The binding of sodium hexadecyl sulphate to poly(N-vinylpyrrolidone) in aqueous solutions. Kinetic and equilibrium studies

Abstract

Binding isotherms and kinetic measurements associated with the adsorption/desorption of the surfactant sodium hexadecyl sulphate (SHS) onto the neutral polymer, poly(N-vinylpyrrolidone)(PVP), at two polymer concentrations are reported. The surfactant monomer concentrations were estimated in situ for all solutions from e.m.f. measurements of a cell containing the surfactant ion-selective electrode. The binding isotherms are very similar to those reported for shorter-chain alkyl sulphates in the presence of PVP. In addition, spectral data associated with the 3 : 1 intensity ratio of the vibronic fluorescence bands of pyrene have also been measured at the same polymer concentrations as a function of increasing SHS. The purpose of these measurements was to use the spectral data on the pyrene as a probe to monitor the hydrophobic environment in the polymer–surfactant formulation. Finally, kinetic (relaxation) data were measured with the pressure-jump relaxation technique and revealed a single relaxation process at all concentrations. The results indicate that the driving force during the binding process is a self-aggregation of the surfactant ions which occur as a result of the initially bound surfactant, creating a nucleus for the promotion of small aggregates on the polymer chain. Application of the linear phenomenological theory to the binding and kinetic data shows clearly that the adsorption rate increases with increasing amount of bound surfactant and that the desorption rate is proportional to the amount of bound surfactant. The desorption rate constant measured for the two formulations agree well. The relaxation data were also analysed using the Aniansson and Wall kinetic treatment associated with monomer aggregate exchange.