A tight squeeze for mobile water.

A tight squeeze for mobile water

Water molecules trapped in a porous material partially filled with water seem more mobile than water molecules in, say, a cup of water. "To our knowledge, this peculiar result has never been observed before," William P. Halperin of Northwestern University in Evanston, Ill., and his co-workers report in the July 3 PHYSICAL REVIEW LETTERS. The finding provides a striking example of how a material's pore structure can alter a liquid's bulk properties.

The researchers performed their experiments on porous silica glass filled so that water takes up different fractions of the space available. Relying on nuclear magnetic resonance  measurements, they found that water molecules move around inside the pores more rapidly than would be expected--so long as the glass sample isn't saturated with water.

The effect seems to stem from the fact that water "wets" the glass, forming a layer on the pores' inner surfaces. The remainder of the pore volume is filled with air saturated with water vapor. The resulting sponge-like system consists of two interpenetrating pore structures--that of the liquid and that of the vapor--each with a different geometry. In such an arrangement, water molecules, instead of following paths through the liquid layer, apparently take shortcuts by going into the vapor phase before reentering the liquid phase. This increases the overall mobility of any water molecules present.

The efficiency of the exchange of water molecules between the liquid and vapor phases depends on how large and how full the pores are. Such a rapid exchange between liquid and vapor doesn't occur when water sits in a cup exposed to air or when the material's pores are very small. The process also seems to reflect details of the porous material's microstructure. "The importance of these effects varies from one kind of material to another," Halperin says.

The findings may have implications for how rapidly liquids diffuse through porous materials, affecting processes such as the hardening of cement and the spread of contaminants in groundwater and materials such as sandstone. "In these cases, transport by diffusion can be much faster than would otherwise have been thought," Halperin says. However, none of these possibilities has been investigated yet.