THE EFFECT OF WATER-VAPOR PRESSURE ON DESOLVATION KINETICS OF CAFFEINE 4/5-HYDRATE
THE EFFECT OF WATER-VAPOR PRESSURE ON DESOLVATION KINETICS OF CAFFEINE 4/5-HYDRATE
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DOI：
10.1016/0378-5173(94)00416-3
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发表时间：
1995-06-16
影响因子：
5.8
通讯作者：
BURGER, A
中科院分区：
文献类型：
--
作者：
GRIESSER, UJ;BURGER, A
在25 ℃下，对两种不同晶体尺寸的晶体制备物在不同相对湿度（RH）下的咖啡因4/5-水合物脱水动力学进行了研究。在五氧化二磷和9、13、24、36和43%的相对湿度下，使用特殊的湿度调节器和低于重量的天平评价重量变化。此外，将样品在0 - 98%的RH下储存8个月。通过分析数据与众多的动力学模型，以及微观调查和各种热分析技术（热显微镜，热重分析，差示扫描量热法）的反应机理的解释。脱水反应的机理被发现是非常复杂的，取决于晶体的大小和水蒸气压。扩散控制以及成核和生长反应被认为是该过程的基本原理。由于Smith-Topley效应的发生，粗结晶批料在0 - 13%相对湿度之间发生恒定的失水速率。小晶体的脱水速率更受水蒸气压变化的影响。在干燥剂中，完全转化为无水物需要大约4小时，是粗晶体所需时间的30分之一。即使在61%RH（25摄氏度）下，咖啡因水合物也会失去结晶水，这在加工和储存条件下应予以考虑。除此之外，本文还表明，需要采用多种分析技术来获得有关结晶水合物脱水特征的足够信息，并避免对固态相变的误解。
The kinetics of dehydration of caffeine 4/5-hydrate at different relative humidities (RH) was studied at 25 degrees C for two crystal preparations with different crystal sizes. The weight change was evaluated over phosphorus pentoxide and relative humidities of 9, 13, 24, 36 and 43%, using special hygrostats and a below-weight balance. Additionally the samples were stored for 8 months at RH between 0 and 98%. A mechanistic interpretation of the reaction was derived by analyzing the data with numerous kinetic models as well as by microscopic investigations and various thermoanalytical techniques (thermomicroscopy, thermogravimetry, differential scanning calorimetry). The mechanism of the dehydration reaction was found to be very complex, depending on the crystal size and water vapor pressure. Diffusion control as well as nucleation and growth reaction are considered as the fundamental principles of the process. Due to the occurrence of a Smith-Topley effect, a constant loss of water rate of a coarse crystalline batch between 0 and 13% relative humidity takes place. The dehydration rate of small crystals is much more affected by changes in water vapor pressure. Over desiccants, complete transformation to the anhydrate requires about 4 h, a 30th of the time required for the coarse crystals. Caffeine hydrate loses the water of crystallization even at 61% RH (25 degrees C), which should be considered during processing and storing conditions. Beyond that, this paper suggests that a variety of analytical techniques are necessary for sufficient information about the dehydration characteristics of a crystalline hydrate and to avoid misinterpretations of phase transformations in the solid state.