Ultrasonic relaxation studies of concentrated surfactant solutions and liquid crystals

Abstract

Ultrasonic relaxation studies have been carried out on non-ionic, anionic and cationic surfactants in the micellar phase and in hexagonal and lamellar liquid crystal phases, over the frequency range 1–105 MHz. For all the surfactants, a relaxation process with similar absorption characteristics occurs in both the concentrated micellar phase and the adjacent liquid crystal phase. For most surfactants studied quantitatively, the relaxation frequency decreases with surfactant concentration and the relaxation process is attributed to the perturbation of a dynamic equilibrium between surfactant-‘bound’ and ‘free’ water molecules, occurring on a time scale of 10^–7 to 10^–8 s.

A model to describe this ‘bound’/‘free’ water-exchange process has been developed which allows the association and dissociation rate constants for the process to be determined. It also gives an estimate of the hydration numbers of the surfactants.

For caesium oleate solution the relaxation rate increases with surfactant concentration. This is tentatively attributed to exchange of free and bound counter-ions; values of the rate constants for the process are derived.