Building microholes into metal-oxide gels.

Inexpensive and easy to build, silica gels serve as the basis of many industrial absorbents, chromatographic-separation agents, bulk fillers and catalyst supports. But their amorphous metal-oxide structure is riddled with holes of various sizes (left diagram). If the holes were the same size -- and very small, on the order of one-billionth of a meter across -- they might usher in a new class of strong, molecular sieves.

In hopes of engineering such filters, Sandia's Douglas A. Loy and his co-workers at the University of California, Irvine, are reformulating the recipes for silica aerogels (SN: 11/17/90, p.316). They substitute small bridging structures (rectangles in right diagram) made from an organic chemical -- such as acetylene (C:C) or a phenyl group ([C.sub.6.H.sub.4]) -- for some oxygen atoms. After anchoring a mat of these new silicon-based molecules together, Loy's team removes the bridges. the goal:a thin film with the structural advantages of molecular sieves -- rigidity, high surface areas and thermal stability to 400[degrees] or 500[degrees] C -- but with tailored, nearly uniform pores.

Acetylene bridges will wash out with an aqueous acid, Loy's team finds. For phenyls and many other organics, Loy now burns his bridges with a room-temperature plasma of atomic oxygen.