Excess properties of aqueous mixtures of methanol: Simulation versus experiment
Diego Gonzales-Salgado and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
We report molecular simulation results for both the excess mixing
and partial molar properties of water-methanol mixtures over the
entire concentration range with the particular emphasis on the low
concentration ends.
It is shown that the mixing properties are very sensitive to
potential models and that the used realistic potentials (TIP4P for
water and OPLS for methanol) give a reasonably good agreement with
experiment only for volumetric properties although the qualitative
trend of the partial molar volume at low concentrations is not
reproduced. As regards excess enthalpy, the results are rather bad
and only its sign is predicted correctly.
2005
Potential of mean force between ions in infinitely diluted simple
short-range models of aqueous electrolytes
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
As an attempt to assess the effect of the long-range electrostatic
interactions in solutions of electrolytes, a simple short-range
model (SSM) of electrolytes made up of primitive water and
primitive ions (i.e., ions whose Coulombic interaction with water
has been replaced by a triangular-well interaction) has been
considered to compute the potential of mean force. The size of the
primitive ions has been set so as to approximate realistic NaCl,
LiI, and CsCl electrolytes. It is shown that despite the missing
long-range Coulombic interaction the model captures the basic
features of real electrolytes and the indirect, i.e. water
mediated, potential of mean force in the SSM is in qualitative
agreement with that of realistic models.
Effect of the range of interactions on the properties of fluids.
Part II. Structure and phase behavior of acetonitrile, hydrogen
fluoride, and formic acid
Ariel A. Chialvo, Matthias Kettler and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
To complete the study on the effect of the long-range part of
Coulombic interactions on properties of complex polar and
associating fluids, we have investigated in detail three compounds
with extreme features: acetonitrile for its unusually large dipole
moment, hydrogen fluoride with very strong hydrogen bonding, and
formic acid for its potential formation of different n-mers in
liquid and gaseous phases.
The effect of the long-range Coulombic interactions on both the
structure and thermodynamics of the homogeneous phase, and on the
vapor-liquid equilibria has been examined using the same
decomposition of realistic potential models into a short-range
part and a residual part as in the previous paper [M. Kettler et
al., J. Phys. Chem. B 106 (2002), 7537-7546].
The present results fully confirm the previous findings that the
properties of polar and associating systems are determined
primarily by the short-range interactions regardless of their
nature, i.e., contributions arising from the long-range
interactions constitute only a small portion of the total
properties, and thus that the short-range potential counterpart of
full realistic models can be used as a convenient reference for a
successful perturbation expansion.
From realistic to simple models of fluids III.
Primitive models of carbon dioxide, hydrogen sulphide, and
acetone, and their properties.
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Recently developed methodology to construct primitive models of
associating fluids as direct descendants of complex realistic
intermolecular potential functions (Vlvcek, L., and Nezbeda,
I., 2004, Molec. Phys., 102, 485) is extended to polar fluids and
applied to three substances of practical importance: quadrupolar
carbon dioxide, and dipolar hydrogen sulphide and acetone. It is
shown that the structural properties (in terms of the site-site
correlation functions) of the primitive models of polar fluids
reproduce very well those of their parent realistic ones but,
nonetheless, they perform worse than in the case of associating
fluids.
A number of thermodynamic properties of the developed models
obtained by computer simulations is also reported (for their later
use in theoretical investigations), and discussed.
Detection and characterization of structural changes in the hard-disk fluid under
freezing and melting conditions
Filip Moucka and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
The fluid of two-dimensional hard disks is investigated over a
range of densities by Monte Carlo simulations in order to detect
and characterize structural changes which take place when the
condition of freezing/melting is approached. A novel method is
proposed based on the use the Voronoi tessellation and a certain
shape factor which turns out to be a clear indicator of the
presence of different underlying sub-structures (domains). Close
to the freezing condition the probability distribution of the
shape factor develops a second distinct maximum corresponding to a
predominant presence of near-regular hexagons, whereas the
original peak, having its origin primarily in pentagons and
distorted hexagons, diminishes and disappears at melting density.
Abstract
It has been traditionally believed that, unlike normal fluids
whose {\it structural} properties are determined primarily by the
intermolecular
short-range repulsive interactions, the properties of polar and
associating fluids are strongly affected by the long-range Coulombic
interactions.
In the course of investigations to find the primary driving forces
governing the behavior of various (non-simple) fluids, and hence
to gain deeper understanding of molecular mechanisms leading to
development of theoretically-based simple models and theory,
extensive and systematic computer simulations have been performed
on typical quadrupolar (carbon dioxide), dipolar (acetone and
acetonitrile), and associating (hydrogen fluoride, methanol, and
water) fluids using available realistic effective pair potentials
and their variants involving forces of different range.
In addition to the main structural characteristics (one- and two-
dimensional
site-site correlation functions, local g-factors, and radial
slices through the full pair correlation function), dielectric
constant and the thermodynamic
properties (internal energy and pressure) of both the homogeneous
liquid and supercritical fluid
phases, and vapor-liquid equilibria have also been considered.
Furthermore, in the case of water, the
diffusion coefficient and viscosity have also been considered along
with water at interface.
All the obtained results lead to the unambiguous conclusion that
the structure, defined in terms of the complete set of the site-site
correlation functions, for both polar and associating pure fluids
is governed by the same molecular mechanism as for normal
fluids, i.e. by the short-range interactions (which however may be
both repulsive and attractive), whereas the long-range part of the
electrostatic forces, regardless of their strength, plays only a
marginal role and may be treated as a perturbation only. The
consequences of these findings for theory and applications are
also discussed.
2004
Thermophysical properties of fluids:
From realistic to simple models and their applications
Ivo Nezbeda and Lukas Vlcek
(back-to-list-of-papers)
Abstract
Recent systematic and extensive computer simulations on realistic
models of polar and associating fluids have shown that the
long-range interactions have only marginal effect on the
properties of these fluids. This finding leads to short-range
models upon which a perturbation theory may be developed.
An attempt to develop a methodology to construct such models
(called primitive models) directly from the parent realistic
models without resorting to (or reducing at least to minimum) {\it
ad hoc} adjustments is presented and exemplified by constructing
models of methanol, water, and carbon dioxide.
It is shown that the structural properties of the
primitive model fluids obtained by means of purely theoretical
considerations compare well with those of their realistic
counterparts.
Abstract
Using available realistic effective pair potentials and their variants
involving forces of different range, the role of the range of interactions
on the structure of pure polar and associating fluids has been investigated.
Systematic computer simulations performed lead to the unambiguous conclusion
that the structural properties of both types of fluids are governed by the
same molecular mechanism as normal fluids, i.e., by the short-range interactions,
whereas the long-range part of electrostatic forces, regardless of their strength,
plays only a marginal role and may be treated as a perturbation only.
Consequences of this finding and their limits are then shortly discussed.
Conformations of homopolymer chains and their phase behavior in a simple
supercritical solvent
Martin Lisal and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Using a simple molecular model and the configurational-bias Monte
Carlo method combined with the parallel tempering technique,
solvent driven changes in conformations of a homopolymer chain in
a simple supercritical solvent are systematically investigated.
The solvent is modelled as a square-well fluid, and two types of
chain are considered: the flexible chain of tangentially touching
(i) hard spheres (purely repulsive chain) and (ii) square-well
spheres (purely attractive chain). The mean square end-to-end
distance and radius of gyration are the main quantities computed
and used to characterize the changes in conformations in
dependence on the temperature and density of the solvent.
It is found that the attractive chain exhibits both the upper and
lower critical solution temperatures, whereas the repulsive chain
exhibits only the upper critical solution temperature.
Thermodynamics of simple models of associating fluids: Primitive models of
ammonia, methanol, ethanol, and water
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Thermodynamic P-V-T properties of primitive models that descend
directly from realistic Hamiltonians and reproduce the structure
of real fluids have been studied both by means of theory and
computer simulations. Analytic expressions for the Helmholtz free
energy of four typical associating fluids, ammonia, methanol,
ethanol, and water, have been derived using the thermodynamic
perturbation theory. Whereas for the models which allow only
single bonding of each site the first-order theory is sufficient,
for models in which some sites may form simultaneously up to two
bonds the theory has to be extended to the second order.
Comparison with simulation data shows that the theory is very
accurate and has therefore been used also to determine
vapor-liquid equilibria. We have found fundamental differences in
the behavior of different models; these differences are linked to
the properties of the hydrogen-bond network that are discussed in
detail.
From realistic to simple models of associating fluids. II.
Primitive models of ammonia, ethanol, and models of water revisited
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Recently developed methodology to construct primitive models of
associating fluids directly from realistic intermolecular
potential functions is applied to ammonia, ethanol, and several
models of water. Hard cores of the molecules are pictured as
fused-hard-sphere bodies defined by composite short-range
repulsions, and the Coulombic repulsions and attractions are
approximated by hard-sphere and square-well potentials,
respectively. Hard sphere diameters are determined directly from
the parent potential using a theoretical route and the range of
the square-well attraction is adjusted using constraints imposed
on hydrogen bonding. It is shown that the developed primitive
models, despite their simplicity and lack of any long-range
interactions, are able to reproduce the structural properties (the
set of the site-site correlation functions) of the parent
realistic models and may thus serve well as a reference in the
perturbation theory.
Vapor-liquid equilibria in five-site (TIP5P) models of water
Martin Lisal, Ivo Nezbeda, and William R. Smith
(back-to-list-of-papers)
Abstract
Using the Gibbs ensemble Monte Carlo simulations, vapor-liquid
equilibria in a new, reparametrized five-site model (TIP5P-E) of
water have been determined and compared with the original TIP5P
model of Mahoney and Jorgensen and TIP4P water. It is shown that
for vapor-liquid equilibria properties the new model provides only
a marginal improvement over the original model and both models are
considerably inferior to the TIP4P model.
On the calculation of the critical temperature from the second virial coefficient
Ivo Nezbeda and William R. Smith
(back-to-list-of-papers)
Abstract
A perturbed virial expansion (expansion in powers of density about
a suitable reference fluid) is used to locate the fluid critical
point. It is shown that, unlike the case for the usual virial
expansion, knowledge of the second virial coefficient alone is
sufficient to obtain a reasonably accurate estimate for the
critical temperature of both model and real fluids.
2003
Thermodynamic perturbation theory of the second-order:
Implementation for models with double-bonded sites
Lukas Vlcek, Jan Slovak and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Contributions of three classes of graphs contributing to the
second-order of the thermodynamic perturbation theory have been
evaluated for two types of qualitatively different primitive
models: (i) MeOH3 model of methanol, and (ii) EPM3 model of water.
It is shown that the contributions of linear chains, i.e. the
graphs satisfying the condition of steric incompatibility, bring
only a marginal improvement over the first-order theory. The most
significant contribution comes from the graph accounting for
double bonding of the oxygen site of the models. Neglecting the
linear chain diagrams and retaining only this graph we derive
general analytic expressions for the thermodynamic properties of
the considered models and find that the theory within this
approximation is in agreement with simulation data.
From realistic to primitive models: A primitive model of methanol
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
An attempt to develop a methodology to construct a primitive model
which descents directly from a parent realistic short-range model
and reproduces its structural properties of has been made. The
realistic three-site OPLS model of methanol has been chosen as a
test case. The primitive model copies geometry of the OPLS model
and pictures thus the methanol molecule as a hard heteronuclear
dumbbell (representing oxygen and carbon atoms) with one embedded
hydrogen site. All sites interact as hard spheres with the
exception of the oxygen-hydrogen pair which may form a hydrogen
bond mimicked by a square-well attraction.
To determine parameters of the model two routes have been followed:
(i) theoretical, based on an effective sphericalized site-site
potentials obtained from the parent potential, and (ii)
semi-theoretical which makes use of the knowledge of the structure
of the dense parent fluid.
Both sets of parameters provide similar results and reproduce the
structure (site-site correlation functions, distribution of
H-bonds, and H-bond geometry) of the parent OPLS fluid reasonably
well.
Thermophysical properties of fluids:
From realistic to primitive models and their application
Ivo Nezbeda and Lukas Vlcek
(back-to-list-of-papers)
Abstract
Recent systematic and extensive computer simulations on realistic
models of polar and associating fluids have shown that the
long-range interactions have only marginal effect on the
properties of these fluids. This finding leads to short-range
models upon which a perturbation theory may be developed.
An attempt to develop a methodology to construct such models
(called primitive models) directly from the parent realistic
models without resorting to (or reducing at least to minimum) {\it
ad hoc} adjustments is presented and exemplified by constructing
models of methanol, water, and carbon dioxide.
It is shown that the structural properties of the
primitive model fluids obtained by means of theoretical
considerations only compare well with those of their realistic
counterparts.
Conformations of Attractive, Repulsive, and Amphiphilic Polymer Chains
in a Simple Supercritical Solvent: Molecular Simulation Study
Martin Lisal and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Using the configurational-bias Monte Carlo method combined with
the parallel tempering technique, solvent driven changes in
polymer conformations in a supercritical solvent are
systematically investigated using simple molecular models. The
solvent is modelled as a square-well fluid, and the polymer is
made up of a flexible chain of tangentially touching hard spheres
and/or square-well spheres. The mean square end-to-end distance
and radius of gyration are computed for various ratios of
square-well and hard-sphere segments and a range of thermodynamic
conditions. The simulation results show that conformation behavior
of amphiphilic chains is rather complex and it is not a simple
combination of the conformation behavior of the attractive and
repulsive chains. The main finding is that the conformations of
amphiphilic chains is determined primarily by attractive
intermolecular interactions between the polymer segments and
molecules of the solvent.
On accuracy of Wertheim's thermodynamic perturbation theory for primitive models of water
Jan Slovak and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Applicability and accuracy of Wertheim's thermodynamic
perturbation theory (TPT) is examined in detail using three
qualitatively different primitive models of water. Theoretical
results for the thermodynamic properties of these models obtained
using the theory in its first order (TPT1) are presented and
compared with new simulation data. Individual approximations used
in the implementation of the TPT1 to get close analytic results
are also assessed by evaluating the appropriate quantities by
computer simulations. It is shown that an approximate description
of the reference system affects the overall performance of the
TPT1 only marginally, if at all, and that the observed
discrepancies between theory and experiment for all models are
rather inherent to the theory itself and result from the neglect
of higher order terms (i.e. certain graphs) contributing to the
excess Helmholtz free energy.
Modeling of aqueous electrolytes at a molecular level.
Simple short-range models and structure breaking and structure enhancement phenomena
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Three different short-range molecular models of infinitely diluted
solutions of electrolytes are considered to investigate structure
enhancement and structure breaking phenomena, and their link to
details of restructuring of the water molecules around the ion in
dependence on the charge. Occurence of the double maximum in entropy
is discussed with respect to both the spatial arrangement around
ions and re-orientation of the individual water molecules.
On independence of the solvation of interaction sites of a water molecule
Milan Predota, Arieh Ben-Naim, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
To support simplifying assumptions used in analytic theories of aqueous
systems, we have used computer simulations to examine correlations in the
bonding of the individual sites of a water molecule using two qualitatively
different extended primitive models, EPM4 and EPM5. We have studied these
correlations not only for the fully interacting water molecule (considered
as a solute) but also for a series of other solutes made from the water
molecule by turning off some of its interaction sites. We have found that
for the EPM5 solvent the local density of water molecules bound to a
specific site is independent of the state of the other sites being turned on
or off; for the EPM4 solvent such an independence does not hold exactly but
the correlations have been found to be very small. These facts fully justify
previously used speculative approximations for the calculation of the
solvation Helmholtz free energy of a water molecule, and lend also support
to the first order thermodynamic perturbation theory of Wertheim.
2002
An examination of the five-site potential (TIP5P) for water
Martin Lisal, Jiri Kolafa, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Parameterization of the five-site model (TIP5P) for water
[M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910
(2000)] has been examined by several computer simulation methods
accounting properly for long-range forces. The structural and
thermodynamic properties at a pressure of 1 atm over the
temperature range (-25C,+75C) and the
vapor-liquid coexistence have been determined. It is shown that
the simple spherical cutoff method used in the original
simulations to find optimized parameters of this five-site model
yields results that differ from those obtained by both the Ewald
summation and reaction field methods. Consequently, the pivot
property to which the parameters were adjusted, the location of
the density maximum at 1 atm, does not agree with experimental
values. The equilibrium properties then show only a fair agreement
with experimental data and are uniformly inferior to those of the
four-site TIP4P water over the entire coexistence range.
Effect of the range of interactions on the properties of fluids.
Phase equilibria in pure carbon dioxide, acetone, methanol, and water
Matthias Kettler, Ivo Nezbeda, Ariel Chialvo, and Peter T. Cummings
(back-to-list-of-papers)
Abstract
The effect of the long-range Coulombic interactions on the
vapor-liquid equilibria properties of polar and associating fluids
has been investigated, by considering typical representatives of
these classes of fluids, namely carbon dioxide, acetone, methanol, and
water, defined by realistic intermolecular pair potential models.
Using the same decomposition of realistic potential models into a
short-range part and a residual part as in previous papers
[Kolafa, J.; Nezbeda, I. Mol. Phys. 2000, 98,
1505-1520; Kolafa, J.; Nezbeda, I.; Lisal, M. Mol.
Phys. 2001, 99, 1751-1764], we carried out Gibbs
ensemble simulations on both the full and short-range models to
determine the thermodynamic properties of the considered compounds
along the vapor-liquid coexistence curve. In addition, we also
considered methanol in two homogeneous phases, liquid and
supercritical, to determine its structure and thermodynamic
properties. We have found that the long-range interactions affect
all considered properties only marginally and that the short-range
system provides a reasonably accurate and reliable zeroth-order
approximation. A simple theoretical analysis has also been made to
explain and estimate the effect of the long-range interactions on
the thermodynamic properties both in the homogeneous phase and at
phase equilibrium.
Size and shape dependence of the hydrophobic hydration
at the level of primitive models
Lukas Vlcek and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
The hydrophobic hydration of apolar solutes of different shapes and
size is studied at an elementary level using two types of extended
primitive models of water and representing the solute by hard spheres,
cylinders, and spherocylinders, respectively. The structure of the first
hydration shell around cylindrical particles is determined and compared
to that around spherical ones and at a hard structurless flat wall. It
is found that while the two studied models of water give the same
hydration structure for small nonpolar particles, larger solutes are
hydrated in a different way.
Whereas one model does not show significant size and shape dependence,
the other predicts significant changes in the orientation and hydrogen
bonding of water molecules in the vicinity of the hydrophobic surface.
These results are in agreement with those found for spherical solutes
and confirm sensitivity of hydration phenomena to details of models of
water, particularly to the strength and geometry of hydrogen bonding.
Hydrophobic hydration at the level of primitive models. II.
Large solutes and water restructuring
Milan Predota, Ivo Nezbeda, and Petr T. Cummings
(back-to-list-of-papers)
Abstract
Details of structural changes that take place in water near an apolar solute
have been studied by Monte Carlo simulations for hard sphere solutes of
increasing size including the limiting case of water at a hard structureless
wall. Water has been modeled by two different types of extended primitive
models, the four-site EPM4 model and five-site EPM5 model. Two different
patterns of the orientational ordering of the water molecules around the
solute as a function of its size have been found. For the EPM5 model, the
structure of water and the orientation of its molecules near an apolar
solute of a finite diameter do not seem to be sensitive to the size of the
solute, and become more pronounced only when the solute becomes a hard wall.
On the contrary, the orientation ordering of the EPM4 molecules gradually
changes with increasing size of the solute and for solutes larger than,
approximately, five times the size of the water molecule it is opposite to
that near a small solute. A novel method to evaluate the excess chemical
potential of large solutes has been implemented and some thermodynamic
quantities for water (distribution of hydrogen bonds and the excess chemical
potential) have also been computed as a function of the distance from the
solute.
2001
Can we understand (and model) aqueous solutions without
any long-range electrostatic interactions?
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
A computer simulation experiment is conducted to study to what extent
long-range Coulombic interactions are indispensable when modeling
aqueous solutions of electrolytes. A simple molecular model, which
accounts explicitly for the molecular structure of water but which does
not incorporate any long-range Coulombic interactions, is employed. The
solvent is primitive water (EPM5-4 model) and the solute molecules are
hard spheres interacting with the interaction sites of the water
molecule by means of either repulsive (like-charge interaction) or
attractive (unlike-charge interaction) short-ranged triangular-well
tails. The structural changes (hydrophobic ordering, structure
breaking, and structure enhancement) which take place in an infinitely
dilute solution upon `charging' the solute are studied, in terms of the
correlation functions and of the orientational distribution functions
and of the average binding energy of the water molecules around the
solute in terms of their dependence on the solute-water oxygen
distance. The main thermodynamic property reflecting these changes is the
residual entropy. This quantity is found to exhibit an asymmetric
double maximum, in agreement with the findings for a realistic
counterpart of this simple model that employs long-ranged Coulombic
interactions.
Effect of short- and long-range forces on the properties of fluids.
III. Dipolar and quadrupolar fluids
Jiri Kolafa, Ivo Nezbeda, and Matin Lisal
(back-to-list-of-papers)
Abstract
Using realistic pair potential models for acetone and carbon dioxide,
both the spatial and orientational structure of these two typical multipolar
(i.e. dipolar and quadrupolar, resp.) fluids is investigated in detail
by computing the complete set of the site-site correlation functions,
multipole-multipole correlation functions, and selected 2D correlation
functions. The effect of the range of interactions on both the
structural and thermodynamic properties of these fluids is studied by
decomposing the potential into short- and long-range parts in the same
manner as for water [Kolafa, J., and Nezbeda, I., 2000, Molec. Phys.,
98, 1505; Nezbeda, I., and L\'{\i}sal, M., 2001, Molec. Phys., 99, 291].
It is found that the spatial arrangement of the molecules is only
marginally affected by the long-range forces. The effect of the
electrostatic interactions is significant at short separations and
cannot be neglected but nevertheless the overall structure of the
short-range and full systems is similar as well as their dielectric
constants.
These findings are also reflected in the dependence of the thermodynamic
properties on the potential range with the short-range models providing
a very good approximation to those of the full system.
A molecular-based theory for the thermodynamic properties
of water
Ulrike Weingerl and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Following the rules set up by molecular theories of fluids, a
perturbed form of the Helmholtz free energy for water has been
developed. The reference term corresponds to short-range water and is
approximated by the properties of a primitive model; the perturbation
term is given by contributions of the dispersion forces and the
dipole-dipole interaction. The used method is first verified by
applying it to TIP4P water and then used for real water without
reference to any specific potential. The parameters of the model are
determined in order to obtain the best representation of the vapor
pressure and coexistence liquid densities from the triple point to
643.15K; no attempt is made to fit the critical region. Despite a number
of approximations employed, the accuracy of the equation of state is
comparable to that of the modified Redlich-Kwong-Soave equation and
SAFT-Yukawa-dipole-dipole equation, and considerably better than
accuracy of SAFT-HS and SAFT-VR equations.
Because of its true molecular footing, the equation remains reliable
also for various thermodynamic properties outside the coexistence
region. It reproduces the anomaly in the isothermal compressibility
locating its minimum at T=38 C (versus the experimental value
T=46 C) for P=1 bar. It also predicts a density maximum but
outside the experimental temperature range (at temperatures below the
triple point temperature).
On molecular-based equations of state: Rigor versus
speculations
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
A general scheme for developing any semi-empirical molecular-based
equation of state is formulated along with several rules which reflect
the essentials of physics of fluids and which should be observed.
Approximations and simplifications used in the implementation of the
scheme are analyzed in the light of these rules and examples showing
superiority of molecular-based considerations over purely intuitive or
empirical ones are presented.
On dispersion force correction terms in perturbed equations of state
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
The first-order correction term for the contribution of dispersion
forces in a perturbation expansion has been analyzed using computer
simulation and perturbation theory results. It turns out that simple
approximations used to make an analytic evaluation of the correction
integral possible may result, ironically, in more complicated (and
erroneous) behavior in comparison with the exact result.
Effect of short- and long-range forces on the thermodynamic
properties of water. A simple short-range reference system
Ivo Nezbeda and Martin Lisal
(back-to-list-of-papers)
Abstract
Three realistic potential models of water, the non-polarizable ST2 and
TIP4P models, and the polarizable TIP4P/P model, were used in computer
simulations to study the effect of the range of intermolecular
interactions on the thermodynamic properties of water. Following the
results of recent studies, a short-range system is constructed to the
full pair potential u(1,2) in such a way that a perturbation
expansion can be formulated in powers of the dipole-dipole interaction
only. Computations of low density properties and computer simulations
performed for several densities on three subcritical and one
supercritical isotherms show that the short-range reference reproduces
not only the structure but approximates also the internal energy and
pressure of water surprisingly well. Differences in the internal energy
between the full and short-range water do not exceed 5 per cent for all
models used over the entire range of the considered thermodynamic
conditions.
Accurate vapor-liquid equilibrium calculations for complex systems using
the reaction Gibbs ensemble Monte Carlo simulation method
Martin Lisal, William R. Smith, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
The Reaction Gibbs Ensemble Monte Carlo computer simulation method
[M. L{\'{\i}}sal, W.R. Smith, I. Nezbeda, J. Phys. Chem. B 103
(1999) 10496-10505] is used to predict the vapour-liquid
equilibrium (VLE) behaviour of binary mixtures involving water,
methanol, ethanol, carbon dioxide, and ethane. All these mixtures
contain molecularly complex substances, and accurately predicting
their VLE behaviour is a considerable challenge for
molecular-based approaches, as well as for traditional engineering
approaches. The substances are modeled as multi-site Lennard-Jones
plus Coulombic potentials with standard mixing rules for unlike
site interactions. No adjustable binary-interaction parameters and
no mixture experimental properties are used in the calculations;
only readily-available pure-component vapour-pressure data are
required. The simulated VLE predictions are compared with
experimental results and with those of two typical semi-empirical
macroscopic-level approaches. These latter are the UNIFAC
liquid-state activity-coefficient model combined with the simple
truncated virial equation of state, and the hole quasi-chemical
group contribution equation of state. The agreement of the
simulation results with the experimental data is generally good
and also comparable with and in some cases better than, those of
the macroscopic-level empirical approaches.
Coexistence properties of higher n-alkanes modelled as Kihara fluids:
Gibbs ensemble simulations
Matthias Kettler, Horst L. Voertler, Ivo Nezbeda, and Martin Strnad
(back-to-list-of-papers)
Abstract
The coexistence vapour-liquid properties of higher n-alkanes of large
anisotropy (pentane, decane, and pentadecane) modelled by rod-like
Kihara fluids were determined using Gibbs-ensemble (GE) and extended
Gibbs ensemble (EGE) Monte Carlo simulations.
We found that for dense, low temperature states the EGE becomes more
accurate and efficient than GE. Comparison of the simulation results with
those of a second-order perturbation theory shows that, surprisingly,
the theory performs reasonably well for models of large elongations
(pentadecane) but disagrees considerably for the model of pentane.
2000
Computer simulation of the thermodynamic properties of
high-temperature chemically-reacting plasmas
Martin Lisal, William R. Smith, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
The Reaction Ensemble Monte Carlo (REMC) computer simulation
method [W.R. Smith and B. Triska, J. Chem. Phys. 100, 3019 (1994)]
is employed to predict the thermodynamic
behavior of chemically reacting plasmas using a molecular-level
model based on the underlying atomic and ionic interactions.
Unlike previous plasma simulation studies, which were restricted
to fairly simple systems of fixed composition, the REMC approach
is able to take into account the effects of the ionization
reactions. In the context of the specified molecular model, the
computer simulation approach gives an essentially exact
description of the system thermodynamics. We develop and apply the
REMC method for the test case of a helium plasma. We calculate
plasma compositions, molar enthalpies, molar volumes, molar heat
capacities, and coefficients of cubic expansion over a range of
temperatures up to 100,000 K and pressures up to 400 MPa. We
elucidate the contributions of the Coulombic forces,
ionization-potential lowering, and short-ranged interactions to
the thermodynamic properties. We compare the results with those
obtained using macroscopic-level thermodynamic approximations,
including the ideal-gas (IG) and the Debye-Hueckel (DH)
approaches. For the helium plasma, the short-ranged forces are
found to be relatively unimportant, but we expect these to be
important for molecular systems. The DH theory is always more
accurate than the IG approximation. The DH theory yields
compositions that slightly underpredict the overall degree of
ionization. For the molar heat capacity and the coefficient of
cubic expansion, the DH theory is accurate at lower pressures, but
at 400 MPa yields results that are up to 40% in error for the
molar heat capacity.
Parallelized sampling of the Gibbs ensemble
Martin Strnad and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
An expression for the probability distribution of NVT-like
sub-ensembles constituting the Gibbs ensemble is derived. Knowledge of
this distribution makes it possible to carry out the simulation without
the explicit exchange of real particles between the simulation boxes
and to evaluate directly any Gibbs ensemble average from a series of
independent simultaneous simulations (Monte Carlo or molecular
dynamics) performed on a set of NVT-like sub-ensembles with the fixed
distribution of particles. An implementation of the method, which is
tailored mainly for complex systems, is exemplified for the square-well
fluid, and its efficiency and results are compared with those obtained
from the conventional Gibbs ensemble simulations.
Effect of short- and long-range
forces on the structure of water. II. Orientational
ordering and the dielectric constant.
Jiri Kolafa and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Effect of short- and long-range interactions on the structure
of water, both spatial and orientational, has been studied in detail by
computing the full pair correlation function, site-site correlation
functions, two-dimensional site-site correlation functions in the
(rOO, rOH)
and (rOO, rHH) planes,
dipole-dipole correlation function, running Kirkwood g-factor, and
the dielectric constant. Two model potentials, the TIP4P and ST2
models, and their short-range versions have been considered at ambient,
elevated, and supercritical conditions.
An analysis of the results shows that although all site-site
correlation functions for the short- and long-range systems are very
similar, the orientational ordering in systems of different range may
be considerably different, this evidence being provided mainly by the
dipole-dipole correlation function: The orientational ordering is only
short-range in long-range systems, whereas in short-range systems the
hydrogen bonding gives rise to a damped long-range regular pattern of
alignment.
Nonetheless, the resulting dielectric constants for the short- and
long-range systems are almost the same.
All findings are more pronounced at low temperatures but they are
otherwise only marginally temperature and density dependent.
Solubility of apolar fluids in water:
A simple molecular model and theory
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
A simple molecular model of dilute aqueous solutions based on an
extended primitive model of water is examined.
Theoretical calculations of solubility of non-polar fluids and
related properties do not make use of any a priori knowledge of
the properties of water and do not resort to computer simulation
results.
It is shown that the model reproduces correctly characteristic
temperature dependencies associated with hydrophobic hydration and
consequently, unlike existing equations of state, also a maximum of the
Henry's law constant at low temperatures. These results are analyzed in
detail and for better understanding compared with those for a simple
fluid solvent.
Molecular Simulation of Multicomponent Reaction and Phase
Equilibria: Reaction-Ensemble Monte Carlo (REMC) Simulation of the
MTBE Ternary System
Martin Lisal, William R. Smith, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Reaction and phase equilibrium in the Isobutene+Methanol+MTBE
ternary system is studied using the Reaction Ensemble Monte Carlo
(REMC) simulation method; the system is modeled at the molecular
level by an OPLS force field. No adjustable binary
cross-interaction parameters or mixture data of any kind are used
in the simulation model, and only vapor pressure data for the pure
components is required as input. The REMC method also computes
excess internal energies and molar volumes as a byproduct of the
simulations. We consider both the non-reacting and the reacting
ternary system over the temperature range of practical interest at
the pressure 5 bar. Our results are compared with our calculations
using two conventional thermodynamic approaches: the Wilson and
the UNIFAC free-energy models for the liquid phase, together with
a truncated virial equation of state for the gas phase in both
cases. We show that our computer simulation results are similar to
those of the thermodynamic approaches, and we argue that they are
likely more accurate.
1999
The Accurate Computer Simulation of Phase Equilibrium
for Complex Fluid Mixtures.
Application to Binaries Involving
isobutene, methanol, MTBE, and n-butane
Martin Lisal, William R. Smith, and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
We have developed a new method, called the Reaction Gibbs Ensemble
Monte Carlo (RGEMC) method, for the computer simulation of the
phase equilibria for multicomponent mixtures, given an
intermolecular potential model for the constituent molecular
species. The approach treats the phase equilibrium conditions as a
special type of chemical reaction, and incorporates knowledge of
the pure-substance vapor pressure data into the simulations.
Unlike macroscopic thermodynamic-based approaches like the Wilson
and the UNIFAC approximations, no experimental information
concerning the mixtures is required. In addition to the PTxy
phase equilibrium data, the volumetric properties of the mixture
are calculated. We developed intermolecular potential models based
on the OPLS potential models of Jorgensen, and used the RGEMC
method to predict phase equilibrium data for the binary systems
isobutene+MTBE and the binaries formed by methanol with isobutene,
MTBE, and n-butane. The predictions are excellent, and of
comparable accuracy to those obtained using the Wilson and the
UNIFAC thermodynamic-based approaches, even though such approaches
use experimental mixture information.
Hydrophobic hydration at the level of primitive models
Milan Predote and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Structural changes which take place in bulk water when a molecule of an
apolar solute is brought therein are studied at a very basic elementary
level. Water is modelled by two different types of extended primitive models
and solute is a hard sphere.
The model mixture, which does not incorporate any water-solute
interaction but the hard-core repulsion and is free of any adjustable
parameter, is shown to predict the site-site correlation functions and
details of the orientational arrangement of water molecules in the first
hydration shell in full agreement with results obtained using complex
realistic potentials.
Abstract
A general extended Gibbs ensemble, obtained by augmenting the standard
Gibbs ensemble by intermediate states in the spirit of the scaled
particle method of Nezbeda and Kolafa [Molec. Simul., 5, 391
(1991)], is introduced. The intermediate states span the states with
different number of particles in the simulation boxes and facilitate
the transfer of particles even in such complex systems as eg
mixtures of very different components, systems of flexible polymeric
molecules, or systems at very high densities.
A general formulation of the ensemble is given and two implementations
are considered in detail.
The method is very general and is exemplified by studying the
fluid-fluid coexistence in a dense binary mixture of the hard-sphere and
square-well fluids. It is found that its efficiency is about by factor
three greater in comparison with the standard Gibbs ensemble
simulations.
Pure fluids of homo- and hetero-nuclear
square-well diatomics. I. Computer simulation study
Martin Lisal and Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Pure fluids of both homo- and hetero-nuclear square-well diatomics have
been studied using Monte Carlo simulations in NVT and Gibbs ensembles.
For the homonuclear models, three values of the elongation have been
considered to complement the existing data for the range of
the square-well interaction lambda=1.5.
The same range has been considered also for
the hetero-nuclear models whose heterogeniety arise from
(i) the difference in the
hard-sphere diameters, and (ii) the difference in the strength of the
site-site interactions.
The computed structural properties include the complete set of the
site-site correlation functions and their low density limits. The
thermodynamic properties include, in addition to the equation of state
and internal energy on three isotherms,
and the 2nd and 3rd virial coefficients, also the
coexistence properties and an approximate location of the critical point.
The reaction ensemble method for the computer simulation of chemical and
phase equilibria: II. The Br2+Cl2+BrCl system
Martin Lisal, Ivo Nezbeda, and William R. Smith
(back-to-list-of-papers)
Abstract
The Reaction Ensemble Monte Carlo (REMC) method [W.R. Smith and
B. Triska, J. Chem. Phys., 100, 3019 (1994)]
is used to study combined reaction
and vapor-liquid equilibrium of the Br2 + Cl2 + BrCl system.
The substances are modelled as nonpolar and dipolar two-site
Lennard-Jones molecules with Lorentz-Berthelot mixing rules for
unlike atoms. No parameters were fitted to any mixture properties in our
calculations.
The simulated data are compared with
experimental results and with previous
simulation data for the mixture, obtained by an indirect semi-grand ensemble approach.
The REMC method calculates the complete phase compositions, whereas only
a limited subset is available experimentally.
The agreement of the simulations with experiment is excellent.
In the course of this work, we used the Gibbs Ensemble Monte Carlo method
to calculate the vapor-liquid equilibrium properties of BrCl;
since this compound is chemically unstable,
such data is experimentally inaccessible.
Fluid-solid boundary of the compressed EXP-6 fluids
Martin Lisal, Ivo Nezbeda, and H. L. Voertler
(back-to-list-of-papers)
Abstract
Using recent computer simulation data and the principle of
corresponding states, the boundary of stability of the
compressed supercritical EXP-6 potential fluids is localized for an
arbitrary potential parameter alpha. All
existing supercritical simulation data of the EXP-6 fluids are then
analyzed and assessed with respect to their potential use for
derivation of an equation of state of the compressed EXP-6 fluids.
Global phase diagrams of model and real binary fluid mixtures. II. Non-Lorentz-Berthelot mixture of attractive hard spheres.
Kolafa J., Nezbeda I., Pavlicek J., Smith W. R.
Abstract
The global phase diagram of a binary mixture of attractive hard spheres
described by the Boublik-Mansoori-Carnahan-Starling-Leland equation of
state with a van der Waals mean field attractive term is investigated in
its dependence on the ratio of the hard-sphere diameters and the
strength of the attraction. Two values of the energetic combining rule
parameter k12, determining the deviation from the geometric mean
rule, are considered. In addition to phenomena reported recently for
k12=1, the shield region of quadruple points is investigated for
k12=0.8, along with its associated azeotropic phenomena. For
k12=1.15, the mixture exhibits a rich set of low-temperature
azeotropic phenomena: cusps on the p--T projections of azeotropic lines,
exchange of branches of azeotropic lines, zero-temperature limited
azeotropy, and double critical azeotropic points. Some of these
azeotropic phenomena are described here for the first time.
Global phase diagrams
of model and real binary fluid mixtures:
Lorentz-Berthelot mixture of attractive hard spheres
Jiri Kolafa, Ivo Nezbeda, Jan Pavlicek, and William R. Smith
(back-to-list-of-papers)
Abstract
The phase behavior and global phase diagram of binary mixtures
of attractive hard spheres described by a hard-sphere mixture equation of
state with a mean field term and the Lorentz-Berthelot combining rule
are examined in detail in dependence on the ratio of (i) the hard-sphere
diameters and (ii) the strength of the mean field attraction.
It is shown that in addition to the usual phenomena the studied mixture
exhibits also the closed liquid-liquid immiscibility loop (Type VI and
VII behavior) and a variety of new azeotropic phenomena.
Topology of phase diagrams is discussed in detail with emphasis on the
boundary states and is compared with that based on the one-fluid van
der Waals equation approach.
Global phase behavior of model mixtures of water and n-alkanols
I. Nezbeda, J. Pavlicek, J. Kolafa, A. Galindo, and G. Jackson
(back-to-list-of-papers)
Abstract
The global phase diagram of model binary mixtures of water and
n-alkanols has been determined using the SAFT equation of state. Closed
regions in the two-dimensional parameter space (the strength of the
hydrogen bond of alkanol versus the length of the alkanol molecule)
where azeotropy and closed liquid-liquid immiscibility loops occur have
been found. When the diagram is transformed to the space of relative
critical temperatures and volumes, no real aqueous mixture of alkanol
lies in these regions; the disagreement is caused by the choice of
parameters in this work.
REVIEW PAPERS
Simple short-ranged models of water and their application. A review
Ivo Nezbeda
(back-to-list-of-papers)
Abstract
Simple molecular (statistical mechanical) models of water and their
properties, with the emphasis on the class of the so called `primitive
models', are reviewed. These models are based on the finding that the
structure of water is determined primarily by short-ranged (both
repulsive and attractive) forces and do
not incorporate thus any long-ranged electrostatic interactions.
Considerable attention is paid to the physical footing of the models and
to their potential in applications and for developing a first-principle
theory of water and aqueous solutions of nonelectrolytes.
