Etude par Microscopie Raman de la Couche Liquide Situee au Contact d'un Cristal de NaNO 3 en Cours de Dissolution dans sa Solution Mère

The efficiency of Raman microscopy coupled with multichannel acquisition of data was tested for studying the dissolution layer of a mineral salt in its undersaturated mother solution. Experimental work was carried out on sodium nitrate solutions at concentrations ranging between 0.05 and 7.8 M . At lower concentrations the existence of three main species, free aquated ions, solvent‐separated ion pairs and contact ion pairs, was confirmed spectroscopically, in addition to a small amount of aggregates (NO 3 − · Na + ) n which occur only at high concentrations ([NO 3 − ]>2 M ). Gaussian‐Lorentzian parameters of each ν 1 ( A ′ 1 ) band characteristic of such individual species were calculated and were found to vary considerably depending on whether the Raman spectra were recorded with a standard scanning system or with a multichannel detector. When the laser beam was focused at the interface between a dissolving NaNO 3 crystal and the sodium nitrate solution ([NO 3 − ]=4.68 M ), the micro‐Raman spectra showed that the concentration of aggregates was considerably increased compared with that determined in the same way in the bulk of the nitrate solution. The technique used to obtain reliable spectra of the dissolution layer is discussed in detail.