We discuss molecular simulation methods for computing the phase coexistence properties of complex molecules. The strategies that we pursue are histogram-based approaches in which thermodynamic properties are related to relevant probability distributions. We first outline grand canonical and isothermal-isobaric methods for directly locating a saturation point at a given temperature. In the former case, we show how reservoir and growth expanded ensemble techniques can be used to facilitate the creation and insertion of complex molecules within a grand canonical simulation. We next focus on grand canonical and isothermal-isobaric temperature expanded ensemble techniques that provide a means to trace saturation lines over a wide range of temperatures. To demonstrate the utility of the strategies introduced here, we present phase coexistence data for a series of molecules, including n-octane, cyclohexane, water, 1-propanol, squalane, and pyrene. Overall, we find the direct grand canonical approach to be the most effective means to directly locate a coexistence point at a given temperature and the isothermal-isobaric temperature expanded ensemble scheme to provide the most effective means to follow a saturation curve to low temperature.

We introduce general Monte Carlo simulation methods for determining the wetting and drying properties of model systems. We employ an interface-potential-based approach in which the interfacial properties of a system are related to the surface excess free energy of a thin fluid film in contact with a surface. Two versions of this approach are explored: a "spreading" method focused on the growth of a thin liquid film from a surface in a mother vapor and a "drying" method focused on the growth of a thin vapor film from a surface in a mother liquid. The former provides a direct measure of the spreading coefficient while the latter provides an analogous drying coefficient. When coupled with an independent measure of the liquid-vapor surface tension, these coefficients enable one to compute the contact angle. We also show how one can combine information gathered from application of the spreading and drying methods at a common state point to obtain direct measures of the contact angle and liquid-vapor surface tension. The computational strategies introduced here are applied to two model systems. One includes a monatomic Lennard-Jones fluid that interacts with a structureless substrate via a long-ranged substrate potential. The second model contains a monatomic Lennard-Jones fluid that interacts with an atomistically detailed substrate via a short-ranged potential. Expanded ensemble techniques are coupled with the interface potential approach to compile the temperature- and substrate strength-dependence of various interfacial properties for these systems. Overall, we find that the approach pursued here provides an efficient and precise means to calculate the wetting and drying properties of model systems.

This work investigates the question if surface capillary waves (CWs) affect interfacial solvation thermodynamic properties that determine the propensity of small molecules toward the liquid-vapor interface. We focus on (1) the evaluation of these properties from molecular simulations in a practical manner and (2) understanding them from the perspective of theories in solvation thermodynamics, especially solvent reorganization effects. Concerning the former objective, we propose a computational method that exploits the relationship between an external field acting on the liquid-vapor interface and the magnitude of CWs. The system considered contains the solvent, an externally applied field (f) and the solute molecule fixed at a particular location. The magnitude of f is selected to induce changes in CWs. The difference between the solvation free energies computed in the presence and in the absence of f is then shown to quantify the contribution of CWs to interfacial solvation. We describe the implementation of this method in the canonical ensemble by using a Lennard-Jones solvent and a non-ionic solute. Results are shown for three types of solutes that differ in the nature of short-ranged repulsive (hard-core) interactions. Overall, we observe that CWs have a negligible or very small effect on the interfacial solvation free energy of a solute molecule fixed near the liquid-vapor interface for the above systems. We also explain how the effects of pinning or dampening of CWs caused by a fixed solute are effectively compensated and do not contribute to the solvation free energy.

We discuss Monte Carlo (MC) simulation methods for calculating liquid-vapor saturation properties of ionic liquids. We first describe how various simulation tools, including reservoir grand canonical MC, growth-expanded ensemble MC, distance-biasing, and aggregation-volume-biasing, are used to address challenges commonly encountered in simulating realistic models of ionic liquids. We then indicate how these techniques are combined with histogram-based schemes for determining saturation properties. Both direct methods, which enable one to locate saturation points at a given temperature, and temperature expanded ensemble methods, which provide a means to trace saturation lines to low temperature, are discussed. We study the liquid-vapor phase behavior of the restricted primitive model (RPM) and a realistic model for 1,3-dimethylimidazolium tetrafluoroborate ([C1mim][BF4]). Results are presented to show the dependence of saturation properties of the RPM and [C1mim][BF4] on the size of the simulation box and the boundary condition used for the Ewald summation. For [C1mim][BF4] we also demonstrate the ability of our strategy to sample ion clusters that form in the vapor phase. Finally, we provide the liquid-vapor saturation properties of these models over a wide range of temperature. Overall, we observe that the choice of system size and boundary condition have a non-negligible effect on the calculated properties, especially at high temperature. Also, we find that the combination of grand canonical MC simulation and isothermal-isobaric temperature expanded ensemble MC simulation provides a computationally efficient means to calculate liquid-vapor saturation properties of ionic liquids.

We study the liquid-vapor saturation properties of room temperature ionic liquids (RTILs) belonging to the homologous series 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) using Monte Carlo simulation. We examine the effect of temperature and cation alkyl chain length n on the saturated densities, vapor pressures, and enthalpies of vaporization. These properties are explicitly calculated for temperatures spanning from 280 to 1000 K for RTILs with n = 2, 4, 6, 8, 10, and 12. We also explore how the identity of the anion influences saturation properties. Specifically, we compare results for [C(4)mim][NTf2] with those for 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]). Simulations are completed with a recently developed realistic united-atom force field. A combination of direct grand canonical and isothermal-isobaric temperature expanded ensemble simulations are used to construct phase diagrams. Our results are compared with experimental data and Gibbs ensemble simulation data. Overall, we find good agreement between our results and those measured experimentally. We find that the vapor pressures and enthalpies of vaporization show a strong dependence on the size of the alkyl chain at low temperatures, whereas no particular trend is observed at high temperatures. Finally, we also discuss the effect of temperature on liquid phase nanodomains observed in RTILs with large hydrophobic groups. We do not observe a drastic change in liquid phase structure upon variation of the temperature, which suggests there is not a sharp phase transition between a nanostructured and homogeneous liquid, as has been suggested in earlier studies.