Relative Permittivity of Dipolar Model Fluids from Molecular Simulation and from the Co-Oriented Fluid Functional Equation for Electrostatic Interactions

The relative permittivity of dipolar fluids is important in many industrial and scientific applications, e.g. whenever electrolytes or electromagnetic fields are present. For non-polarizable model molecules, it is directly linked to the mutual molecular orientation and thereby usually not accessible by equations of state. However, the recently developed Co-Oriented Fluid Functional Equation for Electrostatic interactions (COFFEE) allows for calculating the orientation distribution function of simple polar molecules and thereby establishes a connection between the thermodynamic behavior and the relative permittivity. In this article, we develop an expression to calculate the relative permittivity from the orientation distribution known from COFFEE. Furthermore, we calculate the relative permittivity of simple polar fluids using molecular simulations. We study the original Stockmayer fluid and the shifted Stockmayer fluid, in which the dipole is shifted away from the Lennard-Jones center along the dipole axis. For both fluids, different dipole strengths are investigated. The results from the theoretical expression from COFFEE are compared to the simulation data. Thereby, a possible link between polar equations of state and electric fields or electrolytes is developed.

The magnetization of dense aggregated dipolar fluids

The magnetization for dipolar fluids is studied treated separately at low and high external magnetic field. Canonical ensemble Monte Carlo simulations have been performed in dipolar hard sphere fluid in order to test these theoretical results. New expressions are introduced at low and high external field and ultimately the synthesized formula is given. The main difference in the structure of dipolar liquids at different external field is the average orientation of the formed chains. In case of infinitesimal external magnetic field there is no a well-specified direction, while at high enough external field actually the chains are parallel to this field. The present theory yields good result in the intermediate region as well where the most conspicuous failures are provided by the former theories.

The Initial Magnetic Susceptibility of Dense Aggregated Dipolar Fluids

To correlate the dipole moment and density dependence of the initial magnetic susceptibility on the basis of the former related theories and the probability analysis of chain formation, physically based analytical correlation equation was derived. After the local magnetic field strength and the chaining probability between two particle have been determined the chain and particle distributions came from the geometric distribution. The initial magnetic susceptibility was resulted from the summation of Langevin initial susceptibility of k-length chains. Two particles were considered in a chain if the interaction energy between them was below a certain limit. By varying slightly this energy limit around 70–75 % good agreement has been obtained between the simulation and theoretical data. Monte Carlo simulations were used to calculate the initial magnetic susceptibility of dipolar hard sphere system at different dipole moments and densities.