A study of the micelle formation of short—chain carboxylates by laser—ramon spectroscopy

Abstract

The C-C and C-H vibration regions at 1000–1200 cm −1 and 2800–3000 cm −1, respectively, have been investigated by Laser-Raman spectroscopy for the sodium salts of normal aliphatic fatty acids (C 2-C 8) in aqueous solutions at constant and variable ionic strength. The spectra are compared with the corresponding regions for the crystalline salts, with spectra of an aliphatic alcohol, and with spectra of perdeuterated sodium octanoate and normal sodium octanoate in deuterium oxide. It is shown that the relative intensities of the peaks in these regions are dependent on the concentration. The results can be reconciled with the following model of micelle structure. As the concentration of monomers increases in aqueous solution, the hydrocarbon chain (h.c.) structure changes toward a larger proportion of trans configurations. When micelles are formed, the configuration of the hydrocarbon chains in the micelles remains similar to that of monomers or premicelles in equilibrium with the micelles. The polar character of the environment of the hydrocarbon chains changes gradually toward a nonpolar character; this change continues up to concentrations several times higher than the critical micelle concentration (CMC), indicating considerable binding of water in the micelles. Thus, in the concentration range below the CMC the most important structural change with increasing concentration seems to be the change in the h.c. chain configuration.