ABSTRACT
There exists no surface in nature that is not covered by nanoscopically thin films of water, even in high vacuum. Whereas in the macroworld the presence of such water films is normally totally neglected, there is increasing observational evidence for their importance in the nanoworld, especially in biological systems. In discussing nanoscopic interfacial water layers it is instructive to proceed systematically and to start with the simplest system, the air-water interface. The air-water interface covers more than 70% of the earth’s surface and strongly affects atmospheric, aerosol, and environmental chemistry. According to recent data revealed by isotopic dilution spectroscopy [5] order is found only in the topmost monolayer at
the interface where water molecules present a free OH. In going over to liquid-liquid interfaces the picture becomes more complicated. Precondition for the observation of interfacial phenomena in liquidliquid interfaces is immiscibility, as practically realized between a pure nonpolar liquid in contact with water. Inter facial ordering in such a system can be observed indirectly, for instance, by the order imposed to a third system such as a polymer, forming crystal line nanoneedles [6]—results confirming our own experiments per formed both in stationary and in dynamical interfaces [7]. In dynamical interfaces one component is moving relative to the other-a process that can be associated with electrostatic processes involving the trans fer of electric charge [8], which in turn can induce additional ordering. In passing to liquid-solid interfaces, the degree of complexity of the system again increases, even in the case of water. In contrast to the air-water interface, where the order is limited to one monolayer presenting asymmetric charge distribution, the solid-liquid interface is complicated by the presence of a nanoscopic interfacial water layer, which is structurally different from bulk water. The difference manifests itself primarily in an increase in molecular order reflected by an increase in density and viscosity, depending on the particular nature of the solid surface. The situation is further complicated by the circumstance that the nanoscopic interfacial water layers persist both in air (where under normal relative humidity conditions the thickness of the adsorbed water film easily exceeds that of genuine nanoscopic interfacial water layers) and under water. In practice this means the coexistence of at least four phases: one monolayer of ordered water at the air-liquid interface on top of a layer of normal bulk water covering a nanoscopic film of interfacial water masking the solid surface. In view of the complexity of the situation it is clear that experiments that allow us to analyze the order of nanoscopic interfacial water layers are in no way trivial. Initially the structure of nanoscopic inter facial water layers was probed by X-ray and neutron solution scattering experiments, which coherently indicated an increase in density on hydrophilic surfaces compared to normal bulk water. Analysis by molecular dynamics simulation confirmed the increase in density on hydrophilic surfaces [9].
