Abstract

Phase behavior is a prerequisite for a basic understanding of thermophysical properties. A number of systematic studies focusing on liquid-liquid equilibria were published during the last decades. As some studies have been focused on specific tasks, e.g., extraction, the avoidance of a demixing (fuel technology) or the optimization of specific formulations (pharmaceutics or cosmetics), scientific driven systematic studies are rare. Experimental results are often described in terms of empirical or semi-empirical or group contribution models, sometimes more sophisticated simulations corroborate the results. A critical assessment of the experimental and theoretical data is rather tricky, as different components with their different thermophysical properties and their versatile molecular interactions might result in inextricable contributions to (a) uncertainties of the experimental data and (b) the quantitative determination / calculation of molecular interactions. Therefore, we have performed systematic investigations on the liquid-liquid phase behavior of mixtures of normal alkanes with the molecular solvents ethanol or acetonitrile [1,2]. Both types of mixtures show partial miscibility with upper critical solution temperatures, which are shifted systematically when the chain lengths of one component and, therefore, the molecular interactions are varied gradually. The main objective was to identify systematic trends of the phase behavior. Ethanol as a polar molecule connected with a dipole moment comparable to water is able to form hydrogen bonds. Acetonitrile exhibits a high dipole moment and shows partial miscibility with a couple of common molecular liquids, typically connected with the appearance of upper critical points. Acetonitrile allows for the study of molecular interactions influenced by a dipole moment without the appearance of hydrogen bonds. Recently, in literature experimental LLE of mixtures of acetic acid and alkanes have been reported [3] which exhibit a similar beavior as ACN mixtures. On the molecular level, additionally, hydrogen bonds and dimerization have to be considered.

The different systems show limited miscibility with upper critical solution temperatures (UCSTs), they show variations from a asymmetric shape to an almost symmetrical one. The UCST decreases with decreasing length of the alkyl-chains of the alkane. The systems show the same universal critical behavior. The variations of the parameters are mainly determined by the size of the alkyl-chains. We are discussing the determination of the free enthalpies on basis of a Flory-Huggins like approach, which allows for a description and partially a prediction of UCSTs and critical compositions of the different systems. This approach also gives insights on the molecular interactions.

References

[1] S. Diekmann, E. Dederer, S. Charmeteau, S. Wagenfeld, J. Kiefer, W. Schröer, B. Rathke, J. Phys. Chem. B, 124, 156 (2020).

[2] S. Wagenfeld, W. Schröer, B. Rathke, J. Chem. Eng. Data, 69, 2273 (2024).

[3] H. Matsuda, T. Segawa, T. Tsuji, K. Naito, Y. Kakuta, K. Kurihara, K. Tochigi, J. Chem. Eng. Data, 71, 718 (2026).