Tracer and mutual diffusion of nonionic and ionic mixed micellar systems: octa- and hexa-ethylene glycol monododecyl ether and long-chain ionic surfactants

Tracer diffusion coefficients, Dt, for mixed micelles and mutual diffusion coefficients, Dm, for mixed surfactants were measured in water at 298.15 K for systems of a nonionic surfactant, octaethylene glycol monododecyl ether (C12E8) or hexaethylene glycol monododecyl ether (C12E6), and a long-chain ionic surfactant, octadecyltrimethylammonium chloride (C18TACl) or sodium octadecanesulfonate (C18SO3Na) using the Taylor dispersion method. Aggregation numbers of the mixed micelles were determined by measuring the monomer fluorescence decay of pyrene solubilized in the micelles. When C18TACl or C18SO3Na was added to the 10 mM (1 M=1 mol dm−3) solution of C12E8, Dt values decreased monotonically with increasing fraction of the ionic surfactants. When C18TACl or C18SO3Na was added to the 10 mM solution of C12E6, Dt values increased first and then decreased with increasing fraction of the ionic surfactants, reflecting the decrease in the aggregation number and increase in the micelle-medium electrostatic interactions with increasing ionic surfactant fraction. When C18TACl or C18SO3Na was added to the 10 mM solution of C12E8 or C12E6, the Dm values increased up to a concentration of 2 mM and then reached plateaus with increasing ionic surfactant concentration. Using surfactant and counter ion selective electrodes, concentrations of free ions were measured and ionic charges of the mixed micelles were calculated. From the Dt values of the mixed micelles and counter ions, together with the numbers of free counter ion per micelle, Dm values were calculated and compared with those measured.