Enantiomer separation is big business — worldwide sales of single-enantiomer drugs, for example, reached $147 billion in the year 2001 and enantiomer separation has been used both to obtain enantiopure starting materials and to separate racemic mixtures of the final product. Now, Purnendu Dasgupta from the University of Texas at Arlington and co-workers from Chiang Mai University in Thailand have introduced1 a new enantiomer-separation method — selective permeation through a chirally doped soap film.

Soap films are ultra-thin liquid membranes formed by surfactant molecules, and consist of two layers separated by a solution of micelles. They are known to be gas-permeable, and can dissolve a wide range of substances. Dasgupta and co-workers created soap films doped with α-cyclodextrin — a cyclic oligosaccharide widely used in chiral separations — and then showed that one enantiomer of gaseous α-pinene (a chiral bicyclic hydrocarbon) passed through the film faster than the other, leading to its enrichment. Interestingly, it is the (+)-enantiomer that is enriched despite it being the one that binds most weakly to α-cyclodextrin, showing that, in fact, such binding slows down the permeation of the membrane.

As yet, the selectivity of the enrichment is small — (+)-α-pinene permeates the membrane about 1.6 times faster than the (–)-enantiomer. The demonstration, however, that a stable film containing a chiral dopant can be formed is an important first step, and the enrichment factor corresponds well to the difference in binding constant of the two enantiomers of α-pinene to the chiral dopant. Ultimately, complete chiral separations could be achieved by the use of multiple membranes.