Journal of Molecular Liquids MIXTURE DESIGN AND PHYSICOCHEMICAL CHARACTERIZATION OF AMINO ACID-BASED DEEP EUTECTIC SOLVENTS (AADES) FOR SAMPLE PREPARATION PRIOR TO ELEMENTAL ANALYSIS --Manuscript Draft-- Manuscript Number: Article Type: Full length article Section/Category: Water, aqueous solutions and other hydrogen-bonded liquids Keywords: Mixture design; Sample treatment; Physicochemical properties; ICP-MS; ICP OES Corresponding Author: Mario Gonzalez São José do Rio Preto, Brazil First Author: Taciana G.S. Guimarães Order of Authors: Taciana G.S. Guimarães Daniel F. Andrade Ana P.R. Santana Poliana Moser Sabrina S. Ferreira Iohanna M.N.R. Menezes Clarice D. B. Amaral Andrea Oliveira Mario Henrique Gonzalez Abstract: Amino acid-based deep eutectic solvents (AADES) represent a new subclass of deep eutectic solvents (DES) in which at least one of the components must be an amino acid, offering advantages such as low toxicity, biodegradability and low cost. In this work, β-alanine was used as hydrogen bond acceptor (HBA) in the preparation of a total of 30 AADES mixtures, with the hydrogen bond donor (HBD) being malic acid (AADES 1), citric acid (AADES 2), or xylitol (AADES 3), together with the addition of water. A restricted mixture design was employed to optimize the ideal proportions of the AADES components, which were determined as (% m m-1) 12.50 for β-alanine, 43.75 for the HBD component, and 43.75 for water, with lower values of density and viscosity being the desired responses. Solvents that have low density and viscosity provide greater efficiency in sample preparation procedures, due to faster mass transfer. The highest density and viscosity values were found for AADES 2, due to the greater presence of carboxyl groups in the molecular structure of citric acid, allowing the formation of more hydrogen interactions. The Herschel-Bulkley model provided the best fit to the rheological behavior of the AADES, with AADES 2 showing the highest consistency index. Solvatochromic analyses showed that these solvents had high polarity. Fourier transform infrared (FTIR) spectroscopy analysis revealed hydrogen interactions between the precursor components, confirming formation of the AADES. Thermal analysis revealed the ideal working temperature ranges for applying these solvents in sample preparation, with thermogravimetry (TGA) indicating maximum temperatures of 130 °C for AADES 1 and 150 °C for AADES 2 and AADES 3. Differential scanning calorimetry (DSC) revealed the minimum temperatures at which the solvents remained liquids, which were -13 °C for AADES 1, -22 °C for AADES 2, and -21 °C for AADES 3. Therefore, these AADES were shown to be promising solvents for application in sample preparation, being suitable for the extraction of polar compounds, as well as metals and semimetals. An EcoScale study was carried out, which confirmed that the preparation of the solvents could be considered an excellent green synthesis. Suggested Reviewers: Jose Fernandes Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation josefer@ff.up.pt Expert in deep eutetic solvents Elma Neide Carrilho elma.carrilho@gmail.com Expert in sample preparation Miguel Guardia miguel.delaguardia@uv.es Expert in green analytical chemistry Opposed Reviewers: Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation INSTITUTO DE BIOCIÊNCIAS, LETRAS E CIÊNCIAS EXATAS – DIRETORIA Rua Cristóvão Colombo, 2265 CEP 15054-000 S. J. Rio Preto SP Brasil Tel 17 3221.2410 | Fax 17 3221.2500 - www.ibilce.unesp.br Campus de São José do Rio Preto São José do Rio Preto, 03th, June, 2021. Editor in Chief W. Schröer - University of Bremen Faculty 2 Biology Chemistry, Leobener Str. NW2, 28359, Bremen, Germany Dear Professor W. Schröer, We are submitting the manuscript entitled “Mixture design and physicochemical characterization of amino acid-based deep eutectic solvents (AADES) for sample preparation prior to elemental analysis” for your evaluation and possible publication in Journal of Molecular Liquids. The mixture design was applied for the synthesis of β-alanine used as hydrogen bond acceptor (HBA) for the preparation of a total of 30 AADES mixtures, employing as hydrogen bond donor (HBD) malic acid (AADES 1), citric acid (AADES 2), or xylitol (AADES 3), together with the addition of water. The best AADES proportion was selected based on density and viscosity values. The physicochemical properties of the solvents were also evaluated by rheological and solvatochromic analysis, Fourier transform infrared (FTIR), and thermal techniques. The AADES showed to be a very promising solvent for the extraction of inorganic elements in several types of samples, being remarkably compatible with the plasma-analytical techniques even improving its analytical performance. An EcoScale study was carried out, which confirmed that the preparation of the solvents could be considered an excellent green synthesis. Thank you in advance for your attention, time and editorial assistance. We are looking forward to hearing from you at your earliest convenience. Yours sincerely, Mario Henrique Gonzalez São Paulo State University (UNESP), Department of Chemistry and Environmental Science, 15054-000, São José do Rio Preto, SP, Brazil. Email address: mario.gonzalez@unesp.br Cover Letter Highlights  The best proportion of AADES components based on of β-alanine as HBA was obtained using a mixture design.  The variation of the HBD component directly influences the physicochemical properties.  AADES with high polarities were obtained, promising for the extraction of inorganic analytes. Highlights MIXTURE DESIGN AND PHYSICOCHEMICAL CHARACTERIZATION OF 1 AMINO ACID-BASED DEEP EUTECTIC SOLVENTS (AADES) FOR SAMPLE 2 PREPARATION PRIOR TO ELEMENTAL ANALYSIS 3 Taciana G.S. Guimarães a, Daniel F. Andrade b, Ana P.R. Santana c, 4 Poliana Moser d, Sabrina S. Ferreira a, Iohanna M.N.R. Menezes e, 5 Clarice D. B. Amaral e, Andrea Oliveira e, Mario H. Gonzalez a* 6 7 a São Paulo State University (UNESP), National Institute for Alternative Technologies of 8 Detection, Toxicological Evaluation and Removal of Micropollutants and Radioactives 9 (INCT-DATREM), Department of Chemistry and Environmental Science, 15054-000, São 10 José do Rio Preto, SP, Brazil 11 b Applied Instrumental Analysis Group, Department of Chemistry, Federal University of São 12 Carlos, 13565-905, São Carlos, SP, Brazil 13 c Federal University of Minas Gerais, Department of Chemistry, 31270-901, Belo Horizonte, 14 MG, Brazil 15 d São Paulo State University (UNESP), Department of Food Engineering and Technology, 16 15054-000, São José do Rio Preto, SP, Brazil 17 e Federal University of Paraná, Department of Chemistry, 81531-980, Curitiba, PR, Brazil 18 19 20 *mario.gonzalez@unesp.br 21 22 Manuscript File Click here to view linked References about:blank https://www.editorialmanager.com/molliq/viewRCResults.aspx?pdf=1&docID=36349&rev=0&fileID=519111&msid=9711348b-d213-4f67-9975-4ee8172b0d84 https://www.editorialmanager.com/molliq/viewRCResults.aspx?pdf=1&docID=36349&rev=0&fileID=519111&msid=9711348b-d213-4f67-9975-4ee8172b0d84 Abstract 23 24 Amino acid-based deep eutectic solvents (AADES) represent a new subclass of deep eutectic 25 solvents (DES) in which at least one of the components must be an amino acid, offering 26 advantages such as low toxicity, biodegradability and low cost. In this work, β-alanine was 27 used as hydrogen bond acceptor (HBA) in the preparation of a total of 30 AADES mixtures, 28 with the hydrogen bond donor (HBD) being malic acid (AADES 1), citric acid (AADES 2), 29 or xylitol (AADES 3), together with the addition of water. A restricted mixture design was 30 employed to optimize the ideal proportions of the AADES components, which were 31 determined as (% m m-1) 12.50 for β-alanine, 43.75 for the HBD component, and 43.75 for 32 water, with lower values of density and viscosity being the desired responses. Solvents that 33 have low density and viscosity provide greater efficiency in sample preparation procedures, 34 due to faster mass transfer. The highest density and viscosity values were found for AADES 35 2, due to the greater presence of carboxyl groups in the molecular structure of citric acid, 36 allowing the formation of more hydrogen interactions. The Herschel-Bulkley model provided 37 the best fit to the rheological behavior of the AADES, with AADES 2 showing the highest 38 consistency index. Solvatochromic analyses showed that these solvents had high polarity. 39 Fourier transform infrared (FTIR) spectroscopy analysis revealed hydrogen interactions 40 between the precursor components, confirming formation of the AADES. Thermal analysis 41 revealed the ideal working temperature ranges for applying these solvents in sample 42 preparation, with thermogravimetry (TGA) indicating maximum temperatures of 130 °C for 43 AADES 1 and 150 °C for AADES 2 and AADES 3. Differential scanning calorimetry (DSC) 44 revealed the minimum temperatures at which the solvents remained liquids, which were -13 45 °C for AADES 1, -22 °C for AADES 2, and -21 °C for AADES 3. Therefore, these AADES 46 were shown to be promising solvents for application in sample preparation, being suitable 47 for the extraction of polar compounds, as well as metals and semimetals. An EcoScale study 48 was carried out, which confirmed that the preparation of the solvents could be considered an 49 excellent green synthesis. 50 Keywords: Mixture design, Sample treatment, Physicochemical properties, ICP-MS, ICP-51 OES. 52 53 1. Introduction 54 55 In the last decade, the incorporation of the principles of sustainability in Chemistry, 56 aiming at the reduction of environmental impacts, less risk to humans, and savings of costs 57 and energy, with the implementation of simple and fast synthesis methods, has stimulated 58 research into new sustainable solvents [1]. Scientific interest in the development of 59 sustainable solvents is driven by the still unavoidable use of solvents in chemical analysis, 60 notably in sample preparation, so this has become one of the main lines of research in green 61 analytical chemistry [2]. 62 Ionic liquids (ILs) are non-molecular solvents formed by the combination of an 63 organic cation and an organic or inorganic anion, whose main characteristic is a melting point 64 below 100 °C [3]. They are non-volatile and non-flammable and have been considered 65 sustainable solvents that are ideal candidates to replace potentially toxic traditional solvents 66 [3]. However, the current literature reports that this designation may be erroneous, because 67 in some cases, many steps are necessary to synthesize ILs, using high- cost precursors, in 68 addition to presenting degradation products with high toxicity, low biodegradability, and 69 long lifetimes in the environment [4]. 70 Deep eutectic solvents (DES) are a promising class of sustainable solvents that offer 71 an alternative to ILs [5]. When the DES precursors are naturally occurring substances such 72 as primary metabolites and/or cellular components including amino acids, organic acids, 73 sugars, and choline derivatives, natural deep eutectic solvents (NADES) are formed [6-8]. 74 More recently, NADES based on amino acids received a new classification and a more 75 restricted name, becoming known as amino acids-based deep eutectic solvents (AADES), 76 where at least one of the components of the formulation must be an amino acid [9]. 77 The AADES, as well as DES and NADES, are formed by different interactions 78 (electrostatic, dipole-dipole, van der Waals, and especially hydrogen bond interactions) 79 between an HBA component (hydrogen bond acceptor) and an HBD component (hydrogen 80 bond donor), resulting in a mixture that has a lower melting point than those of its individual 81 constituents [6,10]. The advantages of the DES in general are mainly related to their low 82 toxicity, biodegradability, low cost, high water solubility, low vapor pressure, and high 83 degree of solubilization of various compounds, in addition to their ease of preparation, 84 without the need to use other solvents and without formation of byproducts [11,12]. 85 Variations in the physicochemical properties and stabilities of these solvents are 86 directly dependent on variations in the compositions and molar ratios, producing solvents 87 with different properties, resulting from the displacement of charges caused by 88 intermolecular interactions between the precursor compounds [6,11]. Hence, it is possible to 89 adjust the properties of these solvents, such as viscosity and density, which has led to them 90 being denominated design solvents [6,13]. 91 DES and NADES have been used in several areas of chemistry, such as catalysis 92 processes, organic synthesis, electrochemistry, and chemistry of materials [14]. Another 93 application that is gaining prominence is their use in extraction methods for sample 94 preparation, mainly for extraction of organic compounds, while there are still few studies 95 reporting the use of NADES for extraction of chemical elements [11,15-20]. Studies 96 concerning the applications of AADES are scarce, although these solvents have shown 97 promise in areas including cosmetics, food, medicines, animal feed, and polymer production 98 [9]. They can be used for drug solubilization and transport, biosynthesis, capture of 99 radioactive iodine, oil separation, extraction of bioactive compounds for use in foods and 100 herbal supplements, enzymatic hydrolysis of biomass, and synthesis of bioactive compounds, 101 such as indoles and their derivatives [9]. However, there are still no reports of application of 102 AADES in extraction procedures for inorganic analysis [9]. The amino acids most used for 103 the synthesis and applications of AADES include betaine, lysine, glycine, glutamine, 104 arginine, cysteine, methionine, tryptophan, valine, leucine, and proline [9]. 105 An interesting and promising potential application of these solvents is their use in 106 sample preparation, for which they must be present in the form of a homogeneous and stable 107 mixture [21]. Their viscosity is an important consideration, since most studies reported in the 108 literature describe solvents with high viscosities (>100 mPa.s), due to their applications that 109 require this characteristic, such as in studies of gas solubility [14]. However, when solvents 110 with low viscosity are required, there are several strategies that can be adopted. Among these, 111 increase of the temperature reduces the viscosity, due to the greater kinetic energy of the 112 molecules and, consequently, their vibration, which increases the fluidity of the solvent [14]. 113 In addition, variation of the components (mainly the HBD) and the addition of water during 114 preparation of the solvent can be used to produce solvents with lower viscosities. 115 In addition to the importance of low viscosity solvents in mass transfer processes, 116 they are also required in analyses employing plasma techniques, such as ICP-MS (Inductively 117 Coupled Plasma Mass Spectrometry) and ICP-OES (Inductively Coupled Plasma Optical 118 Emission Spectrometry), where the viscosity influences the processes of formation of the 119 sample aerosol and its transport to the plasma [22]. More viscous solvents form larger and 120 fewer droplets, affecting the formation of spray in the nebulizer and the separation in the 121 nebulization chamber, reducing the sensitivity of the analytical method [20]. 122 In this work, a restricted mixture design was used to optimize the proportions of the 123 AADES components, with β-alanine as the HBA and varying the HBD between organic acids 124 (malic acid and citric acid) and sugar (xylitol), together with the addition of water. The 125 desired responses in this mixture design were lower density and viscosity values. The 126 experimental design considered three components constituting the final product, with the 127 response (the physical properties of the mixture) depending only on the proportions of the 128 components present [23]. A graphical representation of the mixture design is given by an 129 equilateral triangle, where the vertices correspond to the pure components and the central 130 points represent ternary mixtures [23]. In this way, it is possible to evaluate the effect of the 131 proportion of each component used during the preparation and its influence on the properties 132 of the AADES, using a minimum number of experiments. The AADES categorized as ideal 133 were then characterized by rheological measurements, polarity analyses, infrared 134 spectroscopy (FTIR), thermogravimetry (TGA), and differential scanning calorimetry 135 (DSC). 136 To the best of our knowledge, there have been no previous studies reported in the 137 literature concerning the preparation of AADES using a mixture design, with broad 138 characterization based on variation of the HBD component. These solvents have excellent 139 potential for application in sample preparation procedures and the extraction of inorganic 140 analytes. 141 142 143 144 145 146 147 2. Experimental procedures 148 149 2.1. Materials and reagents 150 The materials employed in the experiments were previously decontaminated for 48 h 151 in acid baths containing 10% v v-1 HNO3, followed by washing three times with ultrapure 152 deionized water obtained from a Milli-Q system (ICW-3000, Merck KGaA, Germany). 153 The AADES studied were prepared using β-alanine, DL-malic acid, citric acid, and 154 xylitol (all from Sigma-Aldrich, MO, USA), together with ultrapure water. Information about 155 the reagents, their molecular structures, and their physicochemical properties is provided in 156 Table 1. 157 158 [Table 1] 159 160 2.2. Preparation of AADES using a mixture design 161 Three different AADES were prepared using combinations of the amino acid β-162 alanine and water with malic acid (AADES 1: Ala-MA), citric acid (AADES 2: Ala-CA), 163 and xylitol (AADES 3: Ala-Xyl). The preparation conditions were optimized using a mixture 164 design (Figure 1), with the proportions of the components restricted by lower and upper 165 limits. In this case, the mixture optimization required the presence of all the components to 166 produce suitable AADES, with the minimum (0%) and maximum (100%) proportions being 167 restricted (see Figure 1). 168 169 [Figure 1] 170 171 Table 2 shows the combinations and proportions (% w w-1) of each component in the 172 mixtures used to prepare the AADES, with a total of 30 experiments performed (10 173 experiments for each AADES). The resulting mixtures were heated for 2 h in a water bath at 174 50 °C, under magnetic stirring at 220 rpm (AccuPlate HotPlate Stirrer, LAbNet, USA), 175 according to the methodology proposed by Dai et al. [6]. The AADES obtained were stored 176 in a desiccator to prevent moisture absorption before their subsequent application. 177 178 [Table 2] 179 180 2.3. Characterization of the AADES 181 182 2.3.1. Density and viscosity measurements 183 The responses evaluated for the restricted mixture design were density and viscosity, 184 which were determined by analyses performed in triplicate for all the prepared AADES. The 185 densities of the solvents were analyzed volumetrically, using a pycnometer previously 186 calibrated with water at 25 °C and an analytical balance with accuracy of ±0.0001 g (Model 187 AG200, Gehaka, Brazil). The values were obtained using a simple calculation for density, as 188 shown in Eq. 1, where m (g) is the mass and V (mL) is the volume of the AADES. 189 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 (𝜌) = 𝑚 𝑉 (1) 190 The viscosity measurements of the AADES were performed using a Cannon-Fenske 191 viscometer calibrated with water, at a controlled temperature of 25 °C. The values were 192 determined using Eq. 2, where d1 and t1 are the density and flow time for the AADES, d2 193 and t2 are the density and flow time for water, all at 25 °C, and η is the value of the water 194 viscosity at 25 °C. 195 196 𝑉𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 (µ) = (𝑑1 .𝑡1) (𝑑2 .𝑡2) . 𝜂 (2) 197 198 2.3.2. Rheological measurements 199 200 The rheological measurements were performed using an AR-2000EX rheometer (TA 201 Instruments, Delaware, USA) using concentric cylinder geometry and gap of 5920 μm. After 202 transfer of the sample to the rheometer, it was allowed to rest during a 5 min equilibration 203 time, before starting the flow test. Tests were performed varying the shear rate from 1 to 1000 204 s–1, at 25 °C. Flow curves were obtained and the Newton, Ostwald-de-Waele, Bingham, and 205 Herschel-Bulkley models were fitted to the experimental data. 206 207 2.3.3. Polarity analysis 208 209 Solvatochromic analyses of the AADES were performed using 10 mM Reichardt’s 210 dye solution prepared in methanol, which was subsequently removed from the solution at 211 room temperature. The AADES-dye mixtures were prepared by diluting 1 mL of dye solution 212 in 4 mL of AADES, followed by ultrasonication for 20 min at 25 °C to completely solubilize 213 the dye. Ultraviolet-visible scanning spectra were obtained using a UV-Vis 214 spectrophotometer (UV-2600, Shimadzu, Japan), in the range 200-600 nm. The Dimroth-215 Reichardt parameter was calculated based on the molar energy value, Et (30), in kcal mol-1, 216 as shown in Eq. 3 [24], where λmax is the maximum absorbance wavelength. The analyses 217 were performed in triplicate for each solvent evaluated. 218 219 Et (30) = 28591 𝜆𝑚𝑎𝑥 (3) 220 221 2.3.4. Fourier transform infrared (FTIR) spectroscopy analysis 222 223 Fourier transform infrared spectroscopy (FTIR) analyses were performed using an 224 IRPrestige-21 instrument (Shimadzu, Japan) equipped with an attenuated total reflectance 225 accessory, in order to obtain the spectra of the prepared AADES solvents and their 226 precursors. The instrument was operated at spectral resolution of 4 cm-1, in the range from 227 4000 to 600 cm-1, with each sample being scanned 30 times. 228 229 2.3.5. Thermogravimetry (TGA) and differential scanning calorimetry (DSC) analyses 230 231 The thermal stabilities of the prepared solvents were evaluated by thermogravimetric 232 analysis (Pyris 1TG, PerkinElmer, USA). A mass of approximately 5 mg of the AADES 233 sample was placed on a platinum support and heated from 25 to 500 °C, at 10 °C min-1, under 234 a flow of N2 at 100 mL min-1. 235 The melting points of the AADES were determined using a calorimeter (DSC-60, 236 Shimadzu, Japan). A mass of approximately 10 mg of the AADES sample was placed in an 237 aluminum crucible and heated from -100 to 400 °C, at 10 °C min-1, under a flow of N2 at 100 238 mL min-1. 239 240 3. Results and discussion 241 242 The optimized proportions of the components used to prepare the AADES were 243 selected based on lower values of density and viscosity, in addition to the stability aspects 244 evaluated. According to Espino et al. [11], the solvents should remain as clear and 245 homogeneous liquids, without instantaneous or progressive precipitate formation. These 246 parameters are very important, since solvents with both low density and low viscosity have 247 the advantage of higher extraction efficiency, due to high diffusivity and consequently faster 248 mass transfer [25,26]. 249 To obtain the simultaneous optimization of both responses, the mathematical 250 approach proposed by Derringer and Suich was used [27,28]. This method is based on 251 desirability functions, which is an approach widely used in experiments involving multiple 252 responses. Initially, each response was individually coded from 0 (undesirable response, 253 higher density and viscosity values) to 1 (desirable response, lower density and viscosity 254 values). The individual desirability (di) for each response (yi) was calculated using Eq. 4: 255 𝑑𝑖 = { 1 𝑖𝑓 𝑦 < 𝑇 ( 𝑈−𝑦 𝑈−𝑇 ) 𝑡 𝑖𝑓 𝑇 ≤ 𝑦 ≤ 𝑈 0 𝑖𝑓 𝑦 > 𝑈 (4) 256 where, U is the highest acceptable value for the response, T is the target value (the lowest 257 value for the response), and t is the weight for the desirability function (for example, equal 258 to 1 for a linear desirability function). 259 The overall desirability (D) was calculated, combining the individual desirabilities 260 into a single response, according to Eq. 5: 261 𝐷 = √𝑑1 × 𝑑2 × ⋯ 𝑑𝑚 𝑚 (5) 262 Finally, three models were calculated using the global desirability for each AADES. 263 In this case, ten coefficients were obtained: constant (b0), linear coefficients (b1, b2, and b3), 264 quadratic coefficients (b11, b22, and b33), and interaction coefficients (b12, b13, and b23). 265 The significance of these coefficients was calculated using one-way analysis of variance 266 (ANOVA, 95% confidence level). Figure 2 shows the contour plots obtained for the model 267 after optimizing the AADES preparation, where higher desirability values are represented by 268 the blue color. 269 270 [Figure 2] 271 272 Similar results were observed for the three AADES, with increase of the water content 273 in the preparation mixture resulting in lower density and viscosity values, as reported 274 previously by Santana et al. [26]. A compromise condition for preparation of the three 275 AADES was established at 12.50% w w-1 for β-alanine, 43.75% w w-1 for the HBD 276 component (malic acid, citric acid, or xylitol), and 43.75% w w-1 for water, according to 277 experiment 10 of the mixture design (see Table 2). This proportion was selected based on 278 solvent stability and lower density and viscosity values of the AADES obtained (Table 3). 279 Although the models shown in Figure 2 suggested the maximum contribution of water (75% 280 w w-1), preparation of the AADES using water concentrations above 50% w w-1 weakened 281 the interactions between the HBA and HBD molecules, due to competition for hydrogen 282 bonds [6]. As water was introduced into the system, the hydrogen bonds between the DES 283 precursors were broken, resulting in dissociation of the precursor compounds in water, 284 without interaction between them and changing the physicochemical properties of the solvent 285 [28]. 286 287 [Table 3] 288 289 Density, viscosity, and polarity are considered crucial physicochemical properties for 290 understanding possible applications of AADES [29,30]. Density and viscosity are related to 291 the molecular structures of the precursors employed, as well as to the extensive networks of 292 hydrogen bonds that form AADES, which can result in less mobility between free species in 293 the solvent structure, making it denser and more viscous, with the variation of HBD having 294 the greatest influence [30]. The use of precursors with molecular structures rich in hydroxyl 295 groups increases the sites available for formation of hydrogen bonds, resulting in higher 296 density and viscosity values, due to the greater forces of attraction between the molecules 297 [31,32]. The same occurs with the use of organic acids, where the carboxyl groups are 298 responsible for hydrogen interactions [33]. However, the values of these properties can 299 decrease with variation of the molecular structure of the precursors, related to the size and 300 organization of the molecules, or the presence of bulky groups that can cause steric effects 301 and prevent the formation of hydrogen bonds [34]. 302 For the AADES selected in this work, the density values increased in the order 303 AADES 3 < AADES 1 < AADES 2, with the density of the solvents being directly reflected 304 by the density of the HBD employed (see Table 1 and Table 3) [35]. The highest viscosity 305 was found for AADES 2, due to the greater quantity of hydrogen bond interactions, resulting 306 in decreased movement of free species. AADES 1 presented the lowest viscosity value, 307 because the HBD had a smaller and more organized molecular structure, with the occurrence 308 of van der Waals interactions prevailing [36]. The value obtained for AADES 3 was close to 309 that for AADES 2, despite the fact that xylitol has five hydroxyl groups in its molecular 310 structure, so more hydrogen interactions would be expected, compared to the use of citric 311 acid, which has three sites for formation of these interactions. This lower viscosity could also 312 be attributed to the high structural organization presented by the xylitol molecule, which 313 favors the formation of van der Waals interactions [36]. The Hole Theory presented by 314 Abbott et al. [37] is used to explain the variation of these properties, where the interaction 315 between the HBA and HBD components causes vacancies in the molecular structure. The 316 use of citric acid as HBD acted to reduce the size of the holes (vacancies) in the solvent 317 structure, resulting in strong cohesive energy due to the formation of a strong intermolecular 318 hydrogen network, contributing to less free space and molecular movement. 319 Application of one-way ANOVA to the density and viscosity values determined for 320 the AADES revealed significant differences in most cases, except between AADES 2 and 321 AADES 3 for viscosity (95% confidence level), showing that changing the HBD component 322 directly influenced the physicochemical properties of the solvents, enabling them to be used 323 for different applications. 324 In another study, NADES based on citric acid and malic acid (CA-MA), malic acid 325 and xylitol (MA-Xyl), and citric acid and xylitol (CA-Xyl) were prepared, where the last 326 showed the highest viscosity, followed by the CA-MA NADES [26], corroborating the data 327 presented here, where the use of citric acid resulted in greater viscosity of the solvent. Shafie 328 et al. [32] and Singh et al. [38] obtained similar results for NADES based on citric acid as 329 HBD, with increases of density and viscosity with increase of the molar proportion of this 330 organic acid. This was attributed to an abundance of HBD favoring the formation of hydrogen 331 bonds, resulting in less mobility between free groups in the molecules of the components 332 making up the solvent. 333 The application of DES in extraction processes has been attracting increasing 334 attention, especially focusing on the influence of the solvent viscosity [6,39]. The addition 335 of water during the preparation procedure is one way to reduce viscosity [33]. In this work, 336 the solvents obtained presented lower viscosity than most DES reported in the current 337 literature, due to the 43.75% water content. Huang et al. [40] studied the effect of adding 338 water in NADES based on choline chloride and glycerol, noting that there were reductions 339 in solvent viscosity of 630 mPa.s with 5% water content, 40 mPa.s with 20% water addition, 340 and 30 mPa.s with 30% water addition, for measurements performed at 40 °C. Subsequently, 341 these same NADES were studied as solvents for the extraction of rutin from wheat husk 342 samples, which showed that water addition greater than 20% resulted in lower extraction 343 efficiency [40]. 344 The rheological properties of the AADES were evaluated based on shear stress curves 345 plotted as a function of the shear rate (Figure 3). AADES 2 presented the highest shear stress, 346 followed by AADES 3 and AADES 1. All the rheological models evaluated could be fitted 347 to the experimental data, with high coefficients of determination (R2 >0.999). The best fit to 348 the data was obtained with the Herschel-Bulkley model. The Bingham model is normally 349 used to describe the rheological behavior of DES and NADES [41,42]. However, for the 350 AADES evaluated here, it provided negative yield stress values, which are meaningless from 351 a physical point of view. Altamash et al. [43] also reported that the Herschel-Bulkley model 352 was suitable for describing the flow behavior of DES based on phenylacetic acid. 353 354 [Figure 3] 355 356 The parameters of the Herschel-Bulkley model are shown in Table 4. All the fluids 357 presented n values slightly greater than 1, indicating weak shear thickening behavior (with 358 the shear stress increasing slightly when the shear rate increases). In this work, all the fluids 359 required a yield stress to start flowing. The yield stress indicates the minimum stress required 360 to disrupt the networked structure and achieve flow. This parameter can be correlated with 361 the pumping energy required to initiate flow [42,44]. 362 363 [Table 4] 364 365 AADES 2 presented the highest yield stress and consistency coefficient, while 366 AADES 1 showed the lowest values of these parameters. These results were in agreement 367 with Martinetto et al. [45] and Qin et al. [46], who studied the behaviors of ionic liquids 368 based on polyoxometalate and imidazole compounds, respectively. The shear thickening 369 behavior were determined at room temperature for solvents that presented cations and anions 370 with longer alkyl chains in their structures, associated with the lengths of the alkyl chains of 371 the HBDs used, which were greater for citric acid, xylitol, and malic acid. The results 372 obtained here were consistent with the viscosity data, confirming that the molecular structure 373 of the donor component in the AADES directly influenced the fluid consistency index. 374 Knowing the polarity of AADES is also of fundamental importance for understanding 375 in advance their behavior and capacity for solvation or extraction of different analytes in 376 different sample matrices. However, there have been few studies dedicated to investigating 377 and discussing this physicochemical property [33,47]. 378 The polarity of AADES can be characterized by the spectroscopic responses of 379 solvatochromic absorbance probes, based on changes in the UV-Vis spectrum resulting from 380 the interaction between the probe molecule and the molecules that make up the solvent 381 [48,49]. Reichardt’s dye is a probe based on negative solvatochromism, characterized by a 382 shift of the absorption band to shorter wavelengths, as the solvent polarity increases [49]. 383 Dimroth and Reichardt’s Et (30) scale is normally used to measure solute-AADES 384 interactions, representing the transfer of intermolecular charge of Reichardt’s dye and its 385 absorption band, with higher Et (30) values indicating greater polarity of the solvent [48-50]. 386 In this work, the values of λmax and Et (30) were 290 ± 0 nm and 98.59 ± 0 kcal mol-1 for 387 AADES 1, 304 ± 0.7 nm and 93.90 ± 0.22 kcal mol-1 for AADES 2, and 305 ± 1 nm and 388 93.74 ± 0.31 kcal mol-1 for AADES 3, reflecting solvents with high polarities. The Et (30) 389 value for pure water is 63.1 kcal mol-1 [48]. Application of one-way ANOVA to the Et (30) 390 values for each solvent revealed significant differences between AADES 1 and AADES 2 or 391 AADES 3 (95% confidence level), in agreement with data obtained previously concerning 392 changes in solvent properties when using different HBD components. 393 The nature of the HBD component is one of the main factors influencing the polarity 394 of the solvent [51]. The use of compounds with linear alkyl molecular structures decreases 395 the ability to donate protons, resulting in higher polarity [30,51]. The same behavior is 396 observed when water is added to the mixture. Gabriele et al. [52] found that the addition of 397 between 0 and 30% water in DES mixtures based on choline chloride and glycol linearly 398 increased the polarity of the solvent. 399 The variation in the polarity of AADES influences their ability to extract target 400 analytes, in accordance with the like dissolves like theory [53]. Solvents with high polarities 401 and AADES-water mixtures provide greater efficiency in the extraction of polar compounds, 402 as well as metals and semimetals, due to the ability to form hydrogen bonds with the analytes 403 [48]. A NADES based on choline chloride and maltose, with the addition of water, was 404 studied as solvent for the extraction of phenolic compounds of different polarities from 405 Canajus canjan leaves (pea species) [54]. The results showed that increasing the proportion 406 of water to 20% increased the polarity of the NADES, enabling efficient extraction of 407 compounds of higher polarity [54]. The extraction of phenolic compounds has also been 408 evaluated using NADES based on choline chloride and different alcohols as HBD, with 409 higher extraction efficiency observed when a greater amount of water was used [55]. 410 FTIR spectra of the starting materials and the prepared solvents were obtained for 411 characterization and evaluation of formation of the solvents (Figure 4). The FTIR technique 412 is sensitive to structural changes, enabling its use to identify functional groups and the 413 formation of hydrogen bonds [32]. The FTIR analyses could confirm the formation of 414 AADES by observing the maintenance of characteristic bands of the starting materials used, 415 with small shifts to smaller wavenumbers in the spectra of the prepared solvents. Archer et 416 al. [56] confirmed the formation of NADES based on choline chloride and (R)-3-417 hydroxyacids by observing shifts of wavenumbers and variations in the stretching vibrations 418 of COO˗H and O˗H bonds. 419 420 [Figure 4] 421 422 The β-alanine spectrum showed peaks at 2873 and 2632 cm-1, characteristic of the 423 various hydrogen bonds present in amino acids [57]. In addition, peaks were observed at 424 1568 cm-1, attributed to the carbonyl group (C=O), 846 cm-1, corresponding to the amine 425 group (N˗H), and 651 cm-1, attributed to the carboxyl group (˗COO). The FTIR spectrum for 426 malic acid showed absorption bands at 3487 cm-1, for the free hydroxyl group (˗OH) in the 427 molecule, 1745 cm-1, corresponding to C˗O stretching, 1675 cm-1 (C=O), and 1180 cm-1 428 (˗CH2 stretching) [58,59]. The citric acid spectrum showed absorption bands at 3440 cm-1 429 (free ˗OH groups), 1683 cm-1 (C=O), and 1742 cm-1 (C˗O) [54,56]. For xylitol, absorption 430 bands were observed at 3426, 3367, and 3229 cm-1, corresponding to free ˗OH groups in the 431 molecule, in addition to a peak at 1431 cm-1, corresponding to in-plane bending of ˗OH, and 432 a peak at 743 cm-1, corresponding to out-of-plane bending of ˗OH [58,59]. 433 For the prepared AADES, the FTIR spectra showed broad bands in the region 434 between 3550 and 3000 cm-1, corresponding to free ˗OH groups in the molecules of the 435 starting materials and in water molecules [60]. The AADES 1 spectrum presented a peak 436 shift from 1675 to 1629 cm-1, while the AADES 2 spectrum showed a peak shift from 1683 437 to 1635 cm-1, corresponding to shifts of the C=O group signals to smaller wavenumbers, due 438 to increased electron density in the oxygen atom, indicative of the occurrence of hydrogen 439 bonding [60]. Other wavenumber shifts were also observed in the spectra for AADES 1 (1745 440 to 1728 cm-1, 1675 to 1629 cm-1, and 1180 to 1112 cm-1), AADES 2 (1742 to 1726 cm-1 and 441 1683 to 1635 cm-1), and AADES 3 (1431 to 1411 cm-1 and 743 to 700 cm-1), all attributed to 442 the HBD components employed. 443 In addition, shifts of the peaks corresponding to the amino acid β-alanine were also 444 observed, providing further evidence of hydrogen bond formation between the initial 445 reagents. For all the solvents, no bands corresponding to new functional groups appeared, 446 indicating that there were only physical interactions between the AADES precursors, without 447 any evidence of chemical reactions [61]. 448 The possible applications of AADES are also dependent on the physical conditions 449 under which they remain as homogeneous liquids, without becoming thermally degraded or 450 solidified [62]. Therefore, thermogravimetric (TG) and derivative thermogravimetric (DTG) 451 analyses were performed, together with determination of the melting points of the solvents 452 using differential scanning calorimetry (DSC) (Figure 5). 453 454 [Figure 5] 455 456 All the AADES showed mass losses of approximately 40% at around 100 °C (Figure 457 5A), corresponding to the proportion of water added during the preparation procedure [63]. 458 Thermal decomposition of AADES 1 occurred at around 130 °C, while AADES 2 and 459 AADES 3 showed thermal decomposition temperatures close to 150 °C (Figure 5B). In all 460 cases, the decomposition was complete at around 310 °C, with a 50% mass loss. In previous 461 work by our research group [58], it was found that the HBDs employed here showed thermal 462 decomposition at 234 °C (malic acid), 279 °C (citric acid), and 286 °C (xylitol), while β-463 alanine degrades at around 200 °C [42]. These decomposition temperatures indicated that the 464 solvents formed were less stable than their precursors. This could be attributed to the high 465 proportion of water employed, which weakened the hydrogen interactions between the 466 components forming the solvent, in agreement with the studies carried out by Paveglio et al. 467 [62]. 468 The nature of the HBA and HBD components, the amount of water in the solvent, the 469 formation of intermolecular interactions, and the alkyl chain size of the precursors are some 470 of the factors that can influence the thermal stability of DES [63]. The use of HBD 471 components with larger alkyl chains increases solvent stability, due to the greater possibility 472 of formation of intermolecular interactions between the molecules of the precursors, with 473 greater energy required for them to be broken [64]. The same occurs when using compounds 474 that enable formation of a greater number of hydrogen bonds, such as compounds rich in 475 hydroxyl and carboxyl groups in their molecular structures [65]. 476 Figure 5C shows the DSC curves for the AADES. The melting point corresponds to 477 the temperature at which the hydrogen interactions between the HBA and HBD components 478 are disrupted, with lower molecular weight of the HBD compound being associated with a 479 lower melting point of the prepared solvent [32,66]. The values obtained here were -13 °C 480 (AADES 1), -22 °C (AADES 2), and -21 °C (AADES 3), showing that the melting 481 temperatures of the solvents formed were lower than the melting temperatures of the 482 precursors (Table 1). 483 The values indicated that stronger hydrogen interactions occurred when citric acid 484 was used as the HBD component, since a higher temperature was required for its thermal 485 degradation, together with a lower temperature for solidification of the solvent. It could be 486 concluded from the thermal analysis data that the AADES could be employed in working 487 temperature ranges between -13 and 130 °C, for the AADES produced using malic acid, and 488 between -22 and 150 °C, for the AADES produced using citric acid or xylitol. 489 The classification of solvents as environmentally friendly is relative, where in 490 addition to their main sustainability characteristics, the energy efficiency of their preparation 491 must also be considered [67,68]. The EcoScale was applied to evaluate the ecological 492 character of the AADES preparation methodology employed here, considering yield, initial 493 reagent price, safety for the operator and the environment, setting of the magnetic stirrer 494 technique, temperature, and reaction time. The scores obtained for the preparation procedures 495 were 97% (AADES 1 and AADES 3) and 92% (AADES 2), corresponding to classification 496 as excellent green synthesis methods, since the values were above 75% [69]. 497 498 4. Conclusions 499 Amino acid-based deep eutectic solvents (AADES) were prepared using β-alanine as 500 the HBA component, together with malic acid, citric acid, or xylitol as the HBD component. 501 The proportions of the components were optimized by applying a restricted mixture design. 502 Density and viscosity analyses revealed that the molecular structure of the HBD component 503 had a major influence on the physicochemical properties of the solvents, with the AADES 504 formed using citric acid being denser and more viscous, due to strong hydrogen interactions 505 provided by a structure rich in carboxyl groups. This behavior was corroborated by 506 rheological measurements. Solvatochromic analyses revealed that the solvents presented 507 high polarity and were suitable for the extraction of polar compounds. Infrared spectroscopy 508 analyses confirmed the formation of the solvents, as shown by wavenumber shifts in the 509 solvent spectra, compared to the spectra of the starting materials. Thermal analyses (TGA 510 and DSC) revealed the working temperature ranges in which the solvents could be used, in 511 addition to confirming that stronger interactions with the HBA were exhibited by the citric 512 acid-based AADES. Finally, evaluation using the EcoScale confirmed that the AADES 513 preparation procedure could be considered an excellent green synthesis. 514 Acknowledgments 515 516 The authors are grateful for the financial support provided by the São Paulo State 517 Research Foundation (FAPESP, grant numbers #2015/08893-4, #2015/14488-0, 518 #2016/17304-0, #2017/18531-3, and #2019/22113-8) and the National Institute for 519 Alternative Technologies of Detection, Toxicological Evaluation and Removal of 520 Micropollutants and Radioactives (INCT-DATREM) (FAPESP, grant number 521 #2014/509454; CNPq, grant number #465571/2014-0). T.G.S.G. is grateful for a fellowship 522 provided by Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES, 523 Finance Code 001). We are also grateful to Dra. Márcia Cristina Bisinotti for the provision 524 of facilites at Laboratório de Estudos em Ciências Ambientais (LECA) (FAPESP, grant 525 number #2012/23066-4). 526 527 References 528 [1] P. Anastas, N. Eghbali, Green Chemistry: Principles and Practice. Chem. Soc. Rev. 39 529 (2010) 301-312. https://doi.org/10.1039 /B918763B. 530 [2] S. Armenta, S. Garrigues, M. Guardia, The role of green extraction techniques in Green 531 Analytical Chemistry. TrAC Trend. Anal. Chem. 71 (2015) 2-8. 532 https://doi.org/10.1016/j.trac.2014.12.011. 533 [3] I. Pacheco-Fernández, V. Pino, Green solvents in analytical chemistry. Curr. Opin. 534 Green Sustain. Chem. 18 (2019) 42-50. https://doi.org/10.1016/j.cogsc.2018.12.010. 535 [4] P.D. María, Ionic liquids, switchable solvents, and eutectic mixtures, in: The 536 Application of Green Solvents in Separation Processes. Elsevier, Amsterdam (2017) 139-537 154. https://doi.org/10.1016/B978-0-12-805297-6.00006-1. 538 [5] N. Meine, F. Benedito, R. Rinaldi, Thermal stability of ionic liquids assessed by 539 potentiometric titration. Green Chem. 12 (2010) 1711-1714. 540 https://doi.org/10.1039/C0GC00091D. 541 [6] Y. Dai, J. van Spronsen, G.J. Witkamp, R. Verpoorte, Y.H. Choi, Natural deep eutectic 542 solvents as new potential media for green technology, Anal. Chim. Acta 766 (2013) 61-68. 543 https://doi.org/10.1016/j.aca.2012.12.019. 544 [7] M.A.R. Martins, S.P. Pinho, J.A.P. Coutinho, Insights into the nature of eutectic and 545 deep eutectic mixtures, J. Solution Chem. 48 (2019) 962-982. 546 https://doi.org/10.1007/s10953-018-0793-1. 547 about:blank about:blank https://doi.org/10.1016/j.cogsc.2018.12.010 https://doi.org/10.1016/B978-0-12-805297-6.00006-1 about:blank about:blank about:blank [8] A. Paiva, R. Craveiro, I. Aroso, M. Martins, R.L. Reis, A.R.C. Duarte, Natural deep 548 eutectic solvents - Solvents for the 21st century. ACS Sustainable Chem. Eng. 2 (2014) 549 1063-1071. https://doi.org/10.1021/sc500096j. 550 [9] M.S. Rahman, R. Roy, B. Jadhav, M.N. Hossain, M.A. Halim, D.E. Raynie, 551 Formulation, structure, and applications of therapeutic and amino acid-deep eutectic 552 solvents: An overview, J. Mol. Liq. 321 (2021) 114745. 553 https://doi.org/10.1016/j.molliq.2020.114745. 554 [10] S.C. Cunha, J.O. Fernandes, Extraction techniques with deep eutectic solvents. TrAC 555 Trend. Anal. Chem. 105 (2018) 225-239. https://doi.org/10.1016/j.trac.2018.05.001. 556 [11] M. Espino, M.A. Fernández, F.J.V. Gomez, M.F. Silva, Natural designer solvents for 557 greening analytical chemistry, TrAC Trend. Anal. Chem. 76 (2016) 126-136. 558 https://doi.org/10.1016/j.trac.2015.11.006. 559 [12] A. Abo-Hamad, M. Hayyan, M.A.H. AlSaadi, M.A. Hashim, Potential applications of 560 deep eutectic solvents in nanotechnology, Chem. Eng. J. 273 (2015) 551-567. 561 https://doi.org/10.1016/j.cej.2015.03.091. 562 [13] Y.P. Mbous, M. Hayyan, A. Hayyan, W.F. Wong, M.A. Hashim, C.Y. Looi, 563 Applications of deep eutectic solvents in biotechnology and bioengineering - Promises and 564 challenges. Biotechnol. Adv. 35 (2017) 105-134. 565 https://doi.org/10.1016/j.biotechadv.2016.11.006. 566 [14] Q. Zhang, K.O. Vigier, S. Royer, F. Jérôme, Deep eutectic solvents: Syntheses, 567 properties and applications. Chem. Soc. Rev. 41 (2012) 7108-7146. 568 https://doi.org/10.1039/C2CS35178A. 569 [15] Y. Liu, J. Garzon, J.B. Friesen, Y. Zhang, J.B. McAlpine, D.C. Lankin, S.N. Chen, 570 G.F. Pauli, Countercurrent assisted quantitative recovery of metabolites from plant-571 associated natural deep eutectic solvents, Fitoterapia 112 (2016) 30-37. 572 https://doi.org/10.1016/j.fitote.2016.04.019. 573 [16] C. Bakirtzi, K. Triantafyllidou, D.P. Makris, Novel lactic acid-based natural deep 574 eutectic solvents: efficiency in the ultrasound-assisted extraction of antioxidant polyphenols 575 from common native Greek medicinal plants. J. Appl. Res. Med. Aromat. Plants 3 (2016) 576 120-127. https://doi.org/10.1016/j.jarmap.2016.03.003. 577 [17] H. Lores, V. Romero, I. Costa, C. Bendicho, I. Lavilla, Natural deep eutectic solvents 578 in combination with ultrasonic energy as a green approach for solubilisation of proteins: 579 application to gluten determination by immunoassay, Talanta 162 (2017) 453-459. 580 https://doi.org/10.1016/j.talanta.2016.10.078. 581 [18] A. Shishov, A. Bulatov, M. Locatelli, S. Carradori, V. Andruch, Application of deep 582 eutectic solvents in analytical chemistry: A review. Microchem. J. 135 (2017) 33-38. 583 https://doi.org/10.1016/j.microc.2017.07.015. 584 [19] A.P.R. Santana, D.F. Andrade, J.Á. Mora-Vargas, C.D.B. Amaral, A. Oliveira, M.H. 585 Gonzalez, Natural deep eutectic solvents for sample preparation prior to elemental analysis 586 about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank by plasma-based techniques. Talanta 199 (2019) 361-369. 587 https://doi.org/10.1016/j.talanta.2019.02.083. 588 [20] A.P.R. Santana, D.F. Andrade, T.G.S. Guimarães, C.D.B. Amaral, A. Oliveira, A.R.A. 589 Nogueira, M.H. Gonzalez, Natural deep eutectic solvents (NADES) in sample preparation 590 of phosphate rock and mineral supplement for elementary determination by plasma-based 591 techniques. Quím. Nova XY (2021) 1-7. http://dx.doi.org/10.21577/0100-4042.20170723. 592 [21] M.W. Nam, J. Zhao, M.S. Lee, J.H. Jeong, J. Lee, Enhanced extraction of bioactive 593 natural products using tailor-made deep eutectic solvents: application to flavonoid 594 extraction for Flos sophorae. Green Chem. 17 (2015) 1718-1727. 595 https://doi.org/10.1039/C4GC01556H. 596 [22] R. Thomas. Practical Guide to ICP-MS: A Tutorial for Beginners (2nd edition), CRC 597 Press, New York (2013). 598 [23] R. Leardi, Experimental design in chemistry: A tutorial. Anal. Chim. Acta 652 (2009) 599 161-172. https://doi.org/10.1016/j.aca.2009.06.015. 600 [24] W. Ogihara, T. Aoyama, H. Ohno, Polarity measurements for ionic liquids containing 601 dissociable protons, Chem. Lett. 33 (2004) 1414-1415. 602 https://doi.org/10.1246/cl.2004.1414. 603 [25] A.K. Dwamena, Recent advances in hydrophobic deep eutectic solvents for extraction, 604 Separation 6 (2019) 9-24. https://doi.org/10.3390/separations6010009. 605 [26] A.P.R. Santana, D.F. Andrade, T.G.S. Guimarães, C.D.B. Amaral, A. Oliveira, M.H. 606 Gonzalez, Synthesis of natural deep eutectic solvents using a mixture design for extraction 607 of animal and plant samples prior to ICP-MS analysis, Talanta 216 (2020) 120956. 608 https://doi.org/10.1016/j.talanta.2020.120956. 609 [27] G. Derringer, R. Suich, Simultaneous optimization of several response variables, J. 610 Qual. Technol. 12 (1980) 214-219. https://doi.org/10.1080/00224065.1980.11980968. 611 [28] N.R. Costa, J. Lourenço, Z.L. Pereira, Desirability function approach: a review and 612 performace evaluation in adverse conditions, Chemom. Intell. Lab. Syst. 107 (2011) 234-613 44. https://doi.org/10.1016/j.chemolab.2011.04.004. 614 [29] H. Qin, X. Hu, J. Wang, H. Cheng, L. Chen, Z. Qi, Overview of acidic deep eutectic 615 solvents on synthesis, properties and applications, Green Energy Environ. 5 (2020) 8-21. 616 https://doi.org/10.1016/j.gee.2019.03.002. 617 [30] M. Vilková, J. Płotka-Wasylka, V.Andruch, The role of water in deep eutectic solvent- 618 base extraction, J. Mol. Liq. 304 (2020) 112747. 619 https://doi.org/10.1016/j.molliq.2020.112747. 620 [31] G. García, S. Aparicio, R. Ullah, M. Atilhan, Deep eutectic solvents: Physicochemical 621 properties and gas separation applications, Energy Fuels 29 (2015) 2616-2644. 622 https://doi.org/10.1021/ef5028873. 623 https://doi.org/10.1016/j.talanta.2019.02.083 http://dx.doi.org/10.21577/0100-4042.20170723 https://doi.org/10.1039/C4GC01556H https://doi.org/10.1016/j.aca.2009.06.015 about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank [32] M.H. Shafie, R. Yusof, C.Y. Gan, Synthesis of citric acid monohydrate-choline 624 chloride based deep eutectic solvents (DES) and characterization of their physicochemical 625 properties, J. Mol. Liq. 288 (2019) 111081. https://doi.org/10.1016/j.molliq.2019.111081. 626 [33] L.K. Savi, D. Carpiné, N. Waszczynskyj, R.H. Ribani, C.W.I. Haminiuk, Influence of 627 temperature, water content and type of organic acid on the formation, stability and 628 properties of functional natural deep eutectic solvents, Fluid Phase Equilib. 488 (2019) 40-629 47. https://doi.org/10.1016/j.fluid.2019.01.025. 630 [34] B. Janković, N. Manić, R. Buchner, I. Płotka-Korus, A.B. Pereiro, E. Amado-631 González, Dieletric properties and kinetic analysis of nonisothermal decomposition of ionic 632 liquids derived form organic acid, Thermochim. Acta 672 (2019) 43-52. 633 https://doi.org/10.1016/j.tca.2018.12.013. 634 [35] F. Chemat, H. Anjum, A.M. Shariff, P. Kumar, T. Murugesan, Thermal and physical 635 properties of (Choline chloride + urea + L-arginine) deep eutectic solvents, J. Mol. Liq. 218 636 (2016) 301-308. https://doi.org/10.1016/j.molliq.2016.02.062. 637 [36] M.B. Taysun, E. Sert, F.S. Atalay, Effect of hydrogen bond donor of the physical 638 properties of benzyltriethylammonium chloride based deep eutectic solvents and their usage 639 in 2-ethyl-hexyl acetate synthesis as a catalyst, J. Chem. Eng. Data 62 (2017) 1173-1181. 640 https://doi.org/10.1021/acs.jced.6b00486. 641 [37] A.P. Abbott, R.C. Harris, K.S. Ryder, Application of Hole Theory to define ionic 642 liquids by their transport properties, J. Phys. Chem. B 111 (2007) 4910-4913. 643 https://doi.org/10.1021/jp0671998. 644 [38] A. Singh, R. Walvekar, M. Khalid, W.Y. Wong, T.C.S.M. Gupta, Thermophysical 645 properties of glycerol and polyethylene glycol (PEG 600) based DES, J. Mol. Liq. 252 646 (2018) 439-444. https://doi.org/10.1016/j.molliq.2017.10.030. 647 [39] B.D. Ribeiro, C. Florindo, L.C. Iff, M.A.Z. Coelho, I.M. Marrucho, Menthol-based 648 eutectic mixtures: Hydrophobic low viscosity solvents, ACS Sustainable Chem. Eng. 3 649 (2015) 2469-2477. https://doi.org/10.1021/acssuschemeng.5b00532. 650 [40] Y. Huang, F. Feng, J. Jiang, Y. Qiao, T. Wu, J. Voglmeir, Z.G. Chen, Green and 651 efficient extraction of rutin from tartary buckwheat hull by using natural deep eutectic 652 solvents, Food Chem. 221 (2017) 1400-1405. 653 https://doi.org/10.1016/j.foodchem.2016.11.013. 654 [41] T. Altamash, M.S. Nasser, Y. Elhamarnah, M. Magzoub, R. Ullah, B. Anaya, S. 655 Aparicio, M. Atilahn, Gas solubility and rheological behavior of natural deep eutectic 656 solvents (NADES) via combined experimental and molecular simulation techniques, 657 ChemistrySelect 2 (2017) 7278-7295. https://doi.org/10.1002/slct.201701223. 658 [42] Y. Elhamarnah, H. Qiblawey, M.S. Nasser, A. Benamor, Thermo-rheological 659 characterization of malic acid based natural deep eutectic solvents, Sci. Total Environ. 708 660 (2020) 134848. https://doi.org/10.1016/j.scitotenv.2019.134848. 661 about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank [43] T. Altamash, M. Atilhan, A. Aliyan, R. Ullah, M. Nasser, S. Aparicio, Rheological, 662 thermodynamic, and gas solubility properties of phenylacetic acid-based deep eutectic 663 solvents, Chem. Eng. Technol. 40 (2017) 778-790. https://doi.org/10.1002/ceat.201600475. 664 [44] F.L. Steffe, Rheological methods in food processes engineering, Freeman Press, East 665 Lansing (1996) 418. https://doi.org/10.1177/108201329700300108. 666 [45] Y. Martinetto, S. Basset, B. Pégot, C.R. Marchal, F. Camerel, J. Jeftic, B.C. Boitte, E. 667 Magnier, S. Floquet, Synthesis, physical properties and application of a series of new 668 polyoxometalate-based ionic liquids, Molecules 26 (2021) 496. 669 https://doi.org/10.3390/molecules26020496. 670 [46] J. Qin, G. Zhang, Z. Ma, J. Li, L. Zhou, X. Shi, Effects of ionic structures on shear 671 thickening fluids composed of ionic liquids and silica nanoparticles, RSC Adv. 6 (2016) 672 81913-81923. https://doi.org/10.1039/C6RA12460G. 673 [47] A.R.R. Teles, E.V. Capela, R.S. Carmo, J.A.P. Coutinho, A.J.D. Silvestre, M.G. 674 Freire, Solvatochromic parameters of deep eutectic solvents formed by ammonium-based 675 salts and carboxylic acid, Fluid Phase Equilib. 448 (2017) 15-21. 676 https://doi.org/10.1016/j.fluid.2017.04.020. 677 [48] M.Q. Farooq, N.M. Abbasi, J.L. Anderson, Deep eutectic solvents in separation: 678 Methods of preparation, polarity, and applications in extractions and capillary 679 electrochromatography, J. Chromatogr. A 1633 (2020) 461613. 680 https://doi.org/10.1016/j.chroma.2020.461613. 681 [49] C. Reichardt, Solvatochromic dyes as solvent polarity indicators, Chem. Rev. 94 682 (1994) 2319-2358. https://doi.org/10.1021/cr00032a005. 683 [50] E. Skoronski, M. Fernandes, F.J. Malaret, J.P. Hallet, Use of phosphonium ionic 684 liquids for highly efficient extraction of phenolic compounds from water, Sep. Purif. 685 Technol. 248 (2020) 117069. https://doi.org/10.1016/j.seppur.2020.117069. 686 [51] W. Tang, K.H. Row, Design and evaluation of polarity controlled and recyclable deep 687 eutectic solvent based biphasic system for the polarity driven extraction and separation of 688 compounds, J. Clean. Prod. 268 (2020) 122306. 689 https://doi.org/10.1016/j.jclepro.2020.122306. 690 [52] F. Gabriele, M. Chiarini, R. Germani, M. Tiecco, N. Spreti, Effect of water addition on 691 choline chloride/ glycol deep eutectic solvents: Characterization of their structural and 692 physicochemical properties, J. Mol. Liq. 291 (2019) 111301. 693 https://doi.org/10.1016/j.molliq.2019.111301. 694 [53] X.H. Yao, D.Y. Zhang, M.H. Duan, Q. Cui, W.J. Xu, M. Luo, C.Y. Li, Y.G. Zu, Y.J. 695 Fu, Preparation and determination of phenolic compounds from Pyrola incarnata Fisch. 696 with a green polyols based-deep eutectic solvent, Sep. Purif. Technol. 149 (2015) 116-123. 697 https://doi.org/10.1016/j.seppur.2015.03.037. 698 [54] Z. Wei, X. Qi, T. Li, M. Luo, W. Wang, Y. Zu, Y. Fu, Application of natural deep 699 eutectic solvents for extraction and determination of phenolics in Cajanus cajan leaves by 700 about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank about:blank ultra-performance liquid chromatography, Sep. Purif. Technol. 149 (2015) 237-244. 701 https://doi.org/10.1016/j.seppur.2015.05.015. 702 [55] W. Bi, M. Tian, K.H. Row, Evaluation of alcohol-based deep eutectic solvent in 703 extraction and determination of flavonoids with response surface methodology 704 optimization, J. Chromatogr. A 1285 (2013) 22-30. 705 https://doi.org/10.1016/j.chroma.2013.02.041. 706 [56] L. Archer, B. Jachimska, M. Krzan, M. Szaleniec, E. Hebda, P. Radzik, K. 707 Pielichowski, M. Guzik, Physical properties of biomass-derived novel natural deep eutectic 708 solvents based on choline chloride and (R)-3-hydroxyacids, J. Mol. Liq. 315 (2020) 709 113680. https://doi.org/10.1016/j.molliq.2020.113680. 710 [57] T. Altamash, M.S. Nasser, Y. Elhamarnah, M. Magzoub, R. Ullah, H. Qiblawey, S. 711 Aparicio, M. Atilhan, Gas solubility and rheological behavior study of betaine and alanine 712 based natural deep eutectic solvents (NADES), J. Mol. Liq. 256 (2018) 286-295. 713 https://doi.org/10.1016/j.molliq.2018.02.049. 714 [58] A.P.R. Santana, J.A.M. Vargas, T.G.S. Guimarães, C.D.B. Amaral, A. Oliveira, M.H. 715 Gonzalez, Sustainable synthesis of natural deep eutectic solvents (NADES) by different 716 methods, J. Mol. Liq. 293 (2019) 111452. https://doi.org/10.1016/j.molliq.2019.111452. 717 [59] R.M. Silverstein, F.X. Webster, D.J. Kiemle, Identificação espectrométrica de 718 compostos orgânicos (7th ed.) LTC, Rio de Janeiro (2015). 719 [60] M.F. Arafa, S.A. El-Gizawy, M.A. Osman, G.M. El Maghraby, Xylitol as a potential 720 co-crystal co-former for enhacing dissolution rate of felodipine: preparation and evaluation 721 of sublingual tablets, Pharm. Dev. Technol. 23 (2018) 454-463. 722 https://doi.org/10.1080/10837450.2016.1242625. 723 [61] F.S. Mjalli, M. Al-Azzawi, Aliphatic amino acids as a possible hydrogen bonds donor 724 for preparing eutectic solvents, J. Mol. Liq. 330 (2021) 115637. 725 https://doi.org/10.1016/j.molliq.2021.115637. 726 [62] G.C. Paveglio, F.A.S.C. Milani, A.C. Sauer, D. Roman, A.R. Meyer, L. Pizzuti, 727 Structure-physical properties relationship of eutectic solvents prepared from 728 benzyltriethylammonium chloride and carboxyl acids, J. Braz. Chem. Soc. 32 (2021) 542-729 551. http://dx.doi.org/10.21577/0103-5053.20200208. 730 [63] S. Wang, X. Peng, L. Zhong, S. Jing, X. Cao, F. Lu, R. Sun, Choline chloride/urea as 731 an effective plasticizer for production of cellulose films, Carbohydr. Polym. 117 (2015) 732 133-139. https://doi.org/10.1016/j.carbpol.2014.08.113. 733 [64] H. Ghaedi, M. Ayoub, S. Sufian, B. Lal, Y. Uemura, Thermal stability and FT-IR 734 analysis of Phosphonium-based deep eutectic solvents with different hydrogen bonds 735 donors, J. Mol. Liq. 242 (2017) 395-403. https://doi.org/10.1016/j.molliq.2017.07.016. 736 [65] S. Zhu, H. Li, W. Zhu, W. Jiang, C. Wang, P. Wu, Q. Zhang, H. Li, Vibrational 737 analysis and formation mechanism of typical deep eutectic solvents: An experimental and 738 theoretical study, J. Mol. Graph. Model. 68 (2016) 158-175. 739 https://doi.org/10.1016/j.jmgm.2016.05.003. 740 about:blank about:blank about:blank about:blank about:blank about:blank https://doi.org/10.1016/j.molliq.2021.115637 about:blank about:blank about:blank about:blank [66] T. El Achkar, H. Greige-Gerges, S. Fourmentin, Basics and properties of deep eutectic 741 solvents: a review. Environ. Chem. Lett. (2021). https://doi.org/10.1007/s10311-021-742 01225-8. 743 [67] Z. Yang, Natural deep eutectic solvents and their applications in biotechnology. In: 744 Advances in Biochemical Engineering/ Biotechnology. Springer, Berlin, Heidelberg, 2018. 745 https://doi.org/10.1007/10_2018_67. 746 [68] A. Wypych, G. Wypych, 1 - What does make solvents green? In: Databook of Green 747 Solvents. Chem. Tec. Publishing, 2019. https://doi.org/10.1016/B978-1-927885-43-748 7.50003-2. 749 [69] K.V. Aken, L. Strekowski, L. Patiny, EcoScale, a semi-quantitative tool to select an 750 organic preparation based on economical and ecological parameters, Beilstein J. Org. 751 Chem. 2 (2006) 1-7. https://doi.org/10.1186/1860-5397-2-3. 752 753 https://doi.org/10.1007/s10311-021-01225-8 https://doi.org/10.1007/s10311-021-01225-8 about:blank about:blank about:blank about:blank 754 Tables 755 Reagent CAS number Purity (%) Density (g mL-1) Melting point (°C) Molecular structure β-alanine 107-95-9 98 1.44 207 DL-malic acid 6915-15-7 99 1.61 130 Citric acid 77-92-9 99.5 1.66 153 Xylitol 87-99-0 99 1.52 92 Table 1. Molecular structures and physicochemical properties of the reagents used to prepare 756 the AADES. 757 758 759 760 761 762 763 764 765 766 767 768 Mixture β-alanine (% w w-1) HBD component (% w w-1) a Water (% w w-1) 1 50.00 25.00 25.00 2 25.00 50.00 25.00 3 25.00 25.00 50.00 4 33.33 33.33 33.33 5 12.50 12.50 75.00 6 12.50 75.00 12.50 7 75.00 12.50 12.50 8 43.75 12.50 43.75 9 43.75 43.75 12.50 10 12.50 43.75 43.75 a Malic acid (AADES 1), citric acid (AADES 2), or xylitol (AADES 3). 769 Table 2. Experimental conditions for the synthesis of AADES 1 (Ala-MA), AADES 2 (Ala-770 CA), and AADES 3 (Ala-Xyl). 771 772 773 Mixture AADES 1 AADES 2 AADES 3 Density (g mL-1) a Viscosity (mPa s) a Density (g mL-1) a Viscosity (mPa s) a Density (g mL-1) a Viscosity (mPa s) a 1 1.30 ± 0.01 165.71 ± 2.05 1.31 ± 0.01 237.56 ± 3.00 - d - d 2 1.33 ± 0.01 107.56 ± 2.40 1.36 ± 0.01 252.89 ± 4.19 1.29 ± 0.01 112.38 ± 1.18 3 1.21 ± 0.01 5.95 ± 0.01 1.22 ± 0.01 7.47 ± 0.63 1.18 ± 0.01 6.00 ± 0.29 4 1.29 ± 0.01 45.08 ± 1.14 1.29 ± 0.01 50.84 ± 0.67 1.25 ± 0.01 27.34 ± 1.68 5 1.10 ± 0.01 1.33 ± 0.26 1.10 ± 0.01 1.49 ± 0.00 1.09 ± 0.01 1.48 ± 0.01 6 - b - b - b - b - b - b 7 - b - b - b - b - d - d 8 1.21 ± 9.06.10-5 9.05 ± 0.30 1.23 ± 0.01 17.19 ± 0.32 1.20 ± 0.01 8.59 ± 0.01 9 - c - c - c - c - b - b 10 1.25 ± 0.01 7.332 ± 0.005 1.26 ± 0.01 9.56 ± 0.01 1.21 ± 0.01 9.02 ± 0.29 a Average ± standard deviation (n=3). 774 b The solvent was unstable, with precipitate formation at room temperature. 775 c The solvent showed high density and viscosity, making it impossible to carry out the analyses. 776 d There was no homogeneous solvent formation. 777 Table 3. Density and viscosity values determined for AADES 1 (Ala-MA), AADES 2 (Ala-CA), and AADES 3 (Ala-Xyl), at 25 °C. 778 779 780 Herschel-Bulkley model τ0 (Pa) k (Pa.s) n R2 AADES 1 0.01329 0.00848 1.06 0.99996 AADES 2 0.01753 0.01401 1.05 0.99997 AADES 3 0.01537 0.01132 1.05 0.99997 Table 4. Herschel-Bulkley model rheological parameters for AADES 1 (Ala-MA), AADES 781 2 (Ala-CA), and AADES 3 (Ala-Xyl). τ0: yield stress; k: consistency coefficient; n: flow 782 behavior index. 783 784 Figures 785 786 787 Figure 1. Mixture design delimited by restrictions to optimize synthesis of the AADES. 788 789 790 791 792 793 Figure 2. Contour plots for optimization of the mixture components to synthesize AADES 1 794 (Ala-MA), AADES 2 (Ala-CA), and AADES 3 (Ala-Xyl). 795 796 797 Figure 3. Flow curves for ■ AADES 1 (Ala-MA), ● AADES 2 (Ala-CA), and ▲ AADES 3 798 (Ala-Xyl), obtained at 25 °C. Fits obtained using the Herschel-Bulkley model. 799 800 801 802 803 Figure 4. FTIR spectra of the initial reagents and the prepared solvents: (A) AADES 1 (Ala-804 MA), (B) AADES 2 (Ala-CA), and (C) AADES 3 (Ala-Xyl). 805 806 Figure 5. (A) TGA, (B) DTA, and (C) DSC spectra for the three synthesized AADES. 807 Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Conflict of Interest