Omar A. Karim

The ice/water interface: A molecular dynamics simulation study

The structure and dynamics of the ice/water interface have been investigated using molecular dynamics and the TIP4P model of water. In the bulk liquid phase, the pair correlation functions for this model compare well with recent neutron scattering results, and unlike most water models, the density at room temperature and pressure is close to the experimental value. For the basal plane of ice 1h exposed to liquid water, we have simulated an interface which is stable on the time scale of at least 100 ps. The density profile, molecular orientation, and diffusion constants have been calculated as a function of the distance normal to the interface. The interface is found to be broad, approximately 10 A in extent, with the precise width determined by the particular property under investigation. Finite size effects have been studied. Dynamical molecular trajectories have been used to explore the loss of translational order from the solid through the interface to the liquid.

Sodium chloride ion pair interaction in water: computer simulation

Abstract The thermodynamics and structure of a sodium chloride ion pair in liquid water are studied as a function of the ion pair separation. Distinct minima in the free energy of the system are found for contact and solvent separated ion geometries.

The ice/water interface

Abstract The TIP4P model of water is used to simulate the ice/water interface. A stable interface is constructed with the basal plane of ice 1h exposed to liquid water. Density and orientation profiles, and diffusion constants, are calculated as a function of the distance perpendicular to the interface.

Rate constants for ion pair formation and dissociation in water

Abstract The reactive flux time correlation function for an associating ion pair in water is obtained by computer simulation. The transmission coefficient is found directly from the reactive flux and also through the method of absorbing barriers. Rate constants for the transitions between the contact and solvent-separated states of the ion pair are calculated.

Potential of mean force for an aqueous chloride ion pair : simulation with a polarizable model

The potential of mean force for an aqueous chloride ion pair at room temperature is calculated using a polarizable‐polar model and molecular dynamics computer simulation. A shallow minimum in the potential of mean force is observed near an ion separation of 4.8 A, with approximately two water molecules bridging the chloride ions at this separation. The barrier to further separation is less that 1 kcal/mol. This result does not support the strong association of chloride ion pairs.

Dynamics of an ammonium ion in water: Molecular dynamics simulation

The translational and rotational dynamics of an aqueous ammonium ion are examined using an NVE molecular dynamics computer simulation and a rigid model of water. The linear momentum, angular momentum and reorientational autocorrelation functions of the ammonium ion have been measured. From these, the memory functions for the linear and angular momentum correlation functions are calculated. Reorientation of the ammonium ion about its center of mass shows unambiguous departure from Debye rotational behavior. The translational and rotational diffusion constants have been calculated.

Simulation of an anion in water: effect of ion polarizability

Abstract A polarizable-polar water model is used to study the structure of wate near a chloride ion. A semi-classical description of ion polarizability is included. Significant changes in the solute-solvent distribution functions are observed. When compared with a simulation without ion polarizability, it is found that the hydration number is further decreased when ion polarizability is present.

Ionic Association in Water: From Atoms to Enzymesa

Chemistry and biochemistry are largely concerned with the association and transformation of molecules in water. Theoretical studies of such processes have in the past been hindered by a number of difficulties. Within an aqueous system, there are strong, directional, attractive forces among the water molecules, and often also between solute and solvent molecules, in addition to the excluded volume forces that have made even simple liquids a challenging subject.s2 For a model system comprising a few solute molecules and a few hundred water molecules, the competition among these interactions produces a complicated potential energy surface with many local minima. To calculate structural or thermodynamic properties, one must evaluate averages of certain quantities over a representative set of those configurations that have low enough energy to be thermally populated. To calculate kinetic properties, one must consider motions over energy barriers and, in the case of molecular association, motions corresponding to large displacements over the potential surface. From the perspective of computer simulations, the difficulties that arise in any of the calculations mentioned above are largely associated with the time scales involved. In conventional molecular dynamics simulations, where one solves Newtons equations for the atoms in a model system, the accessible times on conventional computers have been too short for brute-force simulation of many systems. For example, a simulation to generate a fairly representative set of instantaneous hydration structures of a small univalent ion might involve about two hundred molecules and 20 psec of simulation; this would require about 60 hours of CPU time on a VAX 11/780; this is quite manageable. To study the hydration of a moderately large enzyme such as trypsin, however, a 20-psec simulation might require 3500 hrs on a VAX; this is cumbersome a t best. For kinetic properties such as rate constants for barrier crossing or diffusional encounter, the situation can be worse by many orders of magnitude. Happily, advances in the theory underlying computer simulations, in the algorithms used, and in computers themselves, have greatly expanded the range of

A theory of the interionic structure of graphite intercalation synthetic metals: Variations with respect to interactions and state

A statistical mechanical theory is developed and applied to study the structural effects that the thermodynamic state of alkali ions have on graphite intercalation compounds. The system considered is that of second stage Rb–graphite. Two‐dimensional diffraction patterns are computed and compared with experimental measurements. Sensitivity to model parameters are considered. A low order density functional expansion is found to adequately describe the interionic structure of the system modeled as a two‐dimensional one component plasma in an anisotropic external field.

Two-dimensional fluids in a periodic external potential: intercalation in graphite

Abstract An inhomogeneous anisotropic integral equation theory is used to predict the sixfold pattern of intensity in the structure factor for graphite intercalation compounds. The competition of length scales implied by particle size and the underlying lattice through the interplay of the liquid packing structure with the graphitic external field is shown to be responsible for the observed halo-like features in S( k ) . By using a density functional motivated ansatz no explicit truncation of the angular Fourier components of the density distributions was necessary.