Jacqueline Quintana

Grand canonical Monte Carlo and modified singlet integral equations for the density profile of a Yukawa fluid near a planar wall

Results for the density profile for Yukawa molecules near a hard wall and an exponential attractive wall are presented for Grand Canonical Monte Carlo (GCMC) simulations, for the singlet hypernetted chain (HNC) integral equation and for a modified version of the Lovett–Mou–Buff–Wertheim (LMBW-1) which uses the exact contact value theorem. The results of the standard singlet HNC are quite poor. If the LMBW equation is modified (but still using the bulk direct correlation function) the results at high temperature become reasonable. However, the results at low temperatures, close to the bulk coexistence curve, are only a partial improvement. The contact value of the density profile is now quite good but the profile has the incorrect oscillatory behavior predicted by the unmodified approximate equation. The GCMC simulation results show a clear effect of drying at low temperatures.

Phase equilibria of confined liquid crystals

The differences between the phase diagram of the Gay-Berne potential confined by two identical walls versus the corresponding bulk phase diagram have been investigated. A wall-fluid interaction 9-3 Lennard-Jones potential was used. The study was performed in most cases by using the hybrid Monte Carlo method for the μVT ensemble. Several isotherms were analysed where vapour, liquid and smectic phases were observed. The smectic-isotropic coexistence region becomes wider, i.e. the isotropic coexistence line is shifted to lower densities but the smectic coexistence line remains nearly the same. The triple point temperature of the confined system is estimated to be in the vicinity of 0.45 versus 0.40 of the bulk system. For the isotherm at T* = 0.65 an orientational dependence was added to the 9-3 Lennard-Jones potential to model the wall-fluid interaction. For both kinds of walls, 9-3 LJ with and without orientational dependence, confinement was not found to stabilize a nematic phase as found by previous authors.

A COMPARISON OF INTEGRAL EQUATIONS AND DENSITY FUNCTIONAL THEORY VERSUS MONTE CARLO FOR HARD DUMBBELLS NEAR A HARD WALL

A comparison of Percus–Yevick–Pynn–Lado model theory and a density functional (DF) theory of nonuniform fluids of nonspherical particles is performed. The DF used is a new generalization of Tarazona’s theory. The conclusion is that DF theory provides a preferable route to describe the system under consideration. Its accuracy can be improved with better approximation for the direct correlation function (DCF) for bulk system.

On the static dielectric function of the sticky electrolyte model

The static dielectric function of the sticky electrolyte model of electrolyte solutions is studied. We consider a model with bonding distance L=1/2. It contains steric saturation for association at dimer level. At increasing polarity of the diatomic molecules the ordering in the system changes and is manifested in the shape of the static dielectric function e(k). The short ranged order is characterized by the presence of negative branch of e(k).

Orientation of a molecular fluid next to a hard wall: The Percus–Yevick theory

The Percus–Yevick (PY) equation for inhomogeneous hard linear molecules is presented and solved. Calculations of the density profile are shown. Despite earlier failures of the PY approximation to predict some kinds of angular properties in homogeneous systems, here we find predictions which are qualitatively correct and quantitatively acceptable. In particular, we find that the molecules near the wall lie parallel to the wall.

Confinement induced immiscibility of mixtures of enantiomers

The concentration profiles and phase properties of symmetric fluid mixtures confined by parallel planar walls are analyzed for the cases of identical and symmetrically opposed fields at the walls. We focus on the occurrence of phase separation otherwise absent in bulk mixtures and find that this can take place when chemical potentials and molecular interactions are modified at the surfaces. The phenomenon may find application in the resolution of racemic mixtures of enantiomers.

Inhomogeneous hard homonuclear molecules

A review is given of some features of theories for inhomogeneous fluids of nonspherical molecules that take as input the direct correlation function of the corresponding homogeneous system. Two different methods are described for defining the structure of hard homonuclear molecules close to a hard planar wall. A spherical harmonics expanison (SHE) within the integral equation (IE) method is presented and, for comparison, a version of density functional theory for orientable hard bodies. In both cases the Pynn-Lado model is employed and a comparison is made with Monte Carlo data. The results indicate that for hard molecules the IE approach does not always capture the effects of orientation due to the characteristics of the SHE for the step function. This disadvantage is particularly true in the case of the orientationally averaged density profile.

Landau density functional theory for one-dimensional inhomogeneities

We survey a collection of statistical-mechanical problems involving systems inhomogeneous (only) in one spatial direction and seldom discussed by way of a unified treatment. We employ a Landau density functional approach and present the analysis with the help of the equivalent nonlinear dynamical system. The different possible behaviors are visualized in terms of orbits in the corresponding phase portraits. We focus on some exceptional features of inhomogeneous sytems, like the scaling of the order parameter profiles near criticality.

Structure of spherical interphase at high temperatures

Abstract We present an application of a version of the density functional theory to describe the interphase of Lennard-Jones fluid in contact with a large sphere. A comparison of theory and Monte Carlo data is shown. The theory is generally good and becomes more satisfactory as the radius of the large sphere is increased.

Kinetics of phase change in binary mixtures with complete wetting interfaces

Some properties are described of the nucleation and the spinodal decomposition of a binary mixture in the neighbourhood of a three-phase coexistence state that exhibits complete wetting of one phase at the interface of the other two. A square-gradient density functional approach to inhomogeneities in only one spatial direction is applied, and an analysis is attempted with the help of the equivalent nonlinear dynamic system. The different possible types of behaviour are visualized in terms of orbits in the corresponding phase portraits. There are some exceptional structural, prewetting and wetting film, features both for critical nucleation droplets and for spatially periodic stationary states of spinodal decomposition.