In situ Raman spectroscopy for the evaluation of solubility in supercritical carbon dioxide mixtures

Characterization of Water Solubility in n-Octacosane Using Raman Spectroscopy

In this study, we demonstrate the ability of polarization-difference Raman spectroscopy (PDRS) to detect dissolved free water molecules in a n-octacosane (n-C₂₈H₅₈) liquid-rich phase, and thus to determine its solubility, at temperatures and pressures relevant to the Fischer-Tropsch synthesis. Our results for the pure alkane reveal thermal decomposition above a temperature of 500 K as well as an increase of gauche conformers of the alkane chains with an increase in temperature. For binary homogeneous mixtures, raw spectra obtained from two different polarization scattering geometries did not show a relevant signal in the OH stretching frequency range. In contrast, isotropic spectra obtained from the PDRS technique reveal a narrow and tiny peak associated with the dangling OH bonds. Over the complete range of temperatures and pressures, no signature of hydrogen-bonded water molecules was observed in the isotropic Raman scattering intensities. A thorough investigation covering a large range of temperatures and pressures using PDRS signals showed that the higher the fraction of gauche conformers of hydrocarbon, the higher the solubility of water. The proportion of gauche and trans conformers was found to be water-concentration-independent, and the intensity of the OH-dangling peak increased linearly with increasing the vapor partial pressure of water. Therefore, we established a relation between a relevant intensity ratio and the concentration of water obtained from SAFT calculations. Contrary to the results from relevant literature, the calibration factor was found to be temperature-independent between 424 and 572 K. The isotropic Raman scattering intensities are corrected in order to provide a better representation of the vibrational density of states. The influence of correction of the isotropic scattering intensities on the solubility measurements as well as on the analysis of the molecular arrangement is discussed.

Sustainable analytical chemistry—more than just being green

This review article describes analytical chemistry beyond green chemistry and all efforts that contribute to a more sustainable development. A background is given on sustainable development and green chemistry. Examples of “greening” strategies for sample preparation, chromatography, and detection are given. Thereafter, the review discusses how and why a method or a solvent could be claimed as being “green”. Green metrics for analytical chemistry is discussed, including the environment, health, and safety (EHS) index and life cycle assessment (LCA). The choice of solvent and the criteria for a solvent being “green” is also discussed. Finally, sustainable analytical chemistry is described by considering the three important “legs” so as to obtain sustainable development—economic feasibility, societal relevance, and environmental soundness. Hopefully, the review article will stimulate some new perspectives on the difference between greenness and sustainability in analytical chemistry.