Ice mixtures formed by simultaneous condensation of formaldehyde and water: an in situ study by micro-Raman scattering

Abstract

Thin films of formaldehyde–water mixtures are co-deposited at 88 K and 10⁻¹ Torr from gas collected above formaldehyde aqueous solutions of different concentrations (5, 10, 15, 20, 30 mol%). They are analyzed in situ by micro-Raman scattering in the 2700–3800 cm⁻¹ spectral range. The spectral characteristic of H₂CO distributed molecularly in amorphous solid water is obtained under vacuum conditions. As temperature is increased formaldehyde is released during the crystallization of ice between 118 and 138 K. On the other hand, under controlled nitrogen atmosphere, the deposits crystallize in hydrate phases (or solid H₂CO(s)) during annealing. A new phase (metastable FOR-A) of H₂CO(s) (or a low hydrate after rejection by crystallizing ice) can be spectroscopically identified at 138 K before transformation into a hydrate (with molecular H₂CO distributed within the cages of the clathrate FOR-B) takes place at 148 K. This latter phase decomposes between ca. 180 and 200 K. The significant spectral differences between these hydrates and those formed in frozen formaldehyde aqueous solutions reflect the existence of H₂CO-clusters of distinctive structural nature relative to those resulting from important oligomerization process in the liquid. Moreover, the structure, the gas distribution and relative gas population in the formaldehyde clathrate cages are influenced by the relative amount of trapped nitrogen at the surface, which moreover depends on the ice film morphology. The dependence on the crystallization temperature of the deposits is explained by the relative amounts of occluded H₂CO/N₂ and the external pressure conditions. The distinct behavior observed between vacuum and N₂-atmosphere conditions certainly reflects a complex mechanism of surface mediated nucleation in which the transport of the reactants to the hydrate reaction zone is facilitated by the presence of a polar dopant.