Dipolar Fluids Research Articles

Thin liquid films

The concept of the effective interface potential as a framework for describing the formation of thin liquid films in thermal equilibrium is described both for one-component and binary fluids. Heterogeneous substrates give rise to an interesting morphology of such films. The onset of critical fluctuations within films leads to anomalies such as critical adsorption. Attention is paid to thin film helium films close to their superfluid transition which represent a realization of the Casimir effect induced by critical fluctuations. New aspects of the structure of liquid-vapor interfaces of simple and dipolar fluids are discussed. The use of x-rays and neutrons under the condition of grazing incidence allows one to infer quantitative informations about the structural properties of thin films.

Orientational order in simple dipolar fluids: Density-functional theory and absolute-stability conditions.

The formation of ferroelectric liquid crystals by simple dipolar models is investigated using density-functional theory and absolute-stability analysis. It is emphasized that for such systems well-defined results can only be found by specifying exactly how the long-range dipolar interactions are treated. Explicit formal expressions are derived for mean-reaction-field boundary conditions and these are combined with integral-equation approximations in order to obtain numerical results for fluids of dipolar hard and soft spheres. The calculations predict isotropic-to-ferroelectric nematic transitions in qualitative agreement with computer simulations. The quantitative agreement, however, is rather poor.

A mean field theory for fluids of multipolar particles in contact with a polarizable wall

Fluids of multipolar particles in contact with a semi-infinite polarizable hard wall are considered. A mean field theory which reduces the many-body electrostatic wall–solvent interactions to an effective pair potential is described. The effective potential can be employed in conjunction with the reference hypernetted-chain approximation, or some other integral equation theory, to obtain a self-consistent solution for the wall–solvent correlation function and hence the solvent structure at the interface. Explicit results are given for dipolar hard sphere fluids in contact with walls having dielectric constants ranging from 1 to ∞. For this system, it is shown that contributions to the wall–solvent potential from images of other particles are very important and act strongly against the direct ‘‘self-image’’ interaction.

Perturbation theory of polar nonspherical molecule fluids

Perturbation theory of polar nonspherical molecule fluids was applied to systems of dipolar and polarizable dipolar Kihara molecules. Integrals corresponding to the first, second and third order perturbation terms, containing the angle-dependent part of the electrostatic pair potentials, were determined by the numerical methods for a set of parameters characterizing the nonsphericity of molecules, and expressed in the form of Padé approximants. Theoretical expressions for thermodynamic functions were applied to describe the dipolar nonspherical molecule system and compared with the simulation data. Simple models of polarizable quadrupolar and polarizable dipolar Kihara molecule fluids were considered for carbon dioxide and hydrogen chloride and theoretical expressions for the Helmholtz function, compressibility factor and the second virial coefficient were employed to describe the equilibrium behaviour of these systems. Comparison with experimental data is given.

Determination of the transient polarization response of a dipolar fluid

Recent activity in understanding the dynamics of solvation has also renewed interest in the time dependent behaviour of polar liquids. In this article we report a study of the transient polarization response of a diplar liquid to the instantaneous application of an electric field. The time-dependent response of a Stock-mayer system is examined with nonequilibrium molecular dynamics simulations using two different techniques, the substration method of Ciccotti, Jacucci and McDonald and the transient time correlation function formalism of Evans and Morriss. The two approaches are compared at several field strengths and, unlike earlier work with shear flow in atomic fluids, the former is found to be superior at least for the present implementation. This study greatly expands upon the previous calculations of Petersen, de Leeuw and Perram. However, we are unable to find any data from our simulations that supported their conclusion that nonlinear effects become important at smaller fields than previously thoug...

On the use of semiphenomenological closures in integral equations for classical fluids

We have studied two choices of semiphenomenological closures for the Ornstein–Zernike equation, both for a monoatomic Lennard-Jones fluid and a dipolar homonuclear hard diatomic fluid. One of the closures was originally proposed by Verlet for hard-sphere systems, for which is known to yield good results. A second closure is proposed by us in the frame of the reference hypernetted chain (RHNC) theory. We have described the reference systems in this closure by means of Verlet’s approximation and its recent extension to systems of nonspherical particles. This second approach, which we denote by RHNC-VM (Verlet’s modified), turns out to give an excellent description of the structure and thermodynamics of the Lennard-Jones fluid and very accurate predictions for the structure of the dipolar diatomic system. In this latter case the apparent superiority of hypernetted chain results for configurational energies is found to stem from fortuitous cancellation of errors in the integration of the components of the pair correlation function. Nonetheless, an approximation for the bridge function capable of accounting for the particular dielectric behavior of the dipolar diatomic fluid is still lacking.

Structure of a dipolar hard sphere fluid near a neutral hard wall

The position-orientation distribution of a dipolar hard sphere fluid in contact with a neutral hard wall is studied using a density functional theory. Effects of hard sphere exclusions are incorporated in the excess hard sphere free energy density functional. Orientational correlations between the dipoles are taken into account through approximating the pair correlation function of the inhomogeneous dipolar fluid by that of the homogeneous one of the same chemical potential and dipole moment. The latter is obtained by solving reference quadratic hypernetted chain equation. Theoretical results agree well with those of computer simulations which are available at high density. The theory predicts a qualitatively different interfacial structure at low density and high dipole moments from that at high density; the former is very similar to the liquid/vapor interface of dipolar fluids with regard to both the density distribution and orientational order. The theory also demonstrates that different density distributions correspond closely to different orientational orderings of the dipoles at the interface.

Phenomenological behavior of multipolar viscous fluids

A constitutive theory is formulated to describe the flow of viscous fluids: the theory of multipolar fluids exhibits nonlinear relations among the stress tensors and spatial derivatives of the velocity of order greater than one and is compatible with the basic principles of continuum mechanics and thermodynamics. For an isothermal, incompressible, dipolar fluid the velocity profiles for various steady flows, such as proper Poiseuille flow in a circular pipe, are computed; the results are compared with the velocity profiles obtained from the steady Navier-Stokes equations through an application of the Prandtl boundary-layer theory.

Interaction free energy between planar walls in dense fluids: An Ornstein-Zernike approach with results for hard-sphere, Lennard-Jones, and dipolar systems.

The interaction free energy per unit area between planar walls is given as a convolution of wall-solvent pair-correlation functions. This result, derived from the large radius limit of the macrosphere-solvent Ornstein-Zernike equations, and from the hypernetted-chain closure, provides a statistical-mechanical basis for the Derjaguin approximation, and is both generally applicable and computationally tractable. It is found that the interaction between hard walls in a hard-sphere fluid is oscillatory, and in good agreement with simulations. The van der Waals attraction emerges from asymptotic analyses of Lennard-Jones and dipolar fluids, and the full expression allows calculation of this quantity down to molecular separations. This is demonstrated by numerical results for dipolar fluids.

The application of integral equation theories to fluids of nonspherical particles near a uniform planar wall

A general reduction of the Ornstein–Zernike equation is given for molecular fluids near a planar wall. This allows integral equation approximations such as the hypernetted-chain or reference hypernetted-chain (RHNC) theories to be solved numerically for such systems. Dipolar hard sphere fluids near a hard wall are considered in detail and RHNC solutions are obtained. The results are compared with previous calculations for curved surfaces. The RHNC result for the asymptotic behavior of the wall–solvent pair correlation function at large separations is derived and compared with expressions given by classical continuum theory and by exact analysis.

The influence of long range electrostatic forces on static properties of a quasi-Stockmayer fluid

This paper discusses the implementation of the reaction field method previously developed [1] for inhomogeneous fluids, and presents results for selected properties of a dipolar model fluid at a liquid-gas interface. To implement the reaction field, a fit function based on the image charge approach is introduced. The fit function features eight image charges placed at uniquely determined points inside a dielectric slab of infinite extent in the x, y-plane and fixed height in the z-direction in which the molecular dynamics (MD) simulation sample is embedded. In addition to the image charges, a set of seven scaling parameters is introduced to take care of the effect of truncating the image charge expansion. Based upon this approach, MD runs are performed with and without an applied reaction field. These runs reveal that, similar to what is known for dipolar bulk fluids, purely structural properties remain unaffected while dielectric properties are very sensitive to the reaction field.

Collective excitations in a dense dipolar fluid studied by inelastic neutron scattering

The inelastic neutron scattering spectra of liquid sulphur dioxide have been measured for two thermodynamic states and for the momentum-transfer range 0.35<or=Q<or=2 AA-1. The spectra show noticeable inelastic intensities, indicating the presence of propagating modes for Q<or=0.45 AA-1 at T=266 K and for Q<0.70 AA-1 at T=210 K. The spectra have been analyzed following two different approaches: on the one hand, the underlying S(Q, omega ) dynamic structure factor has been reconstructed by means of a deconvolution technique; and, on the other hand, the measured intensities are analysed on the basis of a simplified model folded with the instrumental resolution. The 'dispersion relations' obtained are finally discussed in relation to recent theoretical predictions.

Reaction Field Simulations of Monatomic and Diatomic Dipolar Fluids

Abstract Firstly, the conditions are explored for which molecular dynamics simulations yield reliable thermodynamic results when the reaction field method is applied for dipolar fluids. The influence of the system size, of the scaling method for avoiding the total energy updrift, and of the reaction field dielectric constant are studied. With appropriate conditions, the results for the Stockmayer fluid when compared to previous results show excellent agreement up to very high dipole moments. In the next step the method is applied to two-centre Lennard-Jones plus ideal axial dipole (2CLJD) molecules of elongation L = 0.505. For one state point, the influence of the molecular shape as well as of the dipolar strength on energy and pressure is studied in detail. Finally, in the project of developing a physically based equation of state for polar fluids, thermodynamic results are presented for 2CLJD fluids of elongation L = 0.505 for four different dipole moments at 29 state points in a temperature and density grid. For these 116 runs which all were performed with vectorized codes on a CYBER 205 also the uncertainties in the results are given as have been estimated from the running averages.

Ergodic measures for the simulation of dialectric properties of water

We determine the time interval, t obs, required for effective ergodic convergence of the fluctuations in dipole moment which are needed for the computation of the dialectric properties of water. It is established that the value of t obs can be computed from the scaling relations obeyed by the fluctuation metric, Ω μ( t) associated with the time-average values of the dipole moment of the individual water molecules. The theoretical ideas behind Ω μ( t) are used to compute t obs at several temperatures for the simple point-charge model of water. It is found that t obs increases exponentially with decreasing temperature, the barrier being associated with the difficulty in making collective orientational rearrangements. For the simple point-charge model considered in this paper the barrier for these collective motion is estimated to be 3.5 kcal/mole. This study provides a firm basis for obtaining reliable values of the dielectric properties of dipolar fluids using molecular dynamics simulations.

Clustering and percolation in dipolar hard-sphere fluids.

Clustering and continuum percolation in dipolar fluids are studied through Monte Carlo simulation and connectedness theory with an emphasis on the effect of the dipole-dipole interactions on the size and shape of the clusters and on the threshold percolation density. Two simple models for the dipolar fluid are considered: (i) a system of hard spheres with an embedded point dipole and (ii) a related system of hard spheres in which the dipole-dipole forces are replaced by an angular-averaged dipolar potential. A first-order perturbation theory (for the first model) and the connectedness version of the Percus-Yevick integral equation (for the second one) are used to describe clustering and percolation, and their results are compared with the corresponding Monte Carlo data. Our results show that clusters become larger in size and acquire a stronger mean dipolar moment when the particles' dipolar moments are increased. Far from the percolation transition, the clusters are nonspherical, the eccentricity being favored by the energetics of dipolar orientation. Furthermore, they reveal that larger dipolar strengths imply smaller percolation densities.

Smoluchowski fluctuation theory of dielectric relaxation

A recently proposed theory for transport coefficients in ionic solutions can be reinterpreted so that it becomes applicable to calculating the dielectric relaxation processes in dipolar fluids and in ionic solutions in dipolar solvents. The resulting theory is the same as the Smoluchowski–Vlasov theory which has been derived in various other ways by Munakata, Calef and Wolynes, and Chandra and Bagchi. Here the theory is applied to calculate the time correlation function for the fluctuating polarization density field in simple dipolar fluid, and in an ionic solution in a dipolar solvent. The results are similar to those derived some time ago by Berne using a ‘‘forced diffusion equation’’ as the theory for calculating the response. Comparison is also made with the results of Chandra and Bagchi using the Smoluchowski–Vaslov theory.

Density-functional theory for the interfacial properties of a dipolar fluid

The authors have studied the interfacial properties of a model dipolar fluid using a generalization of the density functional mean-field approximation. The generalization consists in weighting configurations in the mean-field average of the perturbative part of the energy by the low-density approximation of the radial distribution function. This leads to a bulk phase diagram which depends explicitly on the strength of the multipole moments, in contrast with the results of the simpler version of the theory. The calculated surface tension and density-orientational profile are in fair agreement with computer simulation (molecular dynamics) results: the addition of a dipole moment causes the surface tension to increase and there is interfacial ordering induced by purely multipolar forces. An extension of the theory to binary fluid mixtures is also briefly discussed.

New aspects in the simulation and behaviour of polar molecular fluids

A Monte Carlo computer simulation study of a simple model for dipolar molecular fluids is reported. Long-range interactions are treated by the Reaction Field method. A detailed investigation of the density dependence of dielectric, structural and thermodynamic properties is presented. At moderately high densities the dielectric constant is found to exhibit a clear maximum. Special attention has also been addressed to the influence of sample size and cutoff radius on dielectric properties; unlike the case of dipolar hard sphere fluids, such properties are rather sensitive to changes in the sample size, whereas the cutoff radius seemingly plays a minor role.

The reaction potential in anisotropic dense fluids

In Monte Carlo or molecular dynamics calculations for anisotropic dipolar fluids the long-range electrostatic interaction potential poses a serious problem because of the microscopic dimensions of the simulation cells. As in homogeneous bulk fluids these small system sizes necessitate a separation of the entire electrostatic potential into short- and long-range parts. Interactions due to the former are evaluated explicitly while the net effect of the latter can be taken into account via the reaction potential approach. Since short-range interactions are evaluated explicitly the range of these interactions is commonly associated with a cavity characterized by a dielectric constant ϵ = 1 which is embedded in a homogeneous dielectric of ϵ = ϵL. The total electrostatic potential Φ(r) caused by an assembly of molecular dipoles inside the cavity can be split formally into a ‘direct’ part Φ0(r) and a much smaller ‘reaction’ part R(r) due to the polarization of the surrounding dielectric by the molecular dipoles....

An approximate density functional for an inhomogeneous dipolar fluid

The approximate density functional used by Rickayzen and Augousti (1984, Molec. Phys., 52, 1355) in a study of the structure of the inhomogeneous hard sphere fluid is extended to a dipolar fluid. The density and polarization profiles of a fluid of hard spheres with dipoles at their centres confined to a slit in the presence of a weak electric field are obtained. A comparison is made with the results of computer simulation in a Monte Carlo canonical ensemble and with those of the previous density functional theory. The results of simulation are in much better agreement with the present theory than with the previous one.