| Summary: | The UV/Vis absorption energies (<i>ν</i><sub>max</sub>) of different solvatochromic probes measured in co-solvent/water mixtures are re-analyzed as a function of the average molar concentration (<i>N</i><sub>av</sub>) of the solvent composition compared to the use of the mole fraction. The empirical <i>E</i><sub>T</sub>(30) parameter of Reichardt’s dye <b>B30</b> is the focus of the analysis. The Marcus classification of aqueous solvent mixtures is a useful guide for co-solvent selection. Methanol, ethanol, 1,2-ethanediol, 2-propanol, 2-methyl-2-propanol, 2-butoxyethanol, formamide, <i>N</i>-methylformamide (NMF), <i>N</i>,<i>N-</i>dimethylformamide (DMF), <i>N</i>-formylmorpholine (NFM), 1,4-dioxane and DMSO were considered as co-solvents. The <i>E</i><sub>T</sub>(30) values of the binary solvent mixtures are discussed in relation to the physical properties of the co-solvent/water mixtures in terms of quantitative composition, refractive index, thermodynamics of the mixture and the non-uniformity of the mixture. Significant linear dependencies of <i>E</i><sub>T</sub>(30) as a function of <i>N</i><sub>av</sub> can be demonstrated for formamide/water, 1,2-ethanediol/water, NMF/water and DMSO/water mixtures over the entire compositional range. These mixtures belong to the group of solvents that do not enhance the water structure according to the Marcus classification. The influence of the solvent microstructure on the non-linearity <i>E</i><sub>T</sub>(30) as a function of <i>N</i><sub>av</sub> is particularly clear for alcohol/water mixtures with an enhanced water structure.
|
|---|