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/A 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/A to 0.25 V/A; 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. This value is approximately
one third of that estimated in pure water (7.8 S/cm), 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) is in fair agreement with the available experimental
data for a solution molarity of 1.7 M.