Vapor deposited ethanol-H2O ice mixtures investigated by micro-Raman scattering

Solid deposits have been formed at 88 K and 10−1 Torr from ethanol-water gas collected above aqueous solutions of ethanol (EtOH) (0.6, 2, 4.5, 9 and 17 mol%). The composition of different gas mixtures varying between 1:16 and 1:0.8 EtOH:H2O are determined at 295 K using our experimental vapor-liquid equilibrium (VLE) data in combination with the Wilson model [28]. The Wilson constants derived at this temperature are Λ12 = 0.37(4) and Λ21 = 0.58(5). The concentration of EtOH in the ice mixture can be calculated using these data and a kinetic model of condensation. It is found to vary between 9 and 65 mol% EtOH. The ice mixtures are analyzed in situ in a modified cryostage by micro-Raman spectroscopy. The distinct vibrational signatures of pure EtOH, EtOH aqueous solutions and EtOH-ice mixtures are identified in the 400-3800 cm−1 spectral range. Internal vibrational motions of EtOH molecules are affected by temperature and concentration. The presence of amorphous EtOH-ice phases at 88 K is demonstrated by the characteristic vibrational signatures of the νOH stretching modes. The crystallization of an EtOH hydrate is proposed during annealing at ∼140 K of a 65 mol% EtOH-ice mixture. According to our preliminary X-ray diffraction work, this phase has apparently a distinct structure from that of solid EtOH or from EtOH-clathtrate structures usually found in frozen aqueous solutions. For ice mixtures of lower EtOH content, a distinct hydrate phase crystallizes at ∼170 K. These results suggest that ice mixtures obtained by vapor deposition reflect the existence of EtOH clusters of a distinctive structural nature with respect to those encountered in frozen aqueous mixtures.