The Solubilities of Esters of 4-Hydroxybenzoic Acid, Determined Separately and Together, in Aqueous Solutions of 2-Hydroxypropyl-β-cyclodextrin

Abstract

The solubilities of four esters of 4-hydroxybenzoic acid have been investigated singly and together in water and in aqueous solutions of 2-hydroxypropyl-β-cyclodextrin (HPCD). Values obtained for the solubilities of methyl (MP), ethyl (EP), propyl (PP), and butyl (BP) esters in water agreed welt with literature values, when determined separately. When measured together the solubilities of the methyl and butyl esters increased slightly, that of the ethyl ester decreased slightly, and the solubility of the propyl ester decreased by approximately 50%. Association constants for complexes of esters and HPCD determined separately in water agreed well with literature values, declining from methyl to ethyl and then increasing through propyl to butyl. When the association constants were determined for the esters together in solutions of HPCD the same pattern was noted, but the large decreases in association constants may indicate some other process in addition to competition between the esters for the HPCD molecules.