Adsorption of polyvinylpyrrolidone on kaolinite

Abstract

The adsorption of the uncharged polymer polyvinylpyrrolidone (PVP) with molecular weights from 5000 to 600 000 g mol −1 on the non-swelling clay mineral kaolinite has been studied. The effects of the adsorbent concentration, solution pH and PVP molecular weight on the binding isotherms were investigated. The varying affinity character of the binding isotherms with the kaolinite concentration can be related to some molecular weight fractioning of the synthetic polymer samples. An adsorption decrease of about 20% in the pH range 3.5–10.3 indicates that the protonation of kaolinite edge surfaces may also be involved in the adsorption process. At pH 5.5, the maximum amount adsorbed varies with PVP molecular weight ⩽44 000 g mol −1. At higher molecular weights, a levelling-off of the adsorption is found at 1.2 mg m −2. The electrokinetic thickness, δ e , of PVP layers on the kaolinite surface was determined by microelectrophoresis. At ionic strengths ⩽10 −4 M, δ e tends to a limit which can be put equal to the hydrodynamic layer thickness, δ h . The molecular weight dependence of δ h in the presence of a fixed PVP concentration and the adsorbed amount dependence of δ h for various PVP molecular weights are discussed. At high PVP concentrations for surface kaolinite saturation, it can be shown that the molecular weight dependence of the polymer expansion in solution does not change greatly after adsorption. Thus, at high surface coverage, a relatively large increase of δ h up to 100 nm with a PVP molecular weight of 600 000 g mol −1 can be related to a high fraction of segments in loops. On the other hand, at low PVP surface coverages, ⩽0.6 mg m −2, a fairly constant δ h of about 10–14 nm regardless of the PVP molecular weight is found. The build-up of a condensed PVP layer which results from the flattening of the polymer coil structure can be suggested.