Monte Carlo simulation of an 18.45 mol% aqueous ammonia solution

Abstract

An intermolecular potential function for ammonia–water has been developed from ab initio molecular-orbital calculations. The analytical potential function consists of Coulomb interaction terms and exponential terms for short-range interactions. A Monte Carlo statistical-mechanics simulation has been carried out for a system containing 48 ammonia molecules and 168 water molecules, corresponding to a 18.45 mol% ammonia solution in water, at 20.0 °C and 1.0 atm. At this concentration the hydrogen-bonded network among water molecules still exists. Analysis of the solvent structure around water molecules suggests that H₂O/(H₂O)₄, (NH₃)₁ and H₂O/(H₂O)₅, (NH₃)₁ are the dominant units in the solution, regarding water as the central molecule. The ammonia molecule has 14.6 solvent molecules in the first surrounding shell; the dominant compositions are NH₃/(H₂O)₁₂, (NH₃)₂ and NH₃/(H₂O)₁₃, (NH₃)₂. NH₃/H₂O hydrogen-bonded configurations vary considerably in geometry, and H₃N⋯H—NH₂ bonds are hardly visible.