MOLECULAR-DYNAMICS SIMULATIONS OF THE TRANSPORT OF WATER-ETHANOL MIXTURES THROUGH POLYDIMETHYLSILOXANE MEMBRANES

Molecular dynamics (MD) computer simulation studies are conducted to e xamine important aspects of the pervaporation process. As a model syst em the separation of water-ethanol feed mixtures through a PDMS membra ne is taken. Since it is yet not possible to model the whole membrane fully atomistically, two regions-the bulk and the interfacial region-w ere simulated. In the bulk region the movement of the penetrants has t he character of a jump diffusion. The experimental diffusion coefficie nts are well reproduced. The water molecules are faster than the ethan ol molecules. In spite of this, PDMS is a preferentially ethanol perme able membrane because of the great difference in the sorption behaviou r. The simulations in the interfacial region reflect the preferential ethanol sorption. The ethanol molecules accumulate at the polymer-feed interface and take a favourite orientation to the hydrophobic PDMS su rface. The polymer surface shows a swelling, which is dependent on the ethanol concentration of the feed. It can be shown that the behaviour at the interface polymer-feed is dominated by hydrogen bond forces. ( C) 1997 Elsevier Science Ltd.