Question

Is there a membrane that stop liquid water but let Methane gas in?

Answer 1

PDMS?

Explanation:

Yes, there's a few . A common type would be PDMS (polydimethylsiloxane, a silicone polymer). This is widely used as a gas permeable membrane material.

Permeation occurs via a mixture of solution and diffusion, in which the rate of permeation of the gas into the polymer film varies directly as the solubility of the gas (in the polymer) multiplied by the rate of diffusion of the dissolved gas in the polymer (the product of these two factors).

A measure of permeability often used is "permeability coefficient" which can be viewed as "rate of gas permeation per unit area, per unit trans-membrane driving force, per unit membrane thickness". Permeability coefficients are often stated in "barrier" units, where 1.0 barrier unit = `10^-10 cm^3 (STP)· ((cm)/(cm^2)) · s · cm-Hg.`

Methane has a value of around 950 barrier, and PDMS itself is resistant to liquid water.