On the Negative Hydration. A Nuclear Magnetic Relaxation Study

Abstract

The paper is opened by a structural and thermodynamic analysis of the phenomenon of negative hydration (or structure breaking effect). This treatment shows that the state of negative hydration is energetically higher than the state of hydration which is caused by an inert solute particle. Furthermore, in the state of negative hydration the molecular configurations of the hydration sphere are spread over a comparatively wide range. Proton relaxation rates in aqueous solutions of an extended number of ions of different types have been measured in order to investigate which ions show negative hydration. The reorientation time of the water molecule is taken as an indicator for the nature of the hydration. Among others some doubly charged anions were found to be structure breakers. In non‐aqueous solvents generally no negative solvation seems to be present, there are, however, at least two exceptions: ethylene glycol and glycerol. Furthermore, it is demonstrated that the negative hydration is a cooperative effect between many water molecules since the addition of organic molecules to the solution generally destroys this effect. In the concluding discussion it is shown that the structure breaking effect is caused by a weak electrostatic field to be predicted from the rough geometry of the ion considered.