Filtering in Reverse; July 2002; Scientific American Magazine; by Steven Ashley; 1 Page(s)

Filters block the big particles and allow the finer substances through, right? Not necessarily-some filters work in the opposite way. A team of chemical engineers and materials researchers has discovered a method to markedly improve these so-called reverse-selective membranes in a unexpected manner: by adding nonporous filler materials. Rather than stopping up the filter holes, though, the additives enhance the membrane's permeability to large molecules.

This result stems from how these unusual gas filters operate, according to team leader Ingo Pinnau of Membrane Technology and Research in Menlo Park, Calif. A reverseselective membrane first allows compounds to dissolve directly into its matrix; then the molecules diffuse to the other side. Because larger molecules condense into a liquid more readily, they generally tend to dissolve more quickly than smaller constituents. As a result, the proportion of large molecules to small ones can increase on the other side of the membrane. The separation efficiency is limited, however, because large molecules diffuse more slowly through the matrix of the membrane.