Elena N. Brodskaya

Surface tension of water droplets: A molecular dynamics study of model and size dependencies

The applicability of two frequently used interaction potentials for water, the five-site ST2 model and the four-site TIP4P model, is investigated in computer simulations of water droplets of varying cluster size from N=64 to N=512. The orientation of the water molecules in the surface region is investigated for the both models. Surface properties, such as work of cluster formation, local density profiles, kinetic and total energy profiles, and pressure profiles as a function of the droplet size, obtained using the two models are compared. Moreover, the surface potential and the electric potential profiles are calculated. Surface tension is calculated and its dependence on the cluster size is investigated. It is found that surface properties are very sensitive to the used potential models. For example, the water molecules are found to lie differently in the inner region of the surface layer, the ST2 molecules being predominantly perpendicular to the surface, while the TIP4P molecules lie mainly parallel to...

Surface properties of water clusters: a molecular dynamics study

Radial local densities, local energies per molecule, orientational distribution functions, normal component of the pressure tensor and other surface properties of water are calculated, based on molecular dynamics simulations of water clusters at 300 K. Three different water models are evaluated: the rigid five-site ST2 and four-site TIP4P models; and the three-site SPC/E model, which is made flexible with respect to the angle bending. The size of the clusters is varied from 64 to 1000 water molecules. It is concluded that surface properties are highly sensitive to the choice of potential model. On the basis of the dependence of the work of cluster formation on the cluster size, the influence of the water model on the surface tension of the plane surface is discussed. None of the three models considered gives a proper value for the surface tension of water at room temperature.

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.

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...

Molecular dynamics simulation of methanol clusters

Methanol clusters have been studied based on Molecular Dynamics simulations. The methanols are described using the three-site OPLS potential model by Jorgensen. Five separate clusters with sizes up to 512 molecules are treated at two different temperatures, 200 K and 300 K, respectively. Several properties, for example, the local density, electric potential and the normal pressure, are calculated. Also, the surface potential of methanol is computed, based on the radial profiles of the electric potential. It is shown, once again, that the quadrupolar contribution to the surface potential is important to the total value of the surface potential. Using the size dependence of the work of cluster formation, it is found that the surface tension of methanol decreases monotonously upon decreasing the radius of the dividing surface approaching a limiting value. The calculated limiting value for surface tension agrees well with that, observed experimentally for the plane surface of methanol.

Role of Water in the Formation of the Electric Double Layer of Micelles

The surface layer of a direct ionic micelle with explicit solvent was simulated by the molecular dynamics method. Local properties of the system were calculated in the spherical micellar cell. The results for the local electric potential show that the contribution of a polar solvent makes the potential short-ranged, in contrast to predictions of the Poisson-Boltzmann theory.

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...

MOLECULAR DYNAMICS COMPUTATION OF SOLVENT CONTRIBUTION TO WORK OF ION SOLVATION

Clusters of water with various (charged and neutral) solute particles were simulated by molecular dynamics, and the radial profiles of local density, energy, electric potential, normal pressure, and polarization were obtained. The work of cluster formation was calculated. On the basis of an estimate of the surface potential for the vacuum-liquid and liquid-solid interfaces, the linear contribution of the ion charge to the chemical work of solvation was determined. In the case of the K+ ion, the linear contribution to the total work of solvation proved to be practically negligible.

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.