# New Menthol-Based Hydrophobic Deep Eutectic Solvents as a Tool for Lactic Acid Extraction
## 1 Introduction  
In recent years, the demand for lactic acid (LA) has increased considerably as a consequence of the increasing use of LA in the production of polylactic acid and green solvents. The major problem in fermentative LA production is in product separation from fermentation broth and its purification. The classical approach for LA separation is the neutralization of the broth with an alkali reagent, usually Ca(OH)\({}_{2}\)[1]. Precipitated calcium lactate undergoes additional treatment with sulphuric acid to obtain free LA, which is further subjected to purification. The process is complicated and does not assure the necessary purity. Other recovery techniques like membrane separation [2, 3], ion exchange [4], liquid-liquid extraction [5, 6, 7], and aqueous two-phase systems [8] also have been applied for LA separation and purification. Among them, liquid-liquid extraction seems to be the most attractive because it is simple, low energy consuming, easily scaled up, and gives the possibility of in situ operation. Various tertiary amines or quaternary ammonium salts are preferred as extractants because of their selectivity and efficiency [9, 10, 11]. For increasing the solubility of the acid-amine complex and adjusting the physical properties of the extraction phase, the extractant is usually mixed with a diluent and/or modifier. The main drawback of using long-chain amine-based extractants is that they are harmful and toxic to the microorganisms usually used for fermentation. Deep eutectic solvents (DESs) represent a relatively new group of chemicals with very promising properties tobe used as a substitute for high molecular weight aliphatic amines or quaternary ammonium salts in the extraction of organic acids. DES offers many properties, making them an excellent alternative to traditional extractants like low price, ease of preparation, low toxicity, biocompatibility and biodegradability, and possibility of tuning their properties by changing one or both components. The first publication describing the synthesis and properties of such compounds and their characteristic of having lower freezing points than their constituent compounds appeared in 2001 [12]. Later on, a huge number of papers described the synthesis and properties of various DES. The term DES was first introduced by [PERSON] et al. [13] describing the compounds formed between choline chloride and carboxylic acids. Now, for the characterization of DES, the following definition is accepted--a mixture of Lewis and Brensted acids and bases that significantly reduce the freezing point compared with those of the components [14]. DESs are composed of two or more substances--one acting as a hydrogen bond acceptor (HBA) and the other as a hydrogen bond donor (HBD). Various tertiary amines or quaternary ammonium salts are most widely used as HBA and different alcohols, glycols, sugars, or carboxylic acid as HBD. Some chemicals, like menthol, thymol, and some organic acids, may serve as HBA or HBD depending on the second component. DES has broad applications in various fields [15]--metal processing (electrodeposition, electropolishing, and extraction), synthesis, extraction of valuable compounds, etc. In recent years, the number of papers devoted to the extraction of various compounds with DES have constantly increased. Different DESs were used for the extraction of natural bioactive compounds [16; 17; 18], phenolic compounds [19; 20; 21; 22], carotenoids [23], essential oils [24], podophyllotoxins [25], antioxidants [26], etc. Recently, the interest in the application of DESs as extractants, diluents, or modifiers for the extraction of low molecular weight organic acids considerably increased. Different DESs were used in the separation of formic [27], acetic [28], citric [29], phthalic [30], lactic [31; 32], nicotinic [33], palmitic [34; 35], and various carboxylic acids [36; 37]. [PERSON] et al. [38] proposed an original use of DES for the separation of lactic acid--the in situ forming of DES using different lactams as HBA and LA as HBD.