# New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction
## 3 Results and Discussion
### Differential Scanning Calorimetry (DSC)  
In order to confirm the eutectic nature of the menthol/amine mixtures, the melting point of the formulated mixtures was determined by recording the DSC measurements. \begin{table}
\begin{tabular}{l c c c c} \hline
**Eutectic Mixtures** & **First Step** & **Decomposition** & **Second Step** & **Decomposition** \\
**(Ratio)** & **Tdeg** (°C)** & **Degree (\%)** & **Tdeg** (°C) & **Degree (\%)** \\ \hline M/DOA 1:2 ex & - & - & 275 & 100 \\ M/DOA 1:2 re-ex & - & - & 275 & 100 \\ M/TOA 1:1 ex & 220 & 50 & 340 & 50 \\ M/TOA 1:1 re-ex & 220 & 50 & 340 & 50 \\ M/THA 1:1 ex & - & - & 275 & 100 \\ M/THA 1:1 re-ex & - & - & 275 & 100 \\ M/TDDA 1:2 ex & 230 & 41 & 450 & 59 \\ \hline \end{tabular}
\end{table} 
Table 4: Tdeg and decomposition degree after extraction and re-extraction. \begin{table}
\begin{tabular}{l c c c c} \hline
**Pure** & & & & & **First Step Decomposition** \\
**Compound** & **T°C** & **DES** & **Tdeg** (°C) & **Degree (\%)** \\ \hline L-Menthol & 175 & M/DOA 1:1 & 280 & - \\ DOA & 305 & M/DOA 1:2 & 290 & - \\ TOA & 365 & M/TOA 1:2 & 360 & 26.5\% \\ THA & 465 & M/THA 1:1 & 275 & - \\ TDDA & 470 & M/THA 1:2 & 290 & - \\  & & M/TDDA 1:1 & 460 & 30\% \\  & & M/TDDA 1:2 & 460 & 18\% \\ \hline \end{tabular}
\end{table} 
Table 3: Thermal properties of compounds and some eutectic mixtures. The solid\(-\)liquid phase transition was measured from \(-\)90 \({}^{\circ}\)C to 50 \({}^{\circ}\)C with a heating and cooling rate of 10 \({}^{\circ}\)C/min. The selected DSC traces of the menthol: amine(s) (DOA and TDDA) eutectics are shown in Figure 4. The mass fraction of menthol in the mixture with DOA increases as the data are viewed from top to bottom. Menthol, DOA, and TDDA show clear endothermic peaks in the region of 35-42 \({}^{\circ}\)C [43; 44], 11-12 \({}^{\circ}\)C [45], and 15-16 \({}^{\circ}\)C [46], respectively, as their melting temperatures. The thermograms of the various mixtures clearly indicate the formation of binary (or even triple) eutectic mixtures. Multiple thermal transition peaks are expected while heating a mixture of menthol and a suitable conformer to obtain a eutectic solvent. On the other hand, eutectic formation, irrespective of the stoichiometry, is a process accompanied by heat generation. In general, the thermal behavior of the systems studied is marked by transitions between crystalline and amorphous phases which in turn are dependent on the composition and reflect the complex molecular ordering that the systems can adopt [47]. In each of them, the melting points are significantly lower than that of menthol or amine used and hence, qualify the definition of eutectic solvent. The transitions that occur at temperatures that are significantly lower than those observed for the pure menthol and amine compounds suggest the strong interactions that exist between the components. Upon heating, the pre-melting appears sequentially as a broad convolution of several endothermic peaks between about \(-\)25 \({}^{\circ}\)C to 5 \({}^{\circ}\)C, followed up by the final melting from about 8 \({}^{\circ}\)C to 22 \({}^{\circ}\)C depending on the type of amine and its quantity in the eutectic mixture. During cooling from the liquid state at 50 \({}^{\circ}\)C down to \(-\)90 \({}^{\circ}\)C, mixtures exhibit a sharp single crystallization (in the case of M/DOA 2:1) at \(-\)15.73 \({}^{\circ}\)C in the case of M/DOA  
Figure 4: DSC thermograms of menthol/amine: (**a**)–M/DOA 1:2, (**b**)–M/DOA 1:1, (**c**)–M/DOA 2:1, and (**d**) M/TDDA 1:1 mixtures. 2:1 or more than one peak (M/DOA 1:1, 1:2 or M/TDDA 1:1, respectively) occurring at a temperature range from 8.81 \({}^{\circ}\)C to \(-\)20.46 \({}^{\circ}\)C.  
The suggested formation of eutectic mixtures as a result of the hydrogen bonding between the hydroxyl group in menthol and amino group in amine proposed in the discussion of the FTIR results is also consistent with the DSC results.