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Transformation and dehydration kinetics of methylene blue hydrates detected by terahertz time-domain spectroscopy

机译：太赫兹时域光谱法检测亚甲基蓝水合物的转化和脱水动力学

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Methylene blue (MB) as an important dye and medicine, has several different crystalline hydrates. These MB hydrates can transform from one to another at different temperatures or humidities. Determination of these hydrates is a big challenge for traditional spectroscopic technologies like infrared spectroscopy (IR) and Raman spectroscopy due to the component and structure similarities among these hydrates. We present a terahertz (THz) spectroscopy technology to differentiate three typical MB hydrates: pentahydrate, dihydrate and anhydrate. The pentahydrate exhibits prominent THz absorption features at 0.84 and 1.68?THz, and the dihydrate exhibits a broad and weak absorption peak at 0.89 THz, while the anhydrate has no obvious characteristic absorption peak at the measured spectral range of 0.2–2.0 THz. The dehydration kinetics of MB hydrates is investigated according to the variation of one of the main THz characteristic absorption peaks of MB pentahydrate with the heating time at different heating temperatures. A clear relation between the dehydration rate and the heating temperature can be fitted by an Arrhenius equation. The fitted activation energy of 64.5 kJ mol?1 is quite consistent with the enthalpy change due to the transformation of MB pentahydrate to MB anhydrate reported in a previous study.
机译：亚甲基蓝（MB）作为重要的染料和药物，具有几种不同的结晶水合物。这些MB水合物可以在不同的温度或湿度下从一种转变为另一种。由于这些水合物之间的成分和结构相似，因此这些水合物的测定对于诸如红外光谱（IR）和拉曼光谱仪等传统光谱技术而言是一个巨大的挑战。我们提出了太赫兹（THz）光谱技术，以区分三种典型的MB水合物：五水合物，二水合物和无水物。五水合物在0.84和1.68？THz处具有突出的THz吸收特征，二水合物在0.89 THz处具有宽而弱的吸收峰，而无水物在0.2–2.0 THz的光谱范围内没有明显的特征吸收峰。根据MB五水合物的主要THz特征吸收峰之一随加热时间的变化，研究了MB水合物的脱水动力学。脱水率和加热温度之间的明确关系可以通过阿伦尼乌斯方程拟合。 64.5 kJ mol ？1 的拟合活化能与先前研究中报道的五水合MB转变为无水MB所引起的焓变非常一致。

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