(376ai) High-Pressure Phenomena in Adsorbed Films: A New Route to an Experimental Determination of Effective Tangential Pressure

High-pressure phenomena have been observed in many studies of adsorbed films on strongly wetting solid substrates. These include chemical reactions that normally require a high pressure (e.g. 10,000 bar or more), the formation of high-pressure solid phases and spectra indicating strong compression of the film. While it is possible to measure the pressure acting normal to the substrate surface, P_N, no method has been developed so far to measure the tangential pressure, P_T, parallel to the surface. However, this tangential pressure is of particular interest, since it is usually much larger than the normal pressure, and is the main driving force for high-pressure phenomena in these films.

While the normal pressure is uniquely defined for a planar surface, the tangential pressure at a point r is not uniquely defined at the nanoscale. In the commonly used ‘virial-route’ the average of the intermolecular forces acting in the direction parallel to the surface is obtained. The resulting tangential pressure then depends on the fraction of these forces that are assigned to a particular point in space, r. We show that by integrating P_T(r) over a small region of space, roughly the range of the intermolecular forces, we obtain an effective tangential pressure that is unique and well-defined. The grand canonical Monte Carlo simulation results show that this characteristic integration range has negligible correlation with the wetting parameter, which denotes the strength of the solid-fluid interaction, and the characteristic length remains as a constant for large pores. Further investigation reveals that the characteristic length of convergence is related to the density distribution of adsorbed layers, which explains its weak dependence on the wetting parameter. Based on this characteristic length, we propose a new route, the ‘2D-route’, to the effective tangential pressure free from the ambiguities in the definition of the ‘virial-route’ local tangential pressure. The only input parameter for this ‘2D-route’ is the molecular cross-sectional area. In practice, we can obtain the input parameter by combining standard experimental adsorption data with a 2D equation of state derived from statistical mechanics. We expect this ‘2D-route’ to be the first experimental route to the pressure enhancement in the adsorbed layers.