Chemical reactivity and phase behavior of NH4Cl by molecular dynamics simulations. II. The liquid–vapor coexistence curve

Abstract

In order to shed some light on the peculiar liquid–vapor phase transition of ammonium chloride (NH4Cl), we make use of a recently developed model potential (see paper I) accounting for the proton transfer reaction (NH4++Cl−⇔NH3+HCl) to investigate by classical molecular dynamics simulation the phase diagram of NH4Cl in the fluid range. A key result of the simulation is that a phase coexistence is found between a liquid mixture composed of ionic (NH4+,Cl−) and covalent (NH3,HCl) species in nearly equal proportion and a vapor composed exclusively of molecules (NH3), findings which explain the conductivity data of the literature. Although the agreement with the experimental coexistence curve is only semiquantitative, the simulation leads to a decisive improvement compared with current theoretical approaches. In addition, the evolution of the reaction kinetics and of the composition of the fluid with the thermodynamic conditions are evaluated, and the sequence of events which accompany the proton transfer is analyzed in detail.