Dynamic Transitions in a Two Dimensional Associating Lattice Gas Model

TL;DR

This study uses Monte Carlo simulations to explore phase transitions and diffusion behavior in a two-dimensional associating lattice gas model with competing interactions, revealing complex phase diagrams and dynamic transitions.

Contribution

It introduces a detailed phase diagram for the ALG model, identifying multiple critical points and lines, and characterizes the dynamic transitions of diffusivity across these regions.

Findings

Identification of high and low density liquid phases separated by coexistence lines.

Discovery of a bicritical point and tricritical point linking different phase regions.

Observation of fragile-to-strong and strong-to-strong dynamic transitions in diffusivity.

Abstract

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 $\lambda$-line. The high density liquid phase and the fluid phases are separated by a second $\tau$ 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 $\lambda$-line is crossed by decreasing the temperature at a constant chemical potential; and a strong-to-strong transition when the $\tau$-critical line is crossed by decreasing the temperature at a constant chemical potential.