Thermodynamics of Micelle Formation: Model Calculations for Sodium Octanoate

Abstract

A model is presented for the calculation of the standard free energy of transfer of an amphiphilic monomer from the intermicellar solution to a spherical micelle. This model is largely based on previous work by Ruckenstein and Nagarajan¹, Israelachvili, Mitchell and Ninham² and Tanford³. Three main parameters are involved: the interfacial tension between water and a liquid hydrocarbon, the standard free energy of transfer of a hydrocarbon from water to liquid hydrocarbon and the net charge of the polar end groups. The first two parameters are taken from independent experiments using Tanford’s expressions while the last one is adjusted to fit the experimental c.m.c. value of the amphiphile. The model has been used to predict the properties of sodium octanoate micellar solutions (mean aggregation number, activity of counter ions, molar volumes). Good agreement with experimental data is obtained. The agreement is somewhat improved when it is assumed that the hydrocarbon moiety of the micelle extends to between carbon atoms 2 and 3 not up to the carboxylic group.