Ionic Liquids and Deep Eutectic Solvents and Their Use for Dissolving Animal Hair ()
1. Introduction
1.1. 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].
Typical ions in the composition of ILs are: cations of ammonium, phosphonium, pyridinium, 1,3-dialkylimidazolium; N-alkylpyridinium, tetraalkylammonium, tetraalkylphosphonium, trialkylsulfonium and various anions which include chlorides, iodides, bromides, sulfates, hexafluorophosphates, etc.
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.
1.2. Low-Temperature Eutectic Solvents: Versatile Alternatives to Ionic Liquids
Low-temperature eutectic solvents or deep eutectic solvents (DES) [9] [13] [14] [15] are liquid mixtures of a number of organic and (or) inorganic components taken in a certain ratio (eutectic or close to eutectic). DES are considered as a new and very promising class of ionic liquids, which can be used in various fields of industry, including for dissolving various materials [7] [16].
The main advantages of DES, determining the prospects of their use in technology, include such properties as high solubility of metal salts, negligible saturated vapor pressure, availability, low cost and environmental safety [15] [17] [18] [19].
DES can be formed between various quaternary ammonium salts and carboxylic acids [8] [9]. Physical properties largely depend on the structure of the carboxylic acid, but the phase behavior of the mixtures can simply be modeled taking into account the molar fraction of the carboxylic acid in the mixture.
Physical properties, such as viscosity, conductivity, and surface tension of these DES, are similar to ionic liquids at ambient temperature, and understanding the reason for these properties is achieved using hole theory. It was shown that the conductivity and viscosity of these liquids are controlled by the mobility of ions and the presence of voids of suitable sizes, which is consistent with the fluidity of other ionic liquids and molten salts.
An example of DES is a mixture of substituted quaternary ammonium salts, such as hydroxyethyl trimethylammonium chloride (choline chloride), with urea, which gives eutectics that are liquid at ambient temperature and have unusual solvent properties [13] [20].
Eutectic occurs when the molar ratio of urea to choline chloride is 2. The freezing point of the eutectic mixture is 12˚C, which is much lower than the freezing temperature of both components (melting point of choline chloride = 302˚C and urea melting point = 133˚C). This property allows the use of DES as a solvent at ambient temperature. This significant depression of the freezing point should result from the interaction of urea molecules with a chloride ion. In addition to urea, several other derivatives, such as thiourea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, tetramethylurea, acetamide and benzamide, were tested as components of eutectic mixtures.
Using amides of various structures and choline chloride as an example, it was found that only those compounds that are capable of forming hydrogen bonds with chloride ions exhibit the formation of a homogeneous liquid with a significant decrease in the freezing temperature compared to pure amide. It seems that amides with the greatest ability to form hydrogen bonds (i.e. urea and thiourea) exhibit the greatest decrease in freezing point when mixed with choline chloride. According to [21], choline chloride was also assigned a provitamin status (vitamin B4).
However, other sources indicate that choline chloride is not a vitamin in the sense that it cannot be produced by the human body.
It is important that DES are capable of dissolving proteins and amino acids, sugar and polysaccharides. Some DES, such as the eutectic mixture of glycerol and choline chloride sold by Scionix under the trade name Glyceline, are harmless, completely biodegradable and can be used where other solvents cannot, for example, in food and cosmetics.
It is obvious that IL and DES have some unique characteristics that distinguish them from conventional organic solvents, such as the lack of an effective vapor pressure, the wide temperature range of their aggregation state, such as liquids, high polarity and charge density, hydrophobic or hydrophilic characteristics and unique solvating properties.
The British company Scionix Ltd. [21] produces various types of DES based on choline chloride, which is a completely non-toxic product that is produced on a large scale as a feed additive for chickens (Table 1).
1.3. Dissolution of Keratin, as the Main Components of Animal Hair, in ILs and DES
Keratin belongs to a family of fibrous structural proteins. Such materials as hair, feathers, nails, hooves, claws and horns are made of keratin [17] [18] [22] [23] [24] [25] [26]. It is known for its extremely low solubility in water and organic solvents. According to [22] [23] preparation of keratin solution will allow to explore many new applications including manufacturing textile fibers, additives for animal feed, neatening reagents for leather and wool or leather, and many others. It should be noted that many tons of non-spin wool fibers are dumped during wool weaving every year. It means that developing processes for using and reusing the resources are important with an economic and environmental point of view.
Some progress with dissolving keratin by ILs was reported in publications [17] [24] [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%).
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
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Table 1. Composition of the five DES proposed by Scionix Ltd. [21].
viscous for effective dissolution at 130˚C. It was found that these ILs have an ability to disrupt hydrogen bonds in keratin that enhances its solubility. The authors of studies [24] [25] came to the same conclusion using [BMIM]Cl as the solvent for keratin.
According to paper [18] a number of DES based on choline chloride and hydrogen bond donors such as urea, ethylene glycol, citric acid and oxalic acid were tested as solvents for keratin as main component of rabbit hair. It was found that the solubility of rabbit hair evaluated at 98˚C within 2 hours increased in the raw choline chloride-ethylene glycol < choline chloride-urea < choline chloride-citric acid < choline chloride-oxalic acid from 15% - 20%, 19% - 22%, 20% - 27% and 36% - 71%, respectively. As a result, the DES based of choline chloride and oxalic acid (molar ratio 1:1, 1:2 and 1:3) was chosen for more detailed study which showed that solubility of rabbit hair can reach 89% after stirring for 2 hrs at temperature of 120˚C.
In this study, we report the dissolution of keratin from the rabbit hair in 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) in the presence of low concentration of sodium sulfide or ammonium thioglycolate at temperature of 30˚C.
2. Experimental Part
Chemicals
Choline chloride, Cat #C7,9700; Urea, Cat #20,8884; Urea-Hydrogen Peroxide adduct, Cat #289132; Ammonium hydroxide, Cat #211228; Thioglycolic Acid, Cat #T3758; Oxalic Acid, Cat #75688 and Sodium Sulfide, Cat #208043 were purchased from Sigma-Aldrich and used as received. The white rabbit hair (breed White Giant) used for solubility studies was purchased from CREATIVE-EXPORTS LTD (Kharkov, Ukraine).
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].
Procedure for making DES-1 based on choline chloride and urea (1:2 mol).
141.030 grams (1.0 Mol, 99.0% purity) of choline chloride is added to a glass beaker containing 122.57 g (2.0 Mol, 98% purity) of urea. The temperature of the beaker maintained at 50˚C for 30 min. The beaker is agitated with a glass rod. A noticeably viscous visually clear solution of the DES-1 with freezing point about 12˚C is obtained.
Procedure for making DES-2 based on choline chloride and Urea-Hydrogen Peroxide adduct (1:2 mol).
141.030 grams (1.0 Mol, 99.0% purity) of choline chloride is added to a glass beaker containing 193.96 g (2.0 Mol, 97% purity) of Urea-Hydrogen Peroxide adduct. The temperature of the beaker maintained at 35˚C for 30 min. The beaker is agitated with a glass rod. A noticeably viscous visually clear liquid of the DES-HP with freezing point between 10˚C and 15˚C is obtained.
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.
2) Preparing 4 solutions dissolving 5, 10, 15 and 20 mg of washed rabbit hair in 5 g of DES-3 prepared from choline chloride and oxalic acid (1:1 mol) under conditions described in paper [27], i.e. by stirring with magnetic bar for 2 hrs at temperature of 120˚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).
4) Assuming (based on data [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.
3. 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
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Figure 1. Absorbance spectra of rabbit hair in DES-3 at concentrations: 0.4% (1); 0.3% (2); 0.2% (3) and 0.1% (4).
handled reagent. UHP is a non-toxic, odorless crystalline solid. This adduct, which contains 35% of H2O2, sets the hydrogen peroxide free during its application. UHP is used in an efficient solid state oxidation of different organic molecules: hydroxylated aldehydes and ketones (to hydroxylated phenols), sulfides (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
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Table 2. Results on solubility of rabbit hair in eight different compositions consisting of DES-1 and DES-2 with addition of ammonium thioglycolate or sodium sulfide (pH ~12 - 13).
*) Estimated with assumption that starting concentration of H2O2 in UHP is 35% and DES-1 contains 1.348 parts of UHP (M.w. 94.1) for 1 part of choline chloride (M.w.139.6) which leads to concentration of H2O2 in non-diluted DES-2 as high as 20.1%; **) 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 Na2S × 9 H2O contains about 0.05 g of pure Na2S (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.
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Figure 2. Calibration graph used evaluation of rabbit hair’s solubility in DES-based compositions (Table 2).
of ammonium thioglycolate and Na2S was 1.94% and 1% respectively from the amount of DES-1 and DES-HP compositions.
It is obvious that the molar concentration of sulfur in compounds used in existing dehairing process varies in relatively broad range depending on the structure of these compounds. For instance, in molecule of NaHS (M.w. 56) the molar content of sulfur is (32/56) × 100 = 57.1%. However, in molecule of ammonium thioglycolate C2H7O2NS (M.w. 109) which is example of another compounds used for dehairing process, the molar content of sulfur is much less, i.e. (32/109) × 100% = 29.4%.
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 dramatically different in performance compared to the ammonium thioglycolate addition. Considering that cost is a very important issue for the practical application, it would be reasonable to emphasize that the DES-based composition with lower content of DES-HP would be more affordable compared to the composition with a high content of this component.
Figure 3 illustrates the preparation of the keratin film made of solution of rabbit hair in the mixture of DES-1 and DES-2 2:1 (w/w) by “precipitation” technique which includes immersion of keratin’s solution (composition #6 in 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.
4. Conclusions
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 Na2S or 2.96% of Na2S × 9 H2O) with addition of minor amount of ammonium hydroxide (below 1%).
2) All proposed materials are biodegradable, non-toxic, don’t contain Volatile Organic Compounds and comply with MRSL [34], Chem IQ, [35], REACH [36] and TSCA [37] regulations.
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.