No abstract

This study presents a method of external fields to determine thermodynamic characteristics of rigid crystalline phases in the framework of a kinetic Monte Carlo algorithm. The method is based on modeling of the gas−crystal system with explicit accounting for the interface and uses the condition of equal chemical potentials in coexisting phases. Two non-uniform fields are imposed on the gas phase. The first one is the usual external potential, while the other is a proposed-here damping field reducing intermolecular potentials up to zero. This makes the coexisting gas always ideal at any desired density under controlled crystal pressure. It opens an efficient way to reliably determine the chemical potential of very rigid solids. The effect of changing the damping field along the simulation cell is similar to that produced by the temperature gradient, but the condition of equilibrium is not violated. The approach was tested on the model of a self-assembled layer of trimesic acid at specified values of temperature and pressure. In all cases, thermodynamic consistency of the approach was convincingly confirmed. The proposed approach is a promising tool for modeling rigid crystalline structures such as self-assembled monolayers formed by relatively large functional organic molecules.

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.

A technique has been developed for calculating the thermodynamic characteristics of rigid self-assembled organic adsorption layers and the parameters of polymorphic transitions using two types of external fields and the kinetic Monte Carlo method.