23 research outputs found
Gas-liquid critical parameters of asymmetric models of ionic fluids
The effects of size and charge asymmetry on the gas-liquid critical
parameters of a primitive model (PM) of ionic fluids are studied within the
framework of the statistical field theory based on the collective variables
method. Recently, this approach has enabled us to obtain the correct trends of
the both critical parameters of the equisize charge-asymmetric PM without
assuming ionic association. In this paper we focus on the general case of an
asymmetric PM characterized by the two parameters: hard-sphere diameter-,
and charge, , ratios of the
two ionic species. We derive an explicit expression for the chemical potential
conjugate to the order parameter which includes the effects of correlations up
to the third order. Based on this expression we consider the three versions of
PM: a monovalent size-asymmetric PM (, ), an equisize
charge-asymmetric PM (, ) and a size- and charge-asymmetric
PM (, ). Similar to simulations, our theory predicts that
the critical temperature and the critical density decrease with the increase of
size asymmetry. Regarding the effects of charge asymmetry, we obtain the
correct trend of the critical temperature with , while the trend of the
critical density obtained in this approximation is inconsistent with
simulations, as well as with our previous results found in the higher-order
approximation. We expect that the consideration of the higher-order
correlations will lead to the correct trend of the critical density with charge
asymmetry.Comment: 23 pages, 6 figure
Spatial inhomogeneities in ionic liquids, charged proteins and charge stabilized colloids from collective variables theory
Effects of size and charge asymmetry between oppositely charged ions or
particles on spatial inhomogeneities are studied for a large range of charge
and size ratios. We perform a stability analysis of the primitive model (PM) of
ionic systems with respect to periodic ordering using the collective variables
based theory. We extend previous studies [A. Ciach et al., Phys. Rev.E
\textbf{75}, 051505 (2007)] in several ways. First, we employ a non-local
approximation for the reference hard-sphere fluid which leads to the
Percus-Yevick pair direct correlation functions for the uniform case. Second,
we use the Weeks-Chandler-Anderson regularization scheme for the Coulomb
potential inside the hard core. We determine the relevant order parameter
connected with the periodic ordering and analyze the character of the dominant
fluctuations along the -lines. We show that the above-mentioned
modifications produce large quantitative and partly qualitative changes in the
phase diagrams obtained previously. We discuss possible scenarios of the
periodic ordering for the whole range of size- and charge ratios of the two
ionic species, covering electrolytes, ionic liquids, charged globular proteins
or nanoparticles in aqueous solutions and charge-stabilized colloids
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
Field-theoretic description of ionic crystallization in the restricted primitive model
Effects of charge-density fluctuations on a phase behavior of the restricted
primitive model (RPM) are studied within a field-theoretic formalism. We focus
on a -line of continuous transitions between charge-ordered and
charge-disordered phases that is observed in several mean-field (MF) theories,
but is absent in simulation results. In our study the RPM is reduced to a
theory, and a fluctuation contribution to a grand thermodynamic
potential is obtained by generalizing the Brazovskii approach. We find that in
a presence of fluctuations the -line disappears. Instead, a
fluctuation-induced first-order transition to a charge-ordered phase appears in
the same region of a phase diagram, where the liquid -- ionic-crystal
transition is obtained in simulations. Our results indicate that the
charge-ordered phase should be identified with an ionic crystal.Comment: 31 pages, 10 figure
Field theory for size- and charge asymmetric primitive model of electrolytes. Mean-field stability analysis and pretransitional effects
The primitive model of ionic systems is investigated within a field-theoretic
description for the whole range of size-, \lambda, and charge, Z, ratios of the
two ionic species. Two order parameters (OP) are identified, and their
relations to physically relevant quantities are described for various values of
\lambda and Z. Instabilities of the disordered phase associated with the two
OP's are determined in the mean-field approximation.
A gas-liquid separation occurs for any Z and \lambda different from 1. In
addition, an instability with respect to various types of periodic ordering of
the two kinds of ions is found
Ab initio study of the vapour-liquid critical point of a symmetrical binary fluid mixture
A microscopic approach to the investigation of the behaviour of a symmetrical
binary fluid mixture in the vicinity of the vapour-liquid critical point is
proposed. It is shown that the problem can be reduced to the calculation of the
partition function of a 3D Ising model in an external field. For a square-well
symmetrical binary mixture we calculate the parameters of the critical point as
functions of the microscopic parameter r measuring the relative strength of
interactions between the particles of dissimilar and similar species. The
calculations are performed at intermediate () and moderately long
() intermolecular potential ranges. The obtained results agree
well with the ones of computer simulations.Comment: 14 pages, Latex2e, 5 eps-figures included, submitted to
J.Phys:Cond.Ma