Michael Rouha

Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces

The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim(+)][Tf(2)N(-)]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation-anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighting the critical role of charge-induced nanoscale correlations of the RTIL. These observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.

Efficient multiparticle sampling in Monte Carlo simulations on fluids: application to polarizable models.

A novel Monte Carlo simulation scheme based on biased simultaneous displacements of all particles of the system has been developed. The method is particularly suited for systems with nonadditive interactions and its efficiency is demonstrated by its implementation for the polarizable Stockmayer fluid. Performance of the method is compared with both the standard one-particle move method and an unbiased multiparticle scheme by computing the mean squared displacements, rotation relaxation, and the speed of equilibration (translational order parameter). It is shown that the proposed biased method is about a factor of 10 faster, for the system considered, when compared with the other schemes.

Excess properties of aqueous solutions: Hard spheres versus pseudo-hard bodies

The effect of repulsive interactions (geometrical packing of molecules) on mixing properties is examined considering hard sphere solutes of different diameters in the hard sphere and pseudo-hard body solvents. It is shown that the fluid of pseudo-hard bodies, representing repulsive interactions in water, captures, without any parameter adjustment, the experimentally observed decrease of the partial molar volume of nonpolar solutes (hard spheres) at their low concentrations when compared to that in a nonpolar (hard sphere) solvent.

Thermodynamics of pseudo-hard body mixtures

Thermodynamic properties of binary mixtures of hard spheres of various size and pseudo-hard bodies, mimicking the short-range non-additive repulsive interactions in realistic models of water, have been determined over the entire concentration range using standard NVT Monte Carlo simulations. Virial coefficients of the mixture have also been computed. Having no other theoretical tool currently available, a perturbed virial expansion is examined with respect to its potential to estimate/predict the properties of the mixture without resorting to any fitting of simulation data. The perturbed virial expansion is found to perform quite accurately for the mixtures containing larger spheres, whereas for small spheres dissolved in water the result is only qualitatively correct.

Second virial coefficients: a route to combining rules?

ABSTRACT The effect of combining rules on the second virial coefficient is systematically examined for a series of n-alkanes up to octane with the aim to assess the impact of the individual site–site interactions and to find potential general regularities. To describe the interactions of n-alkanes, we use the TraPPE-UA force field and examine the effect of deviations from the Lorentz–Berthelot combining rule, both separately for the energy and length scaling parameters of the Lennard-Jones potential, and for their combined change. The results reveal the different impacts of both site–site interactions and distance and energy scaling parameters on the cross virial coefficient, which can potentially be used in force field development. An attempt is also made to find a scaling that could provide a uniform result.

Extended excluded volume: Its origin and consequences

In contrast to the common intuitive/speculative approach based on an analysis of thermodynamic or structural data of (nonpolar) fluids, the statistical mechanical approach is used to extend the excluded volume concept to all other types of fluids. The (extended) excluded volume incorporates, in addition to common nonelectrostatic interactions defining the shape and size of the molecules, also the short-range part of the repulsive interactions between the embedded Coulombic sites. In this study we show that the extended excluded volume concept correctly predicts the behavior of the partial molar volume (PMV) at infinite dilution in different solvents and, particularly, differences between nonpolar and associating solvents. The concept is then applied to estimate the PMV of methanol in water.