G. Pálinkás

Molecular dynamics simulations of water-methanol mixtures

Abstract Molecular dynamics simulations of two water-methanol mixtures with methanol mole fractions of 0.1 and 0.9 at room temperature have been performed. The interaction potentials are based on flexible three-site models for water and methanol. The structural changes relative to the pure solvents are demonstrated with the help of radial distribution functions and the geometrical arrangement of nearest-neighbor molecules. Differences in thermodynamic properties and in hydrogen bonding between the two mixtures and relative to the pure liquids are discussed.

A Molecular Dynamics Study of Aqueous Solutions. VII. Improved Simulation and Comparison with X-Ray Investigations of a NaCl Solution

The molecular dynamics simulation has been improved by introducing the Ewald summation for ion-ion interactions. On the basis of the Pauling radii new Lennard-Jones parameters have been derived for the halide ions. The results of a simulation of a system consisting of 200 water molecules, 8 sodium and 8 chloride ions are compared with x-ray investigations of a 2 molal NaCl solution through the structure functions. The first neighbor model commonly used in the x-ray analysis of multicomponent liquids is fitted to the experimental structure function and to the structure function calculated from the radial pair correlation functions of the simulation. The parameters of the two fits and the radial pair correlation functions calculated from the fitted first neighbor model are compared. On the basis of this comparison the usefulness and the limitations of the first neighbor model are discussed

Hydration shell structure of the calcium ion

Abstract It has been found from an MD simulation of a 1.1 molal aqueous CaCl 2 solution that the first hydration shell of Ca 2+ consists of either 9 or 10 water molecules. The approximate symmetry of the hydration shells can be described by D 3h or D 4d point groups.

A molecular dynamics study of liquid methanol with a flexible six-site model

Abstract The flexible six-site model employed in the simulation of liquid methanol at 295 K is an extension of the three-site model reported recently. The distinct total pair correlation function is found to be in good agreement with neutron diffraction studies. The ten partial correlation functions which form the contributions of the various atom-atom interactions to the total are also reported. Several thermodynamic properties compare favorably with experimental results after quantum corrections have been applied.

The structure of an aqueous ammonium chloride solution

X‐ray measurements have been performed on a 2 molal NH4 Cl solution at 25u2009°C. The scattering data have been evaluated with the first neighbor model (FNM). These results are compared with molecular dynamics (MD) simulations where the ST2 model for water is employed. The NH4+ is described by a rigid tetrahedron with point charges at the positions of the hydrogen atoms and the Cl− by a Lennard‐Jones sphere with an elementary charge at the center. Good agrement has been found between the measurements and the simulation. Contrary to previous results the nearest neighbor distance for NH4+–water is found to be significantly different from the one for water–water. The structure of the solvent described by the nearest neighbor distance and its root mean square deviation is in the limits of error the same as in pure water and significantly different from the solvent structure in alkali halide solutions.

The structure of the hydration shell of the lithium ion

Abstract Structural data on the hydrated lithium ion are derived from the MD simulation of a 2.2 molal aqueous Lil solution. The results are in good agreement with recent neutron diffraction measurements with isotopic substitution and show additionally the octahedral arrangement of the water molecules around the lithium ion.

Reverse Monte Carlo simulation of liquid water

Abstract Reverse Monte Carlo simulation of liquid water has been carried out on the basis of partial pair correlation functions determined by Soper and Phillips. The configurations obtained from this simulation were analyzed in detail. The results were compared with those obtained from molecular dynamics (MD) simulation in order to interpret the differences between the experimental and the MD partial pair correlation function sets. By evaluating the experimental data we found a more distorted geometry of the hydrogen bonds, and also that a significant fraction of the nearest-neighbour molecules distributes randomly rather than tetrahedrally around a central water molecule.

Ion-Solvent and Solvent-Solvent Interactions. X-ray Study of Aqueous Alkali Chloride Solutions

Abstract The first neighbour model, FNM, used frequently for the description of hydration has been modified by dropping the assumption that the structure of the free solvent is identical with that of the pure solvent. The modified model, FNM2, reproduces the experimental X-ray structure functions of alkali chloride solutions quite well and enables the study of perturbed solvent-solvent interactions. The refined structural parameters reasonably indicate the dependence of the hydration of the alkali cations on their radius and concentration. The hydration of Cl- has been found to be almost independent of both concentration and the type of counter cation. The model assumes regular symmetry for the first neighbour coordination sphere, but the high rms deviations of the water-water distances within the shells of the aggregates indicate con-siderable individual asymmetries. A further modification of the FNM will be discussed in a following paper.

Network of strongly interacting atoms in liquid argon

Abstract An(N,V,T,) Monte Carlo simulation of liquid argon has been carried out in order to study the distribution of clusters formed by the relatively strongly interacting atoms. A set of configurations has been selected for the analysis of connectivity properties. Energetic criterion has been used for the definition of the bond between two atoms. Two atoms were considered to be bonded if their pair interaction energy was deeper than a prefixed value. Varying the value of this limiting energy a bond percolation threshold, at which initially disconnected clusters suddenly transformed to a large space-filling network of connected atoms, was observed. The limiting energy corresponding to this threshold is not far from the minimum value of the pair interaction energy. The similar behavior of connectivity properties in liquid argon to that of water shows that gel formation may take place without orientational forces.

Interactions of Water in Ionic Hydrates

The experimental investigations of water-water interactions in aqueous electrolyte solutions are confronted with the problem of the analysis of the macroscopic data on a molecular level. In addition, the comparison of experimental results is very often difficult as the different kinds of experiments are a measure of different properties of the interactions. This leads very often to different conclusions e.g. on hydrogen bonding. A typical example is the question if water is a uniform or a two state liquid, which has been discussed for many years.