# New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction
## 3 Results and Discussion
### Thermogravimetric Analysis (TGA)  
TGA is employed to study the primary reactions of the heated mixtures and to quantify their degradation. Traditionally, when TGA is performed, the weight loss due to the formation of thermal products is plotted as a function of temperature. The typical decomposition profile is plotted in Figure 3.  
Figure 1: A plot of the transmittance vs. wave numbers of the menthol (blue), TOA (red), and M/TOA eutectic mixtures at 2:1 (green), 1:1 (magenta), and 1:2 (cyan) molar ratio. The inset shows a scale-up in the region below 1150 cm\({}^{-1}\).  
Figure (a)a depicts the temperature dependence curves of the decomposition of the starting components--menthol, corresponding amine (DOA (a)), (TOA (b)), and of the eutectic mixtures before and after extraction and re-extraction.  
The TGA curves of the thermal decomposition of menthol and amines differ significantly. As reported by other authors in the analysis of various eutectic mixtures, the latter show an increased resistance to thermal decomposition compared to that of the individual components [42]. The thermal durability is increased upon conversion to DES as both the inflection point and the endpoint of the thermograms are increased [29]. While the process of degradation of menthol starts as low as about 100 \({}^{\circ}\)C and ends at about 170 \({}^{\circ}\)C, the decomposition of amines starts over 200 \({}^{\circ}\)C and completes at least at 300 \({}^{\circ}\)C (DOA) or at much higher temperatures (TOA--300 \({}^{\circ}\)C; THA and TDDA--460 \({}^{\circ}\)C). The T\({}_{\text{deg}}\) and degree of degradation of eutectic mixtures are shown in Table 3.  
Figure 3: TGA decomposition profile of compounds (gray—menthol, blue—amine (DOA (a); TOA (b))), eutectic mixture (red), after extraction (green), and after re-extraction (black).  
Figure 2: FTIR spectra of the M/TOA eutectic mixtures (blue), after extraction (red) and re-extraction (magenta). The inset shows a scale-up in the region below 1500 cm\({}^{-1}\).  
Analyzing the thermal curves of menthol/amine eutectic mixtures, it is obvious that the process of their degradation proceeds at much higher temperatures compared to menthol degradation but lower than amine degradation. The mixtures of menthol and DOA or THA decompose gradually, which supports the assumption of the formation of menthol/amine complexes in their eutectic mixtures. The noticeable degradation in stages is observed during the process of the thermal degradation of mixtures of menthol, TOA, and TDDA (see Table 3--first step decomposition degree). It should be emphasized, however, that the decomposition temperature of the products in this first stage is higher than that of pure menthol, which implies the absence of unreacted menthol in mixtures with amine. The formation of more than one type of complex with the specified amines is also plausible. The observed differences in the degrees of degradation depending on the type of amine support this assumption that the number and length of the hydrocarbon chains affect the accessibility to the donor center and the manner of implementation of the hydrogen bonds in the complexes.  
Last but not least, it should be pointed out that the degradation of extracted and re-extracted samples follows a similar course of thermal degradation, albeit at slightly lower temperatures, as seen from the data in Table 4.  
Similarly to the thermal behavior of starting mixtures, the two-stage thermal decomposition of eutectic mixtures formed from menthol and TOA or TDDA is observed, and therefore, more than one type of menthol/amine complex is expected to be formed in extracted and re-extracted eutectic mixtures. The increased content of these products implies an occurrence of the process of partial disintegration of the complex resulting from the formation of hydrogen bonds between the donor (amine) and the acceptor (menthol) or the transformation of the type of complexes.