Adsorption of cellulose derivatives on inorganic oxides

Abstract

Adsorption of hydroxyethyl cellulose (HEC) and quaternary ammonium substituted HEC (QNHEC) on silica and titanium dioxide has been investigated as a function of pH and electrolyte (NaCl) concentration. Adsorbed amounts have been determined by means of reflectometry. Adsorption of HEC on SiO 2 is constant up to pH=5. At higher pH the adsorption decreases, which is most pronounced at high (0.5 mol l −1) electrolyte concentration. The thickness of the adsorbed layer, determined by dynamic light scattering, is substantial. This indicates an adsorbed layer with extended conformation having loops and (few) long tails protruding into the solution. Adsorption on TiO 2 in 0.01 mol l −1 NaCl decreases monotonically with increasing pH. In 0.5 mol l −1 NaCl the adsorption is constant up to pH=10, beyond which it decreases rapidly. Mechanisms of binding to both surfaces leading to the observed adsorption behaviour are proposed. Electrostatics dominate the adsorption of QNHEC in 0.01 mol l −1 NaCl on both surfaces. The adsorbed amount increases linearly with pH up to pH=10. A decrease is observed for pH>11. The linearity is interpreted in terms of a molecular condenser which is composed of the surface layer and the polyelectrolyte in the first layer near the surface. In 0.5 mol l −1 NaCl adsorption on SiO 2 is constant up to pH=5. A maximum is observed at pH=10. On TiO 2 the adsorption is low.