A first principles molecular dynamics study of the solvation structure and migration kinetics of an excess proton and a hydroxide ion in binary water-ammonia mixtures

Abstract

We have investigated the solvation structure and migration kinetics of an excess proton and a hydroxide ion in water-ammonia mixed liquids of varying composition by means of ab initio molecular dynamics simulations. The excess proton is always found to be attached to an ammonia molecule to form the ammonium ion. Migration of the excess proton is found to occur very occasionally from one ammonia to the other but no proton transfer to a water molecule is observed during the entire simulations. Also, when the ammonium ion is solvated in water only, its hydrogen bond dynamics and rotation are found to occur at a faster rate than those in water-ammonia mixtures. For water-ammonia mixtures containing a proton less, the defect is found to stay like the hydroxide ion. For these systems, occasional proton transfer is found to occur only through the hydrogen bonded chains of water molecules in these water-ammonia mixtures. No proton transfer is found to take place from an ammonia molecule. The presence of ammonia molecules makes the realization of proper presolvated state of the hydroxide ion to accept a proton a more difficult process and, as a result, the rate of proton transfer and migration kinetics of the hydroxide ion in water-ammonia mixtures are found to be slower than that in liquid water and these rates are found to slow down further with increase of ammonia concentration.