Interactions of Water with Pyridine and Benzene Studied by TOF-SIMS

Abstract

TOF-SIMS is used to characterize the weak intermolecular interactions between heavy water and aromatic molecules such as benzene and pyridine in order to gain insight into solvation. The D2O molecules dissolve in the thin-layer bulk of pyridine above 110 K due to the formation of hydrogen bonds, but they are hardly incorporated in the benzene layer. The benzene molecules dissolve in the D2O layer via hydrophobic hydration for T >120 K, whereas the pyridine remains on the D2O layer up to the desorption temperature of 180 K. Pyridine forms hydrogen bonds with the D2O, but no indication of hydrophobic hydration is observed except at temperatures around 150 K where the mobility of the D2O molecules would be increased due to the phase transition to the cubic ice. From the comparison of the hydration patterns between pyridine and benzene, it is suggested that hydrophobic hydration tends to be quenched on the surface if the hydrophilic hydration coexists. This is because the hydrophobic interaction is energetically a minor process compared to hydrophilic interactions, so that the water structure is determined by the normal O−D···N and O−D···O hydrogen bonds.