Analysis of the Cybotactic Region of Two Renewable Lactone–Water Mixed-Solvent Systems that Exhibit Synergistic Kamlet–Taft Basicity

Kamlet–Taft solvatochromic parameters (polarity, basicity, acidity) of hydrogen bond donor (HBD)/acceptor (HBA) mixed-solvent systems, water (H₂O)−γ-valerolactone (GVL), methanol (MeOH)–GVL, ethanol (EtOH)–GVL, H₂O−γ-butyrolactone (GBL), MeOH–GBL, and EtOH–GBL, were measured over their entire composition region at 25 °C using UV–vis spectroscopy. Basicity of H₂O–GVL and H₂O–GBL systems exhibited positive deviation from ideality and synergism in the Kamlet–Taft basicity values. The cybotactic region around each indicator in the mixed-solvent systems was analyzed with the preferential solvation model. Both H₂O–GVL and H₂O–GBL mixed-solvent systems were found to be completely saturated with mutual complex molecules and to have higher basicity than pure water because water prefers to interact with GVL or GBL molecules rather than with itself. Formation of H₂O–GVL and H₂O–GBL complex molecules via specific hydrogen bond donor–acceptor interactions were confirmed by infrared spectroscopy. In MeOH–GVL or MeOH–GBL mixed-solvent systems, MeOH molecules prefer self-interaction over that with GVL or GBL so that synergistic basicity was not observed. Synergistic basicity and basicity increase for various functional groups of ten mixed-solvent (water–HBA solvent) systems can be quantitatively explained by considering electrostatic basicity and a ratio of the partial excess HBA solvent basicity with the HBA solvent molar volume that correlate linearly with the preferential solvation model complex molecular parameter (f_12/1). Analysis of the cybotactic region of indicators in aqueous mixtures with the preferential solvation model allows one to estimate the trends of mixed-solvent basicity.