Quaternary mutual diffusion coefficients for aqueous solutions of a cationic–anionic mixed surfactant from moments analysis of Taylor dispersion profiles

Abstract

Taylor dispersion is used to measure mutual diffusion in aqueous solutions of dodecyltrimethylammonium bromide (DTAB) + sodium octanoate (NaOct), a cationic–anionic mixed surfactant. Diffusion of the four different constituent ions (DTA+, Br−, Na+, Oct−) constrained only by electroneutrality produces three independent solute fluxes. Mutual diffusion in these solutions is therefore a quaternary process described by nine Dik coefficients. Injecting excess DTAB or NaOct into DTAB + NaOct carrier solutions produces remarkable refractive-index profiles with three maxima and two minima. Least-squares analysis of these profiles failed to converge, prompting the development of a procedure for the evaluation of quaternary Dik coefficients from the heights, areas, first moments, and second moments of dispersion profiles. The reported Dik coefficients describe the coupled diffusion of DTAB(1) + NaOct(2) + NaBr(3) components in solutions containing 40 mmol dm−3 total surfactant at 25 °C and four different DTAB:NaOct ratios. The results show that the unusual profile shapes are caused by strongly coupled diffusion, as indicated by large negative D12, D21 values and large positive D32, D31 values which exceed the magnitude of main-coefficients D11 and D22. Binary mutual diffusion coefficients for aqueous DTAB solutions and critical micelle concentration for DTAB + NaOct solutions are also reported.