It is easy to understand the self-assembly of particles with anisotropic shapes or interactions (for example, cobalt nanoparticles or proteins) into highly extended structures. However, there is no experimentally established strategy for creating a range of anisotropic structures from common spherical nanoparticles. We demonstrate that spherical nanoparticles uniformly grafted with macromolecules ('nanoparticle amphiphiles') robustly self-assemble into a variety of anisotropic superstructures when they are dispersed in the corresponding homopolymer matrix. Theory and simulations suggest that this self-assembly reflects a balance between the energy gain when particle cores approach and the entropy of distorting the grafted polymers. The effectively directional nature of the particle interactions is thus a many-body emergent property. Our experiments demonstrate that this approach to nanoparticle self-assembly enables considerable control for the creation of polymer nanocomposites with enhanced mechanical properties. Grafted nanoparticles are thus versatile building blocks for creating tunable and functional particle superstructures with significant practical applications.

Molecular dynamics simulations have been performed to investigate the structure and dynamics of the ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(4)mim][Tf(2)N]) in the temperature range of 283-460 K. Extensive analysis was carried out to characterize a number of structural and dynamic features. Transport properties were computed using a variety of equilibrium methods that employed the Green-Kubo and Einstein formulations. Nonequilibrium techniques were also used. In general, different methods mostly yielded consistent results, although some differences were observed. Computed self-diffusivities and ionic conductivities tended to be slightly lower than experimental values, while computed viscosities were significantly higher than experiment. Computed thermal conductivities agreed reasonably well with experimental data. Despite these discrepancies, the simulations capture the experimental temperature-dependent trends for all these transport properties. Single ion dynamics were studied by examining diffusional anisotropy, the self-part of the van Hove function, non-Gaussian parameters, and incoherent intermediate scattering functions. It is found that cations diffuse faster than anions and are more dynamically heterogeneous. A clear anisotropy is revealed in cation displacement, with the motion normal to the imidazolium ring plane being the most hindered and the motion along the alkyl chain in the plane of the ring being the most facile. Cations structurally relax faster than anions but they rotationally relax slower than anions. There is a pronounced temperature dependence to the rotational anisotropy of the cations, but only a weak temperature dependence for the anions. The ionic conductivity deviates from the Nernst-Einstein relation due to the correlated motion of cations and anions. The results suggest that the dynamical behavior of this and related ionic liquids is extremely complex and consists of many different modes with widely varying timescales, making the prediction of dynamical trends extremely difficult.

Vaporization enthalpy of an ionic liquid (IL) is a key physical property for applications of ILs as thermofluids and also is useful in developing liquid state theories and validating intermolecular potential functions used in molecular modeling of these liquids. Compilation of the data for a homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C(n)mim][NTf2]) ILs has revealed an embarrassing disarray of literature results. New experimental data, based on the concurring results from quartz crystal microbalance, thermogravimetric analyses, and molecular dynamics simulation have revealed a clear linear dependence of IL vaporization enthalpies on the chain length of the alkyl group on the cation. Ambiguity of the procedure for extrapolation of vaporization enthalpies to the reference temperature 298 K was found to be a major source of the discrepancies among previous data sets. Two simple methods for temperature adjustment of vaporization enthalpies have been suggested. Resulting vaporization enthalpies obey group additivity, although the values of the additivity parameters for ILs are different from those for molecular compounds.

Experimental measurements and molecular dynamics simulations are used to determine the density, heat capacity, self-diffusivity, shear viscosity, and thermal conductivity of six ionic liquids over a range of temperatures. The ionic liquids examined are 1-butyl-3-methylimidazolium bis[(perfluoroethyl). The results of this work suggest that several of these ionic liquids have properties that would enable them to be successful high temperature heat transfer fluids. In particular, their energy storage densities and thermal conductivities are quite favorable when compared to conventional heat transfer fluids. The low temperature viscosities of the ILs are significantly higher than conventional fluids, but the viscosities drop rapidly with increasing temperature. The simulations, which are purely predictive, agree quantitatively with the experimental data for density and qualitatively for other properties. It is shown that the simulated thermal conductivity can be adequately correlated with density and molecular weight of the [Tf 2 N]-based ionic liquids.

We have combined Gibbs ensemble Monte Carlo simulations with the aggregation volume-biased method in conjunction with the Gibbs-Duhem method to provide the first direct estimates for the vapor-solid, vapor-liquid, and liquid-solid phase coexistences of square-well fluids with three different ranges of attraction. Our results are consistent with the previous simulations and verify the notion that the vapor-liquid coexistence behavior becomes metastable for cases where the attraction well becomes smaller than 1.25 times the particle diameter. In these cases no triple point is found.

We show from molecular dynamics simulations that porous graphene of a certain pore size can efficiently separate carbon dioxide from nitrogen with high permeance, in agreement with the recent experimental finding (Koenig et al., Nat. Nanotechnol., 2012, 7, 728-732). The high selectivity is reflected in the much higher number of CO2 passing-through events than that of N2 from the trajectories. The simulated CO2 permeance is on the order of magnitude of 10(5) GPU (gas permeation unit). The selective trend is further corroborated by the free energy barriers of permeation. The predicted CO2/N2 selectivity is around 300. Overall, the combination of high CO2 flux and high CO2/N2 selectivity makes nanoporous graphene a promising membrane for post-combustion CO2 separation.

Effect of bidispersity in grafted chain length on grafted chain conformations and potential of mean force between polymer grafted nanoparticles in a homopolymer matrix The Journal of Chemical Physics 134, 194906 (2011); 10.1063/1.3590275 Modeling the anisotropic self-assembly of spherical polymer-grafted nanoparticles Recent experimental results demonstrated that polymer grafted nanoparticles in solvents display self-assembly behavior similar to the microphase separation of block copolymers and other amphiphiles. We present a mean-field theory and complementary computer simulations to shed light on the parametric underpinnings of the experimental observations. Our theory suggests that such self-assembled structures occur most readily when the nanoparticle size is comparable to the radius of gyration of the polymer brush chains. Much smaller particle sizes are predicted to yield uniform particle dispersions, while larger particles are expected to agglomerate due to phase separation from the solvent. Selected aspects of our theoretical predictions are corroborated by computer simulations.

The vapor-liquid coexistence boundaries of fluids composed of particles interacting with highly directional patchy interactions, in addition to an isotropic square well potential, are evaluated using grand canonical Monte Carlo simulations combined with the histogram reweighting and finite size scaling methods. We are motivated to study this more complicated model for two reasons. First, it is established that the reduced widths of the metastable vapor-liquid coexistence curve predicted by a model with only isotropic interparticle interactions are much too narrow when compared to the experimental phase behavior of protein solutions. Second, interprotein interactions are well known to be "patchy." Our results show that at a constant total areal density of patches, the critical temperature and the critical density increase monotonically with an increasing number of uniformly spaced patches. The vapor-liquid coexistence curves plotted in reduced coordinates (i.e., the temperature and the density scaled by their respective critical values) are found to be effectively independent of the number of patches, but are much broader than those found for the isotropic models. Our findings for the reduced width of the coexistence curve are almost in quantitative agreement with the available experimental data for protein solutions, stressing the importance of patchiness in this context.