Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis

A detailed analysis of the transport of chloroform, isomeric butanols, methanol and water vapors at 40°C through dense silicone rubber membranes has been undertaken by the vapor permeation technique. The variation of fluxes versus upstream activity, coupled to sorption isotherms of the investigated compounds in silicone rubber at 40°C (determined by the swelling technique), offer the possibility to determine the sorption and diffusion steps in the overall permeant flux. It is shown that the chloroform isotherm follows the Flory-Huggins theory, while alcohols require a more sophisticated approach (Koningsveld and Kleinjtens type) in order to fit the isotherms. The water sorption isotherm shows a typical sigmoid shape which could not be fitted correctly by classical sorption equations. The diffusion coefficient of chloroform in PDMS appears to be almost unaffected by solvent concentration in the membrane, similarly to the results obtained with other good solvents in PDMS. Diffusion coefficients of alcohols and water show, however, a common tendency to decrease with their local concentration. An interpretation of the sorption isotherms based on the cluster integral of Zimm and Lundberg suggests some clustering behavior, which seems consistent with the diffusion coefficient variation. The clustering tendency of the solvent molecules in the PDMS matrix was related to their proton donating power, which follows the sequence: water > methanol > 1-butanol > 2-butanol > 2-methyl-2-propanol > chloroform.