<p>Using the accurate quantum chemical calculations, the origin behind the hydrophobicity/hydrophilicity of two similar ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]⁺[PF₆]⁻) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]⁺[BF₄]⁻) is thoroughly investigated. The structure of all the species and their corresponding interaction energies with water are computed by employing the wB97XD functional and 6-311G++(d,p) basis set. The use of the functional and basis set has been benchmarked either considering the experimental data or predictions from the sophisticated <i>ab initio</i> calculations. A continuum solvation model is utilized initially to understand the hydration (solvation) behaviour of the ILs as well as the hydration of the constituent ions. The explicit solvation has also been performed to monitor the effects of each molecular water addition to IL ion pairs and their anionic counterparts. This is done by calculating the interaction energy profiles as well as their charge distributions. It is found that anions and their charges transfer to cations or surroundings play a decisive role in this qualitatively different behaviour of these two ILs towards water. In addition, classical molecular dynamics simulations have been performed to investigate the hydration structure of the ions in the bulk water. The quantum chemical calculations presented here demonstrate the importance of anion-medium interactions in determining the hydrophobicity/hydrophilicity character of these two ILs.</p> Graphical abstract <p>This paper investigates quantum mechanical origin of hydrophobicity by considering two ionic liquids, differing only in anions, as representative systems. The non-ionic interaction was found play an important role. The Gradient isosurfaces and the corresponding RDG plots for both the represenattive ILs in water are also shown</p>