The kinetics and mechanism of water evolution from molten dl lithium potassium tartrate monohydrate
The kinetics and mechanism of water evolution from molten dl lithium potassium tartrate monohydrate
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熔融dl酒石酸锂钾一水合物析水动力学和机理
DOI：
10.1098/rsta.1992.0112
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发表时间：
1992
期刊：
影响因子：
--
通讯作者：
A. Galwey
中科院分区：
文献类型：
--
作者：
S. D. Bhattamisra;G. Laverty;Nikolai A. Baranov;V. B. Okhotnikov;A. Galwey
报道了 dl 酒石酸钾锂一水合物热脱水的动力学和微观研究，并讨论了反应机理。这项工作是一项更全面的研究的一部分，该研究涉及反应物结构对反应性的影响和化学变化的机制。与该盐进行比较的其他水合反应物含有酒石酸阴离子的d和内消旋形式，并以不同的结构结晶。 dl 酒石酸锂钾一水合物在 350-460 K 之间研究的一个主要减速过程中失去了单分子结晶水。反应伴随着熔化，产生残留的玻璃状无水产物，其在 X 射线衍射中是无定形的。最初相对较快地释放水 (6%)，随后是一个减速过程，导致零级反应（在晶体中，在 18% 到 80% 之间延伸），然后大约完成一级反应。粉碎粉末反应物样品的脱水与单晶的不同之处在于相对更快（增加了八倍）；减速过程较长，零阶过程较短（50-85%）。晶体和粉末脱水的活化能为330±30 kJ mol-1。这种动力学行为模式与均相反应的预期不符，速率与反应物浓度项不直接相关。对数据服从适用于固态反应的速率表达式的替代分析同样不成功。因此，我们对速率数据的机械解释先验地考虑了预期参与控制熔体中水析出的因素。结论是玻璃相或熔融相不是均匀的，因此行为与各向同性流体或固体中的反应不同。提出了两个模型来解释我们的观察结果。在两相平衡机制中，假设反应物颗粒由两相组成，水合物区域嵌入脱水材料中，脱水材料保留恒定但比例较小的水。 （这些相参与类似于液体/蒸汽的平衡。）表面边界层模型设想了外围屏障区的初始发展，水扩散的恒定速率通过该屏障区进行速率控制。这类反应在流体中进行，但不添加溶剂，受到的关注相对较少。当前的讨论旨在确定特征行为，并提请注意在讨论可能存在熔融参与的固体反应时考虑此类机制的必要性。
A kinetic and microscopic investigation of the thermal dehydration of dl lithium potassium tartrate monohydrate is reported and the reaction mechanism discussed. This work forms part of a more comprehensive study concerned with the influence of reactant structure on the reactivity and the mechanism of chemical change. The other hydrated reactants with which this salt will be compared contain the d and meso forms of the tartrate anion and crystallize with different structures. dl lithium potassium tartrate monohydrate lost the single molecule of water of crystallization in one predominantly deceleratory process that was studied between 350-460 K. Reaction was accompanied by melting to yield a residual glassy anhydrous product that was amorphous to X-ray diffraction. An initial, relatively rapid release of water (6%) was followed by a deceleratory process that led to a zero-order reaction (that, in crystals, extended between 18% and 80% ) before completion by an approximately first-order stage. Dehydrations of crushed powder reactant samples differed from single crystals in being relatively more rapid (an eight-fold increase); the deceleratory process was long and the zero-order process shorter (50-85%). The activation energy for dehydrations of crystal and of powder was 330 + 30 kJ mol-1. This pattern of kinetic behaviour was not in accordance with expectation for a homogeneous reaction, the rate was not directly related to reactant concentration terms. Alternative analyses of the obedience of data to rate expressions applicable to solid state reactions were equally unsuccessful. Our mechanistic interpretation of the rate data, therefore, considered a priori the factors expected to participate in the control of water evolution from the melt. It is concluded that the vitreous or molten phase is not homogeneous and, therefore, behaviour is different from reactions in an isotropic fluid or in a solid. Two models are proposed to explain our observations. In the two phase equilibrium mechanism it is assumed that the reactant particles are composed of two phases, zones of hydrate are embedded in dehydrated material that retains a constant but small proportion of water. (These phases participate in an equilibrium analogous to that of liquid/vapour.) The surface boundary layer model envisages the initial development of a peripheral barrier zone through which the constant rate of water diffusion is rate controlling. This class of reaction, proceeding in a fluid but the absence of added solvent, has received relatively little attention. The present discussion is intended to identify the characteristic behaviour and draw attention to the necessity to consider such mechanisms in discussions of reactions of solids where there is the possibility of melt participation.