Chemical Transfer Energetics of the −CH2− Group:  A Possible Probe for the Solvent Effect on Hydrophobic Hydration and the 3D-Structuredness of Solvents

Transfer Gibbs energies ΔG°t, and entropies, ΔS°t, of −CH2− have been evolved in aqueous mixtures of methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH), tert-butyl alcohol (t-BuOH), and acetonitrile (ACN) by determining the solubilities of Ag salts of acetate (OAc-), propionate (OPr-), n-butyrate (OBu-), as well as picrate (Pi-) ions from 15 to 35 °C by spectrophotometric measurements. The chemical contributions of these energetics of the ions (i), ΔG°t,ch(i) and TΔS°t,ch(i), at T = 298.15 K have been evolved by subtracting the cavity effect and Born-type and ion−dipole-type electrostatic interaction effects. ΔG°t,ch(i) values of carboxylates (RCOO-) are guided by solvent acidity induced hydrophilic hydration (HlH) of the COO- ion and cosolvent induced hydrophobic hydration (HbH) of the R group and the back-bonding interaction of d10 electrons in the case of Ag+ ion, while TΔS°t,ch(i) values are partly guided by structural effects as well. ΔG°t,ch and TΔS°t,ch values of (−CH2−) are found to be more or less same, indicating their additivity. The increase in ΔG°t,ch (−CH2−)−composition profiles and the “characteristic maximum” of TΔS°t,ch (−CH2−)− composition profiles indicate the effect of increasingly reduced HbH caused by increasing 3D-structure promotion of these alkanols. The decrease in ΔG°t,ch (−CH2−) and the broad minimum in ΔS°t,ch (−CH2−) in aqueous ACN indicate the effect of increased HbH caused by 3D structure breaking of ACN. Thus the chemical transfer energetics and especially entropies of −CH2− reflect not only the solvent effect on HbH but also the 3D-structuredness of aquo-organic cosolvents.