Molecules are not the easiest of traffic to direct. In the nineteenth century, the Scottish physicist James Clerk Maxwell imagined a tiny demon controlling the flow of a gas of randomly moving molecules by opening a trapdoor only to those travelling in one direction. Now physicists at Northwestern University, Evanston, Illinois, have shown that demon traffic controllers are not always necessary. Under the right circumstances, molecular traffic will direct itself.

The researchers are interested in molecular traffic control as a way of separating a gas containing two kinds of molecule -- a situation commonly encountered in chemical engineering. One way of separating molecules of different sizes is to use a 'molecular sieve' -- a solid material perforated with molecule-sized channels or pores.

Materials like this exist naturally. For instance, the minerals called 'zeolites', made largely of aluminium, silicon and oxygen, contain an orderly grid of tiny pores. And now, many artificial zeolites have also been devised to supplement this range of 'molecular sieves'. With luck, one can find a zeolite with pores of just the right size to admit the smaller molecules in a mixture and exclude the larger ones.

But Louis Clark and colleagues at Northwestern University have considered a rather different situation, in which both kinds of molecule in a two-component mixture fit inside the pores of a zeolite.

They have investigated the controversial idea of 'molecular traffic control', suggested in 1980 by E. Derouane and Z. Gabelica. These two chemists proposed that dissimilar molecules might move preferentially down different types of pore. If the two types of pore run in distinct directions, like the north--south and east--west roads of a grid-like city, then the molecules would pass in different directions and become separated.

But the concept of molecular traffic control (or MTC) had not been shown to work. It requires a delicately balanced set of circumstances.

In the case reported by Clark and colleagues, in Physical Review Letters1, the mixture of gases contains 'small' molecules -- xenon, which consists of lone, inert atoms, and 'large' molecules -- sulphur hexafluoride, a composite of seven atoms. As their 'separation grid' the researchers used a zeolite called boggsite, which comprises a network of straight small pores intersected at right angles by straight larger pores.

The researchers supposed that MTC would operate if the xenon atoms moved more quickly along the small pores, while the sulphur hexafluoride molecules moved more freely in the larger pores. Normally, smaller molecules move down any pore more rapidly than larger ones, because the larger molecules have a greater tendency to get jammed.

But corrugations -- regions where the width narrows slightly -- in pores can alter this behaviour. Clarke's group calculated that sulphur hexafluoride molecules should float freely down the large channels of boggsite because they 'feel' the corrugations less than do the smaller xenon atoms, which rattle in the constrictions.

Thus the researchers predicted that the sulphur hexafluoride should become confined largely to the wide pores of boggsite, while the xenon should stay mostly in the narrow pores. So the gases would take different routes. They tested these predictions using computer simulations of a mixture of the two gases diffusing within the criss-crossed pore network.

Sure enough, xenon tended to escape from the holey material along one direction -- from the top and bottom, say -- while the sulphur hexafluoride came out in the perpendicular direction, from the sides. How much easier our city roads might be if cars and lorries could be separated in this way.