Mass transfer of hydrophobic solutes in solvent swollen silicone rubber membranes

Abstract

The mass transfer characteristics of a non-porous silicone rubber membrane contacting an aqueous and an organic phase were determined using a shell and tube mass exchanger. Firstly, the stability of the liquid/membrane interfaces was examined, and when positive aqueous phase transmembrane pressure differentials of up to 2 bar were applied, no bulk transmembrane flow of either liquid was observed. This result was not affected by the addition of surfactants or biomass to the aqueous phase, and therefore it seems that phase breakthrough, common with porous membranes, is avoided. Secondly, the mass transfer characteristics of a range of model solutes were investigated and explained with a resistances in series model. A high membrane/aqueous partition coefficient ( P mem aq) (approximately>25) resulted in the aqueous phase film resistance limiting, whilst a low P mem aq (approximately<5) resulted in the membrane resistance limiting. In contact with organic solvents the silicone rubber swelled to various degrees, and this was solvent dependent. The degree of swelling, and the relative partitioning of the solute into the swelling solvent, impacted on P mem aq and the membrane diffusion coefficient. These two parameters could be increased by using a highly swelling solvent (solvent constituting>50% of the swollen membrane volume) with a high organic/aqueous partition coefficient ( P org aq) for the solute. In this way the membrane resistance for some solutes was decreased, and therefore the overall mass transfer coefficient increased. Finally, the effect of the presence of other hydrophobic solutes on the rate of mass transfer was investigated, and in the case of geraniol and citronellol (two chemically similar solutes with low water solubilities, 0.7 and 0.35 g l −1, respectively) the effect was similar to a “salting out” phenomena. This resulted in an increase of P org aq for both geraniol and citronellol, and therefore a decrease in flux of the solute transferring to the aqueous phase.