Flow Cell to Study Crystallization Processes In Situ Using Terahertz Time-Domain Spectroscopy

Abstract

A versatile setup based on terahertz time-domain spectroscopy was developed to monitor the process of crystallisation and characterise the structure of the crystals as well as the dynamics of the solvent in situ. The setup can be used to investigate crystallisation processes over a wide range of temperature, either under static conditions or during continuous flow. An attached optical imaging probe can be used to monitor the process in parallel to the terahertz spectroscopy investigation. The implementation of terahertz spectroscopy offers a great opportunity to investigate the dielectric relaxation dynamics of the solvent and the vibrational modes characteristic for the crystalline phases simultaneously. The details of the crystallisation setup are demonstrated at the example of the crystallisation process of magnesium sulphate heptahydrate from its aqueous phase.

Measuring Solute Concentration with Terahertz Time-Domain Spectroscopy in Single and Multiphase Systems

“…Aqueous magnesium sulfate solutions were prepared to various defined concentrations from commercial MgSO 4 ·7H 2 O (Sigma-Aldrich, Gillingham, UK). The experimental setup was described in detail in ref and consisted of a liquid cell inserted into a hollow metal sample holder. The temperature was controlled by a recirculating water bath.…”

“…Subsequently, each sample spectrum was calculated from the average of 15 individual waveforms, resulting in the acquisition of one spectrum per 20 s and covering the frequency range from 0.35 to 2 THz. The absorption coefficient was calculated by shifting the acquired reference waveform by about 20 ps and subsequently using the method of Duvillaret et al, , one of a number of transfer matrix approaches …”

“…Subsequently, each sample spectrum was calculated from the average of 15 individual waveforms, resulting in the acquisition of one spectrum per 20 s and covering the frequency range from 0.35 to 2 THz. The absorption coefficient was calculated by shifting the acquired reference waveform by about 20 ps and subsequently using the method of Duvillaret et al, 3,9 one of a number of transfer matrix approaches. 10 Several thermocouples continuously monitored the temperature of the water bath, of the circulating water surrounding the crystallization cell, of the syringe, and of the solution at the inlet of the cell.…”

“…Its use can be extended to study crystallization itself, and it has been shown that even low amounts of crystallinity can be detected in semicrystalline systems . We recently developed a setup that allows for the in situ investigation of crystallization from the liquid phase over a range of concentrations and temperatures …”

“…2 We recently developed a setup that allows for the in situ investigation of crystallization from the liquid phase over a range of concentrations and temperatures. 3 Ultraviolet−visible (UV−vis) spectroscopy has been widely applied for determining the concentration of a range of chemical species in a liquid, such as in the field of solution chemistry and water quality monitoring. 4 By measuring the absorption spectrum of the sample of interest, the identity of a known compound or molecule in solution can be confirmed, and using a calibration curve, the concentration of the compound can be determined quantitatively.…”