Coupled mutual diffusion in solutions of micelles and solubilizates

Abstract

The coupled diffusion of micelles and solubilizates has been studied by measuring ternary mutual diffusion coefficients (D_ik) for aqueous solutions of dodecylsulfobetaine (SB12) with added butanol, pentanol, or hexanol. SB12 micelles solubilize alcohols, so diffusing SB12(1) might be expected to co-transport alcohol(2). Negative values of cross-coefficient D₂₁ indicate, however, that the diffusion of SB12 drives substantial counterflows of alcohol. To help interpret the results, measured decreases in critical micelle concentrations with added alcohol (ROH) are used to evaluate equilibrium constants for the formation of (SB12)_m(ROH)_n mixed micelles. Increasing the concentration of SB12 along a diffusion path raises the concentrations of micelles and solubilized alcohol while lowering the concentration of free alcohol. The resulting flux of relatively mobile free alcohol molecules up SB12 concentration gradients is larger than the flux of solubilized alcohol down SB12 gradients, producing net countercurrent coupled fluxes of alcohol. The measured D_ik coefficients are in close agreement (± 0.05 × 10⁻⁵ cm² s⁻¹) with predictions using self-diffusion coefficients (D_i*) for SB12 and alcohol in solutions at thermodynamic equilibrium and the relations D_ik=∂(C_iD_i*)/∂C_k proposed recently for mutual diffusion in non-equilibrium solutions of associating solutes. Mutual diffusion coefficients for coupled surfactant–solubilizate diffusion are used to evaluate equilibrium constants for the formation of surfactant–solubilizate mixed micelles.