Kinetic studies on adsorption and desorption of surfactants

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The desorption kinetics of surfactant ions at solid/liquid interfaces was investigated by applying the stopped-flow method. A mixing cell of stopped-flow spectrophotometer was modified with platinum electrodes for electric conductivity detection. The change in electric conductivity with time in the desorption process was monitored by a memory-recorder system. Surfactants used were sodium alkyl sultates (C_<10>-C_<16>). The desorption rate of surfactat ions from nylon particles was independent of the surfactant concentration and the rate constants were obtained by applying the first-order reaction scheme, at the concentrations below cmc.Equilibrium adsorption isotherms of the surfactants on nylon particles were obtained from electrokinetic potentials of the particles dispersed in aqueous surfactant solutions at different temperatures. The equilibrium constants of adsorption, K, and the saturated adsorption amount, Ts, were determined from langmuir plots. The values of K and Ts obtained … More by the electro-kinetic method were compared with those by the electiric conductivity method. The values of K by the both agreed with each other. The difference in values of Ts provided information on the slipping plane and the dissociation degree of adsorbed surfactants.Adsorption rate constants were estimated from the desorption rate and equilibrium constants assuming the second-order kinetics. The desorption rate constants were determined in the range of 1 - 6 sec^<-1> and the adsorption rate constants to be in 2 - 8 x 10^4 mol^<-1>dm^3sec^<-1>, the former decreased and the latter increased with increasing number of carbon atoms in akkyl chain of the surfactants.The temperature dependence of desorption rate in the early stage of the process was investigated by an electric conductivity stopped-flow system newly developed. Activation energy of desorption was calculated to be 65.5 kJ/mol form Arrhenius plot, which was almost equal to the enthalpy of adsorption estimated in the same system. Less