E. N. Brodskaya

Molecular dynamics simulations of water clusters with ions at atmospheric conditions

Clustering of water molecules on charged particles has been studied using the method of molecular dynamics simulations. A selected set of model metal and halogen ions, carrying both positive and negative charges, is chosen as nucleation centers for water molecules. The influence of the ion charge, its size, and short-range interactions on the local structure, and kinetic characteristics are investigated for the ion-centered clusters of 20 and 30 water molecules at 200 and 300 K, respectively. It is shown, based on radial densities, energy, polarization profiles, and orientational distribution functions, that the local water structure in the clusters becomes perturbed to a larger degree around negative ions compared to ions carrying a corresponding positive charge. The electric field of an anion is more effectively screened by the first hydration shell, resulting in a weaker dependence of the relaxation processes on the ion field in the second hydration shell. The dependence of the work of cluster formatio...

The molecular dynamics simulation of water clusters

Water clusters containing 15, 27, and 64 molecules have been investigated by molecular dynamics at various temperatures. Profiles for atomic density, the polarization vector, local energy, and the pressure tensor have been obtained. A simple way to calculate the local electric potential as an average Coulomb potential at a sphere of radius r has been offered that resulted in more reliable data. The orientation of water molecules at the surface has been studied in detail. The most probable position of the molecular plane has been shown to be perpendicular and of the dipole vector parallel to the surface. Two models of water molecules with different values of the quadrupole component Qzz along the dipole axis have been considered. The theoretical prediction of the proportionality of the surface potential and Qzz has been confirmed.

Molecular dynamics simulation of AOT reverse micelles

Reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in a nonpolar medium were studied by computer modelling using a coarse-grained approach. For various ratios of water to surfactant substance (SAS), the shape and molecular structure of micelles along with the characteristics of the rotational and translational dynamics were considered. Assuming an ellipsoidal shape for the aggregates, their geometric parameters were estimated. The radial profiles of the partial density, energy, normal component of pressure and local electric potential were calculated. The actual independence of the surface potential on SAS was observed in all aggregates. The influence of the aggregation number on the micelle properties was considered.

Computer simulations of micellar systems

Works published during the last decade devoted to simulation of micellar solutions of different surfactants are reviewed. The main attention is focused on studying the local structure of spherical and cylindrical micelles and their surface layers in solutions, as well as problems relevant to self-aggregation kinetics.

Capillary Condensation and Melting/Freezing Transitions for Methane in Slit Coal Pores

Grand canonical Monte Carlo method has been applied to study the adsorption behavior of methane in slit coal micropores for pore widths from 1.5 to 4.5 nm at temperatures 111–300 K. The adsorbent surface is considered to be inhomogeneous with C, N, S, O and H atoms, as well as vacancies, taken into account. The effect of surface microstructure on mechanism of the pore filling and saturation pressure is found to be more pronounced at relatively low and moderate strength of the adsorption field. It is also shown that irregular impurities and defects at the adsorbent surface contribute to the lowering of the freezing temperature and can change qualitively its dependence on the pore width.

MOLECULAR-DYNAMICS SIMULATION OF WATER CLUSTERS WITH IONS

Clusters containing the K+ or Cl- ion at the centre with 14 or 26 water molecules are investigated by means of computer experiments. For two models of water, the profiles for atomic density, the local energy per molecule, the normal component of the pressure tensor, the polarization vector, the volume and molecular polarizabilities, the local electric potential and the electric field affecting a water molecule are obtained. The most probable configurations of water molecules in the hydration shells are studied in detail. The lifetime τ of a molecule in the first hydration shell and the coefficients of the normal (transversal), D N, and tangential (longitudinal), D T, self-diffusion are estimated. The size dependence of the cluster properties is considered. The grand potential of a liquid cluster around an ion is calculated. The thermodynamic conclusion regarding the difference in the work of formation of clusters on oppositely charged cores is verified.

Molecular simulation of an aerosol OT reverse micelle: 1. The shape and structure of a micelle

The simulation of an Aerosol OT micelle in the apolar environment is performed via the molecular dynamics method in the approximation of a coarse-grain model. The mean size and shape of a micelle, as well as its molecular structure, are determined as functions of the water-surfactant ratio and aggregation number. Geometric parameters of aggregates are estimated through calculation of the inertia tensors of its internal portion under the assumption of an ellipsoidal shape of a micelle. Radial profiles of the partial density and pair correlation functions are obtained, which are used to calculate coordination numbers for water molecules, counterions, and surface-active ions. The most probable arrangement of water molecules and surfactant anions are found on the basis of orientation distribution functions.

Molecular simulation of an aerosol OT reverse micelle: 2. Energy and kinetic characteristics

The effect of the structure of a reverse micelle on the energy and kinetic characteristics of system components is studied via the molecular dynamics method with the use of the coarse-grain model of a surfactant ion. Partial energies are calculated and the behavior of the local electric potential is determined. It is shown that, despite a high ion concentration, the electric potential in the surface layer of a micelle behaves itself as in the surface layer of pure water. Coefficients of translational diffusion of the components are calculated on the basis of the Einstein relation for the dependence of rms displacement on time. Autocorrelation functions of the angular velocity of eigenvectors of water molecules are obtained.

Disjoining pressure in a plane-parallel asymmetric slit with finite sizes

An approach has been developed to calculation of the Irving-Kirkwood pressure tensor inside a circular plane-parallel empty slit between bodies with dispersion forces. The disjoining pressure is determined as the normal component of the pressure tensor on a slit wall and is found as a function of both the width and radius of the slit. It has been shown that, at a preset slit width, the disjoining pressure acquires a value almost equal to that inherent in an infinitely long slit already at a circular slit radius five times larger than the slit width.

Computer simulation of reverse micelles and water-in-oil microemulsions

The literature data are reviewed on molecular simulation of reverse micelles and water-in-oil microemulsions by the molecular dynamics and Monte Carlo methods. Different models of reverse micelles from a spherical cavity with an impenetrable wall to an atomistic ensemble of surfactant molecules are considered. The main structural and thermodynamic properties, as well as the dynamics of micelle components are considered. The results are compared with the data obtained using both a single model and models of different levels.