RESUMEN
A general methodology for determining the thermodynamic characteristics of rigid organic crystals on the atomistic level is presented. The proposed approach is based on a combination of grid interpolation of the precalculated intermolecular potential and kinetic Monte Carlo simulation of the gas-crystal system with an explicit interphase. The two-phase system is stabilized in a wide range of external parameters with an imposed external potential and damping field. The damping field reduces the intermolecular potential at the edges of the crystals and turns it off in the gas phase. To determine the thermodynamic characteristics of a crystal the conditions of equality of chemical potentials in coexisting phases are used. The intermolecular pairwise potential can be calculated on the atomistic or quantum level. In the kinetic Monte Carlo simulations, a grid interpolation of the precalculated potential is performed on each iteration of the algorithm. We have applied the approach to the thermodynamic analysis of a dense monolayer of trimesic acid on a homogeneous surface. The calculated free energy and entropy for the dense "superflower" and filled chicken-wire phases obey the Gibbs-Duhem equation, which confirms the thermodynamic consistency of our approach. Using the proposed approach, we have revealed that the dense "superflower" phase becomes metastable at zero pressure and 470-500 K. Under these conditions, the filled chicken-wire structure with partially released hexagonal cages is thermodynamically favourable. The proposed approach is a potentially universal tool for the thermodynamic analysis of crystals formed by "rigid" organic molecules of any complexity on the atomistic level.
RESUMEN
In this study we present a thermodynamic analysis of several self-assembled molecular layers of trimesic acid (TMA), gas-solid and solid-solid transitions in such layers using the recently proposed Field(s)-supported MultiPhase kinetic Monte Carlo (FsMP/kMC) method. Simulations were performed in an elongated cell comprising a gas-crystal system under an external potential and a damping field imposed on the gas phase and the interphase. The damping field diminishes the intermolecular potentials, which makes it possible to increase the gas phase density by several orders of magnitude. In turn, this provides reliable determination of the chemical potential of the entire system, including the crystal. The effect of temperature and density on entropy and other thermodynamic functions has been thoroughly investigated. At higher temperatures the chicken-wire (CW) structure with the lowest density is the most stable phase. We revealed a phase transition from the close-packed to CW structure at 435 K. Further increase in temperature leads to disassembling CW structure without the appearance of a liquid-like phase. Thermodynamic analysis of the gas phase showed that the critical temperature of the gas is lower than the disassembling temperature. Although the damping field acts like an increase in temperature along the same system, the thermodynamic equilibrium condition is not violated, which is confirmed by the Gibbs-Duhem equation. This makes the FsMP/kMC method a potentially universal tool for the thermodynamic analysis of crystals formed by "heavy" molecules of any complexity.
RESUMEN
A simple lattice model of metal-organic adsorption layers self-assembling on a Au(111) surface and based on pyridyl-substituted porphyrins differing in the number of functional groups and their position has been proposed. The model has been parameterized using DFT methods. The ground state analysis of the considered model demonstrates the variety of surface-confined metal-organic networks (SMONs) containing square, linear, and discrete elements appearing in the adsorption layer depending on the partial pressure of the components. The SMONs comprising more symmetrical molecules with a greater number of pyridyl substituents in the porphyrin core exhibit more diverse phase behavior. Structures of the phase diagrams were verified at nonzero temperatures using Grand Canonical Monte Carlo simulations. It was found that the continuous SMONs have higher thermal stability at relatively low partial pressures of the organic component, while the linear and discrete SMONs are more thermally stable at high pressure. Depending on the partial pressure of the organic component, thermal destruction of continuous SMONs occur either through the formation of defects/islands having structures of the linear SMONs, or through the sublimation of individual structural elements. Melting of linear SMONs reveals the appearance of 2D pores or islands of a purely organic phase. The latter fact is confirmed by the experimentally observed coexistence of these phases.