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The ionization constants of 1,5-diaryl-3-nitroformazans in 20–80%( v/v ) 1,4-dioxane–water mixtures were determined using UV/Vis spectrophotometric measurements. The measurements were performed at 288.15, 298.15, 308.15, and 318.15 K and thus the thermodynamic parameters of ionization and transfer were evaluated. The ionization of formazans has been found to be endothermic and entropically unfavourable. The linear Hammett plot of their  pK  a  values indicates the coplanarity of the molecules of these formazans.  The dependence of the ionization constant and Gibbs energy of ionization (Δ diss  G °) on the mole or volume fraction of 1,4-dioxane and the reciprocal of the dielectric constant of the solvent mixtures is not linear.  The variation of the Δ diss  G ° values (and thus  pK  a  values) obtained over the whole composition range studied can be explained in terms of preferential solvation of formazans in aquo-organic mixtures.  The Δ diss  G ° values are well correlated with the solvent polarity parameter of Dimroth and Reichardt,  E  T (30) or with that of Kosower,  Z .  The linear solvation energy relationships method (LSER) was also applied to correlate the various thermodynamic parameters with the Kamlet and Taft solvatochromic parameters (α, β, π*) of 1,4-dioxane–water mixtures. Thus, the thermodynamic parameters in any 1,4-dioxane–water mixtures up to 80%( v/v ) can be calculated using the obtained equations.  The specific solute-solvent interactions and preferential solvation of the different species present in the ionization equilibria dictate the overall ionization and transfer processes of formazans.

Thermodynamics of Transfer and Solvatochromism of 1,5-Diaryl-3-Nitroformazans in 1,4-Dioxane–Water Mixtures