Moderating effects of alcohols on the interfacial tension in multiphase systems consisting of carbon dioxide and organic solvents

• Applicant: Dr.-Ing. Thomas Manfred Koller
• Subject Area: Technical Thermodynamics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 504217815

Multiphase systems consisting of at least two partially miscible phases can be formed in mixtures of carbon dioxide (CO2) and organic solvents at high pressures and are important in many fields of process engineering. For the interfacial tension (IFT) as key thermophysical property characterizing interfaces, there is still a lack of information in multiphase systems, which is caused by the challenges adherent to corresponding experimental and theoretical methods. The main objective of this project is to contribute to a fundamental understanding of the moderating effects of alcohols on the IFT in multiphase systems containing CO2 and organic solvents. To characterize the phase boundaries, reliable information about their IFTs should be obtained at conditions in the vicinity of the critical point of CO2, where two- or three-phase systems can be found. For developing structure-property relationships, the selected organic solvents given by n-alkanes, n-alkylbenzenes, and 1-n-alkyl-3-methylimidazolium-based ILs imply variations in size, structure, and polarity, while changes in the number of carbon atoms of the moderating 1-alcohols enable to manipulate their amphiphilic properties. For an accurate measurement of IFTs over a broad range down to vanishing values, the SLS method characterized by its contactless working principle at thermodynamic equilibrium has to be further developed with respect to its application for pressures up to 20 MPa. Here, a further objective is to answer the question how the IFTs behave at the transition from vapor-liquid (VL) to liquid-liquid (LL) conditions and how they are related to the VL and LL IFTs at the adjacent vapor-liquid-liquid (VLL) equilibrium. In connection with the saturated densities required to evaluate SLS, measurements with vibrating tube densimetry will be performed for the solvent-rich phase. The experimental data serve also to test MD simulations in predicting IFTs of dense two-phase fluids. From the simulations, also information about the coexisting densities and a close insight into the fluid structure in the bulk and especially at the interface should be obtained, which helps interpret the measured results. By this, it should be identified to which extent the moderating effects of 1-alcohols on the IFT are related to changes in the bulk properties of the coexisting phases and/or to those in the interfacial properties reflected by, e.g., an enrichment of 1-alcohol molecules. Further knowledge about the interfacial composition and phase behavior will be obtained by calculations from density functional theory (DFT) combined with an equation of state (EoS) in the group of the cooperating researcher Prof. Groß for selected systems. Based on the experimental and theoretical results, a generalized prediction model representing the influence of 1-alcohols on the IFTs in mixtures with CO2 and various types of organic solvents with a common alkyl chain functionality should be developed.

• DFG Programme: Research Grants