19 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
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
Liquid polymorphism, order-disorder transitions and anomalous behavior : a Monte Carlo study of the Bell-Lavis model for water
The Bell–Lavis model for liquid water is investigated through numerical simulations. The lattice-gas model on a triangular lattice presents orientational states and is known to present a highly bonded low density phase and a loosely bonded high density phase. We show that the model liquid-liquid transition is continuous, in contradiction with mean-field results on the Husimi cactus and from the cluster variational method. We define an order parameter which allows interpretation of the transition as an order-disorder transition of the bond network. Our results indicate that the order-disorder transition is in the Ising universality class. Previous proposal of an Ehrenfest second order transition is discarded. A detailed investigation of anomalous properties has also been undertaken. The line of density maxima in the HDL phase is stabilized by fluctuations, absent in the mean-field solution
Structure and anomalous solubility for hard spheres in an associating lattice gas model
In this paper we investigate the solubility of a hard-sphere gas in a solvent modeled as an associating lattice gas. The solution phase diagram for solute at 5% is compared with the phase diagram of the original solute free model. Model properties are investigated both through Monte Carlo simulations and a cluster approximation. The model solubility is computed via simulations and is shown to exhibit a minimum as a function of temperature. The line of minimum solubility (TmS) coincides with the line of maximum density (TMD) for different solvent chemical potentials, in accordance with the literature on continuous realistic models and on the "cavity" picture. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4743635]Conselho Nacional de Pesquisas (CNPq)Conselho Nacional de Pesquisas (CNPq) [475039/2010-6, 472210/2011-4]CAPESCapesInstituto Nacional de Ciencia e Tecnologia de Fluidos Complexos (INCTFCx)Instituto Nacional de Ciencia e Tecnologia de FLUIDOS COMPLEXOS (INCT-FCx)Universidade Federal de Santa CatarinaUniversidade Federal de Santa Catarin
Diffusion Anomaly in an Associating Lattice Gas Model
We investigate the relation between thermodynamic and dynamic properties of
an associating lattice gas (ALG) model. The ALG combines a two dimensional
lattice gas with particles interacting through a soft core potential and
orientational degrees of freedom. From the competition between the directional
attractive forces and the soft core potential results two liquid phases, double
criticality and density anomaly. We study the mobility of the molecules in this
model by calculating the diffusion constant at a constant temperature, . We
show that has a maximum at a density and a minimum at a
density . Between these densities the diffusivity
differs from the one expected for normal liquids. We also show that in the
pressure-temperature phase-diagram the line of extrema in diffusivity is close
to the liquid-liquid critical point and it is inside the temperature of maximum
density (TMD) line.Comment: 12 pages, 9 figure
Diffusion Anomaly in a three dimensional lattice gas
We investigate the relation between thermodynamic and dynamic properties of
an associating lattice gas (ALG) model. The ALG combines a three dimensional
lattice gas with particles interacting through a soft core potential and
orientational degrees of freedom. From the competition between the directional
attractive forces and the soft core potential results two liquid phases, double
criticality and density anomaly. We study the mobility of the molecules in this
model by calculating the diffusion constant at a constant temperature, . We
show that has a maximum at a density and a minimum at a
density . Between these densities the diffusivity
differs from the one expected for normal liquids. We also show that in the
pressure-temperature phase-diagram the line of extrema in diffusivity is close
to the liquid-liquid critical point and it is partially inside the temperature
of maximum density (TMD) line
Estudo das propriedades dinâmicas e termodinâmicas em sistemas tipo água
Apesar da água ser o líquido mais comum na natureza, suas características ainda não estão totalmente explicadas. A relação entre a forma do potencial intermolecular efetivo que representa as interações presentes na água, as várias anomalias presentes e a possível existência de dupla criticalidade ainda é uma questão em aberto. Para tentar entender esses comportamentos muitos modelos físicos foram propostos. Alguns levam em conta todas as interações na molécula enquanto outros tratam as mesmas como esferas que interagem através de um potencial efetivo. Recentemente descobriu-se que a água apresenta, além de anomalias termodinâmicas, anomalia na difusão translacional e rotacional. Mostrou-se que, para o modelo computacional SPC/E para água, as anomalias dinâmicas estão conectadas com a temperatura de máxima densidade (TMD) e que as anomalias dinâmicas ocupam uma região maior que a TMD no diagrama de fases pressão vs. temperatura. Além disso, o coeficiente de difusão apresenta uma mudança de comportamento na região super fria do diagrama de fases. Nessa região a difusão segue uma Lei Arrhenius em baixas temperaturas e quando cruza a Linha de Widom passa a se comportar de maneira não-Arrhenius. Essa transição no coeficiente de difusão (chamada transição dinâmica) estaria associada a presença de um segundo ponto crítico na região super resfriada do diagrama de fases da água. Neste trabalho são investigados três modelos simples tipo água: O primeiro é o Gás de Rede Associativo (GRA) em duas dimensões. Neste modelo se estudou a relação entre polimorfismo (presença de diferentes fases com estruturas diferentes) e transição dinâmica. O segundo modelo estudado é uma generalização em três dimensões do GRA, onde foi investigado a relação entre as anomalias na densidade e difusão translacional e qual a relação entre polimorfismo e transição dinâmica. Finalmente o terceiro modelo estudado é o gás de rede bidimensional Bell-Lavis. Este estudo consiste em analisar as propriedades do diagrama de fases do modelo, estudar a relação entre a anomalia na densidade e difusão translacional e a relação entre polimorfismo e transição dinâmica. Os resultados mostram que é possível estudar propriedades de sistemas complexos, como a água e outros líquido estruturados, com modelos simplificados e obter mecanismos genéricos para determinados comportamentos globais.Although water is ubiquitous in nature, its characteristics are not well understood. The relation between the shape of the effective intermolecular potential that represents the interactions present in water, the various anomalies and a possible existence of double criticality is still an open question. In order to understand these behaviors different physical models have been proposed. Some take account all interactions between molecules while others treat molecules as spheres that interact through an effective potential. Recently it was discovered that, besides the thermodynamics anomalies, water also exhibits rotational and translational diffusion anomalous behavior. It was shown that for the computational model SPC/E for water the dynamic anomalies are connected with the temperature of maximum density (TMD) and that the dynamic anomalies appear at the pressure vs. temperature phase diagram in a region outside the TMD. Besides, diffusion coefficient display a change in its behavior in the super cooled region of the phase diagram. In this region diffusivity exhibits an Arrhenius behavior in low temperatures and, crossing the Widom line, it exhibits a non-Arrhenius behavior. This transition is called dynamic transition and would be related to a critical point in the super cooled region of the phase diagram. In this work we investigate three simple water-like models: the first one is the Associating Lattice Gas (ALG) in two dimensions. In this model was studied the relation between polimorfism (the presence of different phases with different structures) and dynamic transitions. The second model is a three dimension generalization of the ALG model, where was studied the relation between density and translational diffusion anomalies and the relation between polimorfism and dynamic transitions. Finally, the third model is the two dimensional lattice gas Bell-Lavis. This work consists in analyze the phase diagram properties of the model, study what is the relation between density and translational diffusion anomaly and the relation between polimorfism and dynamic transitions. The results show that is possible to study some properties of complex systems, like water and other structured liquids, with very simple models and obtain generic mechanisms to some global behaviors
Estudo das propriedades dinâmicas e termodinâmicas em sistemas tipo água
Apesar da água ser o líquido mais comum na natureza, suas características ainda não estão totalmente explicadas. A relação entre a forma do potencial intermolecular efetivo que representa as interações presentes na água, as várias anomalias presentes e a possível existência de dupla criticalidade ainda é uma questão em aberto. Para tentar entender esses comportamentos muitos modelos físicos foram propostos. Alguns levam em conta todas as interações na molécula enquanto outros tratam as mesmas como esferas que interagem através de um potencial efetivo. Recentemente descobriu-se que a água apresenta, além de anomalias termodinâmicas, anomalia na difusão translacional e rotacional. Mostrou-se que, para o modelo computacional SPC/E para água, as anomalias dinâmicas estão conectadas com a temperatura de máxima densidade (TMD) e que as anomalias dinâmicas ocupam uma região maior que a TMD no diagrama de fases pressão vs. temperatura. Além disso, o coeficiente de difusão apresenta uma mudança de comportamento na região super fria do diagrama de fases. Nessa região a difusão segue uma Lei Arrhenius em baixas temperaturas e quando cruza a Linha de Widom passa a se comportar de maneira não-Arrhenius. Essa transição no coeficiente de difusão (chamada transição dinâmica) estaria associada a presença de um segundo ponto crítico na região super resfriada do diagrama de fases da água. Neste trabalho são investigados três modelos simples tipo água: O primeiro é o Gás de Rede Associativo (GRA) em duas dimensões. Neste modelo se estudou a relação entre polimorfismo (presença de diferentes fases com estruturas diferentes) e transição dinâmica. O segundo modelo estudado é uma generalização em três dimensões do GRA, onde foi investigado a relação entre as anomalias na densidade e difusão translacional e qual a relação entre polimorfismo e transição dinâmica. Finalmente o terceiro modelo estudado é o gás de rede bidimensional Bell-Lavis. Este estudo consiste em analisar as propriedades do diagrama de fases do modelo, estudar a relação entre a anomalia na densidade e difusão translacional e a relação entre polimorfismo e transição dinâmica. Os resultados mostram que é possível estudar propriedades de sistemas complexos, como a água e outros líquido estruturados, com modelos simplificados e obter mecanismos genéricos para determinados comportamentos globais.Although water is ubiquitous in nature, its characteristics are not well understood. The relation between the shape of the effective intermolecular potential that represents the interactions present in water, the various anomalies and a possible existence of double criticality is still an open question. In order to understand these behaviors different physical models have been proposed. Some take account all interactions between molecules while others treat molecules as spheres that interact through an effective potential. Recently it was discovered that, besides the thermodynamics anomalies, water also exhibits rotational and translational diffusion anomalous behavior. It was shown that for the computational model SPC/E for water the dynamic anomalies are connected with the temperature of maximum density (TMD) and that the dynamic anomalies appear at the pressure vs. temperature phase diagram in a region outside the TMD. Besides, diffusion coefficient display a change in its behavior in the super cooled region of the phase diagram. In this region diffusivity exhibits an Arrhenius behavior in low temperatures and, crossing the Widom line, it exhibits a non-Arrhenius behavior. This transition is called dynamic transition and would be related to a critical point in the super cooled region of the phase diagram. In this work we investigate three simple water-like models: the first one is the Associating Lattice Gas (ALG) in two dimensions. In this model was studied the relation between polimorfism (the presence of different phases with different structures) and dynamic transitions. The second model is a three dimension generalization of the ALG model, where was studied the relation between density and translational diffusion anomalies and the relation between polimorfism and dynamic transitions. Finally, the third model is the two dimensional lattice gas Bell-Lavis. This work consists in analyze the phase diagram properties of the model, study what is the relation between density and translational diffusion anomaly and the relation between polimorfism and dynamic transitions. The results show that is possible to study some properties of complex systems, like water and other structured liquids, with very simple models and obtain generic mechanisms to some global behaviors