Ionic Liquids and Deep Eutectic Solvents and Their Use for Dissolving Animal Hair

In recent years, one of the priority areas of research in chemistry has become the processes carried out in an environment of liquid organic salts, the so-called ionic liquids (ILs), which are assessed as environmentally friendly or “green” alternatives to conventional organic solvents. ILs are non-volatile, highly polar solvents that dissolve many organic, inorganic, and organometallic compounds. Since they have no detectable vapor pressure, ILs are considered as potential substitutes for volatile organic compounds traditionally used as solvents. So-called deep eutectic solvents (DES) is a group of ILs that are liquid mixtures of a number of organic and (or) inorganic components taken in a certain ratio (eutectic or close to eutectic). DES deserve a special attention due to their negligible saturated vapor pressure, availability, low cost, as well as ability to dissolve at relatively high concentration of metal salts, metal oxides and various polymers. Particularly DES based on a mixture of choline chloride with urea (DES-1) or a mixture of choline chloride and adduct of urea with hydrogen peroxide (DES-2) give eutectics that are liquid at ambient temperature and have unusual solvent properties, including an ability to dissolve an animal hair in the presence of low concentration of sodium sulfide or ammonium thioglycolate. It was found that depending on the ratio between DES-1 and DES-2 in the mixture of two Deep Eutectic Solvents and the nature of sulfur-containing additive, the solubility of rabbit hair under used conditions, varies from 51% to 79%.


Basics of Ionic Liquids (ILs)
ILs are non-volatile, highly polar solvents that dissolve many organic, inorganic, and organometallic compounds. Since they have no detectable vapor pressure, ILs are considered as potential substitutes for volatile organic compounds traditionally used as solvents. That is why ILs are among the most promising solvents in terms of minimal environmental impact, and they provide the development of environmentally friendly technologies for chemical processes. The term "ionic liquids" means substances that are liquids at temperatures below 100˚C and their properties are determined by the structure of ions, in particular, the symmetry or asymmetry of the cation, the nature of the functional groups in the cation, the size of the cation and anion [1]- [6].
The nature of the anion has a great influence on the properties of ILs-melting point, thermal and electrochemical stability and viscosity. The polarity as well as the hydrophilicity or hydrophobicity of ILs can be optimized by appropriate selection of the cation/anion pair, and each new anion and cation provides additional opportunities for varying the properties of ILs.
The increased attention to ILs is due to the presence of the following specific properties: 1) A wide range of liquid state (> 300˚C) and low melting points (Tm < 100˚C). 2) High electrical conductivity. 3) Good solubility in relation to a variety of inorganic, organometallic and organic compounds and polymers [1] of natural and synthetic origin. 4) Catalytic activity, which leads to an increase in the selectivity [2] of organic reactions and the yield of the target product. 5) Non-volatility, reusability. 6) Non-combustibility, lack of explosiveness, non-toxicity and consequent absence of harmful effects on the environment. 7) Unlimited possibilities in the directed synthesis of ILs with desired properties. Qualities 3 and 4 make ionic solvents especially attractive in the synthesis of polymers.
The advantages of ILs in compiling with other classes of substances determine their use as reaction means for synthesis [7], solvents [8], electrolytes in electric batteries [9], plasticizers [10], catalyst [11] [12], etc.  [25]. The authors of study [17] investigated the ability to extract keratin from turkey feather using traditional ILs such as 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-allyl-3-methylimidazolium chloride ([AMIM]Cl), as well as novel IL [choline][thioglycolate]. It was expected that the presence of a thiol functional group in the latter will provide disulfide bridge reducing, and enhancing the keratin solubility. In was found that the process performed within 10 hours at 130˚C under nitrogen atmosphere leads to a significant percentage of solubility (up to 45%).

Low-Temperature Eutectic Solvents: Versatile Alternatives to Ionic Liquids
According to presented experimental data, the chloride ion in ILs appears to be as effective as the thioglycolate ion, and no advantages of [choline][thioglycolate] compared to [BMIM]Cl and [AMIM]Cl in dissolving the keratin was found. In the opinion of the authors of paper [17], it indicates that that disulfide cleavage is not a significant factor in the keratin's dissolution. It was established that around 50 wt% of feather material in all three types of ILs solutions became too Synthesis of DES-1 and DES-2 has been performed following published procedure [13]. Synthesis of DES-3 has been performed according to publication [9].

Synthesis of DES-3 (freezing point around 34˚C) based on choline chloride
and Oxalic acid (1:1 mol) has been performed according to published procedure [9].
Spectrophotometric method for evaluation of rabbit hair's solubility in experiments with different compositions of DES.
The proposed spectrophotometric method includes the following steps.
1) Washing rabbit hair with acetone and drying it overnight at 60˚C. 3) Measuring the absorbance spectra of prepared solutions in the spectral region of 250 -400 nm using Agilent Cary 60 UV-Vis spectrophotometer ( Figure   1). [18]) that the solubility of the rabbit hair at loading 20 mg/5 g DES-3 is ~89%, create the calibration graph which determines the linear relationship between the solubility of hair (%) and intensity of absorbance of investigated solution at wavelength of 277 nm.

Results and Discussion
The main difference between the technology of dissolving hair described in patent [28] and procedure implemented in this study is that instead of using high concentration of strong oxidizers, such as 50% solution of hydrogen peroxide or 32% solution of peracetic acid, we use the mixture of two DES. One of them is DES-1 prepared from choline chloride and urea at molar ratio of 1:2. The second one is DES-2 prepared from choline chloride and urea-hydrogen peroxide adduct (UHP). It should be noted that the UHP is stable, inexpensive and an easily  (to sulfoxides and sulfones), nitriles (to amides) and nitrogen heterocycles (to N-oxides) [29].
UHP is commonly encountered in cosmetic dentistry, where it is used to "bleach" teeth. The active ingredient is hydrogen peroxide, which acts to oxidize interprismatic extrinsic staining within tooth enamel. There are several methods of applying the peroxide gel to the tooth ranging from night-guard application at home or in-surgery application. The bleaching obtained is proportional to the length of time the peroxide is applied to the tooth, and the concentration used.
Concentrations of UHP used for tooth whitening purposes range between 10% and 35%. Higher concentrations carry a higher risk of side effects such as chemical burns. 10% is widely regarded as safe. A 10% solution of UHP in glycerol is used to treat ulcers and other lesions in the mouth. A 6.5% concentration solution is used to loosen and remove earwax [30]. Table 2 summarizes data on composition of mixtures of DES-1 and DES-2 (4 different ratios) used for partial oxidation of rabbit hair which was followed by addition of ammonium thioglycolate or sodium sulfide. In all cases concentration 1%; **) Due to reaction of 0.082 g of Thioglycolic Acid with 0.118 g of 28% aqueous solution of ammonium hydroxide about 0.097 g (1.94% from amount of DESs) is produced; 0.154 g of Na 2 S × 9 H 2 O contains about 0.05 g of pure Na 2 S (1.0% from amount of DESs); ***) Estimated by spectrophotometric method (absorbance at 277 nm) using a calibration graph presented in Figure 2. Note: Diluted solutions of rabbit hair in mixture of DES-1:DES-2 have a strong absorbance with a maximum at wavelength between 272 and 280 nm which is typical for keratin's solutions, as well as for solid keratin [31]. It is obvious that the origin of this maximum is related to the absorption of the aromatic amino acids, such as tyrosine and tryptophan. Advances in Chemical Engineering and Science It should be noted that the reducing of NaHS content from 2% to 1% would be important for practical application. In this case the overall content of sulfur would be changed from 57.1% to 28.55%. In case of using ammonium thioglycolate the overall 28.55% content of sulfur would correspond to 2 × (28.55/29.4) = 1.94% of ammonium thioglycolate.
The following conclusions can be made from the data presented in Table 2: 1) Depending on the ratio between DES-1 and DES-2 and the nature of sulfur-containing additive the solubility of rabbit hair under used conditions, varies from 51% to 79%.
2) An addition of 1.94% of ammonium thioglycolate (as product of reaction between thioglycolic acid and ammonium hydroxide) to the mixture of DES-1 and DES-2 provides slightly improved solubility of the rabbit hair compared to the addition of 1% sodium sulfide. In former case depending on the DES-2 content in the mixture of DES the solubility varies from 58 to 79%, in later case-from 51% to 73%. Note: In two experiments performed at 1:3 (w/w) ratio of DES-1:DES-2 (the results are not shown in Table 2) and the same concentrations of two other components (thioglycolic acid and aqueous solution of ammonium hydroxide), as indicated in Table 2 for experiment #1, the solubility of rabbit hair was estimated within 77% -78%.
3) In general, increasing the content of DES-2 in the mixture of two DES enhances solubility of the rabbit hair in case of addition and ammonium thioglycolate and sodium sulfide.
According to data presented in Table 2, sodium sulfide addition is not dra-   Table 2) placed on glass slide into excess of DI water.
In summary, we investigated a new method for the dissolution of animal hair in the mixture of two deep eutectic solvents. Dissolution in existing technologies [32] [33] is typically performed in aqueous media with the use of sulfur reducing products (sulfides, sulfhydrate or thioglycolate) and strong oxidizers. The sulfur reducing products and oxidizers used at high concentration are very reactive and dangerous to use. Therefore, any attempt to decrease amount of these reagents for regulatory and safety concerns could be of interest for manufacturers involved in dehairing of animal hides. Currently scale-up of proposed method is underway. Figure 3. Formation of film from supernatant prepared from composition #6 (Table 2) by "precipitation" technique: (A) Appearance of supernatant on the surface of glass slide; (B) Glass slide in the petri dish immediately after adding DI water; (C) the same after 3 hrs at room temperature; (D) Appearance of film after taking it off from the glass slide and drying for 5 hrs at 50˚C. Note: The composition #6 ( Table 2) without addition of rabbit hair was placed on the glass slide and subjected to the treatment with DI water under the same conditions as it is shown in (A)-(C). Complete dissolution of the mixture in water was observed.

1)
We propose a novel method of dissolving animal hair that is using reduced amount of sulfur-containing component and oxidizer compared to known methods. After further investigation and scale-up proposed procedure could be used for developing industrial method of removing animal hair and epidermis and it includes the following steps: a) Oxidation of animal hair performed by using the mixture of two deep eutectic solvents (DES) one of which is based on choline chloride and urea and another one is based on choline chloride and adduct of urea and hydrogen peroxide. The content of oxidizer (hydrogen peroxide) in proposed method several times is lower than in known processes which are based on using relatively high content of oxidizers, such as peracetic acid (32% aqueous solution) or 50% aqueous solution of hydrogen peroxide. b) Treatment of hair subjected to oxidation procedure with a reduced amount of sulfur-containing products, such as ammonium thioglycolate (1.94%) of sodium sulfide (1.0% of pure Na 2 S or 2.96% of Na 2 S × 9 H 2 O) with addition of minor amount of ammonium hydroxide (below 1%).
2) All proposed materials are biodegradable, 3) The pH of the reaction is between 12 and 13, the reaction time is 10 hours (or below), the process temperature doesn't exceed 30˚C.