The premise behind a filter is simple enough: smaller objects slip through the holes, leaving the larger ones behind. But imagine that if by increasing the size of the holes, you could actually make the filter more--not less--selective. It sounds paradoxical, but in fact, according to a report in the current issue of the journal Science, researchers have succeeded in creating membranes capable of just that.

The novel nets, developed by Timothy Merkel of the Research Triangle Institute in Research Triangle Park, North Carolina, and colleagues, consist of a glassy, amorphous polymer studded with silica particles. Unlike similar membranes embedded with metal oxides or other nanoparticles, which exhibit reduced permeability, the silica-studded polymer membranes are more permeable. Yet they preferentially allow larger molecules such as butane to pass through, retaining the smaller ones such as methane. "To use an analogy, if you make the holes in a net big enough, ping-pong balls, tennis balls and basketballs will all make it through," team member Richard Spontak of North Carolina State University explains. "With our membranes, we can actually achieve both high permeability and reverse selectivity--in other words, our net preferentially lets basketballs through."

Such membranes could prove useful in seawater desalination, environmental cleanup and other tasks involving molecular separations. These separations are most often accomplished using energy- and capital-intensive technologies such as distillation, absorption and adsorption, the authors note. Nanocomposite membranes, in contrast, are low-cost, energy efficient and environmentally friendly.