Kinetic and equilibrium studies at the solid–liquid interface. The adsorption of sodium hexadecyl sulphate to polystyrene latex

Abstract

Binding isotherms and kinetic measurements associated with the adsorption of the surfactant sodium hexadecyl sulphate onto polystyrene latex particles at two latex concentrations are reported. The surfactant monomer concentrations were estimated in situ for all dispersions from e.m.f. measurements of a cell containing a surfactant ion-selective electrode. Both isotherms are of the classical B.E.T. type. At low surfactant concentration the initial portions of the isotherms (region I) can be fitted by the Langmuir equation and do not depend on the concentration of latex particles. At higher concentrations (region II) the isotherms diverge. Bound surfactant–counter-ion interactions could be responsible for the difference between the binding isotherms. The kinetic data were measured with the pressure jump relaxation technique and revealed a single relaxation process in region II only. The observed relaxation times decrease with increasing surfactant concentration. Application of linear phenomenological theory to the combined equilibrium and kinetic data shows clearly that the adsorption rate coefficient increases with increasing amount of bound surfactant and that the desorption rate is proportional to the amount bound in region II. The rate coefficients measured for the two dispersions agree reasonably well. These findings are consistent with the formation of hemimicelle type aggregates. Application of the Aniansson–Wall kinetic treatment associated with monomer–aggregate exchange to region II gives a linear plot for each solution, but the agreement between the desorption rate coefficients is worse than that obtained using the phenomenological approach.