Adsorption mechanisms of carboxymethyl cellulose on mineral surfaces

The adsorption behavior of carboxymethyl cellulose (CMC) on inorganic surfaces (TiO2 and alpha-Fe2O3) in aqueous solution has been studied systematically. The general trends are that the adsorbed amount decreases with increasing pH, whereas increasing the electrolyte (NaCl) concentration causes the adsorption to increase. The actual values, however, are influenced by the kinetics of the adsorption process. Near the point of zero charge of the oxidic surfaces the adsorption depends linearly on pH. This 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. Adsorption is independent of the number of carboxylic groups per glucose unit (degree of substitution, ds). Neither at low (0.01 mol L-1) nor at high (1 mol L-1) electrolyte concentration does the adsorption depend on the chain length, which indicates a (rather) flat conformation of the adsorbed polymer. The apparent hydrodynamic thickness of the adsorbed layer is found to be substantial. Strong hysteresis is observed with respect to the pH dependence of adsorption: as compared to the adsorption measured directly at specified fixed pH values, at high pH values a substantially higher adsorbed amount can be obtained by initially adsorbing at low pH and subsequently increasing the pH value. Desorption of CMC only takes place after initially low pH values are increased substantially. The arduous desorption originates from a strong interaction with the surface and is enhanced by the chain rigidity of the CMC backbone.