On the heat capacity of adsorbed phases using molecular simulation

The heat capacities of argon, ammonia, and methanol on carbon black at 87.3, 240, and 300 K, respectively, have been investigated. The carbon black surface has been modeled with and without carbonyl groups. Part of this investigation is a decomposition of the heat capacity into its contributions from the different interaction potentials of an adsorption system. All systems show a spectrum of heat capacity versus loading, and this behavior depends on the carbonyl configuration present on the surface. For methanol and ammonia the variation of the heat capacity between the two for the same carbonyl configurations is greater than the variation in the heat of adsorption. Heat capacities of methanol and ammonia are generally dominated by fluid-fluid interactions due to the strong association of fluid particles through hydrogen bonding. The difference in the heat capacity behavior of the two fluids is an indicator of their different clustering behaviors on the carbon black surface. The presence of carbonyl groups reduces the fluid-fluid contributions to the heat capacity. This is due to the compensation of fluid-fluid interactions with fluid-functional group interactions. At 87.3 K a first layer transition to a solidlike state is present for argon and results in a large peak in the heat capacity on a bare surface. The presence of functional groups greatly reduces this peak in the heat capacity by disrupting the packing of argon on the surface and preventing a transition to a solidlike state. (c) 2007 American Institute of Physics

The Interaction of Rare Gas Atoms with Graphite Surfaces. I. Single Adatom Energies

The Gordon-Kim local density method is applied to the calculation of the interaction energy of helium, neon, argon, and krypton with the basal plane of graphite. In all cases, the binding site is found to be above the center of a hexagon, but the barrier to migration to other sites is less than 50 cal/ mole. Comparisons are made with other studies on these systems, and the role of non-two-body additive effects is discussed

Including screening in van der Waals corrected density functional theory calculations: The case of atoms and small molecules physisorbed on graphene

The DFT/vdW-QHO-WF method, recently developed to include the van der Waals (vdW) interactions in approximated Density Functional Theory (DFT) by combining the Quantum Harmonic Oscillator model with the Maximally Localized Wannier Function technique, is applied to the cases of atoms and small molecules (X=Ar, CO, H$_2$, H$_2$O) weakly interacting with benzene and with the ideal planar graphene surface. Comparison is also presented with the results obtained by other DFT vdW-corrected schemes, including PBE+D, vdW-DF, vdW-DF2, rVV10, and by the simpler Local Density Approximation (LDA) and semilocal Generalized Gradient Approximation (GGA) approaches. While for the X-benzene systems all the considered vdW-corrected schemes perform reasonably well, it turns out that an accurate description of the X-graphene interaction requires a proper treatment of many-body contributions and of short-range screening effects, as demonstrated by adopting an improved version of the DFT/vdW-QHO-WF method. We also comment on the widespread attitude of relying on LDA to get a rough description of weakly interacting systems