Adsorption Behavior of Carboxymethylcellulose on Amino-Terminated Surfaces

The adsorption behavior of carboxymethyl cellulose onto amino-terminated surfaces was investigated by means of ellipsometry. The adsorbed amount decreases with ionic strength decrease, in agreement with the screening-reduced adsorption regime. The pH has a strong influence on the adsorption behavior. At pH values lower than 4 the surface charge is high and the segment charge low. The maximum adsorbed amount is obtained at pH 3 for fixed ionic strength. At pH values higher than 4 the adsorption is weak. The protonation of the amino groups on the substrate is unfavored, while the segment charge is high due to the strong dissociation of carboxylate groups. These findings show that the surface charge governs the adsorption behavior and that the driving force is electrostatic in nature. The dependence of the adsorbed amount on the segment charge fits scaling laws. The adsorption kinetics reveal that the adsorption equilibrium is achieved after approximately 45 min. In the first minutes the CMC chains move toward the bare substrate in a fast diffusive process. After this period, the few adsorbed chains must change their conformation in order to make room for the arriving chains. This is a slow process, because the steric and electrostatic hindrances must be overcome. This occurs more slowly in higher ionic strength and at lower pH values. After approximately 45 min the surface is completely covered.