Permeation through nanometer pores was important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. It was found that sub-micrometer-thick membranes made from graphene oxide could be completely impermeable to liquids, vapors, and gases, including helium, but these membranes allow unimpeded permeation of water (H2O permeates through the membranes at least 1010 times faster than He). These seemingly incompatible observations were attributed to low-friction flow of a monolayer of water through two-dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules was blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water.

Unimpeded Permeation of Water through Helium-Leak-Tight Graphene-Based Membranes. Nair, R.R., Wu, H.A., Jayaram, P.N., Grigorieva, I.V., Geim, A.K.: Science, 2012, 335[6067], 442-4