Abstract
A general design strategy for a molecularly selective capillary is proposed, based on the properties of incommensurate intergrowth materials. The strategy is illustrated for urea inclusion compounds, in which
guest molecules are contained within one-dimensional tunnels in a crystalline urea
host structure. It is shown that net transport of guest molecules in one direction along the tunnel can be achieved by inserting new guest molecules at one end of the crystal (by dipping it into the liquid phase of another potential guest), with the original guest molecules expelled from the other end. Such phenomena have considerable potential as the basis of highly selective micro-scale separation techniques, based on discrimination of molecular size, shape and chirality.