Growth of rod-like micelles in anionic surfactant solutions in the presence of Ca2+ counterions

摘要

The experimental data for the growth of rod-like surfactant micelles in the presence of a 2:1 electrolyte (CaCl2) do not comply with the available theory, which was originally developed for a 1:1 electrolyte. To solve the problem we undertook experimental and theoretical investigations with micellar solutions of the anionic surfactant sodium dodecyl polyoxyethylene-a sulfate. Independent dynamic and static light scattering measurements of micelle size demonstrate that the effect of micelle-micelle interactions is negligible for the solutions investigated. Ultrafiltration experiments reveal that a considerable part of the Ca2+ ions are associated with micelles. Since our experiments are carried out at a fixed surfactant-to-Ca2+ ratio, the parameter of micelle growth (the equilibrium constant of micellization) indirectly depends on the surfactant concentration through the electrolyte concentration. That dependence is derived theoretically. The model of micelle growth, extended in this way, compares well with the experimental data. The model provides a quantitative description of the micelle size and charge as functions of the surfactant and electrolyte concentrations. The rod-like micelles have lower surface charge density than the spherical micelles; this makes their growth energetically favorable. (C) 1998 Elsevier Science B.V. All rights reserved. [References: 39]