On the anomalous sorption behaviour of chlorhexidine with poly(2-hydroxyethyl methacrylate)

General observations of solute: plastic interactions would suggest that the sorption of the cationic preservative, chlorhexidine, with poly (2-hydroxyethyl methacrylate) (PHEMA) is not only unexpected but also atypical in its properties. The polymer and preservative were investigated independently and the sorption of PHEMA with simple solutes, namely benzoic acid, benzocaine and aniline found to exhibit conventional properties. The single exception was the uptake of small amounts of benzocaine and aniline cations at acidic pH's. In order to elucidate the sorption characteristics of chlorhexidine a series of N1-(p-chlorophenyl)-N5-alkylbiguanide acetates were synthesized. The anomalous sorption behaviour observed between PHEMA and the bis biguanide, chlorhexidine, was also found to be characteristic of these monofunctional biguanide derivatives. The extent of the interactions increased with increasing alkyl chain length (R = methyl to n-octyl), this being interpreted in terms of an interaction binding mechanism via the biguanide functional group stabilized by van der Waal's forces between the alkyl chain and the polymer backbone. Atypical sorption behaviour was observed for simple organic cations, biguanide and N2-phenylbiguanide acetate with PHEMA, a possible inference being that this is a general characteristic of all cationic sorption with PHEMA.