Dynamics of ethanol and water mixtures observed in a self-adjusting molecularly thin slit pore.

The structure of multicomponent fluids in confined geometries is a key to understanding their properties. However, it remains an experimental challenge to gain molecular-scale resolution information on this structure. Here we show that mono- and multilayers of graphene, conforming to heterogeneous monolayers of molecules in a flexible slit pore between a mica surface and the graphene layers, allow for mapping the phase separation of water and ethanol within such a slit pore. Employing scanning force microscopy, we readily distinguish clusters of ethanol and water molecules due their different sizes, and we show that the phase separated water-ethanol structures become coarser under thicker graphenes. Moreover, we obtain a lower bound for the two-dimensional diffusion coefficient of ethanol in water of D ≥ 2 × 10(-14) m(2) s(-1). Thus, the molecularly thin slit pore provides a powerful tool to control and to investigate mixed fluids in self-adjusting nanopores.