Mass transfer of hydrophobic solutes in solvent swollen silicone rubber membranes

The mass transfer characteristics of a non-porous silicone rubber membrane contacting an aqueous and an organic phase were determined using a shell an d tube mass exchanger. Firstly, the stability of the liquid/membrane interf aces was examined, and when positive aqueous phase transmembrane pressure d ifferentials of up to 2 bar were applied, no bulk transmembrane flow of eit her liquid was observed. This result was not affected by the addition of su rfactants or biomass to the aqueous phase, and therefore it seems that phas e breakthrough, common with porous membranes, is avoided. Secondly, the mas s transfer characteristics of a range of model solutes were investigated an d explained with a resistances in series model. A high membrane/aqueous par tition coefficient (P-aq(mem)) (appriximately>25) resulted in the aqueous p hase film resistance limiting, whilst a low P-aq(mem) (approximately<5) res ulted in the membrane resistance limiting. In contact with organic solvents the silicone rubber swelled to various degrees, and this was solvent depen dent. The degree of swelling, and the relative partitioning of the solute i nto the swelling solvent, impacted on p(aq)(mem) and the membrane diffusion coefficient. These two parameters could be increased by using a highly swe lling solvent (solvent constituting>50% of the swollen membrane volume) wit h a high organic/aqueous partition coefficient (P-aq(org)) for the solute. In this way the membrane resistance for some solutes was decreased, and the refore 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 chemical ly similar solutes with low water solubilities, 0.7 and 0.35 g l(-1), respe ctively) the effect was similar to a "salting out" phenomena. This resulted in an increase of P-aq(org) for both geraniol and citronellol, and therefo re a decrease in flux of the solute transferring to the aqueous phase. (C) 1999 Published by Elsevier Science B.V. All rights reserved.