Ab initio molecular dynamics study of an aqueous NaCl solution under an electric field

We report on an ab initio molecular dynamics study of an aqueous NaCl solution under the effect of static electric fields. We found that at low-to-moderate field intensity regimes chlorine ions have a greater mobility than sodium ions which, being a sort of "structure makers", are able to drag their own coordination shells. However, for field strengths exceeding 0.15 V Å-1 the mobility of sodium ions overcomes that of chlorine ions as both types of ions do actually escape from their respective hydration cages. The presence of charged particles lowers the water dissociation threshold (i.e., the minimum field strength which induces a transfer of protons) from 0.35 V Å-1 to 0.25 V Å-1; moreover, a protonic current was also recorded at the estimated dissociation threshold of the solution. The behaviour of the current-voltage diagram of the protonic response to the external electric field is Ohmic as in pure water, with a resulting protonic conductivity of about 2.5 S cm-1. This value is approximately one third of that estimated in pure water (7.8 S cm-1), which shows that the partial breaking of hydrogen bonds induced by the solvated ions hinders the migration of protonic defects. Finally, the conductivity of Na+ and Cl- ions (0.2 S cm-1) is in fair agreement with the available experimental data for a solution molarity of 1.7 M.