water and urea solutions. Octyl-sulphinylalkanols were used to study the effect of urea on the solvent reorientation associated with the hydrophilic group and alkyltrimethylammonium alkyl sulphates to study alkyl chain effects. Urea modifies micellization in two ways. It lowers the interfacial energy between hydrocarbon and water, thus reducing the free energy differences between the alkyl chains in the solvent and micellar environments and therefore raising the critical micelle concentration. In addition, by changing the degree of solvation of the monomeric species it lowers the compensating heat and entropy terms associated with the solvent reorientation accompanying micellization.