A permselective layer comprising a heat-treated film of a silane compound having a formula R'n Si(OR)4-n in which n is an integer selected from the group consisting of 0, 1, and 2; R' is a hydrocarbon radical comprising 3-12 carbon atoms; and R is a lower alkyl radical comprising 1-4 carbon atoms,said layer having a thickness sufficient to exclude substantially molecules with a molecular weight of