Interaction of p-Hydroxybenzoic Acid Esters with Polyethylene Glycol

Abstract

The interactions of methyl and propyl p-hydroxybenzoate with polyethylene glycol (PEG) 4000 were investigated by means of an ultrafiltration technique. The results show that the interaction is a weak and nonspecific process of large binding capacity, and is a type of partitioning phenomenon between the macromolecule and the aqueous phase. The characteristics of this interaction are quite similar to those of the binding of p-hydroxybenzoates with the polyoxyethylene region of polyoxyethylene dodecyl ether (PDE) micelles. A solubility study of p-hydroxybenzoates in aqueous solutions of PEG-400 was also performed. Assuming that the increase in solubility of the preservatives in the presence of PEG was due to the interaction with PEG, the ratio of bound preservative concentration to PEG concentration was calculated. The ratio increased as the concentration of PEG in the solvent increased, and this was thought to be due in part to a dehydrating effect. A comparison of solubility data with the binding parameters of p-hydroxybenzoates for the secondary binding sites of PDE micelles provided information concerning the micellar hydration. The amount of water trapped by nonionic surfactant micelles was estimated and the values thus obtained were consistent with those reported elsewhere.