Hard-sphere Yukawa fluid near a planar slit

Density functional theory for the microscopic structure of nanoparticles at the liquid-liquid interface

We propose an extension of the density functional approach to study the structure and thermodynamic properties of a system comprising a certain amount of nanoparticles at the interface between two partially miscible liquids. Model calculations have been carried out for a binary symmetric mixture of Yukawa fluids and hard-sphere nanoparticles. Despite its simplicity, the model captures the principal features of this type of system. The results indicate that nanoparticles form layers and the number of layers depends on the amount of nanoparticles and on their diameters. For the systems studied the formation of the layers evidences strong localization of the nanoparticles at the interface.

Two-Yukawa fluid at a hard wall: Field theory treatment

We apply a field-theoretical approach to study the structure and thermodynamics of a two-Yukawa fluid confined by a hard wall. We derive mean field equations allowing for numerical evaluation of the density profile which is compared to analytical estimations. Beyond the mean field approximation, analytical expressions for the free energy, the pressure, and the correlation function are derived. Subsequently, contributions to the density profile and the adsorption coefficient due to Gaussian fluctuations are found. Both the mean field and the fluctuation terms of the density profile are shown to satisfy the contact theorem. We further use the contact theorem to improve the Gaussian approximation for the density profile based on a better approximation for the bulk pressure. The results obtained are compared to computer simulation data.

DNA melting in slit pores: a reaction density functional theory

A reaction density functional theory (R-DFT) is developed for chemical reactions in confined space by integrating reaction thermodynamics and DFT for chain fluids. The theory is applied to investigate DNA melting in slit pores, with nucleotides represented by coarse-grained charged Lennard-Jones particles. Three types of slit pores are considered for DNA melting: repulsive pore, attractive pore, and under electric field. In repulsive pores, the melting temperature increases slightly with reducing pore width, and the increase magnitude is nearly the same for DNA of different chain lengths. The double-strand DNA (dsDNA) and single-strand DNA (ssDNA) are located in the slit center, particularly for long DNA due to the effect of configuration entropy. In attractive pores, the melting temperature increases with increasing wall-fluid interaction. The DNA chains are preferentially adsorbed near the slit walls with a strong wall-fluid interaction. Under electric field, the melting temperature increases slightly and is more distinct for shorter DNA.

Structure and Adsorption of A Hard-Core Multi-Yukawa Fluid Confined in A Slitlike Pore: Grand Canonical Monte Carlo Simulation and Density Functional Study

Because of the increasing interest in studying the phenomenon exhibited by charge-stabilized colloidal suspensions in confining geometry, we present a density functional theory (DFT) for a hard-core multi-Yukawa fluid. The excess Helmholtz free-energy functional is constructed by using the modified fundamental measure theory and Rosenfeld's perturbative method, in which the bulk direct correlation function is obtained from the first-order mean spherical approximation. To validate the established theory, grand canonical ensemble Monte Carlo (GCMC) simulations are carried out to determine the density profiles and surface excesses of multi-Yukawa fluid in a slitlike pore. Comparisons of the theoretical results with the GCMC data suggest that the present DFT gives very accurate density profiles and surface excesses of multi-Yukawa fluid in the slitlike pore as well as the radial distribution functions of the bulk fluid. Both the DFT and the GCMC simulations predict the depletion of the multi-Yukawa fluid near a nonattractive wall, while the mean-field theory fails to describe this depletion in some cases. Because the simple form of the direct correlation function is used, the present DFT is computationally as efficient as the mean-field theory, but reproduces the simulation data much better than the mean-field theory.

Density profile and order parameter of a hard ellipsoidal fluid confined to a slit

The density profile and order parameter of a fluid of hard axially symmetric ellipsoids confined in between two parallel hard walls is obtained by using the density functional theory. The required input direct correlation function of the homogeneous fluid is calculated by the variational method introduced by Marko (1989 Phys. Rev. 39 2050) and the modified closest approach method proposed by Rickayzen (1998 Mol. Phys. 95 393). Here the restricted orientation model, ROM, is extended to study a fluid comprising molecules which can be aligned in more than six directions, making it more representative of a normal fluid. The density profiles, the average number density and order parameter are obtained for different values of density and elongations. The results are in agreement with the previous theory and available Monte Carlo simulation results.

Structures and adsorption of binary hard-core Yukawa mixtures in a slitlike pore: Grand canonical Monte Carlo simulation and density-functional study

The grand canonical ensemble Monte Carlo simulation and density-functional theory are applied to calculate the structures, local mole fractions, and adsorption isotherms of binary hard-core Yukawa mixtures in a slitlike pore as well as the radial distribution functions of bulk mixtures. The excess Helmholtz energy functional is a combination of the modified fundamental measure theory of Yu and Wu [J. Chem. Phys. 117, 10156 (2002)] for the hard-core contribution and a corrected mean-field theory for the attractive contribution. A comparison of the theoretical results with the results from the Monte Carlo simulations shows that the corrected theory improves the density profiles of binary hard-core Yukawa mixtures in the vicinity of contact over the original mean-field theory. Both the present corrected theory and the simulations suggest that depletion and desorption occur at low temperature, and the local segregation can be observed in most cases. For binary mixtures in the hard slitlike pore, the present corrected theory predicts more accurate surface excesses than the original one does, while in the case of the attractive pore, no improvement is found in the prediction of a surface excess of the smaller molecule.

Structure of Inhomogeneous Attractive and Repulsive Hard-Core Yukawa Fluid: Grand Canonical Monte Carlo Simulation and Density Functional Theory Study

A density functional theory is proposed for an inhomogeneous hard-core Yukawa (HCY) fluid based on Rosenfeld's perturbative method. The excess Helmholtz energy functional is derived from a modified fundamental measure theory for the hard-core repulsion and a quadratic functional Taylor expansion for the long-ranged attractive or repulsive interactions. To test the established theory, grand canonical ensemble Monte Carlo simulations are carried out to simulate the density profiles of attractive and repulsive HCY fluid near a wall. Comparison with the results from the Monte Carlo simulations shows that the present density functional theory gives accurate density profiles for both attractive and repulsive HCY fluid near a wall. Both the present theory and simulations suggest that there is depletion for attractive HCY fluid at low temperature, but no depletion is found for repulsive HCY fluid. The calculated results indicate that the present density functional theory is better than those of the modified version of the Lovett-Mou-Buff-Wertheim and other density functional theories. The present theory is simple in form and computationally efficient. It predicts accurate radial distribution functions of both attractive and repulsive HCY fluid except for the repulsive case at high density, where the theory overestimates the radial distribution function in the vicinity of contact.

Adsorption of Lennard-Jones fluid mixture in a planar slit: a perturbative density functional approach

A simple perturbative density functional approach is employed to investigate the adsorption behavior of a model Lennard-Jones fluid confined in a slitlike pore. Adsorption of one-component fluid as well as two-component fluid mixtures in varying pore sizes has been investigated. The results on the density profiles and the excess adsorption obtained from this theory are found to be in overall good agreement with the available computer simulation results. The results are also compared with the same from some recent weighted density based calculations.

Reformulation of density functional theory for generation of the nonuniform density distribution

The concept of universality of the free energy density functional and the weighted density approximation are combined to provide the density distribution profile of nonuniform fluids from the predictions of integral equation theory for the corresponding uniform fluids. To obtain the expression for the free energy as a function of the density distribution, the present formalism expresses the difference of the first order direct correlation function of a nonuniform fluid with respect to its uniform fluid counterpart as a function of the weighted density, which is also a function of the space position. The input parameters used in the present approach are the radial distribution function and the second order direct correlation function of the corresponding uniform fluid. All of these parameters can be easily obtained from numerical solution of Ornstein-Zernike integral equation theory. The present approach is based on the formalism of classical density functional theory (DFT) and its application to two kinds of fluid under different external potentials is presented. The agreement of the theoretical predictions with the corresponding computer simulation data is good. The present formulation of DFT can treat fluids of different interaction potential under nonzero external fields in a unified way.

Transformation from Rogers-Young approximation to the density functional approach for nonuniform fluids: numerical recipe

A numerical recipe is devised to extend the methodology [J. Chem. Phys. 112, 8079 (2000)] to nonhard-sphere nonuniform fluids where analytical expressions for the functional relationship of the bridge function as a function of indirect correlation function do not exist, the numerical recipe is also based on the universality of the free-energy density functional. As an example, the recipe is employed to calculate the density profile of a colloidal suspension near a single charged hard wall and the hard-sphere Yukawa fluid near a single hard wall and a single hard wall with an attractive tail, the agreement of the predictions of the theory with the simulation data is good. The difference of the present methodology from that of the weighted density approximation is discussed.

Density Functional Theory and the Capillary Evaporation of a Liquid in a Slit

Density functional theory is applied to a Lennard-Jones fluid near a single hard wall and in a slit formed by two walls. We use some simplified versions of the Weeks-Chandler-Andersen (WCA) and the Barker-Henderson (BH) theories. Only the most crude mean field version of the WCA theory, in which the hard-sphere correlation function is set equal to unity for all distances, seems useful. Use of the full WCA approximation is impractical because the effective hard-sphere diameter is density dependent. Generally, the best results are obtained using the BH macroscopic compressibility approximation. Our earlier study of "evaporation" of Lennard-Jones molecules in a slit is extended to other densities using the mean field theory. Copyright 2000 Academic Press.