Applications of a preferential–solvation index (PSI) for the comparison of binary mixtures with ionic liquids

•The preferential solvation of dyes in ionic liquid (IL) mixtures was analyzed with a new index.•The preferential-solvation index (PSI) is a simple parameter derived from experimental plots.•Sixty-six examples from the literature illustrate the potential of the PSI approach.•The observed trends were interpreted in terms of the IL structures and interactions.•The approach reveals puzzling trends, raising doubts on traditionally employed PS models.