Chemical Transfer Energetics of the -CH_2- Group: A Possible Probe for the Solvent Effect on Hydrophobic Hydration and the 3D-Structuredness of Solvents

摘要

Transfer Gibbs energies ΔG°_t, and entropies, ΔS°_t, of -CH_2- 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 ℃ by spectrophotometric measurements. The chemical contributions of these energetics of the ions (i), ΔG°_(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 (H_1H) of the COO~- ion and cosolvent induced hydrophobic hydration (H_bH) of the R group and the back-bonding interaction of d~(10) 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 (-CH_2-) are found to be more or less same, indicating their additivity. The increase in ΔG°_(t,ch) (-CH_2-)- composition profiles and the "characteristic maximum" of TΔS°_(t,ch) (-CH_2-)- composition profiles indicate the effect of increasingly 3D-structure promotion of these alkanols. The decrease in ΔG°_(t,ch) (-CH_2-) and the broad minimum in ΔS°_(t,ch) (-CH_2-) in aqueous ACN indicate the effect of increased H_bH caused by 3D structure breaking of ACN. Thus the chemical transfer energetics and especially entropies of -CH_2- reflect not only the solvent effect on H_bH but also the 3D-structuredness of aquo-organic cosolvents.