The Open-Access Journal for the Basic Principles of Diffusion Theory, Experiment and Application Faster by Opposing the Stream

1. Introduction Diffusion, i.e. the irregular movement of molecules, is quantified by measuring the rate of molecular fluxes in the direction of falling concentration. In contrast to intuition, considering methanol diffusion in a novel type of nanoporous material (MOF ZIF-8) this rate has now been found to be enhanced rather than slowed down by an opposing flux of labelled molecules [1,2]. In terms of the key quantities of random particle movement, this result means that the self-diffusivity exceeds the transport diffusivity. It is rationalized by considering the strong inter-molecular interaction and the dominating role of inter-cage hopping in mass transfer in the systems under study.