Deprotonation and transfer energetics of benzoic acid in some quasi-isodielectric ethylene carbonate + water mixtures.

Abstract

Transfer free energies (△<em>G</em><img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>), entropies (△<em>S</em><img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>) and enthalpies (△<em>H</em><img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>) of benzoic acid (HBz) as well as deprotonation energetics of the acid have been determined in quasi-isodielectric aqueous mixtures of 10, 30, 50 and 70 weight (wt) % dipolar aprotic ethylene carbonate (EC) from solubility measurements of the acid and emf measurements of galvanic cells comprising H₂ and Ag - AgCl electrodes respectively at different temperatures. These values when coupled with the previously determined transfer energetics of H⁺ based on the widely used tetraphenylarsonium tetraphenylborate (TATB) reference electrolyte assumption, have yielded the transfer energetics of benzoate ion (Bz^-). The chemical parts of the transfer energetics, △<em>X</em><img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>_,ch are obtained by subtracting the cavity effect as based on the scaled particle theory (SPT) for HBz, and the cavity as well as Born-type and ion-dipole electrostatic effects for Bz. Analysis of △G<img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>_,ch (HBz)- and △G<img src='http://www.niscair.res.in/jinfo/equation30.gif' border=0>,_ch (Bz^-)-composition profiles reveals that while the behaviour of neutral HBz is guided chiefly by the combined stabilizing effects of dispersion, dipole-induced dipole, dipole-dipole and the increased basicity of the mixed solvents, that of Bz^- is guided by the opposing effects of stabilizing dispersion and ion-induced dipole type interactions, and the increased destabilizing effects of dipolar aprotic nature of the cosolvent. T△<em>S</em>_,ch (i)-composition profiles for HBz and Bz^- when examined in the light of Kundu <em>et</em> <em>aI'</em>s four step transfer process reveals that addition of cosolvent induces breakdown of three dimensional (3D) water structure due to the possible formation of H-bonded EC - (H₂O)_x complexes and the packing imbalance at higher compositions. The latter profiles at three different temperatures, viz., 25, 35 and 45°C also reflect the expected temperature induced decreased 3D structuredness of the solvents at water-rich compositions.