We report a molecular simulation study for a model of water adsorption on nonporous and porous activated carbons. The grand canonical Monte Carlo method is used, and the temperature is fixed at 300 K. Water molecules are modeled as a Lennard-Jones sphere with four square-well sites to account for the hydrogen bonding. The carbon surfaces consist of planar graphite sheets, with active chemical sites on the surface modeled as square-well sites. The effect of the density and geometric arrangement of the active sites on the surface is studied. Both macroscopic properties (particularly adsorption isotherms) and molecular configurations are obtained. The adsorption mechanism for water on such surfaces is markedly different from that of simple nonassociating molecules such as hydrocarbons or nitrogen. In contrast to the usual buildup of adsorbed layers on the surface, water adsorption is characterized by the formation of peculiar three-dimensional water clusters and networks, whose formation relies on a cooperative effect involving both fluid−fluid interactions and fluid−solid ones with suitably placed active sites. Both the density and arrangement of the sites on the surface have a pronounced effect on the adsorption. Capillary condensation is observed only for low densities of active sites; for higher densities, continuous filling occurs.

The vapor-liquid phase equilibria of square-well systems with hard-sphere diameters o, welldepths E, and ranges il = 1.25, 1.375, 1.5, 1.75, and 2 are determined by Monte Carlo simulation. The two bulk phases in coexistence are simulated simultaneously using the Gibbs ensemble technique. Vapor-liquid coexistence curves are obtained for a series of reduced temperatures between about T,. = T/T, = 0.8 and 1, where T, is the critical temperature. The radial pair distribution functions g(r) of the two phases are calculated during the simulation, and the results extrapolated to give the appropriate contact values g(a), g(/Za-), and g(;la-I-). These are used to calculate the vapor-pressure curves of each system and to test for equality of pressure in the coexisting vapor and liquid phases. The critical points of the squarewell fluids are determined by analyzing the density-temperature coexistence data using the first term of a Wegner expansion. The dependence of the reduced critical temperature Tr = kT,/q pressure P r = P,o-'/E, number density p: = pE d, and compressibility factor Z = P /(pkT), on the potential range il, is established. These results are compared with existing data obtained from perturbation theories. The shapes of the coexistence curves and the approach to criticality are described in terms of an apparent critical exponent 8. The curves for the square-well systems with il = 1.25, 1.375, 1.5, and 1.75 are very nearly cubic in shape corresponding to near-universal values ofp (flzO.325). This is not the case for the system with a longer potential range; when ;1 = 2, the coexistence curve is closer to quadratic in shape with a nearclassical value of p (PzO.5). These results seem to confirm the view that the departure of fl from a mean-field or classical value for temperatures well below critical is unrelated to longrange, near-critical fluctuations.

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The liquid crystal phases of the Kihara fluid have been studied in computer simulations. The work focuses on the isotropic-nematic-smectic-A triple point region, especially relevant for the understanding of the properties and the design of real mesogens with specific phase diagrams. The Kihara interaction resembles more appropriately than other related models, the shape of elongated polymers and biomolecules, and a closer assertion is provided for the role of the configurational entropy and the dispersive interactions in the behavior of such molecules in dense phases or under macromolecular crowding conditions.

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We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: ͑a͒ hard spherocylinders with an attractive square-well ͑SWSC͒ and ͑b͒ purely repulsive soft spherocylinders ͑SRS͒, in both cases for a length-to-breadth ratio L*ϭ5. Monte Carlo simulations in the isothermal-isobaric ensemble have been performed at a reduced temperature T*ϭ5 probing thermodynamic states within the isotropic ͑I͒, nematic ͑N͒, and smectic A ͑Sm A͒ regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic-nematic transition of the hard spherocylinder ͑HSC͒ fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic-smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I-N and N-Sm A transitions to higher pressures and densities. Moreover, the soft repulsive core of the SRS fluid induces a similar but quite more significant shift of both of these phase boundaries toward higher densities. This latter effect is apparently in contrast with very recent studies of the SRS fluid at lower temperatures, but this discrepancy can be traced back to the different effective size of the molecular repulsive core at different temperatures.

Using molecular-dynamics computer simulation, we study the dynamical behavior of the isotropic and nematic phases of highly anisotropic molecular Auids. The interactions are modeled by means of the Gay-Berne potential with anisotropy parameters~= 3 and~' =5. The linear-velocity autocorrelation function shows no evidence of a negative region in the isotropic phase, even at the higher densities considered. The self-diffusion coefficient parallel to the molecular axis shows an anomalous increase with density as the system enters the nematic region. This enhancement in parallel diffusion is also observed in the isotropic side of the transition as a precursor effect. The molecular reorientation is discussed in the light of different theoretical models. The Debye diffusion mode1 appears to explain the reorientational mechanism in the nematic phase. None of the models gives a satisfactory account of the reorientation process in the isotropic phase.PACS number(s): 61.20.Ja, 61.30.By, 64.70.Md