Katie A. Maerzke

TraPPE-UA force field for acrylates and Monte Carlo simulations for their mixtures with Alkanes and alcohols

An extension of the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field to acrylate and methacrylate monomers is presented. New parameters were fit to the liquid density, normal boiling point, saturated vapor pressure, and (where experimentally available) critical constants of 1,3-butadiene, isoprene, methyl acrylate, and methyl methacrylate using Gibbs ensemble Monte Carlo simulations. Excellent agreement with experiment was obtained for the parametrization compounds and seven additional acrylate and methacrylate compounds, with average errors in liquid density and normal boiling point of approximately 1%. The TraPPE-UA force field also predicts accurate heats of vaporization at 298 K. In addition, Gibbs ensemble Monte Carlo simulations of binary vapor-liquid equilibria for the mixtures methyl acrylate/1-butanol and methyl acrylate/n-decane show that the TraPPE-UA acrylate force field performs well in mixtures with both polar and nonpolar molecules. These simulations also indicate structural microheterogeneity in the liquid phase of these mixtures.

Transferable potentials for phase equilibria-coarse-grain description for linear alkanes.

Coarse-grain potentials allow one to extend molecular simulations to length and time scales beyond those accesssible to atomistic representations of the interacting system. Since the coarse-grain potentials remove a large number of interaction sites and, hence, a large number of degrees of freedom, it is generally assumed that coarse-grain potentials are not transferable to different systems or state points (temperature and pressure). Here we apply lessons learned from the parametrization of transferable atomistic potentials to develop a systematic procedure for the parametrization of transferable coarse-grain potentials. In particular, we apply an iterative Boltzmann optimization for the determination of the bonded interactions for coarse-grain beads belonging to the same molecule and separated by one or two coarse-grain bonds and parametrize the nonbonded interactions by fitting to the vapor-liquid coexistence curves computed for selected molecules represented by the TraPPE-UA (transferable potentials for phase equilibria-united atom) force field. This approach is tested here for linear alkanes where parameters for C(3)H(7) end segments and for C(3)H(6) middle segments of the TraPPE-CG (transferable potentials for phase equilibria-coarse grain) force field are determined and it is shown that these parameters yield quite accurate vapor-liquid equilibria for neat n-hexane to n-triacontane and for the binary mixture of n-hexane and n-hexatriacontane. In addition, the position of the first peak in various radial distribution functions and the coordination number for the first solvation shell are well reproduced by the TraPPE-CG force field, but the first peaks are too high and narrow.

Structure and dynamics of uranyl(VI) and plutonyl(VI) cations in ionic liquid/water mixtures via molecular dynamics simulations.

A fundamental understanding of the behavior of actinides in ionic liquids is required to develop advanced separation technologies. Spectroscopic measurements indicate a change in the coordination of uranyl in the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) as water is added to the system. Molecular dynamics simulations of dilute uranyl (UO2(2+)) and plutonyl (PuO2(2+)) ) solutions in [EMIM][Tf2N]/water mixtures have been performed in order to examine the molecular-level coordination and dynamics of the actinyl cation (AnO2(2+)) ); An = U, Pu) as the amount of water in the system changes. The simulations show that the actinyl cation has a strong preference for a first solvation shell with five oxygen atoms, although a higher coordination number is possible in mixtures with little or no water. Water is a much stronger ligand for the actinyl cation than Tf2N, with even very small amounts of water displacing Tf2N from the first solvation shell. When enough water is present, the inner coordination sphere of each actinyl cation contains five water molecules without any Tf2N. Water also populates the second solvation shell, although it does not completely displace the Tf2N. At high water concentrations, a significant fraction of the water is found in the bulk ionic liquid, where it primarily coordinates with the Tf2N anion. Potential of mean force simulations show that the progressive addition of up to five water molecules to uranyl is very favorable, with ΔG ranging from −52.3 kJ/mol for the addition of the first water molecule to −37.6 kJ/mol for the addition of the fifth. Uranyl and plutonyl dimers formed via bridging Tf2N ligands are found in [EMIM][Tf2N] and in mixtures with very small amounts of water. Potential of mean force calculations confirm that the dimeric complexes are stable, with relative free energies of up to −9 kJ/mol in pure [EMIM][Tf2N]. We find that the self-diffusion coefficients for all the components in the mixture increase as the water content increases, with the largest increase for water and the smallest increase for the ionic liquid cation and anion. The velocity autocorrelation functions also indicate changes in structure and dynamics as the water content changes.

Effects of an applied electric field on the vapor-liquid equilibria of water, methanol, and dimethyl ether.

Gibbs ensemble Monte Carlo simulations are employed to examine the influence of moderately strong electric fields on the vapor-liquid coexistence curves and on structural and energetic properties of the saturated phases of water, methanol, and dimethyl ether. The application of an electric field of 0.1 V/A increases the critical temperature and normal boiling point by approximately 3% compared to the zero field case for all three compounds, whereas the critical density is found to decrease by 1% for methanol and dimethly ether and by 3% for water. For the special case of an electric field applied in only the liquid phase, these effects are magnified with a 4% increase in T(C) and a 13% decrease in rho(C). For the case of an electric field in only the vapor phase, the opposite effect is seen with a 4% decrease in T(C) and a 12% increase in rho(C). Structural analysis shows very little change in the radial distribution functions, but greatly increased orientational ordering with the application of an electric field. The orientational ordering effect is stronger in the liquid phase than in the vapor phase. An examination of the energetics reveals that, in the presence of an electric field, the interactions with the first and second solvation shells become less favorable but these are outweighed by a larger increase in the favorable long-range interactions with more distant molecules and the field.

Self-Consistent Polarization Density Functional Theory: Application to Argon †

We present a comprehensive set of results for argon, a case study in weak interactions, using the self-consistent polarization density functional theory (SCP-DFT). With minimal parametrization, SCP-DFT is found to give excellent results for the dimer interaction energy, the second virial coefficient, the liquid structure, and the lattice constant and cohesion energy of the face-centered cubic crystal compared to both accurate theoretical and experimental benchmarks. Thus, SCP-DFT holds promise as a fast, efficient, and accurate method for performing ab initio dynamics that include additional polarization and dispersion interactions for large, complex systems involving solvation and bond breaking.

Electrostatically embedded many-body method for dipole moments, partial atomic charges, and charge transfer

Fragment methods have been widely studied for computing energies and forces, but less attention has been paid to nonenergetic properties. Here we extend the electrostatically embedded many-body (EE-MB) method to the calculation of cluster dipole moments, dipole moments of molecules in clusters, partial atomic charges, and charge transfer, and we test and validate the method by comparing to results calculated for the entire system without fragmentation. We also compare to calculations carried out by the conventional many-body (MB) method without electrostatic embedding. Systems considered are NH(3)(H(2)O)(11), (NH(3))(2)(H(2)O)(14), [Cl(H(2)O)(6)](-), (HF)(4), (HF)(5), (HF)(2)H(2)O, (HF)(3)H(2)O, and (HF)(3)(H(2)O)(2). With electrostatic embedding, we find that even at the pairwise additive level a quantitatively accurate description of a systems dipole moment and partial charge distribution and a qualitatively accurate description of the amount of intermolecular charge transfer can often be obtained.

Monte Carlo simulations of binary mixtures of nitrotoluene isomers with n-decane.

Previous experimental studies [Sliwinska-Bartkowiak et al., Chem. Phys. Lett. 1983, 94, 609] indicate that o-nitrotoluene/n-decane and m-nitrotoluene/n-decane mixtures are of particular interest because they exhibit anomalous behavior in the nonlinear dielectric effect near the upper critical solution point, and it has been surmised that these effects are due to an increase in cluster formation as each mixture approaches its critical point. In this work, Monte Carlo simulations are performed using the TraPPE force field at multiple temperatures, concentrations, and electric field strengths to examine the structural microheterogeneity of these mixtures and determine the dielectric constants and nonlinear dielectric effect. At low field strength, the simulations indicate substantial structural microheterogeneities, but these persist over a wide range of conditions. At high field strength (an order of magnitude higher than in the experimental work), the simulations indicate a field-induced segregation.