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Updated: May 23, 2026

Methane Hydrate Crystallization on Sessile Water Droplets
Published on: May 26, 2021
The crystalline state of methylene blue: a zoo of hydrates
Timo Rager1, André Geoffroy, Rolf Hilfiker
1Solvias AG, Department for Solid-State Development, Römerpark 2, 4303 KAISERAUGST, Switzerland. timo.rager@solvias.com
Abstract:
A re-investigation of the crystalline state of methylene blue has led to the identification of five different hydrates with clearly distinct structures. These include the already known pentahydrate, a hydrate with 2.2-2.3 equivalents of water, two dihydrates, and a monohydrate. Contrary to older reports, no trihydrate was found. The preparation and characterization of the hydrates as well as the transformations between them are reported. The applied analytical methods include X-ray powder diffraction (XRPD), infrared spectroscopy (ATR-IR), thermogravimetry (TGA), differential scanning calorimetry (DSC), dynamic water vapor sorption (DVS) and solution calorimetry (SolCal). A phase diagram of temperature vs. composition has been established, and the stability domains of the different hydrates as a function of water activity and temperature have been determined based on data from DSC, SolCal and suspension equilibration experiments. Four out of the five hydrates are thermodynamically stable within a certain range of temperature and humidity.
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