217 research outputs found
Diffusion anomaly and dynamic transitions in the Bell-Lavis water model
In this paper we investigate the dynamic properties of the minimal Bell-Lavis
(BL) water model and their relation to the thermodynamic anomalies. The
Bell-Lavis model is defined on a triangular lattice in which water molecules
are represented by particles with three symmetric bonding arms interacting
through van der Waals and hydrogen bonds. We have studied the model diffusivity
in different regions of the phase diagram through Monte Carlo simulations. Our
results show that the model displays a region of anomalous diffusion which lies
inside the region of anomalous density, englobed by the line of temperatures of
maximum density (TMD). Further, we have found that the diffusivity undergoes a
dynamic transition which may be classified as fragile-to-strong transition at
the critical line only at low pressures. At higher densities, no dynamic
transition is seen on crossing the critical line. Thus evidence from this study
is that relation of dynamic transitions to criticality may be discarded
Liquid-state theory of charged colloids
A simple theory of the fluid state of a charged colloidal suspension is
proposed. The full free energy of a polyelectrolyte solution is calculated. It
is found that the counterions condense onto the polyions forming clusters
composed of one polyion and n counterions. The distribution of cluster sizes is
determined explicitly. In agreement with the current experimental and Monte
Carlo results, no liquid-gas phase separation was encountered.Comment: 4 pages, 2 Postscript figures, uses multicol.sty; changed conten
Donnan equilibrium and the osmotic pressure of charged colloidal lattices
We consider a system composed of a monodisperse charge-stabilized colloidal
suspension in the presence of monovalent salt, separated from the pure
electrolyte by a semipermeable membrane, which allows the crossing of solvent,
counterions, and salt particles, but prevents the passage of polyions. The
colloidal suspension, that is in a crystalline phase, is considered using a
spherical Wigner-Seitz cell. After the Donnan equilibrium is achieved, there
will be a difference in pressure between the two sides of the membrane. Using
the functional density theory, we obtained the expression for the osmotic
pressure as a function of the concentration of added salt, the colloidal volume
fraction, and the size and charge of the colloidal particles. The results are
compared with the experimental measurements for ordered polystyrene lattices of
two different particle sizes over a range of ionic strengths and colloidal
volume fractions.Comment: 8 pages, 4 Postscript figures, uses multicol.sty, to be published in
European Physical Journal
Dynamic Transitions in a Two Dimensional Associating Lattice Gas Model
Using Monte Carlo simulations we investigate some new aspects of the phase
diagram and the behavior of the diffusion coefficient in an associating lattice
gas (ALG) model on different regions of the phase diagram. The ALG model
combines a two dimensional lattice gas where particles interact through a soft
core potential and orientational degrees of freedom. The competition between
soft core potential and directional attractive forces results in a high density
liquid phase, a low density liquid phase, and a gas phase. Besides anomalies in
the behavior of the density with the temperature at constant pressure and of
the diffusion coefficient with density at constant temperature are also found.
The two liquid phases are separated by a coexistence line that ends in a
bicritical point. The low density liquid phase is separated from the gas phase
by a coexistence line that ends in tricritical point. The bicritical and
tricritical points are linked by a critical -line. The high density
liquid phase and the fluid phases are separated by a second critical
line. We then investigate how the diffusion coefficient behaves on different
regions of the chemical potential-temperature phase diagram. We find that
diffusivity undergoes two types of dynamic transitions: a fragile-to-strong
trans ition when the critical -line is crossed by decreasing the
temperature at a constant chemical potential; and a strong-to-strong transition
when the -critical line is crossed by decreasing the temperature at a
constant chemical potential.Comment: 22 page
Hydration and anomalous solubility of the Bell-Lavis model as solvent
We address the investigation of the solvation properties of the minimal
orientational model for water, originally proposed by Bell and Lavis. The model
presents two liquid phases separated by a critical line. The difference between
the two phases is the presence of structure in the liquid of lower density,
described through orientational order of particles. We have considered the
effect of small inert solute on the solvent thermodynamic phases. Solute
stabilizes the structure of solvent, by the organization of solvent particles
around solute particles, at low temperatures. Thus, even at very high
densities, the solution presents clusters of structured water particles
surrounding solute inert particles, in a region in which pure solvent would be
free of structure. Solute intercalates with solvent, a feature which has been
suggested by experimental and atomistic simulation data. Examination of solute
solubility has yielded a minimum in that property, which may be associated with
the minimum found for noble gases. We have obtained a line of minimum
solubility (TmS) across the phase diagram, accompanying the line of maximum in
density (TMD). This coincidence is easily explained for non-interacting solute
and it is in agreement with earlier results in the literature. We give a simple
argument which suggests that interacting solute would dislocate TmS to higher
temperatures
Thermodynamic, Dynamic and Structural Anomalies for Shoulder-like potentials
Using molecular dynamic simulations we study a family of continuous
core-softened potentials consisting of a hard core, a shoulder at closest
distances and an attractive well at further distance. The repulsive shoulder
and the well distances represent two length scales. We show that if the first
scale, the shoulder, is repulsive or has a small well, the potential has a
region in the pressure-temperature phase diagram with density, diffusion and
structural anomalies. However, if the closest scale becomes a deep attractive
well the regions in the pressure-temperature phase diagram where the three
anomalies are present shrink and disappear. This result enables us to predict
by the shape of the core-softened potential if anomalies would or would not be
present
Anomalies in a waterlike model confined between plates
Using molecular dynamic simulations we study a waterlike model confined
between two fixed hydrophobic plates. The system is tested for density,
diffusion and structural anomalous behavior and compared with the bulk results.
Within the range of confining distances we had explored we observe that in the
pressure-temperature phase diagram the temperature of maximum density (TMD
line), the temperature of maximum and minimum diffusion occur at lower
temperatures when compared with the bulk values. For distances between the two
layers below a certain threshold ,, only two layers of particles are
formed, for three or more layers are formed. In the case of three
layers the central layer stays liquid while the contact layers crystallize.
This result is in agreement with simulations for atomistic models
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