Membrane Mastery: Nanosize silica speeds up sieve

When separating molecules from one another–whether to purify natural gas or clean wastewater–engineers and scientists usually use expensive techniques that require a lot of energy. A less expensive, environmentally friendly–though not always as efficient–option is to filter molecules with membranes made of polymers, carbon, or ceramics.

Now, a new modification to polymer membranes gives researchers a means to tune certain filters so they separate molecules more quickly and more selectively. The new membranes contain nanoscale particles of silica, Tim Merkel of the Research Triangle Institute in Research Triangle Park, N.C., and his colleagues report in the April 19 Science.

Most membranes contain a microscopic maze of channels and cavities that bar large molecules while letting small ones through. Another type of membrane, called a reverse-selective membrane, permits large molecules to permeate more readily than small ones. Such membranes have passages big enough for large molecules to move through.

Merkel and his colleagues made modified reverse-selective membranes by mixing silica particles, each about 13 nanometers wide, into a polymer and forming the material into membrane films. The membranes’ permeability to butane and other large gas molecules, relative to the permeability to small molecules, increased dramatically with rising concentrations of silica particles, the researchers report.

Adding particles to improve a filter’s permeability seems counterintuitive, says team member Richard Spontak of North Carolina State University in Raleigh. Particles would be expected to clog up the holes through which molecules pass.

But Merkel, Spontak, and their colleagues used a rigid, glassy polymer whose chains of atoms don’t stack together tightly. Unlike a network of flexible polymers, which might wrap around any added silica, this structure was disrupted by the particles. More space opened through which large molecules could move, says Spontak.

“This is a good piece of work all the way around,” comments membrane researcher Henry Foley of the Pennsylvania State University in State College. “They’ve shown the effect of nanoscale engineering on transport through membranes,” he says.