Intermolecular Interactions in Ternary Glycerol–Sample–H₂O: Towards Understanding the Hofmeister Series (V)

摘要

We studied the intermolecular interactions in ternary glycerol (Gly)–sample (S)–H₂O systems at 25 °C. By measuring the excess partial molar enthalpy of Gly, H_GlyEH_{mathrm{Gly}}^{mathrm{E}}, we evaluated the Gly–Gly enthalpic interaction, H_Gly-GlyEH_{mathrm{Gly}mbox{--}mathrm{Gly}}^{mathrm{E}}, in the presence of various samples (S). For S, tert-butanol (TBA), 1-propanol (1P), urea (UR), NaF, NaCl, NaBr, NaI, and NaSCN were used. It was found that hydrophobes (TBA and 1P) reduce the values of H_Gly-GlyEH_{mathrm{Gly}mbox{--}mathrm{Gly}}^{mathrm{E}} considerably, but a hydrophile (UR) had very little effect on H_Gly-GlyEH_{mathrm{Gly}mbox{--}mathrm{Gly}}^{mathrm{E}}. The results with Na salts indicated that there have very little effect on H_Gly-GlyEH_{mathrm{Gly}mbox{--}mathrm{Gly}}^{mathrm{E}}. This contrasts with our earlier studies on 1P–S–H₂O in that Na+, F− and Cl− are found as hydration centers from the induced changes on H_IP-IPEH_{mathrm{IP}mbox{--}mathrm{IP}}^{mathrm{E}} in the presence of S, while Br−, I−, and SCN− are found to act as hydrophiles. In comparison with the Hofmeister ranking of these ions, the kosmotropes are hydration centers and the more kosmotropic the higher the hydration number, consistent with the original Hofmeister’s concept of “H₂O withdrawing power.” Br−, I− and SCN−, on the other hand, acted as hydrophiles and the more chaotropic they are the more hydrophilic. These observations hint that whatever effect each individual ion has on H₂O, it is sensitive only to hydrophobes (such as 1P) but not to hydrophiles (such as Gly). This may have an important bearing towards understanding the Hofmeister series, since biopolymers are amphiphilic and their surfaces are covered by hydrophobic as well as hydrophilic parts.