Can Synergistic Solvation Increase Polarity Beyond Water? An Intriguing Case Study of Aqueous Binary Mixtures of...

In this study, the synergistic behavior of aqueous binary mixtures of 1,2-dimethoxyethane (DME), 2-methoxyethanol (2ME), and ethylene glycol (EG) was investigated using three solvatochromic dyes: coumarin 461 (C461), 4-aminophthalimide (4AP), and para-nitroaniline (pNA) through steady-state UV-visible spectroscopy and fluorescence emission spectroscopy. The absorption maxima of the dyes exhibited extensive bathochromic shifts with varying solvent mixture compositions. In the water-rich region of the mixtures, the absorption maxima displayed significantly larger bathochromic shifts compared with those in pure water. A clear case of synergistic solvation was observed, indicating that the polarity of mixtures exceeds that of pure water. The synergistic effect was pronounced in the water-DME and water-2ME mixtures, while it was weaker in the water-EG mixture. This "hyper-polarity" was analyzed from the molar transition energy variation using a generalized Bosch solvation model. In the water-DME and water-2ME mixtures, the equilibrium constant for synergistic solvation was significantly greater than that for preferential solvation, whereas in the water-EG mixture, the values were comparable. This behavior stemmed from the intermolecular hydrogen bonding between water and cosolvents. The mole fraction of synergistic solvation suggested microheterogeneity around the solute within the mixtures. Notably, the variation in emission maxima of the probes showed no synergistic behavior, implying that solvent reorientation in the excited state disrupts the synergistic effect. IR spectroscopy was also employed to investigate the hydrogen-bonded structures in the binary mixtures. Analytical modeling of -OH and -CH stretching frequency was established, and it revealed that the formation of water-DME and water-2ME hydrogen-bonded aggregates is responsible for the observed synergistic "hyper-polarity" effect.