The Influence of Electrostatic Repulsions on Micellar Diffusivities Obtained by Dynamic Light Scattering: Experiment vs Theory

Abstract

Using the technique of dynamic light scattering, we have examined the effect of interactions (predominantly electrostatic repulsions) on micellar diffusion coefficients, D, for two related cationic surfactants, CTAOH (C₁₆H₃₃N(CH₃)₃0H) and CTACl (C₁₆H₃₃N(CH₃)₃C1), under conditions of little-to-moderate micellar growth -- 0 to 0.5M electrolyte and 0.01 to 0.05M surfactant at 25°C -- where variations in D are due primarily to changes in intermicellar interactions. We obtained systematic plots of D vs [surfactant] with positive slopes and common diffusivities, D_o, extrapolated to the cmc. Diffusivities increase with decreasing [electrolyte] due to inadequate screening of the intermicellar Coulombic repulsions. With no added electrolyte D exceeds D_o by a factor of almost 5 at 0.05M surfactant. Plots of D vs [surfactant] are linear for CTAOH with [NaOH] ≥ 0.03M and for CTAC1 with [NaCl] ≥ 0.01M. At lower [electrolyte] the curvature of the D vs [surfactant] plots is due to screening by anions dissociated from the micelle. Following Corti and Degiorgio we applied linear interaction theory to the D vs [surfactant], [electrolyte] plots, taking into account theormodynamic and hydrodynamic corrections to D, using the DLVO pair interaction potential, which is overwhelmingly repulsive in dilute electrolyte.