O-16/O-18 AND H/D SEPARATION FACTORS FOR LIQUID VAPOR PERMEATION OF WATER THROUGH AN HYDROPHOBIC MEMBRANE

Liquid/vapor permeation of water through commercial grade hydrophobic PTFE membranes at 323 < P(Torr) < 160, shows enhancements in logarithmic separation factors, ln[alpha(i'/i)] = ln[(x'/x)downstream/(x'/x)upstream], which differ markedly for isotope separation of oxygen and hydrogen. The prime refers to the lighter isotope and x is the mole fraction. Thus ln[alpha(16/18)] is found to be as large as 6 times the ratio of the vapor pressure isotope effect, ln(alpha-o = ln (P(o')/P(o)), but ln[alpha(H/D)]/ln[alpha-o(H/D)] is only as large as 1.3. The difference in enhancement factors indicates that membrane transport mechanisms must be different for the two separations. The unusually large separation factor for O-16/O-18 may be of practical interest.