Liquid polymorphism and density anomaly in a lattice gas model

Abstract

We present a simple model for an associating liquid in which polymorphism and density anomaly are connected. Our model combines a two dimensional lattice gas with particles interacting through a soft core potential and orientational degrees of freedom represented through thermal "ice variables." The competition between the directional attractive forces and the soft core potential leads to a phase diagram in which two liquid phases and a density anomaly are present. The coexistence line between the low density liquid and the high density liquid has a positive slope contradicting the surmise that the presence of a density anomaly implies that the high density liquid is more entropic than the low density liquid.

Thermodynamic and dynamic anomalies for a three-dimensional isotropic core-softened potential

“…In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29]. In three dimensions, these potentials do not have dynamic and thermodynamic anomalies but possess a second [30] and sometimes a third [25] critical point, accessible by simulations in the region predicted by the hypernetted chain integral equation [23,26,31].…”

“…In the first case, the potential consists of a hard core, a square repulsive shoulder and, in some cases, an attractive square well [21,22,23,24,25,26,27,28,29,30,31,34]. In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29].…”

“…They possess a repulsive core that exhibits a region of softening where the slope changes dramatically. This region can be a shoulder or a ramp [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36].…”

Structure and anomalous solubility for hard spheres in an associating lattice gas model

“…[7][8][9] Even though there are a number of two and three dimensional lattice models that would in principle exhibit the anomalies present in water, 6,16,[19][20][21] here water is represented by the associating lattice gas (ALG) (Refs. [22][23][24][25][26][27][28]) that has already shown the density and diffusion anomalies described above. The ALG does not have the full complexity of the molecular liquid, however it leads to a network-forming fluid in which directionality plays an important role.…”

Hydration and anomalous solubility of the Bell-Lavis model as solvent

“…Both models present several of the anomalous features of water [14][15][16][17][18][19][20][21][22][23][24][25][26][27]. However, the second kind of model does not involve specific orientation of low-energy pairs of particles: pair energy is controlled by distance, not by orientation.…”