In-Situ Observation of the Structure of Crystallising Magnesium Sulfate Heptahydrate Solutions with Terahertz Transmission Spectroscopy

Terahertz time-domain spectroscopy in a transmission geometry combined with vi- sual analysis was used to investigate the crystallisation process of MgSO4 solution. Careful spectral analysis of both a feature at 1.6 THz and the baseline allowed the ex- traction of information about the liquid phase before and during crystallisation, aiding the investigation of solvation dynamics and the behaviour of molecular species at phase boundaries. The method was reproducibly applied to a number of measurements on a series of solutions of three chosen concentrations at different temperatures. When increasing temperature at the end of the measurement, the dissolution of crystals was observed as well. The temperature-dependent absorption data of the semi-crystalline systems were converted to the solvent concentrations using a recently developed method. Solutions of a series concentrations were also investigated in the temperature range of 4◦C to 25◦C. The results were compared to the theoretical calculated values, and the consistent differences proved the existence of a hydration shell around the salt ions whose behaviour is different from bulk water. Future work will focus on triggering nucleation at specific positions in order to study the very beginning of the crystallisa- tion process. MgSO4 heptahydrate is used as a model system in this study, while the concept and the set-up can be applied to other systems.