A mechanistic and kinetic study: Conversion of ice nanocrystals to nanoparticles of the hydrates of ammonia

Scope and method of study. Two methods were used to prepare FTIR samples: in the first method, an ∼1.0 micron thick network of ice nanocrystals was deposited onto IR transparent windows and then exposed to ammonia vapor in the range 115–123 K. In the second method, the ice particles were mixed with ammonia particles at 90 K and the subsequent reaction was measured in the range 100–112 K. For both methods, the rate of thermal vaporization of ammonia was fast enough to allow measurement of the kinetics for, the conversion of ice nanocrystals to the hydrates of ammonia by monitoring the FTIR spectra as a function of time. However, the first method resulted in formation of the monohydrate of ammonia (NH₃.H₂O) and the second one produced the hemihydrate of ammonia (2NH₃.H ₂O).; Findings and conclusion. It is suggested that which hydrate phase forms is determined by a nucleation process and that the conversion of ice to ether hydrate of ammonia proceeds via a molecular process. Furthermore, it was concluded that the vapor pressure of ammonia is rate limiting for'; the conversion of ice nanocrystals to the monohydrate of ammonia as the activation energy for the monohydrate formation was found to equal the heat of vaporization of NH₃ (s), which is ∼7 kcal/mol. For the conversion of ice nanocrystals to the hemihydrate of ammonia, the rate-limiting step is the diffusion of ammonia through the hemihydrate that forms on the surface of the ice particles. It was shown that the experimental results correlate successfully with the “shrinking (unreacted) core diffusion model”. The diffusion coefficient of ammonia in the amorphous hydrate crust was evaluated in the range 1.72 × 10−18–5.82 × 10 −17 cm/sec at 102–107 K and the activation energy was found as ∼15.13 kcal/mol for the conversion of D₂O ice nanocrystals to the amorphous 2NH₃.D₂O. By comparison, the diffusion coefficient was in the range 6.89 × 10−17–8.81 × 10−16 cm2/sec at 107–112 K and the activation energy was{09}∼12.12 kcal/mol for the conversion of D₂O ice nanocrystals to the crystalline 2NH₃.D ₂O particles.