Influence of ionic strength and composition on the size of mixed micelles of sodium dodecyl sulfate and Triton X-100

Abstract

The dimensions of mixed micelles of sodium dodecyl sulfate (SDS) and Triton X-100 (TX100) were determined, at concentrations well above the critical micelle concentration, by size exclusion chromatography (SEC) and quasielastic light scattering (QELS). The correlation of the apparent hydrodynamic radii obtained by these two methods, i.e., R h SEC and R s QELS, is excellent, indicating that the two techniques are both responding to particle size and not, inter alia particle-stationary phase interactions, or interparticle interactions. The dependence of micelle size on the mole fraction anionic surfactant Y, where Y = [SDS] ([SDS] + [TX100]) −1, is strongly influenced by the ionic strength. Above 0.5 M NaCl, micelle size increases strongly with Y; below 0.2 M NaCl, micelles become smaller with increasing Y; and, at intermediate ionic strengths, a maximum in micelle size is observed near Y = 0.5. The results may be understood in terms of the head group areas of the two surfactants, with the understanding that the effective head group area of SDS includes strong repulsion effects at large Y and low ionic strength.