Threshold criterion for wetting at the triple point

Grand canonical simulations are used to calculate adsorption isotherms of various classical gases on alkali metal and Mg surfaces. Ab initio adsorption potentials and Lennard-Jones gas-gas interactions are used. Depending on the system, the resulting behavior can be nonwetting for all temperatures studied, complete wetting, or (in the intermediate case) exhibit a wetting transition. An unusual variety of wetting transitions at the triple point is found in the case of a specific adsorption potential of intermediate strength. The general threshold for wetting near the triple point is found to be close to that predicted with a heuristic model of Cheng et al. This same conclusion was drawn in a recent experimental and simulation study of Ar on CO_2 by Mistura et al. These results imply that a dimensionless wetting parameter w is useful for predicting whether wetting behavior is present at and above the triple temperature. The nonwetting/wetting crossover value found here is w circa 3.3.Comment: 15 pages, 8 figure

Characterization of Dipeptide-based Sorbent Materials using Combined Thermodynamic and Inelastic Neutron Scattering Techniques

Using a combination of x-ray diffraction, volumetric adsorption and inelastic neutron scattering (INS) the adsorption properties of methane within the channels of L-Isoleucyl-L-Valine (IV) and L-Valyl-L-Alanine (VA) dipeptides have been investigated. These biomaterials have quasi one-dimensional channels of tunable diameters in the range of 3-6 \uc5 and offer possibilities for selective adsorption, as well as, water and gas transport properties. High-resolution volumetric methane adsorption measurements performed near 100K for IV find that this biomaterial exhibits an adsorption capacity of 3c100 m2/g. High-resolution Inelastic Neutron Scattering (INS) measurements were performed at the Spallation Neutron Source using the BASIS spectrometer with 3c 3.5 \u3bceV resolution. The data clearly indicate that at least two spectral features at energy transfers near 100 and 200 \u3bceV are present, which suggests a lowering of the hindering potential for methane reorientation primarily about the three-fold axis within the IV channels. Such features play a key role in understanding details concerning the potential energy surface. These thermodynamic and INS studies suggest that the flexibility and dynamical motion within the dipeptide channels may play a significant role in the adsorption properties

Erratum to: “Melting of thin films of alkanes on magnesium oxide”

In the above article the name of one of the authors has been mistyped. The correct name of the author is D. Fernandez-Cañoto

Effects of melting and ordering on the isosteric heat and monolayer density of argon adsorption on graphite

The aim of this paper is to study the effects of temperature on the state of the adsorbed argon on an uniform graphite surface. We applied the kinetic Monte Carlo scheme to simulate adsorption over a very wide range of temperature, which allows us to model the vapor–solid, the vapor–liquid and the order–disorder transition of the monolayer. The main distinction of our methodology is that it accounts for the lattice constant change with loading in the case of formation of an ordered molecular layer by appropriately changing the simulation box size. To do this we enforced the equality of the tangential pressures obtained by the virial and thermodynamic routes, which corresponds to the minimum Helmholtz free energy of a system at a given number of molecules and volume. This criterion is a consequence of the Gibbs–Duhem equation. A significant result obtained by application of the new simulation method was a sharp contraction of the monolayer just after its completion and the onset of the second layer. It manifests itself in an additional heat release. We re-determined the 2D-melting and 2D-critical temperatures of the molecular layer of argon. We also analyzed the order–disorder transition above the 2D-melting and showed that it could occur at some temperatures above the 2D-critical temperature. In this case, a hexagonal lattice appears at a sufficiently large tangential pressure. The effects of loading on the lattice constant, the 2D-critical temperature of the order–disorder transition and the differential heat of adsorption are thoroughly discussed