Ternary Mutual Diffusion Coefficients and Critical Micelle Concentrations of Aqueous Sodium Dodecyl Sulfate + Lithium Dodecyl Sulfate Solutions at 25 °C

Abstract

Ternary mutual diffusion coefficients (Dik) are reported for aqueous sodium dodecyl sulfate (NaDS) + lithium dodecyl sulfate (LiDS) solutions at 25 °C and (0.020 or 0.040) mol·dm-3 total surfactant. The main diffusion coefficient of each surfactant increases significantly with decreasing solute fraction, and each mole of diffusing surfactant countertransports up to 0.8 mol of the other surfactant. The results are interpreted by considering the limiting cases of diffusion in (a) solutions of NaDS micelles containing traces of LiDS with the counterion exchange equilibrium Li+ + (NaqDSn)q-n ⇌ Na+ + (LiNaq-1DSn)q-n and (b) solutions of LiDS micelles containing traces of NaDS with Na+ + (LiqDSn)q-n ⇌ Li+ + (NaLiq-1DSn)q-n. At fixed total surfactant concentration, the measured Dik coefficients are closely approximated by a simple linear interpolation of the limiting Dik values against solute fraction. Binary mutual diffusion coefficients for aqueous LiDS solutions and critical micelle concentrations for ternary...