N. Galamba

Thermal conductivity of molten alkali halides from equilibrium molecular dynamics simulations

The thermal conductivity of molten sodium chloride and potassium chloride has been computed through equilibrium molecular dynamics Green-Kubo simulations in the microcanonical ensemble (N,V,E). In order to access the temperature dependence of the thermal conductivity coefficient of these materials, the simulations were performed at five different state points. The form of the microscopic energy flux for ionic systems whose Coulombic interactions are calculated through the Ewald method is discussed in detail and an efficient formula is used by analogy with the methods used to evaluate the stress tensor in Coulombic systems. The results show that the Born-Mayer-Huggins-Tosi-Fumi potential predicts a weak negative temperature dependence for the thermal conductivity of NaCl and KCl. The simulation results are in agreement with part of the experimental data available in the literature with simulation values generally overpredicting the thermal conductivity by 10%-20%.

Mapping structural perturbations of water in ionic solutions.

The structure of water in sodium halide aqueous solutions at different concentrations is studied through molecular dynamics. Emphasis is placed on the extent of ionic-induced changes in the water structure, and the concept of kosmotropes/chaotropes is probed, in terms of perturbations to the tetrahedral H-bond network of water. The results show that at low salt concentrations, the halide anions slightly increase the tetrahedrality of the H-bond network of water in the anionic second hydration shell and I(-) is found to be the strongest kosmotrope, contrary to its structure breaker reputation. The sodium cation in turn induces a significant loss of tetrahedrality in the second cationic hydration shell. At higher concentrations, the dominant disruptive effect of Na(+) cancels the anionic effects, even in the anionic second hydration shell. According to a kosmotropes/chaotropes classification of ions, based on the tetrahedrality of the H-bond network of water, halide anions are therefore weak kosmotropes, while Na(+) is a strong chaotrope. However, if this classification is applied to the salts, rather than to the ions, all of the sodium halides are classified as structure breakers even at low concentrations. Further, the effect of pressure on the tetrahedrality of the H-bond network of water is found to be similar to the average effect of the dissolved salts. The present results indicate that the classification of ions in kosmotropes/chaotropes in terms of long-range perturbations to the tetrahedral H-bond network of water is not correlated to the position of the ions in the respective Hofmeister series.

On the Effects of Temperature, Pressure, and Dissolved Salts on the Hydrogen-Bond Network of Water

We study the structure of water through molecular dynamics, specifically the compression/expansion of the hydrogen-bond (H-bond) network, with temperature and pressure, and in salt solutions of alkali chlorides and sodium halides, and relate the observed local spatial perturbations with the tetrahedrality and the average number and lifetime of water H-bonds. The effect of transient H-bonds and transient broken H-bonds on H-bond lifetimes is further investigated, and results are compared with depolarized Rayleigh scattering lifetimes for neat water. A significant electrostriction is observed in the first hydration shell of Li(+) and F(-), while larger ions cause a small expansion of the H-bond network of water instead. However, both alkali cations and halide anions induce a minor contraction of the H-bond network in the second hydration shell. Further, water in the second hydration shell of Li(+), Na(+), and K(+) is less tetrahedral than neat water, resembling water at high pressures, while the H-bond network in the respective hydration shell of halide anions resembles water at low temperatures. Nevertheless, neither ion induced H-bond contraction nor expansion can be exactly mapped onto P or T effects on the local structure of water. Even though the average number and lifetime of H-bonds in the ionic hydration shells of most ions are not very different from those found in neat water, Li(+) and F(-) significantly increase the lifetime of water donor and acceptor H-bonds, respectively, in the first hydration shell. The non-monotonic increase of cation and anion mobility, with ionic size, observed experimentally, is interpreted in terms of the water local tetrahedrality around cations and anions.

Shear viscosity of molten alkali halides from equilibrium and nonequilibrium molecular-dynamics simulations

The shear viscosity of molten NaCl and KCl was calculated through equilibrium (EMD) and nonequilibrium molecular-dynamics (NEMD) simulations in the canonical (N,V,T) ensemble. Two rigid-ion potentials were investigated, namely, the Born-Mayer-Huggins-Tosi-Fumi potential and the Michielsen-Woerlee-Graaf-Ketelaar potential with the parameters proposed by Ladd. The NEMD simulations were performed using the SLLOD equations of motion [D. J. Evans and G. P. Morriss, Phys. Rev. A 30, 1528 (1984)] with a Gaussian isokinetic thermostat and the results are compared with those obtained from Green-Kubo EMD (N,V,T) simulations and experimental shear viscosity data. The NEMD zero strain rate shear viscosity, eta(0), was obtained by fitting a simplified Carreau-type equation and by application of mode-coupling theory, i.e., a eta-gamma(1/2) linear relationship. The values obtained from the first method are found to be significantly lower than those predicted by the second. The agreement between the EMD and NEMD results with experimental data is satisfactory for the two potentials investigated. The ion-ion radial distribution functions obtained with the two rigid-ion potentials for both molten salts are discussed in terms of the differences between the two models.

Insights on Hydrogen-Bond Lifetimes in Liquid and Supercooled Water

We study the temperature dependence of the lifetime of geometric and geometric/energetic water hydrogen-bonds (H-bonds), down to supercooled water, through molecular dynamics. The probability and lifetime of H-bonds that break either by translational or librational motions and those of energetic broken H-bonds, along with the effects of transient broken H-bonds and transient H-bonds, are considered. We show that the fraction of transiently broken energetic H-bonds increases at low temperatures and that this energetic breakdown is caused by oxygen-oxygen electrostatic repulsions upon too small amplitude librations to disrupt geometric H-bonds. Hence, differences between geometric and energetic continuous H-bond lifetimes are associated with large H-bond energy fluctuations, in opposition to moderate geometric fluctuations, within common energetic and geometric H-bond definition thresholds. Exclusion of transient broken H-bonds and transient H-bonds leads to H-bond definition-independent mean lifetimes and activation energies, ~11 kJ/mol, consistent with the reactive flux method and experimental scattering results. Further, we show that power law decay of specific temporal H-bond lifetime probability distributions is associated with librational and translational motions that occur on the time scale (~0.1 ps) of H-bond breaking /re-forming dynamics. While our analysis is diffusion-free, the effect of diffusion on H-bond probability distributions where H-bonds are allowed to break and re-form, switching acceptors in between, is shown to result in neither exponential nor power law decay, similar to the reactive flux correlation function.

Structure and electronic properties of a benzene-water solution

Electronic properties of benzene in water were investigated by a sequential quantum mechanical/molecular dynamics approach. Emphasis was placed on the analysis of the structure, polarization effects, and ionization spectrum. By adopting a polarizable model for both benzene and water the structure of the benzene-water solution is in good agreement with data from first principles molecular dynamics. Further, strong evidence that water molecules acquire enhanced orientational order near the benzene molecule is found. Upon hydration, the quadrupole moment of benzene is not significantly changed in comparison with the gas-phase value. We are also reporting results for the dynamic polarizability of benzene in water. Our results indicate that the low energy behaviour of the dynamic polarizability of gas-phase and hydrated benzene is quite similar. Outer valence Greens function calculations for benzene in liquid water show a splitting of the gas-phase energy levels associated with the 1e(1g)(π), 2e(2g), and 2e(1u) orbitals upon hydration. Lifting of the orbitals degeneracy and redshift of the outer valence bands is related to symmetry breaking of the benzene structure in solution and polarization effects from the surrounding water molecules.

First principles molecular dynamics of molten NaCl.

First principles Hellmann-Feynman molecular dynamics (HFMD) results for molten NaCl at a single state point are reported. The effect of induction forces on the structure and dynamics of the system is studied by comparison of the partial radial distribution functions and the velocity and force autocorrelation functions with those calculated from classical MD based on rigid-ion and shell-model potentials. The first principles results reproduce the main structural features of the molten salt observed experimentally, whereas they are incorrectly described by both rigid-ion and shell-model potentials. Moreover, HFMD Green-Kubo self-diffusion coefficients are in closer agreement with experimental data than those predicted by classical MD. A comprehensive discussion of MD results for molten NaCl based on different ab initio parametrized polarizable interionic potentials is also given.

First principles molecular dynamics of molten NaI: Structure, self-diffusion, polarization effects, and charge transfer

The structure and self-diffusion of NaI and NaCl at temperatures close to their melting points are studied by first principles Hellmann-Feynman molecular dynamics (HFMD). The results are compared with classical MD using rigid-ion (RI) and shell-model (ShM) interionic potentials. HFMD for NaCl was reported before at a higher temperature [N. Galamba and B. J. Costa Cabral, J. Chem. Phys. 126, 124502 (2007)]. The main differences between the structures predicted by HFMD and RI MD for NaI concern the cation-cation and the anion-cation pair correlation functions. A ShM which allows only for the polarization of I- reproduces the main features of the HFMD structure of NaI. The inclusion of polarization effects for both ionic species leads to a more structured ionic liquid, although a good agreement with HFMD is also observed. HFMD Green-Kubo self-diffusion coefficients are larger than those obtained from RI and ShM simulations. A qualitative study of charge transfer in molten NaI and NaCl was also carried out with the Hirshfeld charge partitioning method. Charge transfer in molten NaI is comparable to that in NaCl, and results for NaCl at two temperatures support the view that the magnitude of charge transfer is weakly state dependent for ionic systems. Finally, Hirshfeld charge distributions indicate that differences between RI and HFMD results are mainly related to polarization effects, while the influence of charge transfer fluctuations is minimal for these systems.

Born−Oppenheimer Molecular Dynamics of the Hydration of Na+ in a Water Cluster

The hydration of Na(+) in a water cluster is studied through all-electron Born-Oppenheimer molecular dynamics. The structure, dipole moment, and vibrational spectrum of the sodium cation hydration shells are examined. Emphasis is placed on the extent of the effect of the hydrated cation on the cluster properties. Our results show that hydration of Na(+) takes place in the interior of the cluster leading to significant changes in the hydrogen-bond (H-bond) network beyond the first hydration shell. In particular, we find that single acceptor-only H-bond arrangements increase significantly at the surface of the cluster relative to a neat water cluster. The vibrational spectrum of the first hydration shell of the cation, comprised mostly of H-bond double donor-single acceptor water molecules, is similar to that found for water molecules in the interior of a neat water cluster, although a small blue shift of the OH stretching band is observed. Further, a small reduction of the dipole moment of water molecules in the first hydration shell of the cation relative to a neat water cluster is also observed, and this persists to a minor extent when we move from the interior to the surface of the cluster. The present results indicate that the effect of the Na(+) on the cluster properties, although not pronounced, is not constrained to the first hydration shell. The reason appears to lie mostly in the specific orientation of the water molecules in the first coordination sphere, inducing modifications on the H-bond network topology of the cluster.

A corresponding states approach for the prediction of surface tension of molten alkali halides

The extended corresponding states principle has been applied on the prediction of the surface tension of pure molten alkali halides. The model uses liquid density and vapour pressure data of the salts of interest and of the reference salt, chosen to be NaCl, as the input for the calculation of temperature-dependent equivalent substance reducing ratios (ESRR). The model here described was already applied on the prediction of the viscosity and thermal conductivity of pure molten alkali halides [High Temp.-High Pressures, 2000]. Calculations were also made using the simple two-parameter corresponding states principle, with the melting temperature and corresponding density as scaling factors. Agreement between calculated and experimental data is within 10–15% for most of the salts studied.