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Membranes of highly crystalline PTFE having an average pore size between about 0.01 micron and about 0.5 micron ordinarily reject any passage of water while allowing an organic nonpolar liquid to flow therethrough. By thoroughly wetting an expanded polytetrafluroethylene membrane with isopropyl alco

Membranes of highly crystalline PTFE having an average pore size between about 0.01 micron and about 0.5 micron ordinarily reject any passage of water while allowing an organic nonpolar liquid to flow therethrough. By thoroughly wetting an expanded polytetrafluroethylene membrane with isopropyl alcohol, such a membrane will selectively allow the passage of a polar liquid, for example water, therethrough while rejecting other components, including nonpolar organic liquids, such as oil, kerosene or the like, and dissolved components of fairly large molecular size. Such IPA-wetted membranes are used to separate oil-water mixtures by allowing the water to selectively permeate through the membrane. The membrane may include a 5 mil thick backing layer of nonwoven synthetic fibers. The mixture is supplied to the membrane at a pressure not greater than about 90 psig., and often very low pressures of about 5-20 psig. can be used.

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A method of dewatering a diesel fuel-water mixture, which method comprises wetting a porous membrane consisting essentially of a highly crystalline polytetrafluoroethylene polymer by submersion in a composition consisting of at least about 50 volume percent isopropyl alcohol with any remainder being

A method of dewatering a diesel fuel-water mixture, which method comprises wetting a porous membrane consisting essentially of a highly crystalline polytetrafluoroethylene polymer by submersion in a composition consisting of at least about 50 volume percent isopropyl alcohol with any remainder being water to wet said membrane with isopropyl alcohol, which membrane has an average pore size between about 0.01 micron and about 1.0 micron and a thickness between about 0.5 mil and about 4 mils, and contacting said wetted membrane with a mixture of diesel fuel having a viscosity between about 0.5 centipoise and 1000 centipoisos and water at a pressure not greater then about 90 psig. and a temperature between 0°C. and 100°C. so that water passes through said porous membrane to the substantial exclusion of diesel fuel to thereby deplete such mixture of its water content. A method of dewatering an oil-water mixture, which method comprises wetting a porous membrane consisting essentially of a highly crystalline polytetrafluoroethylene polymer by submersion in a composition consisting of at least about 50 volume percent isopropyl alcohol with any remainder being water to wet said membrane with isopropyl alcohol, which membrane has an average pore size between about 0.01 micron and about 1.0 micron and a thickness between about 0.5 mil and about 4 mils, and contacting said wetted membrane with such oil-water mixture at a pressure between about 5 psig. and about 20 psig. so that water passes through said porous membrane to the substantial exclusion of oil to thereby deplete such mixture of its water content.