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Head-space gas chromatography mass spectrometry (HS-GC-MS) was used for the first time to measure the total vapor pressure of hydrophobic deep eutectic solvents (DESs). The new method was developed as a valid alternative for thermogravimetric analysis (TGA), as TGA did not allow obtaining reliable total vapor pressure data for the hydrophobic DESs studied in this work. The main advantage of HS-GC-MS is that the partial pressure of each DES constituent and the contribution of each DES constituent to the total vapor pressure of the mixture can be measured. The results give a clear indication of the interactions occurring between the DES constituents. Also, activity coefficients, enthalpies of evaporation, and activation energies for fluid displacement were obtained and correlated to the measured vapor pressure data. It was confirmed that the total vapor pressures of the hydrophobic DESs are very low in comparison to vapor pressures of commonly used volatile organic solvents like toluene. The total vapor pressures of the hydrophobic DESs were successfully predicted with perturbed-chain statistical associating fluid theory (PC-SAFT) when using PC-SAFT parameters for the individual DES constituents.

Furfural (FF) and 5-hydroxymethylfurfural (HMF) are intermediates for many products, such as monomers for bioplastics, and can be obtained from various renewable resources. The isolation of these sugar-derived molecules from aqueous solutions is one of the main challenges in biorefinery processes. In the work described in this paper, the separation of FF and HMF from aqueous phases is carried out with hydrophobic deep eutectic solvents (DESs) as new extracting agents. Distribution coefficients of FF and HMF in 10 different hydrophobic DES + water systems have been measured and compared to that of the benchmark extracting agent (toluene). The dependence of the distribution coefficients on the presence of sugars in the system has also been investigated. The hydrophobic DESs were found to selectively extract FF and HMF from aqueous solutions without any co-extraction or precipitation of sugars. Finally, the distribution coefficients have been successfully predicted with PC-SAFT (perturbed-chain statistical associating fluid theory) without the need to fit any parameter to the measured distribution coefficients.

Deep eutectic solvents (DESs) based on terpenes are identified and characterized. 507 combinations of solid components are tested, which results in the identification of 17 new hydrophobic DESs. Four criteria are introduced to assess the sustainability of these hydrophobic DESs from a chemical engineering point of view. These criteria include a viscosity smaller than 100 mPa·s, a density difference between DES and water of at least 50 kg·m-3 upon mixing of the DES and water, low transfer of the DES to the water phase and minor to no pH change. The results show that five new hydrophobic DESs based on natural components satisfy these criteria; thymol and coumarin (2:1), thymol and menthol (1:1), thymol and coumarin (1:1), thymol and menthol (1:2) and 1-tetradecanol and menthol (1:2), and thus are promising DESs. These new DESs can be considered as natural deep eutectic solvents, which have the potential to be environmentally friendly. A selected group of the hydrophobic DESs were used for the extraction of riboflavin from water. They all show higher removal of riboflavin in comparison to decanoic acid:tetraoctylammonium bromide (2:1). The highest extraction efficiency of riboflavin from water, 81.1%, was achieved with the hydrophobic DES DecA:Lid (2:1).