## 3 Results and Discussion
### Extractive Capacity of DES
The composition CE1 yielded over 11 mg of total fatty acids per g of microalgae, while the composition CE0 (without water) only offered 7.9 mg/g microalgae. The addition of water produces a decrease in the viscosity of DES, facilitating fatty acid extraction. However, if water is added in excess, fatty acid capacity can be affected. For instance, the composition CE3, corresponding to a 1:2:3 molar composition with the highest amount of water, yielded 5 mg/g of microalgae in terms of total fatty acid recovery, which is less than half of the total amount of fatty acids obtained with CE1. This excess of water in the DES phases can produce excessive rupture of the hydrogen bonds, reducing the extraction capacity. Thus, in light of these results, it is necessary to find a balance in terms of water content for each DES composition. Moreover, fatty acids, especially those of higher polar character, are bound to other biomolecules (such as proteins and carbohydrates) through electrostatic forces, posing a challenge regarding their extraction. However, it has been reported that water can intervene in the solvent-cellulose interaction, disrupting the bonds between the fatty acids and the biomolecules. This presence of water facilitates ion dissociation and solvation, potentially aiding in the release of fatty acids for more effective extraction [30]. The effect of water is not only seen for the choline chloride-ethylene glycol-based DES but also for the choline chloride-fructose-based phase. As is shown, the recovery of decanoic acid increased from CF1 (0.05 mg/g) to CF2 (0.14 mg/g), but the CF3 mixture was not able to obtain decanoic acid. Due to its advantages as a food supplement, the recovery yield of EPA is of special relevance in the analysis. As seen before, EPA was the second most abundant fatty acid recovered from microalgae with the DES based on ethylene glycol. Figure 6 displays the recovery of EPA with each DES. As can be seen, choline chloride-ethylene glycol-based DES was the only phase with an extraction capacity regarding EPA. The maximum amount that was recovered was obtained with the CE1 composition, with around 3.3 mg/g microalgae,  
Figure 5: Recovery performance of decanoic acid with each DES mixture. which is in line with the results reported for total fatty acids. This was followed by the composition CE2, achieving 2.89 mg/g of microalgae. However, EPA was not found in the samples of the choline chloride-fructose phases. The extractive capacity of the traditional solvent (methanol) was still higher in comparison to the DES phases. It could extract 8.41 mg/g microalgae, which was almost three times the quantity of EPA extracted by CE1. While the extraction results are not yet comparable to those obtained with the traditional organic solvent, they do need to be optimized, and DES could potentially provide a new sustainable pathway for the extraction of fatty acids from microalgae biomass, especially if the recyclability of these solvents is proven. In this work, the extraction of fatty acids from DES phases was not addressed as a research objective. To achieve a fully sustainable process, the development of an optimized re-extraction method represents a critical avenue for future research. It is worth mentioning that the addition of water to the DES phase may be a viable strategy to facilitate these re-extraction methods, as water can weaken the interactions between individual components of the DES. Furthermore, different strategies are being studied to enhance the recyclability of DES. For instance, the utilization of supercritical fluids presents a promising option for the recovery and purification of fatty acids of special interest, such as EPA, as demonstrated in the literature [33], enabling the reuse of the DES phase. This research sets the stage for future investigations into the fine-tuning, recyclability, and optimization of re-extraction methodologies, all of which are essential for advancing the sustainability of DES-based processes.