Looking for a better knowledge concerning water and ionic liquids (ILs) interactions, a systematic study
of the activity coefficients of water in pyridinium, pyrrolidinium and piperidinium-based ILs at 298.2 K is
here presented based on water activity measurements. Additionally, the study of the structural effects of
the pyridinium-based cation is also pursued. The results show that non-aromatic ILs are interacting more
with water than aromatic ones, and among the ortho, meta and para isomers of 1-butyl-methylpyridinium
chloride, the ortho position confers a more hydrophilic character to that specific IL. The physicalchemistry
of the solutions was interpreted based on dissociation constants, natural bond orbitals and
excess enthalpies providing a sound basis for the interpretation of the experimental observations. These
results show that hydrogen bonding controls the behavior of these systems, being the anion-water one of
the most relevant interactions, but modulated by the anionecation interactions.
