Effect of solid-like nitrogen contact layers on graphite: anisotropy of tangential pressure and orientational order

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

“…Such a method involving only the external potential works excellently when the crystal is formed by relatively small molecules of noble gases 73,75,78,79,82 or nitrogen. 74,76,80 However, in the present study, the potential energy of intermolecular interaction in 2D crystals is at least one order of magnitude higher than in the former case. As a consequence, the equilibrium gas density is extremely small, which is a serious obstacle in accurate determination of the chemical potential.…”

“…The external potential can be positive or negative and its variation affects the chemical potential of the whole system and allows for evaluating fundamental dependences such as the dependence of the pressure on the chemical potential. This technique based on the kinetic Monte Carlo simulation in an elongated cell was successfully used in a large number of applications. − The only shortcoming is that in the case of large molecules interacting with each other with a substantially high potential energy, the density of the equilibrium gas phase is too small to determine the chemical potential reliably. The combination of the two external fields resolves this problem because the damping field transforms the gas phase to the ideal gas of any density, while the external potential allows for keeping the crystal at any pressure.…”

“…It allows us to assess the role of the hydrogen bonding in the self-assembly mechanism and more accurately estimate the stability of various polymorphs to changes in the temperature and density of the molecular layer. To develop the thermodynamics of the TMA adsorption layer, we rely on our approach combining the numerical simulation of two-phase systems and equations of state for corresponding homogeneous phases. − The simulation is performed with the kinetic Monte Carlo method − that allows one to determine the chemical potential of crystalline or amorphous SAMs directly during the canonical ensemble simulation without involving any external procedures, such as Widom’s test particle method. An additional calculation of temperature dependences of the potential energy and two-dimensional pressure of the SAMs completes its thermodynamic description.…”

Chemical Potential and Thermodynamic Properties of Self-Assembled Monolayers: A Method of External Fields in a Monte Carlo Simulation

“…This makes the gas phase significantly more statistically representative and substantially reduces statistical errors in the chemical potential, which is especially important when the gas phase is extremely rarefied. The approach was further improved by using a variable external potential imposed on the ends of the simulation cell with the pressure difference between the gas and the liquid/solid placed in the central part of the cell being reliably determined via the gradient of the potential. − This is an additional benefit of the method that allows us to determine the interrelation between the pressure and the chemical potential at constant temperature and check whether the system obeys the Gibbs–Duhem equation. The method was applied to thermodynamic analysis of various adsorption systems.…”

“…Thus, the commensurate–incommensurate phase transition in krypton monolayer on graphite was considered, and the experimentally observed heat spike during nitrogen adsorption on graphite at 77 K (35 degrees above the 2D critical temperature) was first explained by the commensurate crystallization of nitrogen and quantitatively described . The quadrupole–quadrupole interactions of nitrogen molecules were accounted for in the framework of the method to describe the effect of anisotropy of the tangential pressure due to the herringbone orientational order of the commensurate nitrogen monolayer at low temperatures and the long-to-short ordering of the layer on a smooth surface . Also, the kinetic Monte Carlo coupled with the method of external potential was applied to the 3D hard spheres and 12-6 Lennard-Jones fluid .…”