The structure of molecularly thin films of water on mica in humid environments

The structures formed by the condensation and evaporation of water on mica surfaces at room temperature have been imaged with nanometer resolution. The technique used is based on the measurement of the polarization force between a charged AFM tip and the surface. This allowed us to perform non-contact imaging at tip-surface distances of a few hundred angstroms, which is critical to not perturb the surface of liquid layers. We found that at room temperature and up to 40-50% humidity, water condenses forming two-dimensional, molecularly thin films with two distinct phases (I & II). Phase I forms at humidities <20%, while phase II forms between similar to 20 and 40%. Phase II forms polygonal domains in angular epitaxial relationship with the mica crystallographic directions, suggesting that it is solid in character. Above 40% humidity, water adsorbs forming thicker films and droplets.