KINETICS OF POLYMER ADSORPTION AND DESORPTION IN CAPILLARY-FLOW

With streaming potential measurements we performed kinetic and static experiments on the adsorption of poly(ethylene oxide) on glass, in order to obtain the rates of adsorption and desorption and the equilibrium thickness of the polymer layer. The hydrodynamic layer thickness delta(h) is very sensitive to small changes in the adsorbed amount, especially for high molecular weights near saturation. This enabled us to perform measurements of the kinetics and thickness isotherms in the region near saturation with unprecedented accuracy. It was found that both the adsorption and desorption rate are determined by mass transfer between bulk solution and the surface region. In the case of adsorption the observed increase of the hydrodynamic layer thickness with time could be quantitatively predicted by a model which combines the mass-transfer equation with the equilibrium relation between delta(h) and the adsorbed amount GAMMA. For desorption into a flow of pure solvent the rate is proportional to the surface concentration of nonadsorbed molecules. Assuming a local equilibrium between the adsorbed layer and the subsurface region and a logarithmic isotherm, we derived an explicit expression for the adsorbed amount as a function of time: the desorbed amount is proportional to log t. The model predicts that the absolute values of the slopes of the desorption curve GAMMA(log t) and the adsorption isotherm GAMMA(log c) should be the same. This prediction is verified experimentally for molecular weights ranging from 7 X 10(3) to 8.47 X 10(6). Moreover, the absolute value of the observed desorption rate agrees well with that predicted by our model.