Hydronium dynamics in the perchloric acid clathrate hydrate

Abstract

Clathrate hydrates are nanoporous crystalline materials made of a network of hydrogen-bonded water molecules (forming host cages) stabilized by the presence of foreign (generally hydrophobic) guest molecules. The encapsulation of strong acids such as perchloric acid (with an acid to water ratio of 5.5) leads to an ionic clathrate structure formed with perchlorate anions within cationic cages. The excess protons, at the origin of the large protonic conductivity of the compound, are delocalized over the cage sub-structure through a Grotthus mechanism. Such ionic crystal constitutes a good model system for investigating the spatial and time characteristics of the elementary steps of the proton conduction. In this paper, the dynamics of hydronium ions are investigated by means of incoherent quasi-elastic neutron scattering (QENS) experiments performed by tuning the observation time to the timescale of the probed dynamical process met in the perchloric acid clathrate hydrate at 220 K. Thanks to the specific versatility of time-of-flight spectrometer, two elementary processes have been identified for modeling the hydronium ion dynamics: hydronium reorientations coupled to intermolecular proton transfer. The hydronium ions undergo reorientations with a characteristic time of 42 ± 8 ps while the proton transfer within the hydrogen bond between hydronium ion and neighboring water molecule occurs with a characteristic time of 1.4 ± 0.3 ps. A previous high-resolution QENS investigation has shown that water molecules surrounding the hydronium ions undergo reorientations with a characteristic time of 0.7 ± 0.1 ns at 220 K. Such experimental results outline the key role played by the relaxation of the water molecules surrounding the hydronium ions prior to any proton transfer.