Abstract. The thermodynamic liquid-state anomalies and associated structural changes of the Stillinger-Weber family of liquids are mapped out as a function of the degree of tetrahedrality of the interaction potential, focusing in particular on tetrahedrality values suitable for modeling C, H2O, Si, Ge and Sn. We show that the density anomaly, associated with a rise in molar volume on isobaric cooling, emerges at intermediate tetrahedralities (e.g. Ge, Si and H2O) but is absent in the low (e.g. Sn) and high (e.g. C) tetrahedrality liquids. The rise in entropy on isothermal compression associated with the density anomaly is related to the structural changes in the liquid using the pair correlation entropy. An anomalous increase in the heat capacity on isobaric cooling exists at high tetrahedralities but is absent at low tetrahedralities (e.g. Sn). Structurally, this heat capacity anomaly originates in a sharp rise in the fraction of four-coordinated particles and local tetrahedral order in the liquid as its structure approaches that of the tetrahedral crystal.

Molecular dynamics simulations of the Oeffner-Elliot model of germania (GeO(2)) are performed to identify nested regions of anomalous behavior in structural order, diffusivity, and pair entropy in the density-temperature plane, analogous to that seen in BeF(2), SiO(2), and H(2)O. The decreasing constraint of local tetrahedrality in GeO(2), compared to SiO(2) and BeF(2), substantially lowers the onset temperatures for anomalous behavior relative to the experimental melting temperatures (T(m)). Germania resembles water, more strongly than the ionic melts, in terms of temperatures for onset of anomalous behavior as well as in the order maps; for example, the structural anomaly sets in at 3.42T(m) in BeF(2), 3.09T(m) in SiO(2), 1.43T(m) in GeO(2), and 1.21T(m) in H(2)O. The detailed shapes of the anomalous regimes vary for different systems but the relative temperatures of onset for different anomalies are very similar in the different systems. The pair correlation entropy is shown to be a crucial and experimentally accessible quantity for relating structure, entropy, and diffusivity that could be potentially useful for a large class of inorganic ionic liquids.

Fluctuations within the ligand shell of a nanoparticle give rise to a significant degree of anisotropy in effective pair interactions for low grafting densities [B. Bozorgui, D. Meng, S. K. Kumar, C. Chakravarty, and A. Cacciuto, Nano Lett. 13, 2732 (2013)]. Here, we examine the corresponding fluctuation-driven anisotropy for gold nanocrystals densely passivated with short ligands. In particular, we consider gold nanocrystals capped by alkylthiols, both in vacuum and in ethane solvent at high density. As in the preceding study, we show that the anisotropy in the nanoparticle pair potential can be quantified by an angle-dependent correction term to the isotropic potential of mean force (PMF). We find that the anisotropy of the ligand shells is distance dependent, and strongly influenced by ligand interdigitation effects as well as expulsion of ligand chains from the interparticle region at short distances. Such fluctuation-driven anisotropy can be significant for alkylthiol-coated gold nanoparticles, specially for longer chain lengths, under good solvent conditions. The consequences of such anisotropy for self-assembly, specially as a function of grafting density, solvent quality and at interfaces, should provide some interesting insights in future work. Our results clearly show that an isotropic two-body PMF cannot adequately describe the thermodynamics and assembly behavior of nanoparticles in this dense grafting regime and inclusion of anisotropic effects, as well as possibly many-body interactions, is necessary. Extensions of this approach to other passivated nanoparticle systems and implications for self-assembly are considered.

The regime of validity of Rosenfeld excess entropy scaling of diffusivity and viscosity is examined for two tetrahedral, network-forming ionic melts (BeF(2) and SiO(2)) using molecular dynamics simulations. With decrease in temperature, onset of local caging behavior in the diffusional dynamics is shown to be accompanied by a significant increase in the effect of three-body and higher-order particle correlations on the excess entropy, diffusivity, ionic conductivity, and entropy-transport relationships. The signature of caging effects on the Rosenfeld entropy scaling of transport properties is a distinctly steeper dependence of the logarithm of the diffusivity on the excess entropy in ionic melts. This is shown to be true also for a binary Lennard-Jones glassformer, based on available results in the literature. Our results suggest that the onset of a landscape-influenced regime in the dynamics is correlated with this characteristic departure from Rosenfeld scaling. The breakdown of the Nernst-Einstein relation in the ionic melts can also be correlated with the emerging cooperative dynamics.

In order to understand the common features of tetrahedral liquids with water-like anomalies, the relationship between local order and anomalies has been studied using molecular dynamics simulations for three categories of such liquids: (a) atomistic rigid-body models for water (TIP4P, TIP4P/2005, mTIP3P, SPC/E), (b) ionic melts, BeF(2) (TRIM model) and SiO(2) (BKS potential) and (c) Stillinger-Weber liquids parametrized to model water (mW) and silicon. Rigid-body, atomistic models for water and the Stillinger-Weber liquids show a strong correlation between tetrahedral and pair correlation order and the temperature for the onset of the density anomaly is close to the melting temperature. In contrast, the ionic melts show weaker and more variable degrees of correlation between tetrahedral and pair correlation metrics, and the onset temperature for the density anomaly is more than twice the melting temperature. In the case of water, the relationship between water-like anomalies and solvation is studied by examining the hydration of spherical solutes (Na(+), Cl(-), Ar) in water models with different temperature regimes of anomalies (SPC/E, TIP4P and mTIP3P). For both ionic and nonpolar solutes, the local structure and energy of water molecules is essentially the same as in bulk water beyond the second-neighbour shell. The local order and binding energy of water molecules are not perturbed by the presence of a hydrophobic solute. In the case of ionic solutes, the perturbation is largely localized within the first hydration shell. The binding energies for the ions are strongly dependent on the water models and clearly indicate that the geometry of the partial charge distributions, and the associated multipole moments, play an important role. However the anomalous behaviour of the water network has been found to be unimportant for polar solvation.

In addition to the direct attraction, sizable hydrophobes in water experience an attractive force mediated by interfacial water. Using simple geometric arguments, we identify the conditions at which the water-induced interaction between curved hydrocarbon surfaces becomes repulsive. The repulsive contribution arises from the thermodynamic penalty due to the emergence of the liquid/vapor boundary created as water gets expelled between curved hydrophobes. By augmenting the mean field approach with atomistic simulations of pristine and alkyl-coated graphitic nanoparticles in three distinct geometries, spherical, cylindrical and planar, immersed in water, we show the macroscopic thermodynamics remarkably works down to the molecular scale. The new insights improve the prediction and control of wetting and dispersion properties for a broad class of nonpolar nanoparticles.

Molecular dynamics simulations are performed to study the structure and dynamics of the LiF-BeF 2 system over a range of compositions using the transferable rigid-ion model (TRIM). The densities obtained with the TRIM potential are approximately 17-20% lower than the experimental values while polarizable ion models (PIM) give densities within 5% of the experimental value. The TRIM and PIM potentials give essentially identical radial distribution functions (RDFs) for Li-F and Be-F ion pairs though the Be-Be pair correlations differ significantly and reflect the corresponding density differences. The variation in the radial distribution functions with concentration, particularly the anion-anion pair correlation function, reflects the reorganization of the fluoride ions as the addition of BeF 2 in the mixture promotes the formation of the tetrahedral fluoroberyllate network. Along the 67 mol% LiF isopleth, diffusivities and Nernst-Einstein ionic conductivities from simulations using the PIM and TRIM potentials are in good agreement for temperatures up to 925 K. The viscosity data using the PIM model is also found to be in good agreement with the TRIM results presented here along the 873 K isotherm for compositions ranging from 0 to 50 mol% BeF 2. The main conclusion from this study is that the non-polarizable, TRIM provides reasonable results for the structural correlations and transport properties of the LiF-BeF 2 system in comparison with first-principles-based, PIM.

Molecular dynamics simulations of the LiF-BeF(2) molten salt mixture are used to establish relationships between composition, structural order, entropy, and transport properties of multi-component ionic liquids. A sharp rise in tetrahedral order associated with formation of the fluoroberyllate network occurs for compositions with BeF(2) concentrations greater than that of the Li(2)BeF(4)-BeF(2) eutectic. The excess entropy of the liquid in this regime, within the pair correlation approximation, is strongly correlated with the local tetrahedral order. The different degree of participation of beryllium, fluorine, and lithium ions in the cooperative dynamics of the fluoroberyllate network can be related to the degree of deviation from Rosenfeld-type excess entropy scaling, with the lithium ions remaining essentially unaffected by the liquid state network. We demonstrate that the deviations from Nernst-Einstein and Stokes-Einstein behaviour emerge only in temperature-composition regimes where tetrahedral order strongly correlates with the pair entropy. Implications for understanding structure-property relationships in other ionic liquids, such as molten salts, oxide melts, and RTILs are considered.