THE SOLUBILITIES OF ESTERS OF 4-HYDROXYBENZOIC ACID, DETERMINED SEPARATELY AND TOGETHER, IN AQUEOUS-SOLUTIONS OF 2-HYDROXYPROPYL-BETA-CYCLODEXTRIN

The solubilities of four esters of 4-hydroxybenzoic acid have been inv estigated singly and together in water and in aqueous solutions of 2-h ydroxypropyl-beta-cyclodextrin (HPCD). Values obtained for the solubil ities of methyl (MP), ethyl (EP), propyl (PP), and butyl (BP) esters i n water agreed well with literature values, when determined separately . When measured together the solubilities of the methyl and butyl este rs increased slightly, that of the ethyl ester decreased slightly, and the solubility of the propyl ester decreased by approximately 50%. As sociation constants for complexes of esters and HPCD determined separa tely in water agreed well with literature values, declining from methy l to ethyl and then increasing through propyl to butyl. When the assoc iation constants were determined for the esters together in solutions of HPCD the same pattern war noted, but the large decreases in associa tion constants may indicate some other process in addition to competit ion between the esters for the HPCD molecules.