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.

中文翻译：

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用拉曼光谱法表征正十八烷中的水溶性

在这项研究中，我们证明了偏振差拉曼光谱（PDRS）能够检测正辛烷（n -C ₂₈ H ₅₈）液相富集相中的溶解自​​由水分子，从而确定其在温度下的溶解度和与费托合成有关的压力。我们对纯烷烃的研究结果表明，温度超过500 K时会发生热分解，同时纱网的数量也会增加温度升高，烷烃链的构象异构体。对于二元均质混合物，从两种不同的偏振散射几何结构获得的原始光谱在OH拉伸频率范围内未显示相关信号。相反，从PDRS技术获得的各向同性光谱显示出与悬空的OH键相关的狭窄而微小的峰。在整个温度和压力范围内，在各向同性拉曼散射强度中未观察到氢键水分子的特征。彻查覆盖大范围的温度和使用PDRS信号压力的显示的分数越高笨拙烃的构象异构体，较高的水溶解度。薄纱和反式的比例发现构象异构体与水浓度无关，OH悬挂峰的强度随水蒸气分压的增加而线性增加。因此，我们在相关强度比与通过SAFT计算获得的水浓度之间建立了关系。与相关文献的结果相反，发现校准因子在424和572 K之间与温度无关。校正了各向同性拉曼散射强度，以便更好地表示状态的振动密度。讨论了各向同性散射强度校正对溶解度测量以及分子排列分析的影响。