Equilibrium solubility of vanillin in some (ethanol + water) mixtures: determination, correlation, thermodynamics and preferential solvation

Abstract

Equilibrium mole fraction solubility of vanillin in nine aqueous-ethanolic mixtures, as well as in neat water and neat ethanol, was determined at seven temperatures from T = 293.15 to T = 323.15 K. Vanillin solubility in these mixtures was adequately correlated with several well-known correlation models with the mean percentage deviations of 5.9 to 18.3%. Respective apparent thermodynamic functions, i.e. Gibbs energy, enthalpy, and entropy, for the dissolution, mixing and solvation processes, were computed using the van’t Hoff and Gibbs equations. The enthalpy–entropy relationship for vanillin was non-linear in the plot of enthalpy vs. Gibbs energy of dissolution with positive slopes from neat water to the mixture of w1 = 0.10 and the interval 0.50 < w1 < 0.90 but negative in the interval 0.10 < w1 < 0.50 and from w1 = 0.90 to neat ethanol. Accordingly, in the first cases the vanillin transfer from more polar to less polar solvent systems is enthalpy-driven but entropy-driven for the last ones. Moreover, by means of the inverse Kirkwood-Buff integrals is observed that vanillin is preferentially solvated by water molecules in water-rich mixtures but preferentially solvated by ethanol molecules in mixtures of 0.23 < x1 < 1.00.