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

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 the ory, which was originally developed for a 1:1 electrolyte. To solve the pro blem we undertook experimental and theoretical investigations with micellar solutions of the anionic surfactant sodium dodecyl polyoxyethylene-a sulfa te. Independent dynamic and static light scattering measurements of micelle size demonstrate that the effect of micelle-micelle interactions is neglig ible for the solutions investigated. Ultrafiltration experiments reveal tha t 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 pa rameter of micelle growth (the equilibrium constant of micellization) indir ectly depends on the surfactant concentration through the electrolyte conce ntration. That dependence is derived theoretically. The model of micelle gr owth, extended in this way, compares well with the experimental data. The m odel provides a quantitative description of the micelle size and charge as functions of the surfactant and electrolyte concentrations. The rod-like mi celles have lower surface charge density than the spherical micelles; this makes their growth energetically favorable. (C) 1998 Elsevier Science B.V. All rights reserved.