The biogenic choline cation (Ch+) is an essential nutrient for humans and many animals. Its interactions with water and other ions are often crucial for various biochemical processes running in living organisms. Nevertheless, the available information on the features of these interactions is rather scarce. In this paper we present the molecular details of ion hydration and association in water for choline chloride (ChCl) as one of the representatives of choline-based ionic compounds. The features of these processes were studied over a solute concentration range up to 1.6 M using the integral equation method in 1D- and 3D-RISM approaches. Analysis of the obtained data revealed that Ch+ has a spacious, loose hydration shell with large total hydration number. Moreover, as found, only –OH group of Ch+ is able to form H-bonds with solvent. An increasing of solute concentration leads to an appropriate dehydration of both ions and weakening of their H-bonding with water. According to the presented results, the Ch+-Cl− association in aqueous solutions is possible by two ways, namely, via the onium moiety by Coulomb interaction or via hydroxyl group by H-bonding. However, the ion binding way depends on the solute concentration. The presented data add new knowledge on the behavior of this compound in biological media.
