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

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.