Characterization of Water Solubility in n-Octacosane Using Raman Spectroscopy

Abstract

In this study, we demonstrate the ability of polarization-difference Raman spectroscopy (PDRS) to detect dissolved free water molecules in a n-octacosane (n-CH) 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.

Viscosity, Interfacial Tension, and Density of Binary-Liquid Mixtures of n-Hexadecane with n-Octacosane, 2,2,4,4,6,8,8-Heptamethylnonane, or 1-Hexadecanol at Temperatures between 298.15 and 573.15 K by Surface Light Scattering and Equilibrium Molecular Dynamics Simulations

“…The atomistic RDFs from n -hexadecane, on the other hand, show a more defined first peak, followed by a local minimum and then a second peak. This shows the ability of linear molecules to form large structures with a periodic arrangement. − From the CE–CE RDFs, the presence of two peaks within the first maxima, between 0.4 and 0.5 nm, can be seen. The two peaks here represent different configurations due to the trans - or gauche -conformation of the linear alkanes. − In this case, the first peak within the first maxima located at around 0.4 nm represents the trans -conformations, allowing the molecules to interact at very short distances.…”

Diffusivities in Binary Mixtures of n-Hexane or 1-Hexanol with Dissolved CH4, Ne, Kr, R143a, SF6, or R236fa Close to Infinite Dilution

“…Consistent with our previous studies, , no influence of the solute on the spectra of the C 6 H 14 or C 6 H 14 O solvent molecules was found in this work. To evaluate the fluid structure, the CH- and OH-stretching domains are of particular interest as they can give qualitative information on the population of trans and gauche conformers ,, and hydrogen bonding, which both can influence the magnitude of D 11 .…”

“…By analyzing the temporal behavior of the quasi-elastically scattered light intensity, DLS allows the simultaneous determination of Fick diffusion coefficients D 11 and thermal diffusivities a in binary fluid mixtures in macroscopic thermodynamic equilibrium in a direct and absolute way. − The molecular structure can be probed by Raman spectroscopy, relying on the analysis of inelastically scattered light. , In the following, only the main principles of the methods are presented. A comprehensive description of the experimental realization of DLS and Raman spectroscopy applied in the thermophysical property research can be found in refs and .…”

“…In addition to DLS, this study utilizes the PDRS method for retrieving information on the mostly symmetric vibrational motion of molecular bonds, which is linked to the molecular structure and encoded in the isotropic spectra . In the PDRS method, the polarization of the incident light is changed from vertical to horizontal (index V or H), while the detection polarization is kept vertical.…”

“…By subtracting the obtained depolarized ( I VH ) from the polarized ( I VV ) spectra and taking into account the depolarization ratio, the polarization-independent fluorescent background can be minimized, leading to an improved evaluation and interpretation of the peaks observable in the Raman spectra. To further improve the statistics and, thus, the signal-to-noise ratio, for each thermodynamic state, 64 individual isotropic spectra were averaged and normalized using the standard normal variate method . The evaluation and interpretation of the Raman spectra focus on the wavenumber characteristic for the stretching of the CO, CH, and OH dipoles.…”