A computational study of the suppression of ammonia volatility in aqueous systems using ionic additives

Abstract

Controlling vapour pressure is necessary for the viability of aqueous ammonia solutions in commercial applications such as refrigeration. In this study, Gibbs ensemble Monte Carlo (GEMC) simulations were used to calculate the vapour-liquid equilibrium (VLE) of ammonia-water-MCl mixtures, M = Na or Cs, within the isobaric-isothermal- (NpT-) ensemble. The results indicate that in the presence of alkali metal additives, there is a non-negligible 'salting-in' effect for ammonia in the liquid phase. Experimental measurements of the liquid phase concentration of ammonia confirm the GEMC results i.e. the vapour loss rates in systems containing ionic additives is slightly lower. Gibbs ensemble Monte Carlo simulations also indicate that ammonia prefers to solvate aqueous cations as a result of electrostatic interactions. Ab-initio calculations show that the M+-ammonia complex is energetically more stable than the M+-water complex. The difference in the binding free energy Δ(ΔG bind(M+-NH3)-ΔG bind(M+-H2O)) depends on the size of the cation and is highest for the smallest tight cations (e.g. Li+) and lowest for the most polarisable cations (Cs+). © 2013 The Author(s).