# New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction
## 3 Results and Discussion
### Extraction and Stripping of Lactic Acid  
Tests with all the synthesized DESs were performed to determine their ability to extract LA from an aqueous solution. A total of 15 mL of DES was mixed with aqueous LA solution (10 g/L) in a ratio of 1:1. After phase separation, the concentration in the water phase was measured, and this, in the organic phase, was calculated as a difference between the initial and final LA concentration in the aqueous phase. The values of the extraction efficiency and distribution coefficient were calculated for all 12 DESs and the results obtained are presented in Figure 7.  
As can be seen from the figure, the extraction efficiency varied from 43% (for M/THA 1:2) to 86% (for M/DOA 1:2). In general, the best results were achieved with the DES formed by DOA and menthol--from 82 to 86% depending on the ratio. Other DES also showed efficiency over 80%--M/TOA 2:1 and M/THA 2:1. The values of the distribution coefficient varied from 0.8 to 6.2 depending on the amine and component ratio. Best values (about six) were achieved with M/DOA 2:1 and M/THA 2:1. Most values of K are between 1.5 and 4.5. These values are close to those reported by [PERSON] and [PERSON] [9] for LA extraction with different long-chain tertiary amines. [PERSON] et al. [31] reported extraction efficiency of LA of about 30%, 70% for acetic acid, and 90% for oxalic acid when using thymol-menthol-based DES as a modifier for acid extraction from sweet sorghum silage press juice with TOA. Similar results were reported also for other extractants like  
Figure 6: Experimental viscosity measurements of M:DOA DES. Figure 7: LA extraction with DES: (**a**) extraction efficiency; (**b**) distribution coefficient. Aliquat, TOPO, and TBP for the same system [32]. [PERSON] and [PERSON] [28] reported an increase of up to four times in the extraction of acetic acid with DES composed of glycerol and quaternary ammonium salt. An extraction efficiency of over 90% was achieved by [PERSON] et al. [48] in the extraction of 2,4-dichlorophenoxyoxyacetic acid with DES mentholformic acid as a solvent. [PERSON] et al. [37] investigated the extraction of adipic, levulinic, and succinic acids with DES based on TOPO. However, the extraction with pure TOPO was better. [PERSON] et al. [38], applying a new approach for the in situ formation of DES, achieved 99% separation of LA from fermentation broth. [PERSON] [27] used DES formed by menthol and nonanoic acid, decanoic acid, and dodecanoic acid for formic acid extraction. When using DES, the extraction efficiency is about 10-13%, but using DES as diluents for TOA the efficiency increased to 90%. [PERSON] et al. [29] investigated the extraction of citric with DES formed of menthol and TBP. Physical extraction with the studied DES lead to an extraction efficiency of about 38%, while E% increased to 90% when DES were used as a solvent for TOA. [PERSON] et al. [36] studied the extraction of lactic, acetic, and succinic acids with DES formed by amides and geraniol. The values of the distribution coefficient varied from 0.38 to 2.85 depending on DES composition and extracted acid, while extraction efficiency changed between 40 and 80% for LA, 60 and 90% for AA, and 75 and 95% for SA. The DES can be recycled and used in about 15 cycles. A simulation of a continuous extractive column showed about 99% of the LA extraction yield using amide-based hydrophobic DESs (volume ratio of 2.0 and nine stages). In another paper [49] the authors reported similar results for the extraction of lactic malic and tartaric acids with the same DES. [PERSON] et al. [50] investigated various DES formed by decanoic acid and quaternary ammonium salts for the extraction of acetic, propionic, and butyric acids. The extraction efficiencies of the acids that the studied hydrophobic DESs perform better than extraction with TOA. The values of E% varied between 25 and 38% for AA, 45 and 76% for PA, and 74 and 92% for BA. The extraction efficiencies increase with the increasing chain length of the quaternary ammonium salt.