Adsorption kinetics of a semi-flexible polymer

Abstract

The adsorption kinetics of the polysaccharide xanthan from aqueous solution on zirconium oxide were studied as a function of pH and ionic strength. The adsorption was monitored by reflectometry in a stagnation-point flow setup. At intermediate pH and ionic strength, xanthan is present in a helical form and it can be viewed as a semi-flexible polymer under these conditions. By lowering the salt concentration or increasing the pH a helix-coil transition takes place. This transition is caused by the mutual electrostatic repulsion of the short side chains of xanthan. The so-formed coil can be considered as a Gaussian chain, with a large radius of gyration. The conformation of the polysaccharide is roughly reflected in the adsorption behavior. It is, however, deduced that the electrostatic interaction between polymer and surface influences the stability of the helix. The adsorption process can be divided in two regimes. At low surface coverage the rate of adsorption is transport-limited, which in a stagnation-point flow leads to a linear time dependence of the adsorbed amount. The adsorption rate in this regime hardly changes with ionic strength or pH. The time range over which it holds, however, does, which can be understood in terms of electrostatic effects. At higher surface coverage two types of behavior are observed. At low ionic strength and on a highly charged surface the adsorbed amount saturates abruptly. This kind of kinetics resemble those of flexible polymers. In this case the xanthan presumably adsorbs in a coil-like conformation, because the helix becomes unstable in the vicinity of the surface. At higher ionic strength and on a weakly charged surface, the adsorbed amount increases gradually over very long times. Under these conditions, the helix conformation is more stable so that we ascribe this slow process to tentatively rearrangement and alignment processes of the stiff chains on the surface.