Complex formation and microheterogeneity in water–alcohol binary mixtures investigated by solvatochromic study

The water–alcohol binary mixtures have been extensively investigated by experimental and theoretical approaches, since they are extensively used in several industrial processes, being also very important in life sciences. Solvatochromism involves the use of spectrally active molecules (absorbing in the investigated spectral range) as probes introduced in a transparent solvent in small concentrations, so that the intermolecular interactions between its molecules can be neglected. The shift of the absorption spectral band of solute when dissolved is related to the intermolecular interactions between the solute’s molecules and the solvent molecules. Here, we report on the solvatochromic investigation of water–alcohol binary mixtures, based on experimentally measured absorption spectra. For the binary solvents, the composition of the first solvation shell (which have the most important contributions to the total shift of the solute’s spectral band) is different from that one existing in the whole solution. This happens, because the molecules of the active solvent (the one whose molecules stronger interact with the solute’s molecules) will be to a greater extent in the first solvation shell of the solute’s molecule. Three theoretical models were comparatively applied to estimate the composition of the first solvation shell of the solute’s molecule and thus to assess the solvatochromic data: statistical cell model of ternary solutions, Suppan model, and Bosch-Rosés model. Information on the water–alcohol complex formation (by hydrogen bonds) was obtained, as well as on the microheterogeneity of the binary mixture. The obtained data also allow the estimation of the interaction energy between two molecules in pairs solute–solvent.