Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: II. Cetyl Pyridinium Chloride on Methyl and Methyl Ether Surfaces

Abstract

The adsorption of cetyl pyridinium chloride (CPC) in 0.1 M KCl to methyl- and methyl ether-terminated organic monolayers has been studied using a dual-inlet channel flow cell. The surfaces were formed by reaction of H-terminated Si(111) with dodecene and 11-methoxy-1-undecene, respectively. The equilibrium adsorption isotherm, the desorption kinetics, and the adsorption kinetics can be explained self-consistently on the basis of the Frumkin model of adsorption. The equilibrium adsorption isotherms on both surfaces were very close to Langmuirian, indicating that the driving force for adsorption remains roughly constant with coverage. Adsorption was in the mixed diffusion-kinetic regime under the mass transport conditions of the channel flow cell (adsorption times of the order of tens of seconds). Most of the difference in the adsorption energies of the two surfaces appears to be reflected in the adsorption rate constant rather than the desorption rate constant, suggesting a late transition state. Comparing the Frumkin model parameters for the methyl and methyl ether surfaces with those for hydrophilic silica helps clarify and quantify the difference between the surfaces. The nature of the surface has a very-significant effect on the equilibration time for the same initial surfactant distribution because the adsorption rate affects the subsurface concentration and consequently the flux of surfactant to the interface itself. The organic self-assembled monolayers (SAMs) formed by reaction of H-terminated Si(111) with alkenes make robust and versatile substrates for surfactant adsorption experiments. A combination of SAMs with the dual-inlet channel flow cell offers an opportunity for the systematic and quantitative study of the effect of surface chemistry on the adsorption kinetics of surfactants to organic surfaces.