Stop and go. Membranes dosed with fumed silica (inset) strain out small molecules.

Paradoxical Membranes

Industrial efforts to purify water or natural gas, for example, separate desired compounds from mixtures by passing them through membranes pocked with tiny holes. The smaller the holes, the more selective the membrane--in general. But now a team of researchers reports forming membranes with wide-open holes that, paradoxically, allow large molecules through far more readily than smaller ones. The new membranes are both more selective and faster than previous versions and could lead to cheaper and more energy-efficient methods for industrial separations.

Searching for more efficient membranes, a team led by Tim Merkel, a chemical engineer at Research Triangle Institute in Research Triangle Park, North Carolina, and Benny Freeman, a chemical engineer at the University of Texas, Austin, tried spiking a conventional membrane polymer with a type of fine-grained sand called fumed silica. They mixed the fumed silica with rigid polymer chains, each akin to a strand of uncooked spaghetti. The small sand particles acted like meatballs strewn among the stiff spaghetti strands. "That forced the polymer chains apart and increased the permeability" of the membrane, Freeman says. The arrangement gave the membranes an array of gaping holes, which by all accounts should sieve molecules quickly. The researchers braced themselves for the seemingly inevitable influx of chemical intruders.

It never came. In fact, the new membranes allowed large, gaseous organic compounds such as benzene to pass through while straining out smaller gases such as hydrogen. The counterintuitive result, Freeman explains, occurs because molecules move through a membrane in two stages. First they must dissolve into the membrane, and then they must wiggle their way through it. And whereas smaller molecules are faster wigglers, larger molecules are quicker to dissolve. In densely packed membranes, large molecules still get hung up on their way through. But thanks to the wider holes in the new membranes, Freeman says, the bigger molecules have the elbow room they need to take full advantage of their head start and zip across before the smaller molecules.

"This is a very interesting result," which could open new industrial uses for membranes, says Narcan Bac, a chemical engineer and membrane separations specialist at Northeastern University in Boston. Merkel, Freeman, and colleagues are now testing whether their new membranes will separate out unwanted compounds commonly found in natural gas. If so, the hybrid membranes could open the door for energy companies to exploit vast natural gas reserves that currently harbor too many unwanted gases to be useful.

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Freeman's site