Poly(dimethylsiloxane) crosslinked in different conditions - Part II. Pervaporation of water-ethyl acetate mixtures

Dense membranes made of PDMS crosslinked in different conditions were studi ed in vacuum pervaporation of water-ethyl acetate mixtures. The decrease in both water and ester permeation fluxes with the increase in crosslinker/po lymer ratio or in crosslinking temperature can be partly attributed to the decrease in the sorption of the components in the membranes. The selectivit y to ester in the pervaporation of the mixtures is lower than that in sorpt ion, but increases faster with the crosslinking extent. Both water and este r fluxes show similar variations with the eater content in the mixture as w ith the ester sorption extent: increases in the fluxes with the ester conte nt in the homogeneous water-rich region, constant fluxes in the two-phase r egion, and increase in both fluxes in the homogeneous ester-rich region. Th is behaviour indicates that: (i) eater activity is the driving force for ea ter pervaporation and (ii) water permeation is mainly due to an enhanced so rption of water with increased ester contents. Using the solution-diffusion model, we show that the diffusion coefficient of ethyl acetate in a PDMS m embrane is constant, and the flux variation is entirely due to the change i n sorption extent with the ester content. The increase in the permeation fl ux of water when its activity in the mixture decreases is not due to the wa ter-ester transport coupling, but to a synergy in sorption.