Adsorption kinetics in a dual-inlet channel flow cell: I. Cetyl pyridinium chloride on hydrophilic silica

A dual-inlet channel flow cell has been developed for the study of the adsorption kinetics of surfactants to solid-liquid interfaces under hydrodynamic control. This cell, with ellipsometric detection of the adsorbed surfactant, has been used to study the adsorption kinetics of cetyl pyridinium chloride (CPC) to hydrophilic silica in 0.1 M KCl and in pure water. The methodology provides detailed insight into the kinetic parameters and casts light on the adsorption mechanisms. The convection-diffusion behavior in the cell was calculated numerically using the backward implicit finite difference (BIFD) method: the CPC monomer and micelle populations were modeled with constant diffusion coefficients and assumed to equilibrate quickly on the time scale of the experiment. A Frumkin model was used to describe the adsorption behavior at the silica surface: the fitting parameters were determined from the equilibrium adsorption isotherm and kinetically limited desorption measurements. The adsorption kinetics in the cell were then modeled with no free parameters. Adsorption was in the mixed diffusion-kinetic regime under the mass transport conditions of the channel flow cell (adsorption times on the order of tens of seconds). The Frumkin model described well the adsorption of CPC in 0.1 M KCl, but in pure water the fit to the equilibrium adsorption isotherm and the adsorption kinetics was poor. In the presence of 0.1 M KCl, the kinetic parameters suggest a late transition state in the adsorption process. In the absence of salt, both the adsorption and desorption rate constants increase with surface coverage, possibly suggesting a change in mechanism. The experimental methodology can be used to study alternative surfaces and mixed surfactant/polymer systems without modification and can be adapted to study faster kinetics and to incorporate different detection methods.