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WSN: JCP manuscripts on-line
Following preprints (full text + figures) of manuscripts accepted by JCP are
available from JCPExpress (URL: gopher://jcp.uchicago.edu/11/JCP%):
***
Water near a planar interface: atom-based integral equation theory
Michael J. Booth, Der-ming Duh and A. D. J. Haymet
School of Chemistry,
University of Sydney,
NSW 2006 AUSTRALIA
email: haymet@chem.usyd.edu.au
FAX: +61-2-692-3329
The structure and properties of an atom-based model of water next to
a planar interface are solved using an approximate integral equation
theory. The input to the calculations is the structure of the bulk
water in the form of the direct correlation functions. Predictions
from the theory include the oxygen and hydrogen density profiles
perpendicular to the interface, the mean electrostatic potential, the
potential of zero charge and the differential capacitance. The
predicted structure is relatively insensitive to both the surface
potential and the details of the short-range wall-water potentials,
and exhibits a layered structure which extends approximately 15 \AA
into the liquid. For our initial choice of the short-range
wall-water potential, we predict a value of -32 mV for the potential
of zero charge and a differential capacitance of 4.89 $\mu$F cm$^{-2}$ for
pure water at a planar interface. The capacitance is apparently
independent of the surface charge density and the surface potential.
***
Ionic association in electrolyte solutions. A voronoi polyhedra analysis
J.C. Gil Montoro, F. Bresme, and J.L.F. Abascal
Depto. Quimica Fisica
Universidad Complutense de Madrid
Facultad de Ciencias Quimicas
E-28040 Madrid, SPAIN
e-mail: jl@juguete.quim.ucm.es
FAX : + 34 - 1 - 394 41 35
Voronoi Polyhedra (VP) analysis of ionic solutions generated via Monte-Carlo
simulations for a wide range of concentrations and ionic charges are reported.
The properties investigated are the VP volumes, VP surface shared with
unlike/like--ions and VP non--sphericity configurational mean values
distributions. The study shows that high concentrations favor a molten
salt--like structure while low concentrations tend to disrupt such
ordering so small aggregates are more likely. The degree of separation
between these forms is strongly determined by the charge of the ions.
In the limit of low concentrations/high ionic charge, small clusters with
some chain--like character are present which anticipates several features
characteristic of the low density liquid--gas transition of the restricted
primitive model.
***
Studies on free energy calculations. II. A theoretical approach to molecular
solvation.
Haluk Resat and Mihaly Mezei
Department of Biophysics and Physiology
Mount Sinai School of Medicine, CUNY,
New York, NY 10029-6574
Fax: (212) 860-3369
Tel: (212) 241-9457 (Haluk Resat)
e-mail: resat@msvax.mssm.edu (Haluk Resat)
Using the concepts of Scaled Particle Theory, an analytical theory
is developed to investigate the limiting behaviour of solvation free
energies at the particle creation limit. The new theory directly
incorporates the weakly attractive, dispersion interaction terms into
the analytical calculations. For neutral molecular systems, the effects
of longer ranged electrostatic interactions are also incorporated, albeit
in an ad hoc way, and the validity of the utilized assumptions are then
demonstrated with numerical examples. It is shown that it is possible to
blend the numerical and analytical methods to increase the reliability
of quantitative results, and, at the same time, to achieve savings on
computational expenditure for certain types of calculations. Different
methods of performing the Thermodynamic Integration in solvation free
energy calculations are also compared. Studied examples clearly show the
importance of proper treatment of the divergence at the particle creation
limit in obtaining quantitatively reliable results for the solvation free
energies.
<< Enjoy the article, and we welcome any criticism/comments.>>
***
The structure of water at a planar wall. An integral equation approach
with the central force model
M. Vossen, F. Forstmann
Institut fuer Theoretische Physik, Freie Universitaet Berlin
Arnimallee 14, 14195 Berlin, Germany
Phone: + 030/838 3796, FAX: + 030/838 6744
E-mail: vossen@archimedes.physik.fu-berlin.de
The structure of liquid water in contact with a planar wall is calculated for
the central force model by which water is modeled as a stoichiometric mixture
of partially charged hydrogen and oxygen atoms interacting via effective
spherical pair potentials. The density profiles near the wall are obtained
from the Wertheim-Lovett-Mou-Buff (WLMB) equation extended to mixtures with
coulombic interactions. The correlations in the bulk liquid are calculated
by adding bridge functions of appropriate reference hard sphere systems to
the hypernetted chain equation. We have obtained structural (stoichiometry,
hydrogen bonds) and thermodynamic properties in good agreement with
computer simulations. We also expose the water to electric fields due to a
surface charge on the wall.
***
Dynamic Monte Carlo description of thermal desorption processes
Sieghard Weinketz
Instituto de Fisica `Gleb Wataghin'
Universidade Estadual de Campinas (UNICAMP)
Caixa Postal 6165, 13083-970 Campinas, Brazil
e.mail: weinketz@ifi.unicamp.br
Fax number: (55)(192) 39 3127
The applicability of the Dynamic Monte Carlo method of Fichthorn and
Weinberg, in which the time evolution of a system is described in terms of
the absolute number of different microscopic possible events and their
associated transition rates, is discussed for the case of thermal
desorption simulations. It is shown that the definition of the time
increment at each successful event leads naturally to the macroscopic
differential equation of desorption, in the case of simple first- and
second-order processes in which the only possible events are desorption
and diffusion. This equivalence is numerically demonstrated for a
second-order case. In the sequence, the equivalence of this method with
the Monte Carlo method of Sales and Zgrablich for more complex desorption
processes, allowing for lateral interactions between adsorbates, is shown,
even though the Dynamic Monte Carlo method does not bear their limitation
of a rapid surface diffusion condition, thus being able to describe a more
complex `kinetics' of surface reactive processes, and therefore be applied
to a wider class of phenomena, such as surface catalysis.
***
A nonlinear integral equation theory for the optical dielectric
properties of a polarisable fluid
Martyn Winn and Gerhard Kahl
Institut fuer Theoretische Physik, Technische Universitaet Wien,
Wiedner Hauptstrasse 8 - 10, A-1040 Wien, Austria
E-mail: winn@tph12.tuwien.ac.at
In a previous paper, we described a fast and reliable numerical
method for obtaining the optical absorption spectrum of a
fluid of non-polar linearly-polarisable molecules. The fluid is
modelled by a generalisation of the microscopic classical
Yvon-Kirkwood equations, which yields the same dynamic response as the
much-studied quantum Drude oscillator model. Numerical results
were presented based on a linear closure relation to
the central Ornstein-Zernike analogue equation. In the present paper,
we consider a nonlinear closure which includes but goes
beyond the previously-studied linear closure. We display the
absorption spectrum, as implied by the renormalised polarisability
and the dynamic dielectric constant, for both hard sphere and
Lennard-Jones fluids. Comparison with available simulation
results shows that the nonlinear closure performs well
over a wide density range, and in particular corrects
the poor low-density behaviour of the linear theory.
=============
Iosif Vaisman
UNC-Chapel Hill