Thermodynamics of micellization and solubilization in the system water + sodium n-octanoate + n-pentanol at 25°c. Part 1. - Partial molar enthalpies

The molar enthalpies of mixing have been determined for aqueous solutions of sodium octanoate (NaC8) and pentanol in the aqueous solution region as well as for solutions of water in pentanol. From these results, the partial molar enthalpies of the three components have been calculated. The behaviour of sodium octanoate, even at extremely high dilution, is different from that of a simple binary electrolyte, which can be attributed to the interaction between the hydrocarbon chain and water. Ionic solute effects have a dominating influence on the enthalpies in solution. The partial molar enthalpies of pentanol and octanoate increase steeply below the c.m.c. while that of water decreases. Above the c.m.c. there are slower changes in the opposite direction. The transfer of sodium octanoate or pentanol from infinite dilution to a micelle is an endothermic process involving heats of a few kJ mol-1. When pentanol is added to a solution of NaC8 below the c.m.c., micelle formation is induced. When it is added to NaC3 solutions with low concentrations of micelles, the pentanol dissolves almost ideally while the addition at high micellar concentrations appears to be accompanied by changes in the binding of water to the micelles.