Cavities electric charges within

Electric charges within a cavity in a dielectric medium polarize the material outside the cavity. This polarization in turn makes a contribution, called the reaction field, to the electric field within the cavity. In systems containing polar molecules or ions, or both, the reaction field plays an important role in the theory of the dielectric constant e and cannot be disregarded in the study of molecular dynamics. The usual periodic boundary conditions do not give a realistic representation of the actual reaction field for a system of N polar molecules, confined in a volume V and interacting with the dielectric medium outside it. In an attempt to remedy this inadequacy. Barker and Watts have used periodic boundary conditions supplemented by a uniform approximation to the reaction field. ... 

A1r Separation Properties. Self-bound LSX adsorbents have an enhanced ability to selectively adsorb nitrogen from air. For thermodynamically driven adsorption processes, the quantity of a gas adsorbed by a zeolite at a given pressure and temperature Is a function of Its the affinity for the cationic adsorption sites as well as the quantity of sites available for Interaction. Electronic charge balance dictates that the LSX will have the maximum number of cationic sites available for direct Interaction with weakly Interacting adsorbates. The electric field within the zeolite cavity 1s dependent on both structure and the charge density of the extra-framework cation. Small polyvalent cations 1n the dehydrated/dehydroxylated state, especially calcium, show high selectivity for N2 from a1r.(l2)... 

The model rests upon classical electrostatic considerations which have been developed earlier. One evaluates the electrostatic potential created by the liquid surroundings into the region of space occupied by the solute molecule, in order to introduce the perturbation in the hamiltonian of this molecule. This can be achieved by imagining a cavity within a dielectric continuum, in which the molecule is placed. The electrostatic potential arises from the polarization of the continuum by the electric charge distribution of the molecule. This potential in turn keeps the molecule in a polarized state different from its equilibrium state outside the cavity so that the determination of its molecular structure in the liquid must be self-consistent. 

Ire boundary element method of Kashin is similar in spirit to the polarisable continuum model, lut the surface of the cavity is taken to be the molecular surface of the solute [Kashin and lamboodiri 1987 Kashin 1990]. This cavity surface is divided into small boimdary elements, he solute is modelled as a set of atoms with point polarisabilities. The electric field induces 1 dipole proportional to its polarisability. The electric field at an atom has contributions from lipoles on other atoms in the molecule, from polarisation charges on the boundary, and where appropriate) from the charges of electrolytes in the solution. The charge density is issumed to be constant within each boundary element but is not reduced to a single )oint as in the PCM model. A set of linear equations can be set up to describe the electrostatic nteractions within the system. The solutions to these equations give the boundary element harge distribution and the induced dipoles, from which thermodynamic quantities can be letermined. 

The charge density on the surface of the hole, cr(rs), is given by standard electrostatics in terms of the dielectric constant, e, and the electric field perpendicular to the surface, F, generated by the charge distribution within the cavity. 

Within the dielectric continuum model, the electrostatic interactions between a probe and the surrounding molecules are described in terms of the interaction between the charges contained in the molecular cavity, and the electrostatic potential these changes experience, as a result of the polarization of the environment (the so-called reaction field). A simple expression is obtained for the case of an electric dipole, /a0, homogeneously distributed within a spherical cavity of radius a embedded in an anisotropic medium [10-12], by generalizing the Onsager model [13]. For the dipole parallel (perpendicular) to the director, the reaction field is parallel (perpendicular) to the dipole, and can be calculated as [10] ... 

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