Solvation forces and phase separation of water in a thin film of nonpolar liquid between mica surfaces

Abstract

We present direct measurements of the force between two mica surfaces across a thin film of nonpolar liquid (octamethylcyclotetrasiloxane) containing water at activities in the range 0–0.75. The oscillatory solvation force at zero water content has been measured down to molecular contact for the first time. As the water activity is increased there is a reduction in the amplitude of the solvation force at larger separations, but progressively more of the inner oscillations become experimentally inaccessible as the interaction in this regime is replaced by a strong attraction. The adhesion at contact increases dramatically with water activity. We suggest that the results may be explained in terms of two related but separable effects. With increasing activity the increase in surface excess of water disrupts the layering of the molecules of the liquid between the surfaces which smears out the solvation force at long range. Phase separation of water occurs at progressively larger separations, at least for activities of 0.5 and above, and this gives rise to the sudden attraction that replaces the solvation force. The very large adhesion reflects the presence of condensed water around the surfaces at contact.