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

Abstract

Terahertz time-domain spectroscopy in a transmission geometry combined with visual analysis was used to investigate the crystallization process of MgSO 4 solution. Careful spectral analysis of both a feature at 1.6 THz and the overall magnitude of absorption allowed the extraction of information about the liquid phase before and during crystallization, aiding the investigation of solvation dynamics and the behavior 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 semicrystalline systems were converted to the solvent concentrations using a recently developed method. Solutions of a series of concentrations were also investigated in the temperature range of 4–25 °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 behavior is different from bulk water. Future work will focus on triggering nucleation at specific positions in order to study the very beginning of the crystallization process. MgSO 4 heptahydrate is used as a model system in this study, while the concept and the setup can be applied to other systems.

Terahertz Signatures of the Methane Replacement Reaction in Hydroquinone Clathrates

“…However, if even a small proportion of CO 2 molecules are replaced in the clathrate, we expect a measurable impact on the resulting spectra because terahertz spectroscopy is so sensitive to the identity and orientation of the guest molecules. 46,52 Accordingly, since AIMD simulations of neither pure CO 2 nor pure CH 4 fully explain spectral features like the relative heights of the 1.07 THz peak and the 1.21 THz peak, we initialized simulations of mixed-gas states, where some pores are occupied by CO 2 and others by CH 4 molecules. We generated supercells with proportions of two-to-one and oneto-two CH 4 -to-CO 2 , respectively, corresponding to the options for a system with three pores per unit cell.…”

“…The final step in our analysis is informed by recent studies on HQ clathrates, which suggested that CO 2 –CH 4 replacement reactions are generally incomplete, leaving both guest molecules present in the final system. , In fact, ref reports that the replacement reaction from CO 2 to CH 4 resulted in only 6% to 9% of CO 2 molecules replaced. However, if even a small proportion of CO 2 molecules are replaced in the clathrate, we expect a measurable impact on the resulting spectra because terahertz spectroscopy is so sensitive to the identity and orientation of the guest molecules. , …”

“…Subsequently, fully static (0 K) vibrational simulations were performed within the harmonic approximation via finite-differences and numerical differentiation using the crystal23 software package, which yielded vibrational normal modes (eigenvectors) and frequencies (eigenvalues). − crystal23 was chosen for the static-DFT vibrational analyses given its long track record for successfully simulating the terahertz spectra of molecular crystals. − Here, we have attempted to mimic the theoretical parameters used by the cp2k simulations as much as possible given the differences inherent to the two codes, by utilizing the generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) functional coupled with Grimme’s D3 dispersion correction , and the Ahlrichs TZVP basis set . Infrared intensities were determined through the Berry Phase method …”

Dynamical Transition in Dehydrated Proteins

“…Fröhlich postulated the existence of coherent vibrations in the gigahertz to terahertz range that help facilitate the biological function of biomolecules. , In terahertz spectra, such coherent modes appear as peaks in the absorption spectrum. Simultaneously, the absorption coefficient at neighboring frequencies decreases . No such features were found in any measurement (Figure c).…”

“…Simultaneously, the absorption coefficient at neighboring frequencies decreases. 45 No such features were found in any measurement (Figure 1c). Our data hence show no evidence for Froḧlich coherence or the existence of a Froḧlich condensate or other quantum effects in a system that is not actively being pumped.…”