**8. Factors Limiting the Potential of Deep Eutectic Solvents Utilization and How to Overcome Them**

The data in Table 1 and in the previous section documented the pros in the use of DESs for the extraction of value-added substances from phytomass. However, it should be admitted that DESs are not perfect and their use has its limitations. This section deals with the cons of applying DES for extraction purposes and valorization of phytomass.

The use of DESs in the field of biomass pretreatment or extraction of value-added substances has significantly expanded in the 21st century [11]. However, the process of applying faces several limitations from an experimental and commercial point of view.

#### *8.1. Purity*

It is natural that the process of developing new types of solvents and their application at a laboratory scale takes place in glass and using analytical grade chemicals. One of the main limits of subsequent commercial application is the possibility of using chemicals with purity lower than analytical grade. Here, however, problems can arise in terms of the stability of the created system. Even though a DES is formed, crystallization may occur due to long-term storage (sometimes, only a few hours). This effect may be exacerbated by impurities that would be present in the starting chemicals and could initiate crystallization. The reason for using chemicals with lower purity is, of course, that their prices are lower than that of pure chemicals. On the other hand, it should be noted that if the DESs application process is commercialized and expanded, the cost of producing DESs will decrease significantly [40]. In this respect, and given the relatively easy and simple preparation, lower costs would help to expand the use of DES as a new way of exploiting the potential of biomass or biowaste. However, it should be noted that the cost of some conventional organic solvents may be lower.

#### *8.2. Viscosity*

A significant limiting factor associated with the application of DESs is viscosity. Due to the formation and interaction of hydrogen bonds in the DES structure, the viscosity of DESs is relatively high, being 100–1000 times higher than that of water or conventional organic solvents [150]. On the one hand, viscosity presents the limit for penetration into the substrate, and on the other hand, from the point of view of commercial application, there is a problem in terms of the technological steps associated with the preparation of DESs themselves. This is mainly related to handling, mixing, filling, or transportation. Naturally, there are strategies that can partially eliminate this shortcoming, but the price associated with these measures and the consequent effectiveness of the use of DESs in the required operation with it (the goal of using DESs) play an important role here. The easiest way, although not the cheapest, is to increase the temperature and thus, achieve a decrease in viscosity. Another possibility of a simple solution is to add another reagent to the system, either water or another solvent, that will ensure a decrease in viscosity (e.g., alcohols, [113]). However, whether it is water or another type of solvent, it is necessary to realize that from this point of view, there is a change in the whole system, because the addition of another component also changes the behavior of DES. The addition of water into DESs in the process of their formation causes the incorporation of water molecules into the structure of DESs and their fixation by hydrogen bonds; this water can hardly be later fully removed by, e.g., a rotary evaporator. A small addition of water may result in a decrease in viscosity, temperature lowering, and shorter time needed for DES preparation. Water as another component plays an important role for the formation of hydrogen bond donors and acceptors in the DES structure [151,152]. If DES systems contain water or other organic substances as solvents, it is necessary to take water (for example, organic solvents are automatically taken into account) into account as another component of the DES. Therefore, binary systems need to be characterized as ternary.

#### *8.3. Hygroscopicity*

Another challenge or limiting factor is the hygroscopicity of DES. As already mentioned, the addition of water affects the nature of DES in terms of the structure and bonds they form, and water also affects the polarity and ability of DES to extract or solubilize target groups of substances isolated from phytomass. The hygroscopicity of DES must, therefore, not be neglected in the case of ensuring the technological process and its laboratory or commercial use. A more detailed description effect of water vapor from the surrounding air and of these facts is discussed elsewhere [153].

#### *8.4. Long-Term Stability*

In order to ensure better handling, mixing, and transport from DES, we have introduced as one of the options an increase in temperature. Regardless of the economic side of things, however, it should be noted that exposure of DES to higher temperatures for a long time can have adverse consequences [11].

#### *8.5. Acid-Base Properties*

Acidity or alkalinity are other important factors influencing the applicability of DES. Some DESs have significantly low pH, which significantly limits the choice of materials for their commercial use. Laboratory experiments are usually performed in glass, where this effect can be neglected. When using materials containing different types of metals and their compounds, these can cause an undesirable color change and affect the effect of DESs. The absorption of metallic components from the operating equipment is one of the key problems of the commercial use of DESs. Eliminating this problem may require the use of more expensive materials to transport, mix and apply the DESs themselves, which clearly increases input costs and may potentially discourage potential operators from commercializing the use of DESs. In addition, impurities can destabilize DESs and cause them to crystallize, thereby altering their stability.

#### *8.6. Toxicity*

Among the most common issues of research teams, scientists, but also practice and control bodies is the toxicity and recyclability of DES. As for toxicity: at the beginning of the research and application of DESs in 2003, it was very often said that DESs are non-toxic. Over time and the natural evolution of the composition of DESs, this concept has gradually disappeared and currently, DESs are characterized as having low or acceptable toxicity to various biological systems. The toxicity of DESs depends mainly on the toxicity of the starting components, but some DESs may be more toxic than their starting components [153]. The answer to this question about toxicity is a bit unclear. The shortcomings of DESs are gradually emerging, especially in terms of their impact on organisms and the environment; however, the boundaries of the terminology of the impact of DESs and its toxicity are gradually shifting. In general, the toxicity of a substance to organisms and the environment depends on the dose (concentration) and duration of its action. Related to this are the issues of biocompatibility and biodegradability of DESs before applying them to commercial purposes.

#### *8.7. Adsorbable Organic Halides*

A question or possibility of other research activities that still arises from published works or relevant project activities is the ability of chlorine-containing DESs to react with a substrate matrix or extractables, leading to adsorbable organic halides. This issue is extremely important in view of the need to limit the use of these halides and even to reject them on the basis of the 12 principles of environmental chemistry in the field of green technologies. However, it should be emphasized that in assessing the possible negative impacts of chlorine compounds, a distinction must be made between "inorganic, ionic" chlorine in the form of Cl− anions and chlorine bound to a carbon atom in organic compounds. This distinction is important e.g., in waste incineration.

#### *8.8. Recycling*

Given the ongoing research and commercial implementation of DESs processes, recyclability issues also need to be answered. Based on the information from the works that dealt with the careful application of DESs, the following conclusion can be drawn. The most common technique in regenerating DESs is to use an anti-solvent to remove (precipitate) the component from the system in operation, and then, evaporate the anti-solvent from the system, and reuse the DESs. Regeneration and reuse aspects are crucial in assessing sustainability and environmental protection, as well as in reducing the costs of the process [11].

#### **9. Future Trends and Concluding Remarks**

The excellent properties of DESs, such as sustainability, biodegradability, pharmaceutical acceptable toxicity, negligible volatility, and high extractability of compounds with diverse polarity, highlight their potential as green extractants. Several comparisons of the isolation value-added

substances from phytomass performed by DESs and organic solvents have clearly demonstrated, along with ecological advantages, also a higher yield of extracted substances using DESs and thus, cost-related benefits. It can be expected that the valorization of phytomass in the future will focus mainly on the extraction of therapeutically important substances, nutrients, and food supplements. The selection and composition of DESs will be optimized so that whole extracts can be used in practice, without prior separation of DESs and extracted value-added substances. It can be assumed that in the field of research of DESs themselves, mixtures with lower viscosity, predetermined polarity, and acid-base properties, capable of specifically extracting targeted value-added substances, will be sought.

Despite the considerable number of phytomass kinds valorized using DESs, there is still a huge amount of primary phytomass itself, waste of its processing, and food-related waste, which have not been studied from the point of view of isolating value-added substances. Expanding resources is a challenge for both laboratory and industrial workers and can bring many surprising and useful results in the future.

**Author Contributions:** M.J. and J.Š. contributed equally to the conceptualization and design of the work; writing—original draft preparation, M.J. and J.Š. Writing—review and editing; supervision and critical revision of the manuscript, M.J. and J.Š.; and funding acquisition, M.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Slovak Research and Development Agency under the contracts Nos. APVV-15-0052 (50%), and VEGA 1/0403/19 (50%).

**Acknowledgments:** The authors would like to acknowledge the financial support by the Slovak Research and Development Agency.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **Abbreviations**



## **References**


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