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 Delta G(t)degrees, and entropies, Delta S(t)degrees, 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 degrees C by spectrophotometric measurements. The chemical contributions of these energetics of the ions (i), Delta G(t,ch)degrees(i) and T Delta S(t,ch)degrees(i), at T = 298.15 K have been evolved by subtracting the cavity effect and Born-type and ion-dipole-type electrostatic interaction effects. Delta G(t,ch)degrees(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 d(10) electrons in the case of Ag+ ion, while T Delta S(t,ch)degrees(i) values are partly guided by structural effects as well. Delta G(t,ch)degrees and T Delta S(t,ch)degrees values of (-CH2-) are found to be more or less same, indicating their additivity. The increase in Delta G(t,ch)degrees (-CH2-)-composition profiles and the "characteristic maximum" of T Delta S(t,ch)degrees (-CH2-)-composition profiles indicate the effect of increasingly reduced HbH caused by increasing 3D-structure promotion of these alkanols. The decrease in Delta G(t,ch)degrees (-CH2-) and the broad minimum in Delta S(t,ch)degrees (-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.