Étude de l'adsorption du HF à la surface de la glace

This memoire addresses an old problem in fundamental science i.e. the dissociation of hydrogen fluoride in water. Among hydrogen halides, hydrogen fluoride is well-known to behave as a weak acid in water in contrast to the three other hydrogen halides (HC1, HBr and HI). Different hypothesis were proposed to explain the reasons of this singular behaviour. The most important of these hypotheses could be those of Pauling and Giguere. According to Pauling, the electronegativity of the halogen atom A, which is largest for the fluorine atom, determines the relative stability of the molecular species, HA (aq), versus the ionic products, A' (aq> in water at 25 °C. Alternatively, Giguere, on the basis of his infrared spectroscopic studies of concentrated aqueous HF solutions, postulated that the weak acid behaviour of HF is due to the formation of a very stable F'-H30 + contact ion-pair. Recently, Ayotte showed that the physical origin for the weak acid behaviour of dilute HF solutions has a large entropic contribution. He proposed that dilute cryogenic HF solutions in amorphous solid water could behave as strong acids at very low temperatures (T < 125 K). In this study, infrared vibrational spectra are reported for amorphous solid water (ASW) thin films doped with different concentrations of HF at cryogenic temperatures. We studied the adsorption of hydrogen fluoride upon, and its dissolution within thin ice films condensed on a clean Pt (111) substrate. Using surface science techniques and Fourier transform infrared spectroscopy (FTIR), we identified the spectral signature of HF upon its adsorption and its dissolution in ice. The IR spectra obtained in these experiments were compared with spectra from neat ASW films having the same thickness. A strong continuous absorbance in the 1000-3275 cm-1 range was observed. Our experimental results show that HF molecules seem to be extensively dissociated in amorphous and crystalline ice. Moreover, the recombination of HF upon heating seems to be kinetically inhibited, as observations indicate recombination kinetics is slower than the crystallization kinetics of the metastable binary amorphous solid.

Name thesis:degree_name

M. Sc.

Level thesis:degree_level

Maîtrise

Discipline thesis:degree_discipline

Chimie

Grantor dc:publisher

Université de Sherbrooke

Year dc:date.issued

2007

Author dc:creator

• Rafiei, Zohreh

Advisor dc:contributor.advisor

• Ayotte, Patrick

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/11143/4757

OAI identifier oai:identifier

oai:usherbrooke.scholaris.ca:11143/4757