Étude par spectroscopie de diffusion raman de la relaxation vibrationnelle de l'ion nitrate en solution aqueuse dans les états liquides et vitreux

Laboratoire de Spectroscopie Infrarouge (UA 124), Université de Bordeaux I, 351, cours de la Libération, 33405 Talence, France.

Résumé

A partir de l'analyse des profils de diffusion Raman de l'anion nitrate en solution aqueuse, il est possible de déterminer des grandeurs caractéristiques de sa dynamique vibrationnelle. En suivant le comportement dynamique de cet anion en fonction de la temperature, de la concentration en sel et de la pression on peut obtenir des informations sur l'évolution de la structure locale de sa couche de solvatation. L’étude du spectre Raman isotrope de la vibration v, (1 050 cm^-1) de l'ion NO^-₃ dans les solutions LiO₃(1M)/LiCI(xM)/H₂O pour 0<x<8 permet d'atteindre la distribution (M2) et les fluctuations temporelles (τ₍₁₎) de l'environnement immédiat de l'anion.

Abstract

From the analysis of the Raman scattering profiles of the nitrate anion in aqueous solution, characteristic vibrationnal dynamical parameters can be obtained. Such a study of the dynamical behaviour of this anion with varying temperatures, salt concentrations and pressures gives important information on the changes in the local order of its solvation sheat. The study of the isotropic Raman profile of the vibrational v₁ (1 050 cm_-1) mode of NO^-₃ion in LiO₃(1M)/LiCI(xM)/H₂O (0<x<8) solutions, allows for the determination of the distribution (M2) and the temporal fluctuations (τ₍₁₎) of the close environment of this anion.

Without LiCI, profiles are symmetric and raising the temperature leads to a broadening of the distribution of the nitrate environments and to an increase in their fluctuation rates. Globally, modulation becomes faster and the process tends to the homogeneous limit.

Adding LiCI allows for the study of the spectra in the liquid and in the vitreous states. Into the glass phase (77 K), the slow modulation limit is almost reached for phase relaxation and there is evidence for a small contribution coming from vibrational energy relaxation (on a 10 picoseconds time scale). Into the liquid phase (243-353 K), increasing LiCI concentration, raising the temperature and decreasing the pressure give rise to a progressive incorporation of lithium cation into the solvation sphere of the anion. The profiles, which are now unsymmetrical, show that the equilibrium :

[math]

(external sphere) (internal sphere)

is characterized by a ΔH<0 and a ΔV>0, in agreement with the already known thermodynamic properties of the lithium cation solvation