# Behavior of ionic salts in solution

The electrons of an isolated sodium chloride bound pair in vacuum reside at a semi-classical level in their ground state, so that the Born-Oppenheimer approximation applies, and the 'molecule' acts like a sort of rigid rotor, whose relevant degrees of freedom are translational momentum and orbital angular momentum. In solution, however, one might expect things to become a bit more complicated. Electrons and holes from water, H+ and OH- and other metastable arrangements would tend to perturb the electronic structure of the NaCl molecule, and it seems that this could greatly impact its effective behavior (after integrating out the water molecules, say.) It does not seem totally unreasonable that the NaCl pair, once in solution, would spend most of its time partially dissociated, and that the constitutive ions would tend to be far enough from each other to be described reasonably well by a 'semi-classical' Langevin dynamical model. However, if one simulates overdamped Langevin dynamics of two charged ions separated by distances that are within the screening length, one finds that the ions tend to approach each other arbitrarily closely. Because of this, it would seem that the ions must also spend an appreciable time in some sort of quantum mechanical bound state. My question is, to what extent is the structure of this bound state known (beyond that it probably resembles the bound state in vacuum), and if so, what are the relevant 'soft' degrees of freedom?

• Sorry ! The molecule NaCl does not exist. The salt NaCl is made of an arrangement of Na+ and Cl- ions. But there is never a real NaCl molecule. Dissolution into water has only one effect, that is to separate the ions from one another. – Maurice Sep 2 at 20:01
• – Mithoron Sep 2 at 20:20
• Still OP here seems to like jargon way too much, still not enough to hide things like "Electrons and holes from water" - seriously? Not to mention "weave themselves into the electronic structure" isn't exactly proper description of single no so lofty word "hydration" that the post lacks. – Mithoron Sep 2 at 20:27
• Rather all the time fully dissociated, ions hydrated and moving independently. Even in gaseous state, it is rather a ion pair than a molecule. – Poutnik Sep 2 at 20:57
• In the vapour phase NaCl is a molecule, albeit with a lot of ionic nature. The vibrational frequency $v=0$ is $\approx 380$ wavenumbers. In solution, if you repeat the calculation with a large dielectric constant (80 for water) you should see that the ions will separate as the attraction is comparable or smaller than thermal energy. – porphyrin Sep 3 at 20:50

• Note that the solid NaCl does not contain NaCl molecules either, but is a collection of alternating Na+ and Cl- ions, where each ion is surrounded by 6 equivalent opposite ions. When dissolved, is interacts mainly with neutral water molecules, as H+ and OH- ions are far from abundant, having concentration $\pu{10^-7 mol/L}$. Each ions is then dissolved separately by being hydrated by water molecules. – Poutnik Sep 3 at 6:38