Influence of solute structure on deviations from the log-linear solubility equation in propylene glycol:water mixtures

The solubilities of the methyl, ethyl, propyl, and butyl esters of p-hydroxy- and p-aminobenzoates have been determined in propylene glycol:water mixtures. The log of the observed solubility data in propylene glycol:water mixtures was examined for deviations from the following equation: In Xi = f In (Xc) + (1 - f) In (Xw), where Xi is the calculated mole fractional solubility of the solute, f is the volume fraction of cosolvent, Xc is the observed mole fractional solubility in the neat cosolvent, and Xw is the solubility in water. In each case, the deviations from the predicted solubilities demonstrated a characteristic pattern. Positive deviations were observed at high volume fractions of cosolvent, while negative deviations were observed at low volume fractions. The magnitude of the deviations at low volume fractions of cosolvent was related to the carbon chain length within each group of esters. A similar phenomenon was not observed at high volume fractions of cosolvent; however, the magnitude of the deviations was dependent on the nature of the polar group on the ester. The data are interpreted in terms of the possible effects of solvent structure on the solubility of the solutes.