Andrei V. Egorov

Solid-liquid phase transition in small water clusters: a molecular dynamics simulation study

Water clusters, (H2O) n , of varying sizes (n = 8, 12, 16, 20, 24, 28, 32, 36, and 40) have been studied at different temperatures from 0 to 200 K using molecular dynamics simulations. Transitions between solid and liquid phases were investigated to estimate the melting temperature of the clusters. Although the melting temperatures showed non-monotonic behaviour as a function of cluster size, their general tendency follows the classical relationship T m ∝ n −1/3 to the cluster size n. Moreover, it was observed that the liquid-solid surface tension decreased with the cluster size in a similar way to the liquid-vapour surface tension in bulk water. Upon cooling, ice-like crystals were formed from the smaller clusters with n up to 20, while the larger clusters were transformed to glassy structures. The decrease in the glass transition temperature with the cluster size was observed to be much less than the corresponding melting temperature. The mutual order of the melting and glass-transition temperatures were found to be reversed compared with that observed for bulk water.

Molecular dynamics simulation study of glycerol-water liquid mixtures.

To study the effects of water on conformational dynamics of polyalcohols, Molecular Dynamics simulations of glycerol-water liquid mixtures have been carried out at different concentrations: 42.9 and 60.0 wt % of glycerol, respectively. On the basis of the analysis of backbone conformer distributions, it is found that the surrounding water molecules have a large impact on the populations of the glycerol conformers. While the local structure of water in the liquid mixture is surprisingly close to that in pure liquid water, the behavior of glycerols can be divided into three different categories where roughly 25% of them occur in a structure similar to that in pure liquid of glycerol, ca. 25% of them exist as monomers, solvated by water, and the remaining 50% of glycerols in the mixture form H-bonded strings as remains of the glycerol H-bond network. The typical glycerol H-bond network still exists even at the lower concentration of 40 wt % of glycerol. The microheterogeneity of water-glycerol mixtures is analyzed using time-averaged distributions of the sizes of the water aggregates. At 40 wt % of glycerol, the cluster sizes from 3 to 10 water molecules are observed. The increase of glycerol content causes a depletion of clusters leading to smaller 3-5 molecule clusters domination. Translational diffusion coefficients have been calculated to study the dynamical behavior of both glycerol and water molecules. Rotational-reorientational motion is studied both in overall and in selected substructures on the basis of time correlation functions. Characteristic time scales for different motional modes are deduced on the basis of the calculated correlation times. The general conclusion is that the presence of water increases the overall mobility of glycerol, while glycerol slows the mobility of water.

The effect of ions on solid–liquid phase transition in small water clusters. A molecular dynamics simulation study

Small water clusters, containing ions, have been studied using molecular dynamics simulations at temperatures ranging from 0 to 250 K. The simulations are carried out systematically by varying the ion size, shape, and charge as well as the cluster size and the initial configuration. Transitions between solid and liquid phases are followed to study the effects of the ions on the cluster melting temperature, compared to pure water clusters of the same size. The effect of the ion on the ice-cluster melting appears to be a complicated process which depends simultaneously on a variety of factors, such as the initial cluster configuration and the ion position inside the cluster as well as the ion mass, size and its charge. In the case of monovalent cations the most important characteristics for the cluster evolution is the ion mass, while for divalent cations the ion charge is the most dominant factor. In the case of negatively charged ions the main factor of the system evolution is the ion size. Two principall...

Microstructure and dynamics of electrolyte solutions containing polyatomic ions by NMR relaxation and molecular dynamics simulation

The NMR relaxation method has been used for determing coordination numbers of oxygen-containing polyatomic ions in diamagnetic aqueous solutions. The results, obtained for 7 ions, make it possible to define the rules which govern the formation of ion hydration shells. Probably the developed model can be used for the estimation of results of quantum-chemical calculations and computer simulations. As an example, the molecular dynamics simulation of the sulfate dianion hydration shell has been analyzed.

Computer modeling of melting of ionized ice microcrystals

Ionized water clusters, OH−(H2O)N and H3O+(H2O)N, of different sizes (N=19 and 26) have been studied at temperatures ranging from 10 to 200 K using molecular dynamics simulations. The solid–liquid phase transitions are investigated to estimate the effects of the presence of an ion on the melting temperature of the clusters. It was found that the behavior of the aggregates during the melting is determined mainly by water–water interactions. Compared to corresponding pure water clusters, the observed changes in the melting temperature, Tm, are small and within the statistical uncertainty of the simulations. A weak trend can be observed with the hydroxyl ion reducing the Tm, while there is a slight tendency for an increase of Tm for clusters containing the hydronium ion. In general, the ions disrupt the hydrogen bond network and at the same time, the formation of a strong hydration shell contributes to a decrease of the mobility of the molecules. These two phenomena affect the solid–liquid phase transition t...

Influence of temperature on the microstructure of the lithium-ion hydration shell. A molecular dynamics description

Abstract The microstructure of lithium cation hydration shell has been studied by means of molecular dynamics simulations. The aqueous LiCl solution has been analyzed in the temperature range from −30 to 120°C using the SPC and ST2 water models for a set of model Lennard-Jones parameters of intermolecular potentials. The results of simulations have been compared to NMR-relaxation data and concern the reorganization of the lithium-cation hydration shell with temperature variations. The obtained results show that it is necessary to modify the water model.

Molecular Dynamics Simulation Study of Solid‐Liquid Phase Transition in Water Clusters. The Effect of Cluster Size

Solid SPC/E and TIP4P water clusters of varying sizes from 8 to 216 molecules have been studied over a temperature range from 0 to 200 K using Molecular Dynamics computer simulations. Solid‐to‐liquid phase transitions were investigated to estimate the effect of cluster size (n) on its melting temperature. Simulations demonstrate that water model geometry is crucial for description of the solid cluster phase behaviour. For solid clusters with n>24 molecules the three‐site SPC/E water model gives higher melting temperatures than the four‐site TIP4P. For smaller clusters with n<24 the situation is diametrically opposed. The analysis of the effect of cluster size on its melting temperature shows that classical liquid drop approximation is useful for both SPC/E and TIP4P clusters. In the case of three‐site SPC/E water the classical relation is valid even for as small clusters as n=12, in the case of the four‐site TIP4P model, it is valid only for n≥20.

Substitution behavior of square-planar and square-pyramidal Cu(II) complexes with bio-relevant nucleophiles

Abstract Substitution reactions of [CuCl2(en)] and [CuCl2(terpy)] complexes (where en = 1,2-diaminoethane and terpy = 2,2′:6′,2″-terpyridine) with bio-relevant nucleophiles such as inosine-5′-monophosphate (5′-IMP), guanosine-5′-monophosphate (5′-GMP), L-methionine (L-Met), glutathione (GSH) and DL-aspartic acid (DL-Asp) have been investigated at pH 7.4 in the presence of 0.010 M NaCl. Mechanism of substitution was probed via mole-ratio, kinetic, mass spectroscopic and EPR studies at pH 7.4. In the presence of an excess of chloride, the octahedral complex anion [CuCl4(en)]2− is formed rapidly while equilibrium reaction was observed for [CuCl2(terpy)]. Different order of reactivity of bio-molecules toward Cu(II) complexes was observed. Mass spectrum of [CuCl2(terpy)] in Hepes buffer has shown two new signals at m/z = 477.150 and m/z = 521.00, assigned to [CuCl(terpy)]+-Hepes fragments of coordinated Hepes buffer. These signals also appear in the mass spectra of ligand substitution reactions between [CuCl2(terpy)] and bio-molecules in molar ratio 1:1 and 1:2. According to EPR data, L-Met forms the most stable complex with [CuCl2(en)] among the ligands considered, while [CuCl2(terpy)] complex did not show significant changes in its square-pyramidal geometry in the presence of the buffer or bio-ligands.