Multiple-contact adsorption of target molecules by heteropolymer gels

We examined adsorption of target molecules through a multiple-contact interaction in a thermo-sensitive heteropolymer gel which can undergo a volume transition at 34 °C in water. Multi-valent anionic target molecules were adsorbed electrostatically by monovalent cationic adsorbing sites in the gel. The overall affinity (SK) between the gel and the target molecule was calculated from the initial slope of the Langmuir adsorption isotherm: [T_ads] = KS[T_sol]/[1+ K[T_sol]), where S, K, [T_sol] and [T_ads] represent the number of adsorption sites per unit volume of the gel, the effective binding constant, the equilibrium target concentration in the external solution, and the target concentration adsorbed in the gel, respectively. The affinity for the collapsed gel at 60 °C was studied in terms of the concentrations of three factors: the adsorber ([Ad]), the coexistent salt (replacement ion, [Re]), and the cross-linker ([Xl]). We found that the relationship between the affinity and these factors can be summarized by the following formula first suggested by T. Tanaka: Affinity, SK ∝([Ad]/[Re])^pexp[-c(p-1)[Xl]/[Ad]^2/3]. Here c is a constant and p is the number of the contact points between the adsorber units and the target molecule.