  When two binary solutions are separated by a permeable barrier, the
individual species typically diffuse and mix, dissipating their chemical
potential gradients. However, we use model lattice simulations to show that
single-file molecular-sized channels (such biomembrane channels and zeolites)
can exhibit diffusional pumping, where one type of particle uses its entropy of
mixing to drive another up its chemical potential gradient. Quantitative
analyses of rates and efficiencies of transport are plotted as functions of
transmembrane potential, pore length, and particle-pore interactions. Our
results qualitatively explain recent measurements of ``negative'' osmosis and
suggest new, more systematic experiments, particularly in zeolite transport
systems.
