Adsorption isotherms of poly(vinyl alcohol) on silicon oxide

Poly(vinyl alcohol), PVA, is a water-soluble polymer that is made up of repeating alcoholic and acetate units. Because this polymer is sometimes applied to soils, it is advantageous to fully understand the relative impact on the adsorption behavior of this polymer by each of the functional groups on the polymer. Using adsorption isotherm data, our objective was to elucidate the effect of the alcoholic and acetate groups on the macromolecule's affinity for a Si oxide surface. Seven PVA polymers were chosen for this study to cover a range of polymer sizes (10 000-128 000 g mol(-1)) and degree of hydrolysis (78-99%). As the percentage of hydrolysis of the polymer increases (increase in the alcoholic groups, decrease in the acetate groups), the adsorption of the polymer decreases on a mass basis. We propose that the acetate functional groups on the PVA chains encourage the formation of loops by the polymer on the Si surface, which also increases the amount adsorbed on a mass basis. The larger polymers also adsorbed more than the smaller ones. Conformity of the polymer to the Langmuir isotherm indicates that the adsorption is site-specific. The Langmuir equilibrium constant (K) was determined for the polymer above low surface coverage conditions because the energy of adsorption was stronger at low surface coverage. As the hydrolysis percentage of the polymer was lowered, K increased; this was semi-independent of the polymer size. Consequently, the acetate functional groups on the PVA chains also enhance the affinity of the polymer toward the surface.