Thermodynamics of Rigid Self-Assembled Crystals: Molecular Simulation of Two-Phase Systems under External Fields

A general methodology for determining the thermodynamic characteristics of orientationally ordered rigid crystals is presented. The basic problem here is associated with a very small flux of primary molecules that are released from a narrow interface and carry main information on thermodynamic properties of the crystal. The proposed approach is based on the kinetic Monte Carlo simulation of the gas-crystal system with an external "damping field" that reduces the intermolecular potential at the crystal edges and switches it off in the gas phase. Such a technique increases the primary molecular flux by several orders of magnitude, which is crucial for accurate determination of thermodynamic functions. In this study, we applied the approach to the thermodynamic analysis of the trimesic acid monolayer, explicitly accounting for hydrogen bonds, the dispersion, and electrostatic potentials. We considered equations of state, heat capacities, Helmholtz free energies, and entropies of three polymorphous structures: honeycomb, flower-like, and hexagonally close-packed structures in a wide range of temperatures and pressures. The calculated free energy and entropy excellently obey the Gibbs-Duhem equation, which confirms the thermodynamic consistency of our approach. The role of hydrogen bonds in the stability of different phases, as well as the condition of phase transitions, was also considered.