Rate study of ice particle conversion to ammonia hemihydrate: Hydrate crust nucleation and NH3 diffusion

Reaction rates for the conversion of ice nanocrystals within 3-D arrays, to the hemi- and monohydrates land deuterates) of ammonia, have been determin ed for temperatures in the range 100 to 128 K. The loss of ice and the grow th of the hydrate product, as a function of time, temperature, and the acti vity of ammonia at the surface of the particles, has been monitored using t ransmission FT-IR spectroscopy. Though this study has focused on the ammoni a-ice system, the results may provide general insights to the low-temperatu re formation of hydrates from ice particles. The ammonia hydrate formation follows a nucleation stage that occurs only after saturation of the ice sur face with ammonia molecules; the propagation of the reaction depends on amm onia diffusion, not within the ice but through a hydrate crust that quickly envelops the particles. Apparently, adsorbed ammonia molecules do not achi eve a free energy consistent with the nucleation of a new (hydrate) phase u ntil saturation of the low free energy ice surface sites is complete. After nucleation occurs, diffusion of ammonia through the hydrate crust may be r ate controlling, the determining parameters being the chemical activity of the ammonia adsorbed on the particle (hydrate) surface and the thickness of the hydrate crust. A diffusion coefficient for ammonia in the amorphous "h emihydrate" has been determined as 2.8 x 10(-19) cm(2)/s at 102 K with E-a = 15 kcal/mol, while the coefficient found for the crystalline hemihydrate was 1.1 x 10(-17) at 107 K with E-a estimated as 12 kcal/mol.