193 research outputs found
Gas-liquid critical point in ionic fluids
Based on the method of collective variables we develop the statistical field
theory for the study of a simple charge-asymmetric primitive model (SPM).
It is shown that the well-known approximations for the free energy, in
particular DHLL and ORPA, can be obtained within the framework of this theory.
In order to study the gas-liquid critical point of SPM we propose the method
for the calculation of chemical potential conjugate to the total number density
which allows us to take into account the higher order fluctuation effects. As a
result, the gas-liquid phase diagrams are calculated for . The results
demonstrate the qualitative agreement with MC simulation data: critical
temperature decreases when increases and critical density increases rapidly
with .Comment: 18 pages, 1 figur
Statistical field theory for simple fluids: the collective variables representation
An alternative representation of an exact statistical field theory for simple
fluids, based on the method of collective variables, is presented. The results
obtained are examined from the point of another version of theory that was
developed recently by performing a Hubbard-Stratonovich transformation of the
configurational Boltzmann factor [J.-M. Caillol, Mol. Phys. 101 (2003) 1617].
The analytical expressions for the pressure and the free energy are derived in
two-loop approximation for both versions of theory and it is shown that they
are indeed equivalent.The results yield a new type approximation within an
untested approximation scheme
Ionic fluids: charge and density correlations near gas-liquid criticality
The correlation functions of an ionic fluid with charge and size asymmetry
are studied within the framework of the random phase approximation. The results
obtained for the charge-charge correlation function demonstrate that the
second-moment Stillinger-Lovett (SL) rule is satisfied away from the gas-liquid
critical point (CP) but not, in general, at the CP. However in the special case
of a model without size assymetry the SL rules are satisfied even at the CP.
The expressions for the density-density and charge-density correlation
functions valid far and close to the CP are obtained explicitely
Asymmetric Primitive-Model Electrolytes: Debye-Huckel Theory, Criticality and Energy Bounds
Debye-Huckel (DH) theory is extended to treat two-component size- and
charge-asymmetric primitive models, focussing primarily on the 1:1 additive
hard-sphere electrolyte with, say, negative ion diameters, a--, larger than the
positive ion diameters, a++. The treatment highlights the crucial importance of
the charge-unbalanced ``border zones'' around each ion into which other ions of
only one species may penetrate. Extensions of the DH approach which describe
the border zones in a physically reasonable way are exact at high and low
density, , and, furthermore, are also in substantial agreement with
recent simulation predictions for \emph{trends} in the critical parameters,
and , with increasing size asymmetry. Conversely, the simplest
linear asymmetric DH description, which fails to account for physically
expected behavior in the border zones at low , can violate a new lower bound
on the energy (which applies generally to models asymmetric in both charge and
size). Other recent theories, including those based on the mean spherical
approximation, have predicted trends in the critical parameters quite opposite
to those established by the simulations.Comment: to appear in Physical Review
Equilibrium solvation in quadrupolar solvents
We present a microscopic theory of equilibrium solvation in solvents with
zero dipole moment and non-zero quadrupole moment (quadrupolar solvents). The
theory is formulated in terms of autocorrelation functions of the quadrupolar
polarization (structure factors). It can be therefore applied to an arbitrary
dense quadrupolar solvent for which the structure factors are defined. We
formulate a simple analytical perturbation treatment for the structure factors.
The solute is described by coordinates, radii, and partial charges of
constituent atoms. The theory is tested on Monte Carlo simulations of solvation
in model quadrupolar solvents. It is also applied to the calculation of the
activation barrier of electron transfer reactions in a cleft-shaped
donor-acceptor complex dissolved in benzene with the structure factors of
quadrupolar polarization obtained from Molecular Dynamics simulations.Comment: Submitted to J. Chem. Phys., 20 pages and 13 figure
Thermo-statistical description of gas mixtures from space partitions
The new mathematical framework based on the free energy of pure classical
fluids presented in [R. D. Rohrmann, Physica A 347, 221 (2005)] is extended to
multi-component systems to determine thermodynamic and structural properties of
chemically complex fluids. Presently, the theory focuses on -dimensional
mixtures in the low-density limit (packing factor ). The formalism
combines the free-energy minimization technique with space partitions that
assign an available volume to each particle. is related to the
closeness of the nearest neighbor and provides an useful tool to evaluate the
perturbations experimented by particles in a fluid. The theory shows a close
relationship between statistical geometry and statistical mechanics. New,
unconventional thermodynamic variables and mathematical identities are derived
as a result of the space division. Thermodynamic potentials ,
conjugate variable of the populations of particles class with the
nearest neighbors of class are defined and their relationships with the
usual chemical potentials are established. Systems of hard spheres are
treated as illustrative examples and their thermodynamics functions are derived
analytically. The low-density expressions obtained agree nicely with those of
scaled-particle theory and Percus-Yevick approximation. Several pair
distribution functions are introduced and evaluated. Analytical expressions are
also presented for hard spheres with attractive forces due to K\^ac-tails and
square-well potentials. Finally, we derive general chemical equilibrium
conditions.Comment: 14 pages, 8 figures. Accepted for publication in Physical Review
Modelling colloids with Baxter's adhesive hard sphere model
The structure of the Baxter adhesive hard sphere fluid is examined using
computer simulation. The radial distribution function (which exhibits unusual
discontinuities due to the particle adhesion) and static structure factor are
calculated with high accuracy over a range of conditions and compared with the
predictions of Percus--Yevick theory. We comment on rigidity in percolating
clusters and discuss the role of the model in the context of experiments on
colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in
colloidal systems with short-range attractive forces", Messina, December
2003
Liquid-liquid equilibrium for monodisperse spherical particles
A system of identical particles interacting through an isotropic potential
that allows for two preferred interparticle distances is numerically studied.
When the parameters of the interaction potential are adequately chosen, the
system exhibits coexistence between two different liquid phases (in addition to
the usual liquid-gas coexistence). It is shown that this coexistence can occur
at equilibrium, namely, in the region where the liquid is thermodynamically
stable.Comment: 6 pages, 8 figures. Published versio
A diagrammatic formulation of the kinetic theory of fluctuations in equilibrium classical fluids. VI. Binary collision approximations for the memory function for self correlation functions
We use computer simulation results for a dense Lennard-Jones fluid for a
range of temperatures to test the accuracy of various binary collision
approximations for the memory function for density fluctuations in liquids. The
approximations tested include the moderate density approximation of the
generalized Boltzmann-Enskog memory function (MGBE) of Mazenko and Yip, the
binary collision approximation (BCA) and the short time approximation (STA) of
Ranganathan and Andersen, and various other approximations derived by us using
diagrammatic methods. The tests are of twotypes. The first is a comparison of
the correlation functions predicted by each approximate memory function with
the simulation results, especially for the self longitudinal current
correlation function (SLCC). The second is a direct comparison of each
approximate memory function with a memory function numerically extracted from
the correlation function data. The MGBE memory function is accurate at short
times but decays to zero too slowly and gives a poor description of the
correlation function at intermediate times. The BCA is exact at zero time, but
it predicts a correlation function that diverges at long times. The STA gives a
reasonable description of the SLCC but does not predict the correct temperature
dependence of the negative dip in the function that is associated with caging
at low temperatures. None of the other binary collision approximations is a
systematic improvement upon the STA. The extracted memory functions have a
rapidly decaying short time part, much like the STA, and a much smaller, more
slowly decaying part of the type predicted by mode coupling theory. Theories
that use mode coupling commonly include a binary collision term in the memory
function but do not discuss in detail the nature of that term. ...Comment: 18 pages, 10 figure
Simple Fluids with Complex Phase Behavior
We find that a system of particles interacting through a simple isotropic
potential with a softened core is able to exhibit a rich phase behavior
including: a liquid-liquid phase transition in the supercooled phase, as has
been suggested for water; a gas-liquid-liquid triple point; a freezing line
with anomalous reentrant behavior. The essential ingredient leading to these
features resides in that the potential investigated gives origin to two
effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur
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