Solvent effects on chemical processes. Part 6. The phenomenological model applied to the solubility of naphthalene and 4-nitroaniline in binary aqueous–organic solvent mixtures

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Abstract
The phenomenological theory of solvent effects on solubility partitions the free energy of solution into contributions from solvent–solvent interactions (the general medium or solvophobic effect), solvent–solute interactions (the solvation effect), and solute–solute interactions (the intersolute effect). The theory relates the solvent effect on solubility to the mole fractions of the solvent components and the model parameters K1 and K2(which are stepwise solvation exchange constants) and gA(the general medium parameter, where g is a curvature correction factor to the surface tension, and A is the nonpolar surface area of the solute). Solubility data for naphthalene and 4-nitroaniline are given over the entire composition range in aqueous–organic mixtures, the organic cosolvents being methanol, ethanol, propan-2-ol, propane-1, 2-diol, ethylene glycol, acetone, dimethyl sulfoxide and acetonitrile. The data are quantitatively described by the model. Cosolvents are classified, on the basis of their polarities (as measured by their octan-1-ol–water partition coefficient P), into a less polar class whose solvation exchange constants increase with log P, and a more hydrophilic class with the opposite tendency. The gA parameter can be reasonably predicted with the empirical relationship gA=–33.4 log PM+ 13.4 log PR(Å2 mol–1), where M and R refer to the organic cosolvent and the solute, respectively. The product g(γ2–γ1), where γ2 and γ1 are the surface tensions of pure cosolvent and water, respectively, is close to the dispersion surface energy component of water and many other solvents.

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