The influence of permeant properties on the sorption step in hydrophobic pervaporation

Liquid solubility measurements were performed by immersion of poly(octyl)methylsiloxane (POMS) in dilute aqueous solutions of homologous series of aliphatic esters and alcohols with compound specific concentrations of 20, 40, 80 and 120 ppm. Measurements were performed at 20 °C with both single- and multi-compound mixtures and linear sorption isotherms were obtained. By fitting experimentally determined molar solubility coefficients to their liquid molar volumes, exponential relationships were obtained for compounds within the same chemical group. The solubility coefficients of alcohols were smaller than those obtained for esters of approximately the same molecular size due to the hydrophilic hydroxyl group in the former. There was evidence of some coupling effects; in the case of larger esters the solubility was higher when they were sorbed from multi-compound solutions of esters. The solubility of all the alcohols except n-octanol was the same regardless of whether they were sorbed from single- or multi-compound solutions. As far as the n-octanol was concerned, the solubility decreased when other alcohols were present in the solution. One possible explanation of this is that the ability of alcohols to form clusters through hydrogen bonding modifies the polymer phase by reducing its ability to take up n-octanol.