E. Lomba - Academia.edu (original) (raw)

Papers by E. Lomba

ABSTRACT The fluid-solid equilibrium of a system of hard spheres with embedded classical Heisenbe... more ABSTRACT The fluid-solid equilibrium of a system of hard spheres with embedded classical Heisenberg spins is studied by means of computer simulation and various combinations of density functional theories with the spin-spin contributions treated at the level of first-order perturbation theory. The phase boundaries are determined from the simulation data using free energy calculations in the ferromagnetic and isotropic phases. Estimates of the location of the Curie line in the solid are extracted from inspection of the evolution of magnetization with temperature. The agreement between theory and simulation is relatively good for the ferromagnetic transition and merely qualitative as far as the fluid-solid transition is concerned. The theoretical approach investigated here tends to underestimate the stability of the liquid phase.

Ornstein-Zernike equations and simulation results for hard-sphere fluids adsorbed in porous media... more Ornstein-Zernike equations and simulation results for hard-sphere fluids adsorbed in porous media. Enrique Lomba, James A. Given, and George Stell Department of Chemistry, State Univeristy of New York at Stony Brook, Stony Brook, New York 11794-3400. ...

... Jean-Jacques Weis Laboratoire de Physique Theorique et Hautes Energies, Batiment 211, Univers... more ... Jean-Jacques Weis Laboratoire de Physique Theorique et Hautes Energies, Batiment 211, Universite de Paris-Sud, 91405 Orsay Cedex, France Noe G. Almarza ... Our data preclude the existence of a tricritical point, but there is evidence that the line of Curie points may end up in ...

Physical Review E, 2000

We model a disordered planar monolayer of paramagnetic spherical particles, or ferrofluid, as a t... more We model a disordered planar monolayer of paramagnetic spherical particles, or ferrofluid, as a twodimensional fluid of hard spheres with embedded three-dimensional magnetic point dipoles. This model, in which the orientational degrees of freedom are three dimensional while particle positions are confined to a plane, can be taken as a crude representation of a colloidal suspension of superparamagnetic particles confined in a water/air interface, a system that has recently been studied experimentally. In this paper, we propose an Ornstein-Zernike integral equation approach capable of describing the structure of this highly inhomogeneous fluid, including the effects of an external magnetic field. The method hinges on the use of specially tailored orthogonal polynomials whose weight function is precisely the one-particle distribution function that describes the surface-and field-induced anisotropy. The results obtained for various particle densities and external fields are compared with Monte Carlo simulations, illustrating the capability of the inhomogeneous Ornstein-Zernike equation and the proposed solution scheme to yield a detailed and accurate description of the spatial and orientational structure for this class of systems. For comparison, results from density-functional theory in the modified mean-field approximation are also presented; this latter approach turns out to yield at least qualitatively correct results.

Condensed Matter Physics, 2001

The use of inhomogeneous Ornstein-Zernike equations to analyze phase transitions and ordered phas... more The use of inhomogeneous Ornstein-Zernike equations to analyze phase transitions and ordered phases in magnetic systems is explored both in bulk three dimensional disordered Heisenberg systems and in a simple model for a two dimensional ferrofluid monolayer. In addition to closures like the Mean Spherical Approximation, Hypernetted Chain and Zerah-Hansen approximation, the inhomogeneous Ornstein-Zernike equation must be complemented by a one-body closure, for which the Born-Green equation has been used in this paper. The results obtained prove that the proposed approach can furnish accurate estimates for the paramagneticferromagnetic transition in the three dimensional Heisenberg spin fluid, reproducing reliably the structure of the isotropic and ordered phases. In two dimensions, the results are fairly accurate as well, both for the dipolar film alone and in the presence of external perpendicular fields. At high densities/dipole moments the equation seems to predict a transition to a phase in which the dipoles lie mostly in the plane and are aligned into vortex-like structures. Evidence of this new phase is found in the simulation at somewhat higher couplings.

Physical Review E, 2007

The classical XY model describes particles in three-dimensional space that carry magnetic moments... more The classical XY model describes particles in three-dimensional space that carry magnetic moments or spins whose motion is restricted to rotations in a plane. Introduction of an external magnetic field lying in the same plane then generates a system that is anisotropic in the azimuthal angle . We use numerical simulations and integral equation techniques to study this system, producing in the latter case a formalism that is identical to that of the simpler isotropic version having no external field. The basis for this simplification is a generalization E m ͑͒ of the ordinary exponential basis set e im that restores orthogonality in the presence of the external field. We display results of sample calculations obtained with two integral equation closures, reference hypernettedchain and soft mean-spherical approximation, both coupled to the Lovett-Mou-Buff-Wertheim relation, along with results from the numerical simulations for comparison. Construction of the E m ͑͒ is described in an Appendix.

Physical Review E, 2005

Using extensive Monte Carlo simulations and a simple approximation in density functional theory, ... more Using extensive Monte Carlo simulations and a simple approximation in density functional theory, we study the phase behavior of a fluid of nematogenic molecules with centers of mass constrained to lie in a plane but with axes free to rotate in any direction, both with and without an external disorienting field perpendicular to the plane. We find that simulation predicts the existence of an order-disorder phase transition belonging to the Berezinskii-Kosterlitz-Thouless type, along with a low temperature gas-liquid transition. In contrast to the simulation results, density functional theory predicts a first-order orientational phase transition coupled continuously with a first-order gas-liquid transition. The approximate theoretical approach qualitatively reproduces the field dependence of the order-disorder and gas-liquid transitions but is far from quantitative.

Molecular Physics, 2003

ABSTRACT By means of extensive grand canonical Monte Carlo simulations and replica Ornstein-Zerni... more ABSTRACT By means of extensive grand canonical Monte Carlo simulations and replica Ornstein-Zernike integral equation calculations, we explore the thermodynamics and dielectric behaviour of a dipolar fluid confined in disordered matrices. Different matrix topologies are modelled using, on the one hand, quenched hard-sphere configurations and, on the other, randomly positioned spheres. This illustrates the influence of the pore size and shape on the properties of the adsorbed fluid. For the same purpose, various sizes of the matrix particles have been considered. The integral equation calculations in the hypernetted chain approximation agree quantitatively with the simulation results. As in other studies on quenched disorder, one observes that the effect of confinement when considering hard-sphere matrices is rather limited, exhibiting however a tendency to facilitate the transition to ferroelectric states, as well as favouring the gas-liquid transition. Additionally, one observes that the sensitivity of the fluid properties to changes in the size of the matrix particles is considerably larger when these are randomly positioned.

Journal of Physics: Condensed Matter, 1994

ABSTRACT

Journal of Molecular Liquids, 2004

Following a well-established method that uses integral equations for the study of anisotropic flu... more Following a well-established method that uses integral equations for the study of anisotropic fluids, we analyze the behavior of a simple model of nematogens confined to plane in the presence of a disorienting field. Extensive computer simulation calculations support the validity of the integral equation results, which predict an isotropic-nematic transition at low temperatures and zero field and an in-plane order-disorder transition in the presence of a disorienting field at somewhat higher temperatures. ᮊ

Journal of Chemical Theory and Computation, 2012

ABSTRACT The stability of complexes of a recently synthetized (Scott et al. J. Am. Chem. Soc. 201... more ABSTRACT The stability of complexes of a recently synthetized (Scott et al. J. Am. Chem. Soc. 2011, 134, 107) opened nanocontainer C50H10 with several guest molecules, H-2, N-2, CO, HCN, H2O, CO2, CS2, H2S, C2H2, NH3, CH4, CH3CN, CH3OH, CH3CCH, 2-butyne, methyl halides, and with noble gas atoms, has been examined by means of symmetry adapted perturbation theory of intermolecular interactions, which fully incorporates all important energy components, including a difficult dispersion term. All complexes under scrutiny have been found stable for all studied guests it 0 K, but entropic effects cause many of them to dissociate into constituent molecules. under standard conditions.. The estimation of temperature At which the Gibbs free energy Delta G = 0 revealed that the recently observed (Scott et al J. Am. Chem. Soc. 2011, 134, 107) complex. CS2@C50H10 is the most stable. at room temperature while the corresponding complexes with HCN and Xe guests should decompose at ca. 310 K and that with CO2 at room temperature (ca. 300 K). In agreement with the Delta G estimation, molecular dynamics simulations performed in vacuum for the CS2@C50H10 complex predicted that the complex is stable but decomposes at ca. 350 K The MD simulations in CHCl3 solution showed that the presence of solvent stabilizes the CS2@C50H10 complex in comparison to vacuum. Thus, for the complexes obtained in solution the CO2 gas responsible for the greenhouse effect could be stored in the C50H10 nanotube.

The Journal of Chemical Physics, 2008

The properties of a simple one-dimensional lattice model with two repulsive ranges are studied in... more The properties of a simple one-dimensional lattice model with two repulsive ranges are studied in terms of its analytic solution. Its phase behavior is characterized by the presence of a disorder-order-disorder transition ͑or a fluid-solid-fluid transition in lattice gas language͒. A similar situation was discussed by Hemmer and Stell ͓Phys. Rev. Lett. 24, 1284 ͑1970͔͒ when considering the purely repulsive version of their ramp potential. The melting of the solid phase, when pressure is increased along an isotherm, is a feature common to both models and one of the characteristic features of water.

The Journal of Chemical Physics, 2007

The behavior of a two-dimensional neutral Coulomb fluid in the strong association regime ͑low den... more The behavior of a two-dimensional neutral Coulomb fluid in the strong association regime ͑low density, high ionic charge͒ is explored by means of computer simulation and the hypernetted chain integral equation. The theory reproduces reasonably well the structure and thermodynamics of the system but presents a no-solution region at temperatures well above the computer simulation estimates of the metal-insulator transition. In contrast with hypernetted chain predictions for the three-dimensional Coulomb fluid, here the breakdown of the solution is not accompanied by divergences in any physical quantity.

The Journal of Chemical Physics, 2003

Structural, thermodynamic, and dielectric properties of a dipolar fluid confined in a charged ran... more Structural, thermodynamic, and dielectric properties of a dipolar fluid confined in a charged random matrix are studied by means of grand canonical Monte Carlo simulation and replica Ornstein-Zernike integral equations in the hypernetted chain approximation. The fluid is modeled by a system of dipolar hard spheres. Two matrix topologies are considered: a frozen restricted primitive model matrix and a frozen hard sphere fluid with randomly distributed negative and positive charges. Both models lead to similar results in most cases, with significant deviations from the behavior of the corresponding equilibrated mixtures. The dielectric behavior is particularly interesting, since the effect of partial quenching on the equilibrated mixture recovers the electrostatics of the pure dipolar fluid but with the presence of Coulomb tails in the dipole-dipole total correlations. Differences between the two matrix models arise more vividly in the low density regime, in which the matrix with randomly distributed charges tends to enhance dipole association around the matrix particles. The integral equation results are in relatively good agreement with the computer simulation estimates.

The Journal of Chemical Physics, 2001

Liquid hydrogen fluoride is a well-known hydrogen bonded substance, in many aspects related to li... more Liquid hydrogen fluoride is a well-known hydrogen bonded substance, in many aspects related to liquid water, and for which a wide variety of interaction models have recently been proposed. We have studied two of these models by means of a reference hypernetted chain equation in order to assess the ability of this latter approach to describe the properties of this highly associative system. Our calculations, when compared with molecular dynamic results, show that the integral equation reproduces quantitatively both the structure and the thermodynamics of liquid hydrogen fluoride over a wide range of thermodynamic states. However, the integral equation approach is apparently unable to produce estimates for the phase diagram since the low-density ͑gas phase͒ side of the binodal curve lies inside the nonsolution region of the equation. This failure can be understood as the result of the inability of standard integral equation theories to account for the behavior of low density strongly associative systems like highly charged electrolytes or, in this case, the gaseous phase of hydrogen fluoride.

The Journal of Chemical Physics, 2010

A pillared interlayered clay is represented by a two-dimensional quenched charged disordered medi... more A pillared interlayered clay is represented by a two-dimensional quenched charged disordered medium, in which the pillar configuration is produced by the quench of a two-dimensional electrolyte and the subsequent removal of the anions ͑that act as a template͒. The cation charge is counterbalanced by a neutralizing background that is an ideal representation of the layer's negative charge in the experimental system. In this paper we investigate the adsorption of electrolyte particles in this charged disordered medium resorting both to the use of the replica Ornstein-Zernike equation in the hypernetted chain approximation and grand canonical Monte Carlo simulations. The theoretical approach qualitatively reproduces the simulated behavior of the adsorbed fluids. Theoretical estimates of the material porosities obtained for various types of pillar distributions are in good agreement with the simulation. We investigate the influence of the matrix on correlation functions and adsorption isotherms.

The Journal of Chemical Physics, 2009

The microporous structure of pillared interlayered clays is determined by their interlayer separa... more The microporous structure of pillared interlayered clays is determined by their interlayer separation and the distribution of the pillars that separate their layers. The pillars provide stability to these quasi-two-dimensional high surface area materials. In this work we present a topological analysis of available and accessible volumes within various simple models of pillared interlayered clays. Each model is characterized by a distribution of pillars. Both fully ordered structures and disordered pillar distributions with either attractive or repulsive interpillar correlations are considered. Particular attention is paid to the problem of accessibility. In systems with similar degrees of porosity, even when cavities within each model might be able to host the same adsorbate molecules, their accessibility will strongly depend on the pillar distribution. The theoretical analysis presented in this work may facilitate the interpretation of experimental results, pointing out those quantities that are key to describe the texture of the porous material.

Fluid Phase Equilibria, 1996

Physical Review E, 1998

We develop a general method to study inhomogeneous liquids in an external field using orthogonal ... more We develop a general method to study inhomogeneous liquids in an external field using orthogonal polynomials tailored to the one-body density. The procedure makes integral equation calculations of these systems no more difficult than those of ordinary homogeneous molecular fluids. We apply this method to the ferromagnetic Heisenberg spin fluid in an external magnetic field using both the reference-hypernetted chain