The solid state dehydration of d lithium potassium tartrate monohydrate is complete in two rate processes II. The nucleation and growth second reaction and dehydration mechanism
The solid state dehydration of d lithium potassium tartrate monohydrate is complete in two rate processes II. The nucleation and growth second reaction and dehydration mechanism
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d酒石酸锂钾一水合物的固态脱水在两个速率过程II中完成。
DOI：
10.1098/rsta.1994.0043
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发表时间：
1994
期刊：
影响因子：
--
通讯作者：
V. B. Okhotnikov
中科院分区：
文献类型：
--
作者：
A. Galwey;G. Laverty;Nikolai A. Baranov;V. B. Okhotnikov
对一水酒石酸锂钾脱水过程中连续发生的两个速率过程中的第二个成核和生长反应进行了动力学和机理研究。部分反应晶体解理表面的电子显微镜观察表明，有由无水产物的小晶体组成的三维晶核的发展，在450K以上有核内熔化的证据。与该模型一致，第二反应遵循Avrami-Erofe‘ev方程{[-ln(1-α)]1/2=kt}。单晶样品和粉碎粉末样品的总体脱水速率非常相似。第一反应和第二反应以及单晶和粉末状反应物的脱水活化能均为150-160kJ·mol-1。在反应物中加入产物微晶大大缩短或消除了成核和生长过程的诱导期。从动力学特征和显微观察到的织构变化来看，我们的结论是以下机理很好地解释了我们的结果。第一个反应进行到所有晶体表面的脱水，代表着大约10微米厚度的层的水分损失。这种减速过程最初发生在类似于反应物的结构中，但后来水位空位浓度的增加导致反应物无序增加，并可能包括外层的融合。当第一反应水的析出速度减慢时，无水产物的结构在有限数量的位置发生再结晶，以产生有效地充当核生长的种子晶体的胚核。在第二个反应中，反应物-产物接触界面被确定为扩散失水区，类似于第一个反应的描述。然而，在这里，产物的微晶促进了脱水物质的重组，从而打开了水逃逸的通道，并不断暴露出新的水合物表面，在这些表面上脱水继续进行。这种产物重结晶能够保持核界面的推进，因此第一和第二反应的速率都受到来自反应物活性边界的水的扩散损失的控制。产品重组通过引入失水的逃逸通道来消除累积产物层的抑制特性，从而使界面继续推进，尽管是痉挛的，但这是以恒定的平均线性速率向前迁移的。这项工作很有意义，因为已经获得了对整个反应有贡献的两个连续速率过程的动力学测量。两者的控制显示非常相似。本文提出的反应模型提供了对脱水界面的结构和水分释放机理的洞察。
A kinetic and mechanistic study has been undertaken of the nucleation and growth reaction that is the second of the two consecutive rate processes that occur during the dehydration of d lithium potassium tartrate monohydrate. Electron microscopic examinations of the cleaved surfaces of partly reacted crystals show the development of three-dimensional nuclei that are composed of small crystals of the anhydrous product and above 450 K there is evidence of intranuclear melting. Consistent with this model, the second reaction obeys the Avrami-Erofe’ev equation {[ — ln (1 — α)]1/2 = kt}. Overall rates of the dehydrations of single crystals and of crushed powder samples were closely similar. The activation energy for dehydration was 150-160 kJ mol-1 for both first (reported in part I, preceding paper) and second reactions and for both single crystal and crushed powder reactants. The addition of product crystallites to the reactant reduced sharply, or eliminated, the induction period to the nucleation and growth process. From consideration of the kinetic characteristics, together with the textural changes observed microscopically, we conclude that the following mechanism very satisfactorily accounts for our results. The first reaction proceeds to the dehydration of all crystal surfaces, representing water losses from a layer ca. 10 µm thickness. This deceleratory process occurs initially in a structure resembling that of the reactant but later the increasing water site vacancy concentration results in increasing reactant disorder and possibly includes fusion of the outer layer. When the first reaction water evolution has slowed, recrystallization to the structure of the anhydrous product occurs at a limited number of sites to generate germ nuclei that effectively act as seed crystals for nucleus growth. During the second reaction the reactant—product contact interface is identified as a zone of diffusive water loss, similar to that described for the first reaction. Here, however, the product crystallites promote reorganization of dehydrated material, thereby opening channels for water escape and continually exposing new hydrate surfaces at which dehydration continues. This product recrystallization enables advance of the nucleus interface to be maintained, so that rates of both first and second reactions are subject to control by diffusive loss of water from an active boundary of the reactant. Product reorganization removes the inhibiting character of accumulated product layer by introducing escape channels for water loss so that interface advance continues and, although spasmodic, this migrates forward at a constant average linear rate. The work is of interest because kinetic measurements have been obtained for both of the consecutive rate processes that contribute to the overall reaction. The controls of both are shown to be closely similar. The reaction model proposed here provides insight into the structure of the dehydration interface and the mechanism of water release.