3.2.1. σ-Profiles

The σ-profile is the most important molecule-specific property because it indicates the probability distribution of the surface area of molecules that have charge density. The σ-profiles of all contaminants and DESs are presented in Figure 5. In the diagram, the range of surface area over charge density is between −0.025 and 0.025 eÅ−2. This range can be divided into three segments, i.e., the non-polar region (−0.0084 eÅ−<sup>2</sup> < σ < 0.0084 eÅ−2), the hydrogen bond acceptor (HBA) region (−0.025 eÅ−<sup>2</sup> < σ < 0.0084 eÅ−2), and the hydrogen bond donor region (HBD) (0.0084 eÅ−<sup>2</sup> < σ < 0.025 eÅ−2). The HBA and HBD regions indicate the potential of the studied molecules to form strong hydrogen bonds. The results indicate that the σ-profile of ChCl:U (1:2) and ChCl:OA (1:2) almost overlap each other. This means that both DESs have similar properties with regard to molecular interaction. In both DESs, much larger peaks can be observed around negative values, compared to peaks around positive values, which shows more presence of HBA than HBD. The peaks of all siloxanes assume a similar shape and most of the areas are located in the non-polar area (−0.0084 eÅ−<sup>2</sup> < σ < 0.0084 eÅ−2), and there are small fragments of siloxane peaks in the HBD region. The opposite results can be observed for carbon dioxide, which may be a hydrogen bond acceptor to a small extent. Similar small areas of hydrogen sulfide peaks are found in the HBA and HBD parts. The σ-profile results show that the siloxanes have more negative activity coe fficient values compared to CO2 and H2S.

**Figure 5.** σ-profiles of ChCl:OA (1:2), ChCl:U (1:2), H2S, CO2, and siloxanes (L2, L3, D4).
