Transfer energetics of tetraalkylammonium ions in aquo-organic systems and the solvent effect on hydrophobic hydration

The solvent effect on hydrophobic hydration has been studied by measuring the solubilities of some tetraalkylammonium tetraphenylborate (R4N+Ph4B-) salts with R = methyl (Me), ethyl (Et), n-propyl (Pt), and n-butyl (Bu) spectrophotometrically at 25-degrees-C in some aqueous mixtures of tert-butyl alcohol (TBA) and acetonitrile (ACN). The observed results, which yield standard free energies of transfer (DELTA-G(t)-degrees) of R4N+Ph4B- from reference solvent water to these cosolvents, have been dissected into individual ion contributions using the preevaluated values of Ph4B- which are based on the widely used tetraphenylarsonium tetraphenylborate (TATB) reference electrolyte (RE) assumption. The DELTA-G(t)-degrees (i) values of R4N+ cations were then combined with the previously evaluated standard enthalpies of transfer (DELTA-H(t)-degrees (i)) based on the same RE assumption, to give standard entropies of transfer (DELTA-S(t)-degrees (i)). The DELTA-G(t)-degrees (i) values were analyzed in light of the cavity effect and electrostatic effect and the rest in light of the hydrophobic hydration (HH) effect, which was found to decrease with cosolvent. The corresponding DELTA-S(t,HH)-degrees values when analyzed in light of Kundu et al.'s four-step transfer process and the marked influence on water structure by the three-dimensional (3D) structure promoter TBA and 3D structure breaker ACN molecules reflect the respective solvent effect on hydrophobic hydration as induced by R4N+ cations.