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| - Adsorption Kinetics in a Dual-Inlet Channel Flow Cell: I. Cetyl Pyridinium Chloride onHydrophilic Silica
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| - A dual-inlet channel flow cell has been developed for the study of the adsorption kinetics of surfactants tosolid−liquid interfaces under hydrodynamic control. This cell, with ellipsometric detection of the adsorbedsurfactant, has been used to study the adsorption kinetics of cetyl pyridinium chloride (CPC) to hydrophilicsilica in 0.1 M KCl and in pure water. The methodology provides detailed insight into the kinetic parametersand casts light on the adsorption mechanisms. The convection−diffusion behavior in the cell was calculatednumerically using the backward implicit finite difference (BIFD) method: the CPC monomer and micellepopulations were modeled with constant diffusion coefficients and assumed to equilibrate quickly on thetime scale of the experiment. A Frumkin model was used to describe the adsorption behavior at the silicasurface: the fitting parameters were determined from the equilibrium adsorption isotherm and kineticallylimited 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 channelflow cell (adsorption times on the order of tens of seconds). The Frumkin model described well the adsorptionof CPC in 0.1 M KCl, but in pure water the fit to the equilibrium adsorption isotherm and the adsorptionkinetics was poor. In the presence of 0.1 M KCl, the kinetic parameters suggest a late transition state in theadsorption process. In the absence of salt, both the adsorption and desorption rate constants increase withsurface coverage, possibly suggesting a change in mechanism. The experimental methodology can be used tostudy alternative surfaces and mixed surfactant/polymer systems without modification and can be adapted tostudy faster kinetics and to incorporate different detection methods.
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