Title: Nuclear quantum effects on the structural properties of solids
Author(s): Scivetti I., Hughes D., Gidopoulos N., Caro A., Kohanoff J.,
AIP Conference Proceedings, 963, pp. 212-223 (SEP 25 2007)Abstract
Title: Competition of charge density waves and superconductivity in Sulfur
Author(s): Degtyareva O., Magnitskaya M.V., Kohanoff J., Profeta G., Scandolo S., McMahon M.I., Gregoryanz E.,
Physical Review Letters, 99, Art. No. 155505 (OCT 15 2007)
A one-dimensional charge-density wave (CDW) instability is shown to be responsible for the formation of the incommensurate modulation of the atomic lattice in the high-pressure phase of sulfur. The coexistence of, and competition between, the CDW and the superconducting state leads to the previously observed increase of Tc up to 17 K, which we attribute to the suppression of the CDW instability, the same phenomenology found in doped layered dichalcogenides.
Title: Simulations of ionic liquids, solutions, and surfaces
Author(s): Lynden-Bell R.M., Del Popolo M.G., Youngs T.G.A., Kohanoff J., Hanke C.G., Harper J.B., Pinilla C.C.
Accounts of Chemical Research, 40, No. 11, pp. 1138-1145 (OCT 2007)
We have been using atomistic simulation for the last 10 years to study properties of imidazolium-based ionic liquids. Studies of dissolved molecules show the importance of electrostatic interactions in both aromatic and hydrogen-bonding solutes. However, the local structure strongly depends upon ionion and solutesolvent interactions. We find interesting local alignments of cations at the gasliquid and solidliquid interfaces, which give a potential drop through the surface. If the solid interface is charged, this charge is strongly screened over distances of a few nanometres and this screening decays on a fast time scale. We have studied the sensitivity of the liquid structure to force-field parameters and show that results from ab initio simulations can be used in the development of force fields.
Title: Clusters, liquids, and crystals of dialkyimidazolium salts. A combined perspective from a initio and classical computer simulations
Author(s): Del Popolo M.G., Kohanoff J., Lynden-Bell R.M., Pinilla C
Accounts of Chemical Research, 40, No. 11, pp. 1156-1164 (NOV 2007)
We summarize results obtained by a combination of ab initio and classical computer simulations of dialkylimidazolium ionic liquids in different states of aggregation, from crystals to liquids and clusters. Unusual features arising from the competition between electrostatic, dispersion, and hydrogen-bonding interactions are identified at the origin of observed structural patterns. We also discuss the way Bronsted acids interact with ionic liquids leading to the formation of hydrogen-bonded anions.
Title: Polarization relaxation in an ionic liquid confined between electrified walls
Author(s): Pinilla C., Del Popolo M.G., Kohanoff J., Lynden-Bell R.M.
Journal Of Physical Chemistry B, 111, No. 18, pp. 4877-4884 (MAY 10 2007)
The response of a room temperature molten salt to an external electric field when it is confined to a nanoslit is studied by molecular dynamics simulations. The fluid is confined between two parallel and oppositely charged walls, emulating two electrified solid-liquid interfaces. Attention is focused on structural, electrostatic, and dynamical properties, which are compared with those of the nonpolarized fluid. It is found that the relaxation of the electrostatic potential, after switching the electric field off, occurs in two stages. A first, subpicosecond process accounts for 80% of the decay and is followed by a second subdiffusive process with a time constant of 8 ps. Diffusion is not involved in the relaxation, which is mostly driven by small anion translations. The relaxation of the polarization in the confined system is discussed in terms of the spectrum of charge density fluctuations in the bulk.
Title: Neutral and charged 1-butyl-3-methylimidazolium triflate clusters: Equilibrium concentration in the vapor phase and thermal properties of nanometric droplets
Author(s): Ballone P., Pinilla C., Kohanoff J., Del Popolo M.G.
Journal Of Physical Chemistry B, 111, No. 18, pp. 4938-4950 (MAY 10 2007)
Ground state energy, structure, and harmonic vibrational modes of 1-butyl-3-methylimidazolium triflate ([bmim][Tf]) clusters have been computed using an all-atom empirical potential model. Neutral and charged species have been considered up to a size (30 [bmim][Tf] pairs) well into the nanometric range. Free energy computations and thermodynamic modeling have been used to predict the equilibrium composition of the vapor phase as a function of temperature and density. The results point to a nonnegligible concentration of very small charged species at pressures (P similar to 0.01 Pa) and temperatures (T >= 600 K) at the boundary of the stability range of [bmim][Tf]. Thermal properties of nanometric neutral droplets have been investigated in the 0 <= T <= 700 K range. A near-continuous transition between a liquidlike phase at high T and a solidlike phase at low T takes place at T similar to 190 K in close correspondence with the bulk glass point T-g similar to 200 K. Solidification is accompanied by a transition in the dielectric properties of the droplet, giving rise to a small permanent dipole embedded into the solid cluster. The simulation results highlight the molecular precursors of several macroscopic properties and phenomena and point to the close competition of Coulomb and dispersion forces as their common origin.