Recent Advances

 

First principles Monte Carlo simulations of aggregation in the vapor phase of hydrogen fluoride

M.J. McGrath, J.N. Ghogomu, C.J. Mundy, I-F.W. Kuo, and J.I. Siepmann,' First principles Monte Carlo simulations of aggregation in the vapor phase of hydrogen fluoride, ' Phys. Chem. Chem. Phys. 12, 7678-7687 (2010).

Particle-based Monte Carlo simulations were employed to examine the molecular-level effects of bonding density on the retention of alkane and alcohol solutes in reversed-phase liquid chromatography. The simulations utilized octadecylsilane stationary phases with various bonding densities (1.6, 2.3, 2.9, 3.5, and 4.2 μmol/m2) in contact with a water/methanol mobile phase. In agreement with experiment, the distribution coefficient for solute transfer from mobile to stationary phase initially increases then reaches a maximum with increasing bonding density. A molecular-level analysis of the solute positional and orientational distributions shows that the stationary phase contains heterogeneous regions and the heterogeneity increases with increasing bonding density. Above, the distribution of n-butane in the x-y plane (the plane parallel to the silica surface) is shown for molecules within 10 Å of the silica surface (top row) and for molecules more than 10 Å from this surface but within the Gibbs dividing surface (bottom row). Black circles represent the location of the residual silanol groups and white circles indicate where the dimethyl octadecylsilane chains are tethered to the substrate.

 

Development of the TraPPE force field for ammonia

L. Zhang and J.I. Siepmann, 'Development of the TraPPE force field for ammonia,' Collect. Czech. Chem. Commun. 75, 577-591 (2010).

The transferable potentials for phase equilibria (TraPPE) force field is extended through the development of a non-polarizable five-site ammonia model. In this model, the electrostatic interactions are represented by three positive partial charges placed at the hydrogen position and a compensating partial charge placed on an M site that is located on the C3 molecular axis and displaced from the nitrogen atom toward the hydrogen atoms. The repulsive and dispersive interactions are represented by placing a single Lennard-Jones site at the position of the nitrogen atom. Starting from the five-site model by Impey and Klein (Chem. Phys. Lett. 1984, 104, 579), this work optimizes the Lennard-Jones parameters and the magnitude of the partial charges for three values of the M site displacement. This parameterization is done by fitting to the vapor-liquid coexistence curve of neat ammonia. The accuracy of the three resulting models (differing in the displacement of the M site) is assessed through computation of the binary vapor-liquid equilibria with methane, the structure and the dielectric constant of liquid ammonia. The five-site model with an intermediate displacement of 0.08 for the M site yields a much better value for the dielectric constant, whereas differences in the other properties are quite small.


 

Exploring the discrepancies between experiment, theory, and simulation for the homogeneous gas-to-liquid nucleation of 1-pentanol

R.B. Nellas, S.J. Keasler, J.I. Siepmann, and B. Chen, 'Exploring the discrepancies between experiment, theory, and simulation for the homogeneous gas-to-liquid nucleation of 1-pentanol,' J. Chem. Phys. 132, art. no. 164517/9 pages (2010).

Using an efficient Monte Carlo approach known as Aggregation-Volume-bias Monte Carlo with self-adaptive Umbrella Sampling and Histogram Reweighting (AVUS-HR), we obtained the nucleation free energy profile of 1-pentanol at various temperatures from 220 to 360 K. From these profiles, differences between the free energy barrier heights obtained from our simulations and those predicted by the classical nucleation theory (CNT) were calculated. Our results strongly support that the logarithm of the nucleation rate ratio between simulation (or experiment) and CNT increases almost linearly with the inverse temperature. Among the various factors that contribute to the discrepancy between simulation and CNT nucleation rates, the nonzero surface free energy of the monomer included in the CNT makes the largest contribution. On the molecular level, the simulations indicate that a gas-phase cluster of 1-pentanol molecules is relatively compact and can contain multiple hydrogen bonded aggregates of various sizes and that this aggregate size distribution depends strongly on temperature and also on the overall size of the cluster system.

 

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