The salt-induced sphere-rod transition of micelles of dodecyltrimethylammonium bromide in aqueous NaBr solutions as studied by the ultrasound velocity measurements

Abstract

The density and adiabatic compressibility of aqueous NaBr solutions of dodecyltrimethylammonium bromide have been determined by means of pycnometry and ultrasound velocity measurements at 25°C. Phenomenological equations of density and adiabatic compressibility of micellar solutions have been given as functions of the concentration of the spherical micelle, to which the apparent molar volume and the apparent adiabatic compressibility are assigned. These equations have been further extended in such a way to be applicable for solutions containing both spherical and rod-like micelles, by introducing the apparent molar volume and the apparent adiabatic compressibility of a rod-like micelle. In the actual application of the present treatment, the concentration of the rod-like micelle has been obtained as a function of total micelle concentration by referring to the previous results of light scattering. Actually the density and adiabatic compressibility have changed linearly with total micelle concentration, even in the presence of both kinds of micelles. In this way the apparent molar volumes and the apparent adiabatic compressibilities of dodecyltrimethylammonium bromide in both spherical and rod-like micelles have been derived. Its apparent molar volume is slightly larger for the rod-like micelle than for the spherical one. Its apparent adiabatic compressibility is negative in the monomeric form, and its apparent adiabatic compressibility in spherical micelles changes in a somewhat complicated way with increasing NaBr concentration, while that in rod-like micelles increases less sharply with increasing NaBr concentration.