Joan E Curry
Publications
Abstract:
One- to five-layer cyclohexane and octamethyltetracyclosiloxane (OMCTS) films confined between mica-like surfaces are studied to elucidate changes in the lattice type and composition of the films. Grand canonical ensemble Monte Carlo computer simulations are used to study the laterally confined film. In contrast to previous studies, solid-like order is induced primarily by the strong fluid-solid interaction and is largely a function of pore width. Solid-like order within the layers causes the composition of the pore fluid to shift from the bulk composition, favoring either cyclohexane or OMCTS, depending on the pore width. A shift in the relative alignment of the surfaces perturbs the solid-like fluid structure but does not cause the sudden shear melting transition associated with epitaxial alignment of the fluid atoms with the surface.
Abstract:
The adsorption behavior of n-pentane and cyclohexane in mica slits at room temperature has been studied as a function of chemical potential and gap width with multiple-beam interferometry. The measured film thicknesses close to saturation for large slit widths (effectively isolated surfaces) range up to 7 nm with n-pentane (at a relative vapor pressure of 0.9996) and 3 nm with cyclohexane (at a relative vapor pressure of 0.995). The thickness of these adsorbed wetting films is slightly larger than that predicted by van der Waals theory. The difference may be accounted for by thermal fluctuations of the adsorbed liquid-vapor interface. At smaller slit widths a capillary condensation transition occurs as the slit fills up with liquid. The separation at which this occurs is in good agreement with a film-thickening mechanism due to van der Waals forces across the gap only for the thickest films (t ≥ 6 nm). For thinner films the capillary condensation transition occurs at larger than expected slit widths, and the deviations are large for t ≤ 3 nm. We speculate that these larger-than-expected condensation separations are related to a fluctuation-enhanced film thickness in this regime. The work demonstrates the utility of measurements in a system consisting of a single slit-pore, without the complications of polydispersity and connectivity of pore networks. The results show that vapor adsorption isotherms can be measured with multiple-beam interferometry, i.e., in the surface force apparatus.
Abstract:
The linear Poisson-Boltzmann equation is solved with a variable dielectric constant to account for dielectric saturation. Two different spatially dependent forms of the dielectric constant are compared. It is shown that the electrostatic repulsion between two flat plates is decreased by dielectric saturation and that it depends in a complicated way on both the dielectric constant and the electric field. © 1992.