Hydration and ion association of aqueous choline chloride and chlorocholine chloride

The choline ion (Ch + ) is ubiquitous in nature and also its synthetic homologue, chlorocholine (ClCh + ), is widely used. Nevertheless, surprisingly little information on the hydration and counter-ion binding of these cations can be found in the literature. In this contribution we report effective hydration numbers, determined by dielectric relaxation spectroscopy, and ion-pair association constants with Cl − , determined by dilute-solution conductivity measurements. In combination with RISM calculations the obtained data suggest that for Ch + water is bound to the hydroxy group via hydrogen bonds whereas for ClCh + a rather stiff clathrate-like shell around the chlorine atom seems to be formed. With Cl − both cations form contact ion pairs with association constants of only ∼2 to 3 M −1 . Choline hydration occurs predominantly via its hydroxyl group, and weak contact ion pair formation with Cl − is via the onium moiety.