The origin of interparticle potential of electrorheological fluids

The particles of electrorheological fluids can be modelled as dielectric spheres (DS) immersed in a continuum dielectric. When an external field is applied, polarization charges are induced on the surfaces of the spheres and can be represented as point dipoles placed in the centres of the spheres. When the DSs are close to each other, the induced charge distributions are distorted by the electric field of the neighbouring DSs. This is the origin of the interaction potential between the DSs. The calculation of this energy is very time consuming, therefore, the DS model cannot be used in molecular simulations. In this paper, we show that the interaction between the point dipoles appropriately approximates the interaction of DSs. The polarizable point dipole model provides better results, but this model is not pair-wise additive, so it is not that practical in particle simulations.Comment: 10 pages, 5 figure

Monte Carlo simulation of the electrical properties of electrolytes adsorbed in charged slit-systems

We study the adsorption of primitive model electrolytes into a layered slit system using grand canonical Monte Carlo simulations. The slit system contains a series of charged membranes. The ions are forbidden from the membranes, while they are allowed to be adsorbed into the slits between the membranes. We focus on the electrical properties of the slit system. We show concentration, charge, electric field, and electrical potential profiles. We show that the potential difference between the slit system and the bulk phase is mainly due to the double layers formed at the boundaries of the slit system, but polarization of external slits also contributes to the potential drop. We demonstrate that the electrical work necessary to bring an ion into the slit system can be studied only if we simulate the slit together with the bulk phases in one single simulation cell.Comment: 11 pages, 8 figure

Energetics of ion competition in the DEKA selectivity filter of neuronal sodium channels

The energetics of ionic selectivity in the neuronal sodium channels is studied. A simple model constructed for the selectivity filter of the channel is used. The selectivity filter of this channel type contains aspartate (D), glutamate (E), lysine (K), and alanine (A) residues (the DEKA locus). We use Grand Canonical Monte Carlo simulations to compute equilibrium binding selectivity in the selectivity filter and to obtain various terms of the excess chemical potential from a particle insertion procedure based on Widom's method. We show that K$^{+}$ ions in competition with Na$^{+}$ are efficiently excluded from the selectivity filter due to entropic hard sphere exclusion. The dielectric constant of protein has no effect on this selectivity. Ca$^{2+}$ ions, on the other hand, are excluded from the filter due to a free energetic penalty which is enhanced by the low dielectric constant of protein.Comment: 14 pages, 7 figure

Monte Carlo simulation of an ion-dipole mixture as a model of an electrical double layer

Canonical Monte Carlo simulations were performed for a nonprimitive model of an electrical double layer. The ions and the solvent molecules are modeled as charged and dipolar hard spheres, respectively, while the electrode as a hard, impenetrable wall carrying uniform surface charge. We found that the ion-dipole model gives a reasonable description of the double layer for partially charged ions with small to moderate dipole moments, or equivalently for an "effective" dielectric constant. Density, polarization and mean electrostatic potential profiles are reported. Strong layering structure, and at higher charges, charge inversion in the second layer were found. With appropriate choices of charge and solvent parameters, states corresponding to the primitive or the solvent primitive model can be produced, and the results agreed well with literature data. At higher effective charges and dipole moments, the dipolar solvent has difficulties in preventing the ions from clustering. More realistic models of water and other solvents are necessary to study the double layer. © 1998 American Institute of Physics.published_or_final_versio

Monte Carlo study of the capacitance of the double layer in a model molten salt

Monte Carlo simulations are reported for charged hard spheres at high density near a charged wall. This system is a simple model for a molten salt double layer. Unfortunately, the reduced temperatures that correspond to experiment are very small. This results in a large Boltzmann factor. As a result, we are unable to obtain meaningful results for such low values and report results only for moderately low values of the reduced temperature. Even so, our results should be a useful benchmark. Further, we are able to give a qualitative answer to an interesting question. We find that at low temperatures the capacitance near the point of zero charge increases with increasing temperature. This agrees with experiment for molten salts and disagrees with the behavior of double layer in dissolved salts, which can be modeled with low density and high temperature charged hard spheres near a wall. This also disagrees with the predictions of the Gouy-Chapman theory and the mean spherical approximation. It appears that it is the approximations, and not the charged hard sphere double layer model, that are at fault for describing double layers in molten salts. © 1999 American Institute of Physics.published_or_final_versio

The mean spherical approximation for a dipolar Yukawa fluid

The dipolar hard sphere fluid (DHSF) is a useful model of a polar fluid. However, the DHSF lacks a vapor-liquid transition due to the formation of chain-like structures. Such chains are not characteristic of real polar fluids. A more realistic model of a polar fluid is obtained by adding a Lennard-Jones potential to the intermolecular potential. Very similar results are obtained by adding a Yukawa potential, instead of the Lennard-Jones potential. We call this fluid the dipolar Yukawa fluid (DYF). We show that an analytical solution of the mean spherical approximation (MSA) can be obtained for the DYF. Thus, the DYF has many of the attractive features of the DHSF. We find that, within the MSA, the Yukawa potential modifies only the spherically averaged distribution function. Thus, although the thermodynamic properties of the DYF differ from those of the DHSF, the MSA dielectric constant of the DYF is the same as that of the DHSF. This result, and some other predictions, are tested by simulations and are found to be good approximations. © 1999 American Institute of Physics.published_or_final_versio

Thermodynamics and structural properties of the dipolar Yukawa fluid

We report computer simulations and a theoretical study of the thermodynamics and structure of a dipolar Yukawa system. A comparison between the analytical mean spherical approximation (MSA) solution, perturbation theory and Monte Carlo simulation data of pressure, internal energy and dielectric constant is given. In the perturbation theory, the MSA solution of hard core Yukawa fluid is used as a reference system. It was found that the MSA solution is reasonable only at lower dipole moments, while the perturbation theory gives good results at low and high values of dipole moment. Liquid-vapor coexistence data of dipolar Yukawa fluid are also obtained by Monte Carlo simulation and by both MSA and perturbation theory. It was found that at high dipole moments the liquid-vapor equilibrium disappears while chain-like structures appear in the low density fluid phase. The appearance of chain-like structures of dipolar Yukawa fluid is discussed in comparison with the Stockmayer fluid. © 1999 American Institute of Physics.published_or_final_versio

Magnetization and susceptibility of ferrofluids

A second-order Taylor series expansion of the free energy functional provides analytical expressions for the magnetic field dependence of the free energy and of the magnetization of ferrofluids, here modelled by dipolar Yukawa interaction potentials. The corresponding hard core dipolar Yukawa reference fluid is studied within the framework of the mean spherical approximation. Our findings for the magnetic and phase equilibrium properties are in quantitative agreement with previously published and new Monte Carlo simulation data.Comment: 8 pages including 4 figure

Spontaneous polarisation of the neutral interface for valence asymmetric coulombic systems

In this paper, we discuss the phenomenon of a spontaneous polarisation of a neutral hard planar interface for valence asymmetric coulombic systems. Within a field theoretical description, we account for the existence of non trivial charge density and electric potential profiles. The analysis of the phenomenon shows that the effect is related to combinatorics in relation with the existence of the two independent species cations and anions. This simple and basic feature is related to the quantum mechanical properties of the system. The theoretical results are compared with numerical simulations data and are shown to be in very good agreement, which a fortiori justifies our physical interpretation.Comment: 12 pages, 11 figure

Dielectric constant of the polarizable dipolar hard sphere fluid studied by Monte Carlo simulation and theories

A systematic Monte Carlo (MC) simulation and perturbation theoretical (PT) study is reported for the dielectric constant of the polarizable dipolar hard sphere (PDHS) fluid. We take the polarizability of the molecules into account in two different ways. In a continuum approach we place the permanent dipole of the molecule into a sphere of dielectric constant Ɛ∞ in the spirit of Onsager. The high frequency dielectric constant Ɛ∞ is calculated from the Clausius-Mosotti relation, while the dielectric constant of the polarizable fluid is obtained from the Kirkwood-Frohlich equation. In the molecular approach, the polarizability is built into the model on the molecular level, which makes the interactions non- pairwise additive. Here we use Wertheim’s renormalized PT method to calculate the induced dipole moment, while the dielectric constant is calculated from our recently introduced formula [22]. We also apply a series expansion for the dielectric constant both in the continuum and the molecular approach. These series expansions ensure a better agreement with simulation results. The agreement between our MC data and the PT results in the molecular approach is excellent for low to moderate dipole moments and polarizabilities. At stronger dipolar interactions ergodicity problems and anizotropic behaviour appear where simulation results become uncertain and the theoretical approach becomes invalid.Приведено результати систематичного вивчення методом Монте Карло (МК) та теорії збурень (ТЗ) діелектричної сталої рідини поляризаційних дипольних твердих сфер. Поляризація молекул враховується двома способами. У підході неперервного середовища постійний диполь молекули поміщається у сферу з діелектричною сталою Ɛ∞, так як це робиться Онзагером. Високочастотна діелектрична стала Ɛ∞ рахується зі співвідношення Клаузіса-Мозотті, тоді як діелектрична стала поляризаційної рідини отримується з рівняння Кірквуда-Фройліха. У молекулярному підході поляризація враховується на молекулярному рівні, внаслідок чого взаємодії не є парно адитивними. Для розрахунку індикованого дипольного моменту використовується ренормалізаційний метод ТЗ Вертхайма, а для діелектричної сталої використовується оригінальна формула [22]. Ми також застосовуємо групові розклади для діелектричної сталої в обох підходах. Групові розклади забезпечують кращу узгодженість з комп’ютерними результатами. Узгодженість між МК даними і результатами ТЗ є дуже доброю у діапазоні низьких та середніх дипольних моментів та поляризації. Для сильних дипольних взаємодій проявляються проблеми ергодичності та анізотропної поведінки і комп’ютерні дані стають сумнівними, а теоретичні результати – необгрунтованими