Simulation of preferential cation solvation in aqueous ammonia

Abstract

The energy surface for Na⁽⁺⁾/NH₃ has been evaluated by means of ab initio SCF calculations at extended basis-set level, and a six-center Na⁽⁺⁾/NH₃ analytical pair potential has been derived, which consists of Coulomb interaction terms, exponential and 1/r² terms. A Monte Carlo simulation has been carried out for an ammonia–water mixture (18.45 mol% NH₃) containing one Na⁽⁺⁾ ion at 20.0°C and 1.0 atm. From the resultant total radical distribution functions, two solvation shells were recognized. The first shell contains 4.0 ammonia and 2.4 water molecules. The ammonia molecules point with the lone-pair site mostly towards the ion; the angular orientation of the water molecules, however, varies significantly owing to the influence of neighbouring ammonia molecules. Intermediate peaks appear between the first and second shell in Na–H₂O radical distribution functions, containing 3.4 water molecules. The second solvation shell contains ca. seven water molecules, whereas only two ammonia molecules are found. Ammonia molecules in the second solvation shell are independent from the central ion and are influenced in their molecular orientation mostly by neighbouring water molecules.