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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Rate Study of Ice Particle Conversion to Ammonia Hemihydrate: Hydrate Crust Nucleation and NH_3 Diffusion
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Rate Study of Ice Particle Conversion to Ammonia Hemihydrate: Hydrate Crust Nucleation and NH_3 Diffusion

机译：冰粒转化为半水氨的速率研究：水合物结壳成核和NH_3扩散

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摘要

Reaction rates for the conversion of ice nanocrystals within 3-D arrays, to the hemi- and monohydrates (and deuterates) of ammonia, have been determined for temperatures in the range 100 to 128 K. The loss of ice and the growth of the hydrate product, as a function of time, temperature, and the activity of ammonia at the surface of the particles, has been monitored using transmission FT-IR spectroscopy. Though this study has focused on the ammonia-ice system, the results may provide general insights to the low-temperature formation of hydrates from ice particles. The ammonia hydrate formation follows a nucleation stage that occurs only after saturation of the ice surface with ammonia molecules; the propagation of the reaction depends on ammonia diffusion, not within the ice but through a hydrate crust that quickly envelops the particles. Apparently, adsorbed ammonia molecules do not achieve a free energy consistent with the nucleation of a new (hydrate) phase until saturation of the low free energy ice surface sites is complete. After nucleation occurs, diffusion of ammonia through the hydrate crust may be rate 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 "hemihydrate" has been determined as 2.8 * 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 * 10~(-17) at 107 K with E_a estimated as 12 kcal/mol.
机译：已经确定了温度范围为100至128 K时，将3D阵列中的冰纳米晶体转化为氨的半水和一水合物（和氘代）的反应速率。冰的损失和水合物的生长产品的时间，温度和颗粒表面氨活性的函数已使用透射FT-IR光谱仪进行了监测。尽管此研究集中在氨冰系统上，但该结果可能为从冰粒中低温形成水合物提供一般见解。氨水合物的形成遵循一个成核阶段，该阶段仅在冰表面被氨分子饱和后才会发生。反应的传播取决于氨的扩散，而不是取决于冰中的氨扩散，而是取决于迅速包裹颗粒的水合物外壳。显然，直到低自由能冰表面位点的饱和完成之前，吸附的氨分子才能获得与新（水合物）相成核相一致的自由能。在成核之后，可以通过速率控制氨通过水合物外壳的扩散，确定的参数是吸附在颗粒（水合物）表面上的氨的化学活性和水合物外壳的厚度。氨在无定形“半水合物”中的扩散系数经确定为在102 K时E_a = 15 kcal / mol时为2.8 * 10〜（-19）cm〜2 / s，而结晶半水合物的扩散系数为1.1 *在107 K下为10〜（-17），E_a估计为12 kcal / mol。

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