Interaction of cetyltrimethylammonium chloride micelles under static and hydrodynamic conditions

Abstract

Self-diffusion coefficients (D_m) of cetyltrimethylammonium chloride (CTAC) micelles have been measured as a function of surfactant concentration (C_s) under static and hydrodynamic conditions using cyclic voltammetry (CV) and rotating disk electrode (RDE) voltammetry with ferrocene acting as an electroactive probe. The diffusion coefficients obtained from both techniques were found to be an inverse function of surfactant concentration with the behaviour conforming to the relationship D_m=D^o_m/[1 +ƒ(C_s– c.m.c.)], having a common value, D^o_m, at the critical micelle concentration (c.m.c.). This behaviour has been interpreted in terms of an inter-micellar interaction parameter ƒ. At surfactant concentrations above the c.m.c., clear differences between the statically measured (CV) and the hydrodynamically measured (RDE) diffusion coefficients were observed, with significantly higher values being obtained with the RDE, giving ƒ= 13.5 ± 0.2 dm³ mol^–1 under static conditions and ƒ= 7.9 ± 0.2 dm³ mol^–1 under hydrodynamic conditions, i.e. inter-micellar interaction is a function of the experimental conditions. The difference between the hydrodynamically and statically measured diffusion coefficients is shown to be a function of inter-micellar distance.