1,846 research outputs found
Effects of weak surface fields on the density profiles and adsorption of a confined fluid near bulk criticality
The density profile and Gibbs adsorption of a near-critical fluid confined
between two identical planar walls is studied by means of
Monte Carlo simulation and by density functional theory for a Lennard-Jones
fluid. By reducing the strength of wall-fluid interactions relative to
fluid-fluid interactions we observe a crossover from behaviour characteristic
of the normal surface universality class, strong critical adsorption, to
behaviour characteristic of a 'neutral' wall. The crossover is reminiscent of
that which occurs near the ordinary surface transition in Ising films subject
to vanishing surface fields. For the 'neutral' wall the density profile, away
from the walls, is almost constant throughout the slit capillary and gives rise
to an adsorption that is constant along the critical isochore. The same
'neutral' wall yields a line of capillary coexistence that is almost identical
to the bulk coexistence line. In the crossover regime we observe features in
the density profile similar to those found in the magnetisation profile of the
critical Ising film subject to weak surface fields, namely two smooth maxima,
located away from the walls, which merge into a single maximum at midpoint as
the strength of the wall-fluid interaction is reduced or as the distance
between walls is decreased. We discuss similarities and differences between the
surface critical behaviour of fluids and of Ising magnets.Comment: 34 pages, 10 figures, submitted to the Journ. Chem. Phy
Liquid-vapor interface of a polydisperse fluid
We report a Grand Canonical Monte Carlo simulation study of the liquid-vapor
interface of a model fluid exhibiting polydispersity in terms of the particle
size . The bulk density distribution, , of the system
is controlled by the imposed chemical potential distribution . We
choose the latter such that assumes a Schulz form with
associated degree of polydispersity . By introducing a smooth
attractive wall, a planar liquid-vapor interface is formed for bulk state
points within the region of liquid-vapor coexistence. Owing to fractionation,
the pure liquid phase is enriched in large particles, with respect to the
coexisting vapor. We investigate how the spatial non-uniformity of the density
near the liquid-vapor interface affects the evolution of the local distribution
of particle sizes between the limiting pure phase forms. We find (as previously
predicted by density functional theory, Bellier-Castella {\em et al}, Phys.
Rev. {\bf E65}, 021503 (2002)) a segregation of smaller particles to the
interface. The magnitude of this effect is quantified for various via
measurements of the relative adsorption. Additionally, we consider the utility
of various estimators for the interfacial width and highlight the difficulties
of isolating the intrinsic contribution of polydispersity to this width.Comment: 9 pages, 10 Fig
Freezing line of the Lennard-Jones fluid: a Phase Switch Monte Carlo study
We report a Phase Switch Monte Carlo (PSMC) method study of the freezing line
of the Lennard-Jones (LJ) fluid. Our work generalizes to soft potentials the
original application of the method to hard sphere freezing, and builds on a
previous PSMC study of the LJ system by Errington (J. Chem. Phys. {\bf 120},
3130 (2004)). The latter work is extended by tracing a large section of the
Lennard-Jones freezing curve, the results for which we compare to a previous
Gibbs-Duhem integration study. Additionally we provide new background regarding
the statistical mechanical basis of the PSMC method and extensive
implementation details.Comment: 18 pages, 6 figure
Three-body interactions in complex fluids: virial coefficients from simulation finite-size effects
A simulation technique is described for quantifying the contribution of
three-body interactions to the thermodynamical properties of coarse-grained
representations of complex fluids. The method is based on comparing the third
virial coefficient for a complex fluid with that of an approximate
coarse-grained model described by a pair potential. To obtain we
introduce a new technique which expresses its value in terms of the measured
volume-dependent asymptote of a certain structural function. The strategy is
applicable to both Molecular Dynamics and Monte Carlo simulation. Its utility
is illustrated via measurements of three-body effects in models of star polymer
and highly size-asymmetrical colloid-polymer mixtures.Comment: 13 pages, 8 figure
Review of Model Defamation Provisions – Discussion Paper: Submission to Council of Attorneys-General
Liquid-gas coexistence and critical point shifts in size-disperse fluids
Specialized Monte Carlo simulations and the moment free energy (MFE) method
are employed to study liquid-gas phase equilibria in size-disperse fluids. The
investigation is made subject to the constraint of fixed polydispersity, i.e.
the form of the `parent' density distribution of the particle
diameters , is prescribed. This is the experimentally realistic
scenario for e.g. colloidal dispersions. The simulations are used to obtain the
cloud and shadow curve properties of a Lennard-Jones fluid having diameters
distributed according to a Schulz form with a large (40%) degree of
polydispersity. Good qualitative accord is found with the results from a MFE
method study of a corresponding van der Waals model that incorporates
size-dispersity both in the hard core reference and the attractive parts of the
free energy. The results show that polydispersity engenders considerable
broadening of the coexistence region between the cloud curves. The principal
effect of fractionation in this region is a common overall scaling of the
particle sizes and typical inter-particle distances, and we discuss why this
effect is rather specific to systems with Schulz diameter distributions. Next,
by studying a family of such systems with distributions of various widths, we
estimate the dependence of the critical point parameters on . In
contrast to a previous theoretical prediction, size-dispersity is found to
raise the critical temperature above its monodisperse value. Unusually for a
polydisperse system, the critical point is found to lie at or very close to the
extremum of the coexistence region in all cases. We outline an argument showing
that such behaviour will occur whenever size polydispersity affects only the
range, rather than the strength of the inter-particle interactions.Comment: 14 pages, 12 figure
Enthalpies of formation of lanthanide oxyapatite phases
A family of lanthanide silicates adopts an oxyapatite-like structure with structural formula Ln9.33∎0.67(SiO4)6O2 (Ln 4 La, Sm, Nd, Gd, ∎ = vacancy). The enthalpies of solution, DHS, for these materials and their corresponding binary oxides were determined by high-temperature oxide melt solution calorimetry using molten 2PbO·B2O3 at 1078 K. These data were used to complete thermodynamic cycles to calculate
enthalpies of formation from the oxides, ΔHs f-oxides (kJ/mol): La9.33∎0.67(SiO4)6O2 = −776.3 ± 17.9, Nd9.33∎0.67(SiO4)6O2 = −760.4 ± 31.9, Sm9.33∎0.67(SiO4)6O2 = −590.3 ± 18.6, and Gd9.33∎0.67(SiO4)6O2 = −446.9 ± 21.9. Reference data were used to calculate the standard enthalpies of formation from the elements, ΔH0 f (kJ/mol): La9.33∎0.67(SiO4)6O2 = −14611.0 ± 19.4, Nd9.33∎0.67(SiO4)6O2 = −14661.5 ± 32.2, Sm9.33∎0.67(SiO4)6O2 = −14561.7 ± 20.8, and Gd9.33∎0.67(SiO4)6O2 = −14402.7 ± 28.2. The formation enthalpies become more endothermic as the ionic radius of the lanthanide ion decreases
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