Quantum effects turn pore diffusion on its head

Separating isotopes is difficult business: Because an element’s various isotopes share similar size and shape, separation methods such as thermal diffusion and centrifugation tend to be time and energy intensive. Now, however, a team led by Suresh Bhatia (University of Queensland, Australia) has shed new light on what may prove an attractive alternative—nanoporous materials known as molecular sieves. Normally, molecular sieves aren’t particularly effective isotope separators. Take diatomic hydrogen and its iso-topic relative deuterium. As one would expect, the lighter H₂ molecules diffuse through the porous molecular sieve faster than the heavier D₂, but the difference is slight. That picture changes, though, if the temperature is low enough—and the pores small enough—for quantum effects to set in. If the pore size is on the order of the molecules’ de Broglie wavelength, the molecules’ zero-point energy becomes the important barrier for pore diffusion and small mass becomes a disadvantage. Not only does D₂ then diffuse faster than H₂, it can do so by a substantial margin. Using quasi-elastic neutron scattering and carbon molecular sieves with 3-å-diameter pores, Bhatia and company directly measured those diffusivities, which differed by nearly an order of magnitude at the coldest temperatures. Although quantum sieve effects—first predicted nearly 15 years ago—had been previously seen in equilibrium adsorption experiments, the research team’s findings represent the first microscopic observations of the kinetic phenomenon. (T. X. Nguyen, H. Jobic, S. K. Bhatia, Phys. Rev. Lett. 105 , 085901, 2010 http://dx.doi.org/10.1103/PhysRevLett.105.085901 .)