Surface viscosity in simple liquids.

The response of Newtonian liquids to small perturbations is usually considered to be fully described by homogeneous transport coefficients like shear and dilatational viscosity. However, the presence of strong density gradients at the liquid/vapor boundary of fluids hints at the possible existence of an inhomogeneous viscosity. Here, we show that a surface viscosity emerges from the collective dynamics of interfacial layers in molecular simulations of simple liquids. We estimate the surface viscosity to be 8-16 times smaller than that of the bulk fluid at the thermodynamic point considered. This result can have important implications for reactions at liquid surfaces in atmospheric chemistry and catalysis.

Time correlation functions of simple liquids: A new insight on the underlying dynamical processes.

Extensive molecular dynamics simulations of liquid sodium have been carried out to evaluate correlation functions of several dynamical quantities. We report the results of a novel analysis of the longitudinal and transverse correlation functions obtained by evaluating directly their self- and distinct contributions at different wavevectors k. It is easily recognized that the self-contribution remains close to its k → 0 limit, which turns out to be exactly the autocorrelation function of the single particle velocity. The wavevector dependence of the longitudinal and transverse spectra and their self- and distinct parts is also presented. By making use of the decomposition of the velocity autocorrelation spectrum in terms of longitudinal and transverse parts, our analysis is able to recognize the effect of different dynamical processes in different frequency ranges.

Collective dynamics of supercooled water close to the liquid-liquid coexistence lines.

We report molecular dynamics simulation results for the collective dynamical properties of supercooled bulk water at 180 K at three different densities, corresponding to different phases whose coexistence has recently been discovered in the supercooled regime. In this study, we focus on the behaviour of the longitudinal and transverse current correlation functions and their relative spectra, which we analyze in detail to understand the dynamical processes responsible for the main features observed. Despite the considerable differences in the structure and densities of the three thermodynamic states considered, the obtained current correlation functions show rather similar behaviour in every case. We show that the longitudinal spectra can only be described in terms of three Lorentzian lines, while the accurate reproduction of the transverse spectra requires at least four separate spectral lines. In fact, the behaviour of the peak frequency of the modes necessary to reproduce the spectra as a function of the wavevector indicates in a clear way the nature of the dynamical process. We demonstrate the presence of collective modes associated with the propagation of both longitudinal and transverse sound along with the important contribution of "optical-like" modes, which point out the relevance of localized motions for a right interpretation of the spectral line shape. The wavevector dependence of the relative contributions of the various modes to the total spectral area is also discussed.

Dynamical properties of supercooled water close to the liquid-liquid coexistence lines, and their relation to those at ambient conditions.

We report results of molecular dynamics simulations of supercooled bulk water at 180 K, close to the liquid/liquid coexistence lines recently discovered in the supercooled regime, both experimentally and by computer simulations. Despite the considerable differences in the densities of the three states considered, the obtained velocity autocorrelation functions display very similar behaviour in every case. On the other hand, the corresponding spectra show the presence of three well-defined modes. The two modes at higher frequencies are assigned to the symmetric and asymmetric stretching motions, respectively, whereas the lowest frequency mode to the bending and torsional motions in clusters of five hydrogen-bonded water molecules. A careful fitting procedure demonstrates the presence of such clusters also in the normal liquid phase of water at ambient conditions.

GM1 ganglioside embedded in a hydrated DOPC membrane: a molecular dynamics simulation study.

A long molecular dynamic simulation of a fully hydrated DOPC bilayer, containing one GM1 ganglioside molecule embedded in each of the two leaflets, has been performed. The location and conformation of the GM1 molecules as well as their effect on the properties of the membrane are investigated in detail. The simulation results reveal that the GM1 molecules are present in two equilibrium arrangements, differing in the orientation of one of their two headgroup branches. The existence of these two equilibrium arrangements of GM1 in the membrane is clearly demonstrated, although their relative population, and hence their free energy difference, cannot be inferred from the present results. A condensing effect on the membrane due to the presence of the GM1 molecules is observed, and the local changes in surface density are analyzed using Voronoi polygons. Although the DOPC molecules are packed more closely in proximity of the gangliosides, the analysis of the deuterium order parameter shows that the DOPC tails are less ordered when close to a GM1.

Structural and thermodynamic properties of different phases of supercooled liquid water.

Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs I(h), III, and VI exists.

Short-range structure of a GM3 ganglioside membrane: comparison between experimental WAXS and computer simulation results.

The local structure of a GM3 ganglioside bilayer, whose wide-angle X-ray spectrum is reconstructed from molecular dynamics simulations, is found to compare quantitatively well with the experimental one. By separating inter- and intramolecular contributions, correlations between distinct head groups are shown to contribute in a substantial way to the total scattering intensity. This finding supports the hypothesis of a strong local head group order as recently formulated on the basis of calorimetry and X-ray experimental data.

Structure of coexisting liquid phases of supercooled water: analogy with ice polymorphs.

The structural changes occurring in supercooled liquid water upon moving from one coexisting liquid phase to the other have been investigated by computer simulation using a polarizable interaction potential model. The obtained results favorably compare with recent neutron scattering data of high and low density water. In order to assess the physical origin of the observed structural changes, computer simulation of several ice polymorphs has also been carried out. Our results show that there is a strict analogy between the structure of various disordered (supercooled) and ordered (ice) phases of water, suggesting that the occurrence of several different phases of supercooled water is rooted in the same physical origin that is responsible for ice polymorphism.

Development of a new polarizable potential model of hydrogen fluoride and comparison with other effective models in liquid and supercritical states.

Development of a new polarizable potential of hydrogen fluoride through the reparametrization of the JV-P model is presented: The length of the H-F bond has been shortened and the other parameters of the model have been readjusted accordingly. The structural, thermodynamic, and liquid-vapor equilibrium properties of the new model are compared with those of other effective potential models of HF as well as with experimental data in a broad range of thermodynamic states, from near-freezing to supercritical conditions. It is found that although the reparametrization does not change the structural properties of the HF model noticeably at the level of the pair correlations, it improves the reproduction of the thermodynamic properties of hydrogen fluoride over the entire range of existence of a thermodynamically stable liquid phase and also that of the vapor-liquid coexistence curve. However, the new model, which still overestimates the close-contact separation of the HF molecules, underestimates the density of the coexisting liquid phase and overestimates the saturation pressure, probably due to the too steep repulsion of the potential function.

Diffusion of water in confined geometry: the case of a multilamellar bilayer.

The diffusion of water confined in a stack of GM3 ganglioside bilayers is studied by computer simulation. A theoretical analysis of the behavior of the mean square displacement parallel and perpendicular to the bilayer surface is also provided in terms of diffusion equations in a very long time interval, between 2 ps and 2 ns. Such an analysis has allowed us to identify two different time regimes, to clarify the nature of nonlinear time dependence of the mean square displacement, and to give an interpretation to the origin of the often used time dependent diffusion coefficient. Both the effects of spatial inhomogeneities and boundary conditions are demonstrated to be the key points for the interpretation of all these results within a consistent theoretical framework.

Liquid-vapor and liquid-liquid phase equilibria of the Brodholt-Sampoli-Vallauri polarizable water model.

Liquid-vapor and liquid-liquid phase equilibria of the polarizable Brodholt-Sampoli-Vallauri water model have been investigated by Gibbs ensemble Monte Carlo computer simulations. The coexisting liquid and vapor densities and energy of vaporization of the model is found to be in a reasonable agreement with experimental data in the entire temperature range of liquid-vapor coexistence. The critical temperature and density of the model are found to be 615 K and 0.278 gcm(3), respectively, close to the experimental values of 647.1 K and 0.322 gcm(3). In the supercooled state two distinct liquid-liquid coexistence regions are observed. The existence of liquid-liquid phase separation of a polarizable water model is demonstrated for the first time.

Free volume properties of a linear soft polymer: a computer simulation study.

Molecular dynamics simulation of a linear soft polymer has been performed and the free volume properties of the system have been analyzed in detail in terms of the Voronoi polyhedra of the monomers. It is found that there are only small density fluctuations present in the system. The local environment of the monomers is found to be rather spherical, even in comparison with liquids of atoms or small molecules. The monomers are found to be, on average, eight coordinated by their nearest geometric neighbors, including intra-chain and inter-chain ones. The packing of the monomers is found to be rather compact, in a configuration of 1900 monomers there are, on average, only three voids large enough to incorporate a spherical particle as large as a monomer, indicating that the density of the large vacancies in the system is considerably, i.e., by a few orders of magnitude lower than in molecular liquids corresponding to roughly the same reduced densities.

Temperature of maximum density line of a polarizable water model.

Monte Carlo simulations of liquid water were performed with a polarizable model in a narrow range of temperatures around the temperature of maximum density (TMD) and a broad pressure range, extended also to negative pressures. In order to infer features of the phase diagram relevant for the explanation of the anomalous behavior of water the pressure dependence of the TMD line was analyzed. The TMD is found to increase with decreasing pressure up to 274.8 K at -32 bar, and decrease upon further stretching. This behavior excludes the possibility of a reentrant liquid-gas spinodal line.