Fruit sugar-based deep eutectic solvents and their physical properties

Thermochimica Acta 541 (2012) 70–75 Contents lists available at SciVerse ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Fruit sugar-based deep eutectic solvents and their physical properties Adeeb Hayyan a,b , Farouq S. Mjalli a,∗ , Inas M. AlNashef c , Talal Al-Wahaibi a , Yahya M. Al-Wahaibi a , Mohd Ali Hashim b a b c Petroleum and Chemical Engineering Department, Sultan Qaboos University, Muscat 123, Oman Department of Chemical Engineering, Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysia Chemical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia a r t i c l e i n f o Article history: Received 18 February 2012 Received in revised form 23 April 2012 Accepted 25 April 2012 Available online 4 May 2012 Keywords: Fructose Monosaccharides Deep eutectic solvents Ionic liquids a b s t r a c t In this study, a novel fructose-based DES of choline chloride (2-hydroxyethyl-trimethylammonium) has been synthesized at different molar ratios. The physical properties such as density, viscosity, surface tension, refractive index and pH were measured and analyzed as function of various temperatures (25–85 ◦ C). The analysis of these physical properties revealed that these new DESs have the potential to be utilized for possible industrial applications involving processing and separation of food constituents. The suggested DESs have many desirable characteristics, e.g. they have low vapor pressure, inflammable, biodegradable, and made from renewable resources. The use of these DESs will positively affect the environment and make use of available renewable resources. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Industrial products based on abundant agricultural bioresources such as sugars are addressed as one of the essential products for the sustainability of human life. Sugars industry research and development is gaining increasing concern due to the impact of increased consumption of refined sugar on human health [1]. Numerous types of sugar and syrups are available to domestic and industrial users [2,3]. As a consequence of pressure from conservationists and local communities, sugar industry is continuously refining its production methodologies and technologies to cope with environmental considerations. The most important factors to improve the sugar industry are finding new and available sources associated with economical extraction and purification processes to produce high quality sugars. Al-Eid et al. [4] considered the nutritional value of date syrups and sugars as well as their chemical composition. The study reported that there is higher percentage of fructose than glucose in date syrup. Consequently, by separating fructose, more profitable value-added products could be achieved. Palm date is a rich raw material for producing fructose in high abundance of supply and perennial availability [5]. Fructose is a highly important product in the food industry as well as the pharmaceutical industries. These industries require a continuous and cheap sustainable supply of sugars. On a dry weight basis, palm dates contain 65–80% equal ∗ Corresponding author. Tel.: +968 24142558; fax: +968 24141354. E-mail address: farouqsm@yahoo.com (F.S. Mjalli). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.04.030 amounts of glucose and fructose. Currently, only about 30% of the usable produced dates are utilized for human consumption and the remaining quantity contributes as an ingredient serving the food industry. Recently, AlNashef et al. [6] patented the use of ionic liquids ([dimethylimidazolium dimethylphosphate] and [1-ethyl3-methylimidazolium ethylsulfate]) for the separation of monosaccharides from their aqueous and solid mixtures. The patent claimed that ionic liquids work as selective agent that can separate glucose and fructose under ambient conditions. There are many advantages and favorable merits for using ILs in many industrial applications. Examples include, the undetectable vapor pressure, liquidity at a wide temperature range, the high solubility for a wide range of chemical compounds, as well as their less toxicity [7,8,9]. In recent years, deep eutectic solvents (DES) were introduced as a promising class of room temperature ionic liquids that lend themselves as efficient alternatives for conventional ionic liquids with better cost effectiveness. Their simple synthesis and the flexibility in choosing their constituent components facilitate their use over complex and expensive ILs. This encourages their utilization in food processing applications. DESs are relatively new class of ionic liquids that are simply synthesized via mixing of salt with a hydrogen bond donor compound [9,10]. They have many of ILs merits such as their biodegradable components, non-flammability due to their low or none measurable vapor pressure and low toxicity [8,10,11]. DESs were introduced in many industrial applications as attractive alternatives to ILs such as the synthesis of zeolite analog [12], solvent extraction of aromatics from naphtha [13], removal A. Hayyan et al. / Thermochimica Acta 541 (2012) 70–75 Table 1 Compositions and abbreviations for d-fructose based DESs. Molar ratio 1:1 1.5:1 2:1 2.5:1 Abbreviation DES1 DES2 DES3 DES4 71 Table 2 Experimental uncertainties in measurements. Appearance at room temperature Liquid Liquid Liquid Liquid of excess glycerol from biodiesel fuel [14], synthesis of shape controlled catalyst nano-particles [15] and their use in electrochemical applications [16,17]. DESs as new types of ILs can be used in the fractionation and separation of monosaccharides such as fructose and glucose. There are limited physical properties reported in the literature for monosaccharides based DES. Providing sufficient data in terms of physical properties will increase the possibility of utilizing these DESs in many future applications. Hence, the main objectives of this study is to synthesis fructose based DES and study their important physical properties such as the density, viscosity, surface tension, refractive index and pH. 2. Methods and materials 2.1. Chemicals Choline chloride (2-hydroxyethyl-trimethylammonium), dfructose anhydrous with purity (98%) and, pH buffer solutions were supplied by Merck Chemicals (Darmstadt, Germany). Chemicals were dried in a vacuum oven prior to use to eliminate moisture contamination. 2.2. Technical methodology DESs samples were synthesized as different ratios of choline chloride to d-fructose as given in Table 1. Because of its hygroscopic nature, choline chloride was treated by drying in a vacuum dryer at 80 ◦ C for 6 h before utilization. The salt (choline chloride) and the hydrogen bond donor (d-fructose) were mixed in an incubator shaker (Brunswick Scientific Model INNOVA 40R). The mixture of choline chloride and d-fructose was shaken at 400 rpm and 80 ◦ C for a period of 2 h until the DESs becomes homogenous and stable with no apparent precipitate. DES samples were synthesized at atmospheric pressure and under tight control of moisture content. In this study, the temperature range of all physical properties was 25–85 ◦ C. All samples were prepared in a moisture controlled environment and kept in well-sealed vials after preparation. Fresh samples were used for analysis to avoid any structure changing and to avoid humidity effects from the environment which may affect the physical properties of DES. The viscosities of the DESs were measured using a rotational viscometer (Anton Paar Rheolab QC). The temperature was controlled using external water circulator (Techne-Tempette TE-8A). The densities of all samples of DESs were measured using a liquid densitometer (Anton Paar DMA4500M). The surface tension of samples was measured using an automatic tensiometer (Krüss K10ST classification B with Du Noüy ring method). An Abbe type refractometer (model 60/ED equipped with a sodium D1 line) was used to measure the DESs refractive indices. The temperature was controlled in the refractometer using Techno TE-8D water circulator. Deionized water was used for calibration before each experiment. The temperature of each sample was controlled using a water circulator (Julabo Labortechnik). Table 2 shows the experimental uncertainties in the measurement of each physical property. Property Estimated uncertainty Density of solid phase Density Viscosity (relative) Surface tension Refractive index pH ±0.001 g cm−3 ± 0.0001 g cm−3 (3–5) percent of measured value ±0.1 mN m−1 0.007 0.05 3. Results and discussion Choline chloride and d-fructose were used to prepare 4 samples of DESs as shown in Table 1 along with their abbreviations and our observations during the preparation stage. The ratio was reported as different molar amount of d-fructose and fixed amount of choline chloride. DES1, DES2, DES3, and DES4 appeared as transparent liquid phase with very viscous form. Based on this, the study covered the physical properties for DES1, DES2, DES3, and DES4 in liquid phase and other unsuccessful ratios were discarded. At the early stage of DES preparation, the mixtures appeared as a white viscous gel within the first 30 min. After 60 min of mixing, a liquid phase started to appear with some yellow precipitate. Consequently, the time of mixing was extended to 120 min in order to get a homogenous liquid phase (DES). The long time of shaking at high temperature results in the yellowish color of the d-fructose based DESs. This is mainly due to the oxidation of sugar due to the caramelization phenomena which increases with temperature. Therefore, special care was taken in the preparation stage to avoid any change in the structure of DES. After 120 min there was no apparent precipitation in the mixing flask which indicated that all d-fructose molecules were physically bonded to choline chloride. It was reported that the lowest DES melting temperature depends on the molar ratio of salt to hydrogen bond donor [9]. However, in the case of d-fructose sugar-based DES, the cyclic form of the sugar molecule results in an angle of interaction (between the chlorine anion and the hydroxyl group) that is more in favor to allow 2 choline chloride molecules to form hydrogen bonds [18]. This could be the reason for having the eutectic point (of DES3) at the molar ratio 2:1. The freezing points of d-glucose based DESs are shown in Table 3 [18]. As can be inferred from the data, the freezing points of studied DESs range between 10 and 37 ◦ C. Table 3 indicates that the eutectic point was 10 ◦ C for DES3 at molar ratio (1:2). Due to this low freezing point, DES3 has promising potential for separation or reaction applications. Consequently, further physicochemcial properties should be taken into consideration for DES3. As shown in Table 3 the highest freezing points are for DES4 and DES1 while the lowest freezing points were for DES2 and DES3. Density is a very important property for chemical materials and their processing. It is well known that density is a function of temperature. The increase in temperature results in more molecular activity and mobility. This increases the solution molar volume which eventually reduces density. For many application, it is very important to know the temperature effect on density. The studied DESs density measurements were conducted as a function of Table 3 Experimental values of freezing points of the salt (choline chloride) and d-fructose DES at pressure p = 0.1 MPa [18].a Freezing point/◦ C DES1 DES2 DES3 DES4 a Standard uncertainty is ±1 ◦ C. 20 13 10 37 72 A. Hayyan et al. / Thermochimica Acta 541 (2012) 70–75 Table 4 Density–temperature model parameter. 1.36 1.34 DES1 DES2 DES3 DES4 1.30 b −5.6182 −5.6189 −5.8889 −5.3350 1.3513 1.3179 1.2932 1.2725 1.28 1.26 1.24 1.22 20 40 60 80 o t/ C Fig. 1. Densities, , of d-fructose based DESs as a function of temperature t. 䊉, , , and △ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (1). temperature in the range (25–85 ◦ C). The effect of temperature on densities of different d-fructose based DES at different ratios is depicted in Fig. 1. Measured densities of DESs at all molar ratios were less than 1.34 g cm−3 . The reduction in density was linear for all studied DESs. It should be noted that the density of the DES increases as the salt molar ratio increases The highest density was that of DES1 with the molar ratio 1:1, which reaches a maximum of 1.3370 g cm−3 at room temperature and a minimum of 1.2269 g cm−3 at 85 ◦ C. On the other hand, DES4 has the lowest density (1.2115 g cm−3 at the highest temperature of 85 ◦ C). The results attained in this work were compared to Kareem et al. [9] work for the physical properties of phosphonium-based DES. It was found that DES2 have approximately similar values of density range to the DES made of [methyltriphenylphosphonium bromide:glycerol] at a ratio of 1:1.75. In addition, density of ILs were compared with the results reported in this study and it was found that [1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate] has density 1.20 g cm−3 which is very close to the density value of DES4 at room temperature. The highest value of DES density was 1.33692 g cm−3 at room temperature which is very close to the that of phosphonium based ILs such as [1,11-di(tripropylphosphonium)-3,6,9-trioxaundecane bis(trifluoromethane) sulfonamide] and slightly higher than density of ammonium based ILs such as [1,3-dimethyl-2-(Nmethyl-N-butyl ammonium) imidazolidine hexafluorophosphate] which has density value of 1.33 g cm−3 [19]. A high density of 1.32 g cm−3 [1-amyl-3-methylimidazolium hexafluorophosphate] was reported. [19] This matches the measured value of DES1 at room temperature. However ILs may have much higher density, an example is a value of 2.40 reported for the ionic liquid [(CH3 )3 S]+ [Al2 Br7 ]− .[8] To summarize we can say that the d-fructose based DESs have similar densities to other reported DESs and ILs. The density values of the studied DESs were modeled as a function of temperature as follows: /g cm −3 ◦ = a(t/ C) + b (1) where  is the density, t is the temperature, a and b are constants that depend on the molar ratio of DES. The values of a and b for the studied DESs are presented in Table 4. Viscosity data of DES is very important for the design stage of industrial processes, fluid flow systems and in the selection of suitable applications. Viscosity data can be used for the selection of optimum ratio of salt and hydrogen bond donor. In general, at atmospheric pressure, temperature has a profound effect on viscosity. The increase in temperature, results in increasing the average speed of the molecules in the liquid which decreases the average intermolecular forces and consequently reduces the fluid resistance to flow which is termed as the viscosity. It was reported that DES being liquid at room temperature makes them easy to handle and applicable to many chemical processes and industrial applications [9]. In this study, the highest viscosity measurement was attained at room temperature (25 ◦ C) for DES4 (17645.5 mPa s) followed by DES2 (14347.4 mPa s). Certain chemical applications such as liquid–liquid extraction and reactions of liquid phases utilizing fluids such as DESs [13] may need high pumping energy requirements for the case of viscous fluid. Therefore, pre-heating is a very simple and efficient technique that can be used to reduce the viscosity before processing. Fig. 2 shows that the viscosities of DES1, DES2, DES3 and DES4 have decreased with increasing temperature. The lower values of viscosity at 85 ◦ C for all DESs are 129.30 mPa s for (DES1), 182.6 mPa s for (DES4), 236.10 mPa s for (DES3) and 280.6 mPa s for (DES1). It was noted that the lowest viscosity at room temperature (25 ◦ C) and at high temperature belongs to eutectic composition of DES3. It can be said that physical properties such as viscosity reflects the eutectic point for different ratios of DESs and consequently this highlights the importance of the eutectic point for such solvents. d-fructose based DES is very sensitive to the increase in temperature as shown in Fig. 2. There is significant reduction in the viscosity for all DESs within the temperature range of 45 ◦ C and 55 ◦ C. For example the reduction of viscosity for DES4 in that temperature range was from 4461.0 mPa s to 1267.5 mPa s. While for DES3, which has the lowest freezing point, the reduction was from 2457.0 mPa s to 1261.7 mPa s at the same temperature range. It can be concluded that the d-fructose based DES has high viscosity at room temperature and therefore it is recommended to 20000 18000 16000 14000 12000 μ / mPa.s ρ/g.cm -3 1.32 a × 10−4 10000 8000 6000 4000 2000 0 0.0028 0.0030 0.0032 0.0034 t-1/K-1 Fig. 2. Dynamic viscosity, , of d-fructose -based DESs as a function of temperature. 䊉, , , and △ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (2). A. Hayyan et al. / Thermochimica Acta 541 (2012) 70–75 Table 6 Surface tension–temperature model parameters. Table 5 Viscosity–temperature model parameters. (E /R)/K−1 o /mPa s −6 DES1 DES2 DES3 DES4 73 6.442 × 10 3.393 × 10−6 9.234 × 10−7 7.534 × 10−7 6421.44 6579.41 6940.45 7121.50 DES1 DES2 DES3 DES4 heat the DES to higher temperatures such as 45–55 ◦ C or even to 65 ◦ C in order to reduce the viscosity of the DESs. Other studies did not investigate the effect of molar ratios on the viscosity of a particular type of DES. d-fructose based DESs are much viscous than other reported phosphonium-based DESs by other studies [9]. The viscosity of the tested DESs was modeled with an Arrhenius form model as shown in Eq. (2):  = o e[−E /RT ] (2) where  is the viscosity, o is a pre-exponential constant, E is the activation energy, R is the gas constant, and T is the temperature in Kelvin. Values of o and E are given in Table 5. It is worth mentioning that the uncertainty in viscosity measurements is higher at the upper range of measurements. This resulted in a non-smooth scattering of the measured data. This behavior could be attributed to the complex physical bonding existing between the salt and HDB which cannot be explained fully by the conventional Arrhenius type model used. Surface tension is a basic fluid property which is defined as the energy required to increase its surface per unit area. This energy is caused by the effect of intermolecular forces at the interface. It is mostly used for calculations involving emulsions and surfactants in chemical, biochemical and pharmaceutical applications. Surface tension data is rarely reported for DES liquids. The measured values of surface tension at room temperature for DES1, DES2, DES3, and DES4 were 70.4, 75.6, 74.0, and 75.0 mN m−1 , respectively. Fig. 3 presents the relationship between the temperature and the surface tension of DESs. DES2 and DES3 have very close surface tension values at different temperatures while DES1 is far beyond other DESs. It was noted that DES4 has the highest surface tension values due to the high ratio of salt to hydrogen bond donor. The possible reason for that is due to high viscosity of d-fructose based DES as well as the higher number of hydrogen bonding for the higher molar ratios DES. As the ratio of d-fructose to salt increases, the surface tension also increases as shown in a b −0.1843 −0.2079 −0.2000 −0.2000 74.6214 78.8607 79.0000 80.0000 Fig. 3. The measured surface tension of the studied DESs were compared to the corresponding conventional ILs data available in the literature [16]. The majority of ILs reported in the literature has surface tension in the range 3–55 mN m−1 . Ammonium based ILs such as [2-hydroxyethylammonium formate] relatively has high surface tension (65 mN m−1 ) [16] which is the same value of that for DES2 at 65 ◦ C. such as [1-allyl-1-methylpyrrolidinium ILs bis(trifluoromethanesulfonyl)imide] has surface tension of (57 mN m−1 ) [16]. Surface tension behavior was fitted linearly for each DES according to the following relationship: /mN m−1 = a(t/◦ C) + b (3) where is the surface tension, t is the temperature, and a and b are constants that depend on the salt: HBD molar ratio in the DES. The values of a and b for the studied DESs are shown in Table 6. Refractive index (RI) is a material property that expresses the ratio of the speed of light in vacuum relative to that in the considered medium. It is one of the important properties that has many applications such as checking the purity of materials and measuring the concentration of solutes in solutions [9]. Similar to other DES physical properties, refractive index data was not covered extensively in literature and especially that of d-fructose based DESs. Refractive index of the considered DESs was measured as a function of temperature and shown in Fig. 4. It was noted that the RI of all DESs lies within the range of 1.5071–1.5228 for the temperature range of 25–85 ◦ C. At room temperature, RI values lie within the range 1.5198–1.5228. It is well known that the RI is proportional to the square root of electrical permittivity and magnetic permeability. Both properties change nonlinearly with temperature, and hence RI has a nonlinear behavior with temperature, in general. Previous studies reported that the DES refractive index does not have a simple relationship with temperature [9]. In case of d-fructose based DES, the RI decreased with increasing temperature as shown in Fig. 4. 76 1.524 74 1.522 72 1.520 1.518 68 1.516 nD γ/m.N.m -1 70 66 1.514 64 1.512 62 1.510 60 1.508 58 1.506 20 40 60 80 o t/ C Fig. 3. Surface tension, , of fructose-based DESs as a function of temperature. 䊉, , , and △ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (3). 20 40 60 80 o t/ C Fig. 4. Refractive indices, nD , of d-fructose based DESs as a function of temperature. 䊉, , , and △ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (4). 74 A. Hayyan et al. / Thermochimica Acta 541 (2012) 70–75 Table 7 Refractive index–temperature model parameters for the studied DESs. DES1 DES2 DES3 DES4 a × 10−4 b −1.9432 −2.0729 −2.0921 −1.9379 1.5280 1.5262 1.5250 1.5264 based DESs. DES1 has a pH value of 6.1 at room temperature which is similar to the corresponding value of the phosphonium based DES formed between [benzyltriphenylphosphonium chloride] and ethylene glycol in the ratio of (1:3) [9]. The temperature–pH relationship was linearly fitted according to the following relationship pH = a(t/◦ C) + b Table 8 pH–temperature model parameters for the studied DESs. DES1 DES2 DES3 DES4 a b −0.0309 −0.0100 −0.0306 −0.0116 6.9568 7.1757 7.5120 7.3893 where t is temperature in ◦ C, and a and b are constants that vary according to the type of DES: Table 8 shows the values of these two parameters. 4. Conclusion It is worthy to mention that the eutectic composition of DES2 has the lowest RI. The highest value of RI is that for DES1. Values for DES2 and DES4 are very close to each other. The RI for the studied DESs are higher than those reported by pervious studies [9]. Imidazolium based ILs such as [1-octyl-3-methylimidazolium chloride] has refractive index of 1.505 [16] which is very close to DES3 at 85 ◦ C. The refractive index–temperature relationship was fitted linearly for all samples of d-fructose based DESs according to the following relationship: RI = a(t/◦ C) + b (5) (4) ◦ C, and a and b are constants that vary where, t is temperature in according to the type of DES. Parameters a and b are unitless. Table 7 shows the values of refractive index parameters a and b for Eq. (4). pH is a physical property that has essential impact on the selection of metal type in many industrial applications to minimize corrosion problems. It is also important for biochemical reactions. In this study, pH was measured for the different types of d-fructose based DESs. Because the considered DES is composed of non-toxic and environmental friendly constituents, it can be used in biological application hence it is very important to know the pH of such DES systems. pH of d-fructose based DES has values in the range of 6.1 (DES1) to 7.1 (DES4) at room temperature (25 ◦ C). As shown in Fig. 5 at high temperature the d-fructose based DES tend to be more acidic gradually with the increase in temperature. The new pH measurements were compared to the reported phosphonium 7.5 In this study the physical properties of d-fructose based DESs including density, viscosity, surface tension, refractive index and pH were measured and analyzed as a function of temperatures (25–85 ◦ C). The room temperature measurements of d-fructose based DES properties revealed that, they have high viscosity, density and surface tension. Hence, it is recommended to heat these DESs before processing. It was also found that as salt mole ratio increases, the density of the DES increases accordingly. The lowest measured viscosity is that of the eutectic composition (DES3), which indicates that such property can be used to identify eutectic composition. The lowest RI value belongs to DES3 while the highest RI value was 1.5228. The pH of all DESs tested in this study tends to be more acidic with the increase of temperature. The physical properties of d-fructose based DESs indicated that this type of fluids has a practical potential use in many industrial applications involving reactions, pharmaceutical applications and as a solvent in extraction processes (for example the fractionation and separation of fructose from monosaccharide mixtures). Acknowledgments This research was funded by the Petroleum and Chemical Engineering Department, Sultan Qaboos University, Sultanate of Oman research grant number IG/ENG/PCED/11/04and Deanship of Scientific Research at King Saud University, Saudi Arabia through group project No. 10-ENV1010-02 and in collaboration with the Centre for Ionic Liquids (UMCiL) University of Malaya, Malaysia. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tca.2012.04.030. 7.0 References 6.5 pH 6.0 5.5 5.0 4.5 4.0 20 40 60 80 o t/ C Fig. 5. pH for fructose-based DESs as a function of temperature. 䊉, , , and △ refer to DES1, DES2, DES3,and DES4, respectively. Solid lines, Eq. (4). [1] O.D. Cheesman, Environmental Impacts of Sugar Production, CABI, Publishing, UK, 2004. [2] T. Mallawaarachchi, R.K. Blamey, M.D. Morrison, A.K.L. Johnson, J.W. 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