Salting away guest molecules.

Salting away guest molecules

Preserving perishables by mixing them with salt has a long history. Now a group of chemists has come up with a modern, microscopic version of this ancient practice. They have prepared tiny salt crystals that incorporate neutral molecules to create a new kind of composite material.

The idea is to store volatile or otherwise unstable molecules within, say, an alkali halide such as common salt, says Josef Michl, now at the University of Texas at Austin. Chemists can then study these "guest" molecules under controlled conditions. In the past, they had to use frozen gases like solid nitrogen or argon as hosts, which vaporized too easily for high-temperature experiments. Michl and his colleagues report their findings in the Oct. 29 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.

The researchers prepare their samples by condensing alkali halide vapor along with molecules of the chosen guest compound on a surface at 77 kelvins. When the resulting crystals are brought up to room temperature, the molecules are now trapped in their salt "storage boxes." Guest molecules make up as much as a few percent of the final product.

"The permanency of the incorporation is astonishing," the chemists say. They found, for example, that naphthalene stays trapped within cesium iodide crystals even after two hours at 450[deg.]C. By itself, naphthalene, a common mothball ingredient, vaporizes easily and melts at 80.5[deg.]C.

In most cases, the guest molecules seem to form small clusters within the host crystals. Michl and his group are now looking for ways to keep guest molecules isolated from one another. "If we can get the molecules singly isolated," says Michl, "it creates a very unusual environment for these molecules." Benzene, for instance, surely doesn't want to be in the middle of a highly polar, ionic environment like that, he says. The researchers want to study the photochemical properties of molecules under these conditions.

Michl, who already has a patent on this technique, is also considering possible applications. Using salt as a way of storing organic compounds, he says, could be a convenient way of getting drugs to cows, which like licking salt but often dislike medicinal preparations.

This method could also be used to generate "molecular imprints" on surfaces. Any guest molecules that evaporate from a surface may leave behind a hollow. "If this footprint is stable," says Michl, "you'll have a shape that will presumably accept the same sort of molecule that left and not others that are bigger." This could lead to useful detectors for specific compounds.