Transfer thermodynamics of p-nitroaniline in aqueous solutions of some ionic and non-ionic cosolvents and the structuredness of the solvents

Abstract

Standard free energies [Formula: see text] and entropies [Formula: see text] of transfer of p-nitroaniline (pNA) from water to aqueous mixtures of some ionic and non-ionic cosolvents like KBr, urea (U), propylene glycol (PG), glycerol (GL), dioxane (D), and 1,2-dimethoxyethane (DME) have been determined from solubility measurements at different temperatures. The observed [Formula: see text]–composition profile in KBr solution appears to result from the well known salting-out effect of the salt and those in aqueous poly-ols and ethers are the result of the combined effects of dispersion and "acid–base" type interactions but that in aqueous urea solutions is governed by the "salting-out" type interaction by the predominant zwitterionic form of urea. The observed [Formula: see text]–composition profiles as well as those obtained after correcting for the "cavity-effect", as estimated tentatively by the scaled particle theory (SPT), were examined in the light of Kundu et al.'s semi-quantitative theory proposed earlier. The latter profiles suggest that unlike isopropyl alcohol (IPA), the poly-ols and ethers as well as KBr and urea induce structure breaking of three dimensional (3D) water structure right from the initial compositions. Moreover, the observed "roller-coaster" type behaviour of the profile in aqueous urea is indicative of the effect of urea–water aggregation around 4–10 m urea. Also, the observed graded reduction of structure-promoting effect of IPA in PG and GL cosolvent systems confirms that structuring and destructuring ability of a cosolvent depends on the ratio of hydrophobicity to hydrophilicity of the cosolvent.