Horst L. Vörtler

Simulation of chemical potentials and phase equilibria in two- and three-dimensional square-well fluids: finite size effects.

We study the simulation cell size dependence of chemical potential isotherms in subcritical square-well fluids by means of series of canonical ensemble Monte Carlo simulations with increasing numbers of particles, for both three-dimensional bulk systems and two-dimensional planar layers, using Widom-like particle insertion methods. By estimating the corresponding vapor/liquid coexistence densities using a Maxwell-like equal area rule for the subcritical chemical potential isotherms, we are able to study the influence of system size not only on chemical potentials but also on the coexistence properties. The chemical potential versus density isotherms show van der Waals-like loops in the subcritical vapor/liquid coexistence range that exhibit distinct finite size effects for both two- and three-dimensional fluids. Generally, in agreement with recent findings for related studies of Lennard-Jones fluids, the loops shrink with increasing number of particles. In contrast to the subcritical isotherms themselves, the equilibrium vapor/liquid densities show only a weak system size dependence and agree quantitatively with the best-known literature values for three-dimensional fluids. This allows our approach to be used to accurately predict the phase coexistence properties. Our resulting phase equilibrium results for two-dimensional square-well fluids are new. Knowledge concerning finite size effects of square-well systems is important not only for the simulation of thermodynamic properties of simple fluids, but also for the simulation of models of more complex fluids (such as aqueous or polymer fluids) involving square-well interactions.

The Lennard-Jones Fluid Revisited: Computer Simulation Results

Abstract Pseudoexperimental data of high accuracy on the pressure and the internal energy of the Lennard-Jones fluid have been generated both by the Monte Carlo and molecular dynamics methods for five subcritical and three supercritical isotherms. Values of the chemical potential of the Lennard-Jones fluid computed by a new version of the gradual insertion particle method for two isotherms up to very high densities are also reported and discussed, and compared with existing data.

MC simulation results for a hard core model of carbon tetrachloride

Monte Carlo simulations on a hard tetrahedron fluid (hard core model of CCl4) have been performed. The average site-site correlation functions, their generalized (1, 0, 0) spherical harmonic expansion coefficients, equation of state, and virial coefficients have been calculated and compared with theoretical methods currently available. The RISM equation is less accurate for the model studied than for simpler models considered so far. For the equation of state the best results are obtained from the Boublik-Nezbeda equation which agrees with the simulation results throughout the density range considered.

Monte Carlo simulation of fluid phase equilibria in pore systems: square-well fluid distributed over a bulk and a slit-pore

Abstract An extension of the Gibbs ensemble simulation technique is presented, which permits the simultaneous treatment of multiple fluid-phase equilibria involving a bulk and a confined fluid. The method incorporates separate phase simulation boxes for the two regions and possible coexisting phases within each region. We perform three-box Gibbs simulations for subcritical bulk square-well fluids at equilibrium between a bulk and a hard planar slit region. We compare our results with grand canonical simulation data on supercritical conditions when each region contains only a single phase, and calculate coexistence densities for three-phase states at bulk gas-liquid equilibrium.

Computer simulation studies of hard body fluid mixtures. II

Four different equimolar binary mixtures of hard nonlinear triatomic molecules with heteronuclear dumbbells and linear symmetric triatomics, respectively, have been studied by Monte Carlo simulation using an NPT ensemble. Average site-site correlation functions and the equation of state have been evaluated and the assumption of ideal mixing has been tested. It is shown that both the ideal mixing assumption and Boublik-Nezbeda equation of state are very accurate for all four studied mixtures. Great attention has also been paid to the error analysis and a method to estimate a priori the errors of the simulation is proposed.

MONTE CARLO SIMULATION OF PORE SYSTEMS : PRIMITIVE MODEL OF WATER IN SLIT-LIKE PORES

Abstract A primitive model of associating fluids due to Nezbeda and Smith has been applied to water confined to planar slits with hard walls. It describes the molecules by hard bodies with additional short-range off-center attractive forces mimicking hydrogen bonding. Monte Carlo simulations have been performed on this model. Both the density profile across the slit and the pressure normal to the walls have been estimated for slits of different width. A significant change in structure and a decrease in pressure at the wall have been found in comparison to the underlying pure hard-sphere fluid. For comparison, some simulation results for confined square-well fluids have been included.

Monte Carlo simulation of primitive water in slit-like pores: networks and clusters

The structure of water in smooth slit-like pores is studied by means of a primitive molecular model of associating fluids due to Nezbeda and Smith [W.R. Smith, I. Nezbeda, J. Chem. Phys. 81 (1984) 3694]. MC simulations in an NVT ensemble are performed for different slit widths L ranging from one to about three diameters of the water molecule. Detailed information about cluster and network formation of the associating fluid confined in the pores is obtained. The change of the aggregate topology from linear chains in case of quasi two-dimensional narrow slits into tetrahedral bulk-like structures in wider slits is studied by the increase of L. The research provides basic knowledge for a theoretical understanding of hydration phenomena at interfaces at a molecular level.

Fluid-solid boundary of the compressed EXP-6 fluids

Abstract Using recent computer simulation data and the principle of corresponding states, the boundary of stability of the compressed supercritical EXP-6 potential fluids is localized for an arbitrary potential parameter α. All existing supercritical simulation data of the EXP-6 fluids are then analyzed and assessed with respect to their potential use for derivation of an equation of state of the compressed EXP-6 fluids.

Monte Carlo simulation of hard sphere fluids in planar slits with molecularly rough surfaces

Abstract The structural and thermodynamic behaviour of hard-sphere fluids confined in planar slit-like pores with hard rough walls is investigated by canonical Monte Carlo simulations. The surfaces of the pores are modelled by means of parallel impenetrable walls. Additionally a molecular “roughness” of the pore surfaces is introduced using several models of randomly distributed hard spheres located (fixed or restricted in motion) at the walls. The density distribution of the hard spheres in the pore and the pressure normal to the walls are estimated and compared with the behaviour of fluids in slit-like pores with smooth hard walls. The results are discussed in the context of experimental studies of fluids confined in the polar interfaces of bilayers.

Computer simulation of chemical potentials of primitive models of water

Abstract We report Monte Carlo simulations of chemical potentials on short ranging primitive models (PM) of water due to Nezbeda and co-workers. Since Widom’s test particle method fails for network forming PM water, we simulate chemical potentials by gradual particle insertion. From chemical potential versus packing fraction isotherms, we estimate densities of coexisting phases by means of a Maxwell equal-area rule. Site–site correlation functions, cluster size distributions, and mean square displacements of coexisting phases are analysed. The presented chemical potentials and coexistence properties should be considered as important reference data for PM based molecular theories of associating fluids.