Kinetic studies of surfactant-aspirin micellization by means of ultrasonic relaxation spectroscopy

Abstract

We report on the dynamics of the Sodium dodecyl sulfate (SDS) – aspirin micellization process taking place in aqueous environment by combining vibrational and ultrasonic spectroscopies. The experimental vibrational spectra have been compared with the corresponding theoretically predicted spectra obtained by density functional theory electronic structure calculations. The formation of a swollen SDS@aspirin micelle above a specific surfactant concentration significantly affects several physical properties of the system including surface tension and isentropic compressibility allowing the estimation of the critical micelle concentration. In the acoustic spectra, a single ultrasonic relaxation process has been detected in SDS-aspirin-water solutions at concentrations slightly above critical micelle concentration. The variation of the sound absorption coefficient and velocity as a function of frequency at different concentrations reveals the presence of a relaxation process attributed to the perturbation of the monomer-micelle dynamic equilibrium. We progress on the comprehensive understanding of the micellization kinetics, and the results are discussed in the context of several theoretical models of the field.