Pyridine and its methyl derivatives form complexes with water due to hydrogen bonds. The co-operative nature of the hydrogen bonds leads to the association of the complexes and to various modes of hydration. The degrees of association of monohydrate complexes have been calculated for liquid dilute aqueous solutions of pyridine, 2-, 3-, 4-methylpyridine, and 2,6-dimethylpyridine at temperatures from 273 K to 268.5 K. The association number increases with an increase of the amine concentration. Positive correlation was found between the degree of association of the 1:1 water-amine complexes and the size of microheterogeneities. It was shown that the mechanics of this process involves hydrogen bonding and van der Waals interactions.

Phase properties, molar volumes, isentropic compressions and isobaric thermal expansions of the system 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N]+2,6-dimethylpyridine+ water were studied and interpreted in terms of molecular interactions. The solvation of [C4mim][Tf2N] diluted in the binary solvent consists most probably in the accommodation of the ions between pyridine rings of the hydrogen-bonded hydrates of 2,6-dimethylpyridine (C7H9N...H–OH) n . Cations and anions are located in the neighbouring voids forming ionic pairs. That process is dependent on the electric permittivity of the solvent, The higher is the permittivity, i.e. the lower is the concentration of 2,6-dimethylpyridine, the larger are limiting partial compression and volumes of [C4mim][Tf2N] due to weakened Coulomb forces that act between ions. That leads to gradual decay of the ion pairs. The effect on compression is particularly pronounced, while that on volume is small, but evident. Solvation of ions causes that the limiting partial expansion of [C4mim][Tf2N] is equal to zero or at least close to that value. With increasing concentration of the ionic liquid, the solvation shells undergo disruption that leads to the phase separation.