An idealized four-site ionic liquid model having characteristics approximating those of 1-butyl-3-methylimidazolium hexafluorophosphate ([Im(41)][PF(6)]) is introduced as a low-cost alternative to existing all-atom models for purposes of simulating solute-based dynamics over nanosecond and longer time scales. The structural and energetic properties of the model are in reasonable agreement with those of [Im(41)][PF(6)] and similar ionic liquids, but the dynamics are unrealistically slow. A temperature shift of approximately 100 K is required to produce agreement between the viscosity and diffusion coefficients of the model and experimental values. Several aspects of the ion dynamics such as subdiffusive translational motions, non-Gaussian van Hove distributions, and jumplike displacements in both positions and orientations, are similar to behavior observed in supercooled liquids. Translational diffusion coefficients and rotational correlation times show roughly the proportionalities to viscosity expected from hydrodynamic models, and slip hydrodynamic calculations provide reasonable accuracy in some cases. But anomalously high rotational diffusion coefficients which decouple from viscosity at low temperature are also observed. These anomalies are explained in terms of the prevalence of 180 degrees rotational jumps coupled to the presence of marked heterogeneity in rotational motions, especially about one molecular axis. Comparisons between the dynamics observed in the ionic liquid (IL) model and a neutral mixture (NM) counterpart help to explain the origins of the distinctive dynamics in ionic liquids compared to conventional solvents. The requirement for balancing electrostatic interactions in the IL leads to uniform and interleaved distributions of cations and anions resembling a distorted ionic lattice, similar to the structure of molten NaCl. The resistance to reorganizing this structure is what leads to the slow dynamics of ionic liquids. The coupling among large collections of ions is presumably responsible for the similarity of ionic liquids to supercooled conventional liquids.

Computer simulations of realistic models of the two solutes diphenylbutadiene ("DPB") and hydroxymethylstilbene ("HMS") in supercritical CO 2 (310 K, 0.25-2.0F c ) are used to explore the interplay between local density augmentation and friction in supercritical solvents. Good agreement is found between the extent of local density augmentation observed in these simulations and that deduced from experimental measurements of electronic spectral shifts (also reported here). To test the accuracy of the solvent model for treating solute friction, the viscosity and self-diffusion constants of the neat solvent were simulated and compared to experiment and good agreement was also found. The rotation times of these solutes are compared to results of prior experimental measurements made by Anderton and Kauffman (J. Phys. Chem. 1995, 99, 13 759). In the case of DPB, the simulated times are larger than experimental estimates by as much as 30% at higher densities (F ≈ 1.8F c ). The origins of this disagreement are presently unknown. The density dependence of the simulated friction on both rotational and translational motions is curious in that it appears insensitive to the considerable density augmentation present in these systems. This insensitivity is shown to result from a cancellation of static and dynamic aspects of the friction, both of which are individually responsive to the excess density in the neighborhood of the solute.

The authors report a simulation study of the interaction between gold nanoparticles stabilized with both linear and branched alkane chains in supercritical ethane. In agreement with experimental and previous theoretical work, the authors find that increasing solvent density and making ligands more branched make the nanoparticle interaction more repulsive. These findings are analyzed in terms of the extent of the chain interdigitation and chain-solvent interaction energy.

We present a density functional theory study of colloidal interactions in a concentrated polymer solution. The colloids are modeled as hard spheres and polymers are modeled as freely jointed tangent hard sphere chains. Our theoretical results for the polymer-mediated mean force between two dilute colloids are compared with recent simulation data for this model. Theory is shown to be in good agreement with simulation. We compute the colloid-colloid potential of mean force and the second virial coefficient, and analyze the behavior of these quantities as a function of the polymer solution density, the polymer chain length, and the colloid/polymer bead size ratio.

A first theoretical study of surfactant-stabilized carbon nanotube dispersions is presented. Density functional theory is used to compute potential of mean force between nanotubes in an aqueous solution of cationic surfactant n-decyltrimethylammonium chloride. In agreement with experimental results, it is found that stable dispersions can be prepared for surfactant bulk concentrations below the critical micelle concentration. Computed density profiles of head and tail segments indicate that surfactants adsorb on nanotube surfaces in a random fashion rather than form cylindrical micelles, which is also in agreement with recent small-angle neutron scattering measurements.