Recent Advances

Application of the TraPPE force field for predicting the Hildebrand solubility parameters of organic solvents and monomer units

N. Rai, A.J. Wagner, R.B. Ross, and J.I. Siepmann, 'Application of the TraPPE force field for predicting the Hildebrand solubility parameters of organic solvents and monomer units,'
J. Chem. Theory Comput., 4, 136-144 (2008).

Configurational-bias Monte Carlo simulations in the isothermal-isobaric and Gibbs ensembles using the transferable potentials for phase equilibria (TraPPE) force field were carried out to compute the liquid densities, the Hildebrand solubility parameters, and the heats of vaporization for a set of 32 organic molecules with different functional groups at a temperature of 298.15 K. In addition, the heats of vaporization were determined at the normal boiling points of these compounds. Comparison to experimental data demonstrates that the TraPPE force field is significantly more accurate than predictions obtained from molecular dynamics simulations with the Dreiding force field [Belmares et al. J. Comput. Chem. 2004, 25, 1814] and an equation of state approach [Stefanis et al. Fluid Phase Equil. 2006, 240, 144]. For the TraPPE force field, the mean unsigned percent errors for liquid density, the Hildebrand solubility parameter, and the heat of vaporization at 298.15 K are 1.3, 3.3, and 4.5%, respectively. The figure above shows a comparison of the predicted solubility parameters at T = 298.15 K with experimental data. The red, green, and black circles represent the solubility parameters computed with the TraPPE force field, the Dreiding force field with ESP charges, and the Dreiding force field with Mulliken charges. The correspondingly colored lines show the linear least-squares fits, and the blue line is the ideal correlation (y = x).

Size effects on the solvation of anions at the aqueous liquid-vapor interface

B.L. Eggimann and J.I. Siepmann, 'Size effect on the solvation of anions at the aqueous liquid-vapor interface,' J. Phys. Chem. C, 112, 210-218 (2008).

The presence of certain anions at the aqueous liquid-vapor interface is often attributed to polarization effects. This work shows that the size of the ion also plays an important role in driving monovalent inorganic ions to the surface. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were performed to investigate the liquid-vapor interface for neat water and ionic solutions at 298 K. The total ion concentration is about 1.2 M, but the solutions contained a mixture of anions of varying size all having the same fixed charge and Lennard-Jones well depth (i.e., the same polarizability). Results show that as the size of the anion increases so does the propensity for interfacial solvation. The largest anions are consistently found closer to the interface than smaller anions. We also find that large anions, which may prefer a surface location on their own, can be excluded from the interfacial region by inclusion of even larger anions in a mixture. Above, the absolute number densities for mixtures of smaller (left column) and larger (right column) anions is shown above. A histogram bin width of 0.2 A was used for each mixture. The water density profile is shown with a solid black line, with all anion profiles scaled by 1528/4 to account for the difference in numbers of molecules between water and anions. Anion types are represented, from largest to smallest, by gray-blue, brown, maroon, purple, cyan, green, yellow, orange, and red lines. For reference, a snapshot of the liquid box with similarly colored anions is also included (cations appear as the darker blue color). The Gibbs dividing surface is shown as a solid vertical line, with the corresponding interfacial width demarcated by the dashed vertical lines.

Free Energies of Formation of Metal Clusters and Nanoparticles from Molecular Simulations: Al_n with n = 2-60

Z.H. Li, D. Bhatt, N.E. Schultz, J.I. Siepmann, and D.G. Truhlar, 'Free Energies of Formation of Metal Clusters and Nanoparticles from Molecular Simulations: Al_n with n = 2-60,' J. Phys. Chem. C, 111, 16227-16242 (2007).

Efficient simulation methods are presented for determining the standard Gibbs free energy changes for the reactions, M + M_n-1 ↔ M_n (R1), involved in the formation of atomic clusters and nanoparticles (also called particles) in the vapor phase. The standard Gibbs free energy of formation (Δ_fG°) of a particle is obtained from these Gibbs free energy changes (ΔG°) by a recursion relationship using the experimental Δ_fG° of the monomer. In the present study, this method has been applied to reactions involving Al_n particles with n = 2-60. This method has been validated for n = 2, where the experimental thermodynamic properties of Al₂ have been recompiled using the latest available experimental or highly accurate theoretical data. For n = 2-4, two completely different approaches, a Monte Carlo configuration integral (MCCI) integration of partition functions and a Monte Carlo direct simulation of the equilibrium constants (MCEC), employing four well-validated potential energy functions have been used to calculate ΔG° of R1. Excellent agreement is observed for these two methods. Although different potential energy functions give different stage-1 results for n ≤ 10, three high-level correction (HLC) terms, namely, a correction for the potential energy difference of the global minima, another for the electronic excitation contribution, and a third based on calculating isomeric-rovibrational contribution, have been applied to mitigate deficiencies in the potential energy functions. For n = 2, good agreement has been found between the corrected simulation results and experimental data. For larger n, the more efficient MCEC method has been used. Finally, accurate ΔG° of R1 and thus Δ_fG° of Al_n particles with n = 2-60 have been determined. This is the first example of the determination of nanoparticle free energies of formation.