Bulk simulation of polar liquids in spherical symmetry.

Molecular simulations of strongly coupled dipolar systems of varying size have been carried out, using particles confined inside a dielectric cavity and an image charge approach to treat the dielectric response from the surroundings. A simple method using penalty functions was employed to create an isotropic and homogeneous distribution of particles inside the cavity. The dielectric response of the molecular system was found to increase as the number of particles N was increased. Nevertheless, a significant surface effect remained even for the largest systems (N=10,000), manifesting itself through a decrease in the dielectric constant of the system as the confining surface was approached. The surface effect was significantly reduced by using a negative dielectric constant of the surrounding dielectric medium, although accomplishing a full dielectric solvation of the molecular system was not possible

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АПТЕКИИСТОРИЯ МЕДИЦИН

Nondielectric long-range solvation of polar liquids in cubic symmetry

Long-range solvation properties of strongly coupled dipolar systems simulated using the Ewald and reaction field methods are assessed by using electric fluctuation formulas for a dielectric medium. Some components of the fluctuating electric multipole moments are suppressed, whereas other components are favored as the boundary of the simulation box is approached. An analysis of electrostatic interactions in a periodic cubic system suggests that these structural effects are due to the periodicity embedded in the Ewald method. Furthermore, the results obtained using the reaction field method are very similar to those obtained using the Ewald method, an effect which we attribute to the use of toroidal boundary conditions in the former case. Thus, the long-range solvation properties of polar liquids simulated using either of the two methods are nondielectric in their character. (C) 2009 American Institute of Physics. [doi:10.1063/1.3250941

Structural Anisotropy in Polar Fluids Subjected to Periodic Boundary Conditions

A heuristic model based on dielectric continuum theory for the long-range solvation free energy of a dipolar system possessing periodic boundary conditions (PBCs) is presented. The predictions of the model are compared to simulation results for Stockmayer fluids simulated using three different cell geometries. The boundary effects induced by the PBCs are shown to lead to anisotropies in the apparent dielectric constant and the long-range solvation free energy of as much as 50%. However, the sum of all of the anisotropic energy contributions yields a value that is very close to the isotropic one derived from dielectric continuum theory, leading to a total system energy close to the dielectric value. It is finally shown that the leading-order contribution to the energetic and structural anisotropy is significantly smaller in the noncubic simulation cell geometries compared to when using a cubic simulation cell

Formation of ferroelectric domains observed in simulation of droplets of dipolar particles

In this work it is shown that domains of ordered dipoles are formed in large droplets made from dipolar particles provided that the dipole-dipole interaction between nearest neighbors is larger than the thermal energy. The size of the domains grows almost linearly with the size of the droplets for droplets containing 1000-30 000 particles. The largest domains occupy around 25-35% of the droplet volume. The total dipole moment of a domain is of the order of 3-10% of the maximum dipole moment possible if all dipoles in the domain were parallel. The finding offers an explanation to the observation that different boundary conditions yield different long-range order for dipolar liquids and challenges the present view of a short-range dipolar order in polar solvents

On the effective interaction between an ion and a hydrophobic particle in polar solvents. A step towards an understanding of the hofmeister effect?

It has been shown that the generally accepted assumption that hydrophobic molecules and ions repel each other in polar solvents is only valid for moderately polar solvents. When the dipole moment of the medium molecules is increased, then ions and hydrophobic particles start to attract each other. For particles with the size and dipole moment of a water molecule one can expect that the effective potential curve between the two types of particles is almost flat. This opens up for an explanation of the 114 years old unexplained Hofmeister effect