Journal of Molecular Liquids 178 (2013) 137–141

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Journal of Molecular Liquids

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Glucose-based deep eutectic solvents: Physical properties

Adeeb Hayyan a,b, Farouq S. Mjalli a,⁎, Inas M. AlNashef c, Yahya M. Al-Wahaibi a,Talal Al-Wahaibi a, Mohd Ali Hashim b

a Petroleum and Chemical Engineering Department, Sultan Qaboos University, Muscat 123, Sultanate of Omanb Department of Chemical Engineering, Centre for Ionic Liquids (UMCiL), University of Malaya, Kuala Lumpur 50603, Malaysiac Chemical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia

⁎ Corresponding author. Tel.: +968 2414298 1558; faE-mail address: farouqsm@yahoo.com (F.S. Mjalli).

0167-7322/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.molliq.2012.11.025

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 April 2012Received in revised form 22 November 2012Accepted 23 November 2012Available online 10 December 2012

Keywords:GlucoseMonosaccharidesDeep eutectic solventsIonic liquids

Deep eutectic solvents (DESs) are considered nowadays as green ionic liquid (IL) analogues. Despite their rel-atively short period of introduction as a special class of ILs, they have been under an increasing emphasis bythe scientific community due to their favorable properties. In the present study, a glucose based DES of cho-line chloride (2-hydroxyethyl-trimethylammonium chloride) with the monosaccharide sugar D-glucose an-hydrous was synthesized at different molar ratios. The physical properties of density, viscosity, surfacetension, refractive index, and pH were measured and analyzed as function of temperature in the practicaltemperature range of 298.15–358.15 K. The analysis of these physical properties revealed that these novelDESs have the potential to be utilized for several possible industrial applications involving processing andseparation of food constituents, pharmaceutical applications, as well as mediums for chemical reactions.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids (ILs) have received escalating attention in organicsynthesis, due to their unique physicochemical properties and theiravailability compared with conventional solvents. ILs have manyfavorable merits that distinguish them from conventional solvents.Not to mention all, ILs are known to have undetectable vapor pres-sure, wide liquid temperature range, special solubility for manyorganic or inorganic compounds, and less toxicity[1–3]. ILs havebeen used in many chemical applications such as CO2 capture [4],battery development [5], electrochemical applications [6] and bio-logical applications such as biocatalysis [7]. ILs can be applied inmany chemical and industrial processes [8]. Recently, ILs were ap-plied in the separation of food constituents such as separation ofsugars from natural fruits [9]. This is due to the fact that their physi-cal and chemical properties can be tailored by the judicious selectionof their basic building block (cation, anion and substituent) [9].

Deep eutectic solvents (DESs) are relatively new class of ionic liquidsthat can be simply synthesized via mixing of a salt with a hydrogenbond donor compound. A common example is the DES between cholinechloride and urea [10,2]. Due to their favorable properties, DESs werereported in many industrial applications as attractive alternatives toILs. DESs share with ILs many of their intrinsic merits such as their bio-degradable components, non-flammability due to their low or nonemeasurable vapor pressure, and low toxicity [1,2,11]. Additionally,DESs are much cheaper than ILs which makes them readily available

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for large scale industrial applications. DESs were introduced in manyapplications and product preparations. Examples of such are in the syn-thesis of zeolite analogues [12], liquid–liquid extraction for aromaticseparation from naphtha [13], mediums for the deposition of specificmetals in the electro and electroless plating of metals [14,15] and inthe removal of excess glycerol from biodiesel fuel [16]. Moreover, itwas found that DES can be used as template-delivery agents in a con-trolled manner for the synthesis of materials [17].

The sugar industry sector is a very promising fieldwhich contributesmuch to the food and pharmaceutical industry. Research and develop-ment in the field of sugar separation and fractionation from naturalbio-resource materials such as local fruits are necessary to improvethe sugar industry and utilize the abundant bio-resource. Conventionalsugar separation and fractionation technologies are expensive, compli-cated and sometimes not suitable to deal with sensitive food ingredi-ents. There exists a great need for improving these technologies andintroducing new separation methods. The key concept that contributesin this direction is the adoption of green engineering methodologieswhich improve the sustainability, efficiency, biodegradability, and envi-ronmental friendliness [2,18–20]. Recently, AlNashef et al. [9] patenteda novel process for the separation of sugars from mixtures of fructoseand glucose from a liquid phase or a solid mixture containing the fruc-tose and glucose using ionic liquids. The patent claimed that ionic liquidswork as selective agents that can separate glucose and fructose underambient conditions. For example solubility of 1,3-dimethylimidazoliumdimethylphosphate is 2–6 times higher than that of fructose [9].

The next step in improving this technology is by reducing its cost tomake it practically available for industrial scale applications. DESs cancontribute much in this area. These new and environmental friendly

Table 1Compositions and abbreviations for the studied DESs.

Molar ratio Abbreviation Appearance at room temperature

1:1 DES1 Colorless liquid1.5:1 DES2 Colorless liquid2:1 DES3 Colorless liquid2.5:1 DES4 Turbid white liquid1:1.5 DES5 White semisolid1:2 DES6 White semisolid1:2.5 DES7 White semisolid

Table 2Experimental uncertainies in measurments.

Property Estimated uncertainty

Density at solid phase ±0.001 g cm−3

Density ±0.0001 g cm−3

Viscosity (relative) (3 to 5) % of measured valueSurface tension ±0.01 mN m−1

Refractive index 0.0001pH 0.05

Table 3Freezing points of the salt (choline chloride) and D-glucose [21].

Freezing point/K

DES1 304.15DES2 297.15DES3 288.15DES4 317.15

138 A. Hayyan et al. / Journal of Molecular Liquids 178 (2013) 137–141

solvents can be used as successful alternatives to ILs in the separation ofsugars. In a similar analogy to the separation of glycerol from biodie-sel [16], where a glycerol-based DES was utilized as a solvent, asugar-based DES can be used to capture selectively sugar moleculesfrom sugar mixtures. Since most natural fruits contain monosaccha-ride sugars (mainly glucose, fructose or both), sugar-based DESsshould be synthesized from the same type of sugars. Harris [21]studied the freezing points of glucose based DES without further in-vestigation of physical properties such as density, viscosity, surfacetension, refractive index, and pH. To initiate research in this area, thephysical properties of sugar-based DESs must be known and analyzed.In a recent study, the physical properties of phosphonium-based DESwere studied by Kareem et al. [2]. In another work, Abbott et al. [10]reported the physical properties of DES formed between choline chlo-ride and carboxylic acids. Physical properties of quaternary ammoniumsalt such as choline chloride and hydrogen bond donor of naturalsources such as D-glucose were not yet studied. Therefore, the firstobjective of this study is to synthesize a new class of DESs based onammonium salts at different ratios with a sugar as a hydrogen bonddonor such as D-glucose. The other objective of this study was to inves-tigate themost important physical properties such as density, viscosity,surface tension, refractive index, and pH as functions of temperature.This study investigated the effect of themole ratios of salt and hydrogenbond donor on these physical properties. The findings of this studyhighlight the characteristics, nature/features of these DESs as probablenew solvents. The availability of physical property data of such newclasses of DESs will help investigating further related applications andin the design of chemical processes involving these DESs.

2. Experimental

2.1. Chemical

Choline chloride (2-hydroxyethyl-trimethylammonium) and D-glucose anhydrous with purity (>98%), were supplied by MerckChemicals (Darmstadt, Germany). Both chemicals were used with-out any further purification.

2.2. Preparation of glucose based DES

In this study, DES samples were synthesized in different ratios ofcholine chloride to D-glucose as shown in Table 1. An incubator shaker(Brunswick Scientific Model INNOVA 40R) was used to mix cholinechloride salt and the hydrogen bond donor (D-glucose). The mixture ofcholine chloride and D-glucose was shaken at 400 rpm and 353.15 Kfor a period of 2 h until a homogenous transparent colorless liquid wasformed. DES samples were synthesized at atmospheric pressure andunder tight control of moisture content.

2.3. Physical properties measurement

All samples were kept in well-sealed vials after preparation andfresh samples were used for analysis to avoid any structure changingand to avoid humidity effects from the environment which may affect

the physical properties of DES. In this study, the temperature range ofall physical properties was between 298.15–358.15 K.

The densities of choline chloride and D-glucose in solid powder formwere measured at room temperature using a Quantachrome instrumentBoyNTon. Choline chloride and D-glucose were dried under vacuumovernight in order to eliminate the moisture in both powders beforemeasuring the density. The densities of all samples of DESs were mea-sured using Anton Paar DMA4500M while the viscosities of the DESsweremeasured using Anton Paar Rheolab Qc. The temperature was con-trolled using externalwater-circulator type Techne-Tempette TE-8A. Thesurface tension of DES samples was measured using an automatedtensiometer Krüss K10ST classification B with Du Noüy ring method.Refractive indices were obtained using a Bellingham and Stanley AbbeRefractometer (model 60/ED) with a sodium D1 line. The temperaturewas controlled in the refractometer using Techno TE-8Dwater circulator.Deionized water was used for calibration before each experiment. pH ofsynthesized DESswasmeasured using Thermo Scientific 3 star pH Benchtop. The pH meter was calibrated using a standard pH buffer. The tem-perature of each sample was controlled using a water circulator (JulaboLabortechnik). Table 2 shows the estimated uncertainties for the exper-imental measurement of each physical property.

3. Results and discussion

Different molar ratios of choline chloride to D-glucose were usedto prepare 7 samples of DESs and were described in Table 1 alongwith their abbreviations and our observations during preparationstage.

The different DES composition ratios were prepared by varyingD-glucose composition at a fixed amount of salt. DES5, DES6, and DES7were not successful as the two components did not mix properly andthe products were in a white semisolid form. The probable reason forthese ratios to be in solid phase after mixing, is that the amount of saltwas not enough to build hydrogen bonding for all available sugar. Con-sequently, the mixture was saturated with sugar and no further reduc-tion in freezing pointwas achieved. The high concentration of hydrogenbond donor compared to salt makes the mixture heterogeneous andeven after long time of shaking under high temperature the mixturestayed as semisolid phase. After cooling DES5, DES6, and DES7 toroom temperature they transform to white solid phase.

This leads to the conclusion that these ratios are not recommendedfor sugar based DES synthesis and therefore the three samples of DESwere discarded from further consideration in this study. On the otherhand, the DES1, DES2, DES3, and DES4 appeared as colorless liquidphase and easy to handle although DES4 was close to a semisolid form

T /K

300 310 320 330 340 350 360

ρ / g

.cm

-3

1.20

1.22

1.24

1.26

1.28

1.30

1.32

Fig. 1. Densities, ρ, of glucose-based DESs as a function of temperature T.●,○,▼, and Δrefer to DES1, DES2, DES3,and DES4, respectively. Solid lines, Eq. (1).

T/K

300 310 320 330 340 350 360

μ / m

P.s

0

2000

4000

6000

8000

10000

12000

Fig. 2. Dynamic viscosity, μ, of glucose-based DESs as a function of temperature T withthe model predictions.●,■,▼, and▲ refer to DES1, DES2, DES3,and DES4, respectively.Solid lines, Eq. (2).

139A. Hayyan et al. / Journal of Molecular Liquids 178 (2013) 137–141

at room temperature but after little heating (308.15–318.15 K) it formsa liquid phase which is easy to handle. Based on this, the study focusedon measuring the physical properties (density, viscosity, surface ten-sion, refractive index, and pH) for DES1, DES2, DES3, and DES4 in liquidphase. During the synthesis stage, DES samples (DES1, DES2, DES3, andDES4) were formed in a white viscous gel within the first 30 min. After60 min of mixing, a liquid phase started to appear with some precipita-tion. Therefore, the period of mixing was extended to 120 min in orderto get a homogenous liquid phase DES.

It was reported that compounds forming the most useful ionic liq-uids are those with the lowest freezing points [19]. The DES whichhas the lowest melting temperature depends on its molar ratio ofsalt to hydrogen bond donor [2]. Harris [21] reported the freezingpoints of D-glucose based DESs as shown in Table 3. It is found thatthese DESs have freezing points between 286.15–291.15 K. Table 3indicates that the eutectic point was 288.15 K for DES3 at molarratio (2:1). D-glucose is in cyclic formwith the angle of interaction be-tween the chlorine anion and the hydroxyl group is more favorable toallow 2 choline chloride molecules to form hydrogen bonds [21]. Thiscould be the reason for having an eutectic point at the molar ratio 2:1of DES3. This particular ratio will be more practical to use in liquid–liquid system applications. In this case the system should be with atemperature as far as possible away from freezing.

Density measurement is essential for working on many fluidmechanics and mass transfer calculations and to design chemical pro-cesses. The studiedDESdensitymeasurementswere conducted as a func-tion of temperature in the range (298.15–358.15 K). D-glucose basedDESs at different ratios were colorless even after heating at high temper-ature (358.15 K). This indicated that carmelization reactions did notoccur at this temperature. Fig. 1 depicts the effect of temperature on thedensities of different DESs. All recorded densities of synthesized DESswere above 1.20 g cm−3. These values lie between the room tempera-ture densities of pure salt (choline chloride) of 1.1856 g cm−3 andthat of pure D-glucose of 1.5345 g cm−3. The reduction in densitywas linear for all studied DESs as shown in Fig. 1. The highest densi-ty was that of DES1 with the molar ratio of 1:1, which reaches amaximum of 1.2978 g cm−3 at room temperature and a minimum

Table 4Density-temperature model parameter.

a b.104

DES1 −5.3334 1.4570DES2 −5.1600 1.4210DES3 −5.1250 1.3950DES4 −5.1860 1.4060

of 1.2659 g.cm−3 at 358.15 K. On the other hand, DES3 has the lowestdensity (1.2115 g cm−3 at high temperature of 358.15 K) comparedto other DESs and recalling that it achieved the lowest freezing pointas well. Results of this study support the investigation by Harris [21]were the eutectic point occurred at the molar ratio of 2:1 of DES3 be-cause the density of DES3 is the lowest among other DESs as shown inFig. 1.

However due to the eutectic phenomena, the variation of DES den-sities is not linearly dependent on their molar ratios. This is clear fromthe varying increments between the density lines in Fig. 1.

The results of this work were compared to those reported [2] forthe physical properties of phosphonium-based DES. It was foundthat DES1 has approximately similar values to the DES made of[methyltriphenylphosphonium bromide:glycerol] at a ratio of 1:1.75.DES2, DES3, and DES4 have density values similar to the DES formedfrom [methyltriphenylphosphonium bromide:glycerol] at a ratio of 1:4.On the other hand, density of ILs such as 1-hexyl-1-methyl-pyrrolidiniumbis (trifluoromethylsulfonyl) imide was reported to be slightly higherthan that of DES1 (1.34 g cm−3 at 293.15 K) [22]. Other DESs (DES2,DES3 and DES4) have similar densities compared to imidazoliumand pyrrolidinium based ILs such as 1-ethyl-3-methylimidazoliummethylsulfate and 1-butyl-1-methylpyrrolidinium trifluorometh-anesulfonate which have density values of 1.28 and 1.298.15 g cm−3

at 293.15 K respectively [22]. It is concluded that the glucose-basedDESs have comparable densities to other studied DESs as well as com-mon ILs. The density values of the studied DESs were fitted as a functionof temperature by a linear relationship as follows:

ρ=g cm−3 ¼ α þ b T=Kð Þ ð1Þ

where ρ is the density, T is the temperature, and a and b are two con-stants for the DES molar ratio under consideration. The values of a andb for the studied DESs are presented in Table 4.

Very few studies reported the viscosity of ILs [10]. Moreover, DESphysical properties are very scarce. Having knowledge of viscosity ofDES may lead to the proper selection of the optimum ratio of salt andhydrogen bond donor that is suitable to a particular application and

Table 5Viscosity-temperature model parameters.

μo/mPa s (−Eμ/R)/K

DES1 4.5663×10−5 5701.36DES2 5.7716×10−10 9019.85DES3 4.6978×10−13 11142.67DES4 2.7575×10−7 7283.67

T /K300 310 320 330 340 350 360

γ / m

N.m

-1

68

69

70

71

72

73

74

75

76

Fig. 3. Surface tension, γ, of glucose-based DESs as a function of temperature T. ●,○,▼,and Δ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (3).

T /K300 310 320 330 340 350 360

n D

1.656

1.658

1.660

1.662

1.664

1.666

Fig. 4. Refractive indices, nD, of glucose-based DESs as a function of temperature T. ●,○, ▼, and Δ refer to DES1, DES2, DES3, and DES4, respectively. Solid lines, Eq. (4).

140 A. Hayyan et al. / Journal of Molecular Liquids 178 (2013) 137–141

consequently save material as well as energy. DES being liquid atroom temperature makes them easy to handle and applicable tomany industrial routes [2].

The general trend of DES viscosity profiles as a function of tempera-ture is very similar to that of common ILs. It is noted that the highest vis-cosity value at room temperature belongs to DES4 (10910.00 mPa s)followed by DES1 (9037.10 mPa s). High viscosity for DES is notrecommended for certain applications such as liquid–liquid extraction.In addition, viscous solvents need preheating before processing andneed more pumping energy as well. As shown in Fig. 2, viscosities ofall DESs decreased with increasing temperature.

The lower bounds of viscosity for all studied DESs for DES1, DES2,DES3, and DES4 are 209.3, 130.7, 72.0, 108.3 mPa s respectively at a tem-perature of 358.15 K. The viscosity upper bounds of 9037.1, 8000.0,8045.1 and 10910.0 mPa s were at 298.15 K. This indicates the high sen-sitivity of the viscosity of these solvents to temperature. It is worth men-tioning that the viscosity of the eutectic composition of DESs (molar ratioof 2:1) is also the lowest. This highlights the importance of the eutecticpoint for such solvents. Other studies reported in the literature did notinvestigate the effect of molar ratios on the viscosity of a particular typeof DES. D-glucose-based DESs are much viscous than other reportedphosphonium-based DESs especially at low temperatures (16.6 mPa sfor [methyltriphenylphosphonium bromide:glycerol] DES in the ratio of1:1.75 and 5.4 mPa s for [methyltriphenylphosphonium bromide:ethyl-ene glycol] DES in the ratio of 1:4 at 338.15 K)[2].

The viscosities of the tested DESs were fitted using an Arrheniusmodel as shown below:

μ ¼ μoe−Eμ

RT

� �ð2Þ

where μ is the viscosity, μo is a pre-exponential constant, Eμ is theactivation energy, R is the gas constant, and T is the temperature inKelvin. Values of μo and Eμ are shown in Table 5.

Surface tension is a very important property which is used in manyareas especially in emulsions and surfactants applications. Similar to

Table 6Surface tension-temperature model parameters.

a b.102

DES1 85.02 −4.036DES2 85.03 −4.193DES3 87.19 −5.236DES4 93.02 −6.086

viscosity, very few studies have been reported for the surface tensionof ILs [10]. The values of surface tension at room temperature forDES1, DES2, DES3, and DES4 were 73.1, 72.7, 71.7, and 75.0 mN m−1

respectively. The variation of DES surface tension with temperature isshown in Fig. 3. It was noted that the mole ratio of 2:1 has the lowestsurface tension at the same time this ratio has the lowest freezingpoint as reported by Harris [21].

The high values of surface tension for DES4 were due to the highmolar ratio of salt. Compared to surface tension of ILs such as then-butyl-methylimidazolium tetrafluoroborate (which has a surfacetension value of 38.4 mN m−1 at 336.15 K)[20], DESs in this studywere higher. Similarly, surface tension data of DESs in this studywere higher than those for carboxylic acids-based DES reported ear-lier [10]. This is due to the higher number of hydrogen bonding forthe samemolar ratios of DES. Surface tension behavior was fitted lin-early for each DES according to the following relationship:

γ=mNm−1 ¼ aþ b T=Kð Þ ð3Þ

where γ is the surface tension, T is the temperature, and a and b aretwo constants for the molar ratio of DES under consideration. Thevalues of a and b for the studied DESs are showed in Table 6.

Refractive index (RI) is an important property that is involved inmany applications including the optical identification of particular sub-stances, checking the purity of materials and in measuring the concen-tration of solutes in solutions [2]. Refractive indices of glucose-basedDESs as a function of temperature were measured and are shown inFig. 4. In general, refractive indices ofmaterials decreasewith increasingtemperature due to the reduction of density values. It was reported thatrefractive index does not have a simple relationship with temperature[2]. At room temperature, the highest refractive index value of 1.6661was recorded for DES3. DES4 at a mole ratio of 2.5:1 salt to hydrogenbond donor exhibited the lowest value of 1.6574 at 358.15 K. The re-fractive indices for the studiedDESs are higher than those reported else-where for phosphonium based DESs [2] which were below 1.5800. This

Table 7Refractive index-temperature model parameters for the studied DESs.

a b.105

DES1 1.671 −1.596DES2 1.678 −4.554DES3 1.692 −8.411DES4 1.704 −1.309

T /K300 310 320 330 340 350 360

pH

5.8

6.0

6.2

6.4

6.6

6.8

7.0

7.2

7.4

Fig. 5. pH for glucose-based DESs as a function of temperature T. ●,○,▼, and Δ refer toDES1, DES2, DES3,and DES4, respectively. Solid lines, Eq. (4).

141A. Hayyan et al. / Journal of Molecular Liquids 178 (2013) 137–141

is expected because of the high content of sugarwhichhas a high refrac-tive index value due to its high density. Refractive index behavior wasfitted linearly for the studied DESs according to the following relation-ship:

RI ¼ aþ b T=Kð Þ ð4Þ

where RI is refractive index, T is temperature in K, and a and b are con-stants that vary according to the type of DES. As refractive index is aunitless property, a and b are unitless parameters. Table 7 shows valuesof refractive index a and b for Eq. (4).

pH is an important liquid property which helps in the selection of thetype of pipe constructionmaterial and corrosion related design aspects. Italso has influence on conducting reactions and especially bioreactions. Inthis study, the pH values of glucose-based DES as function of temperaturevaried in the range of 6.03–7.11. The room temperature measurement ofpH for all studied DESs was around 7 indicating neutral mixtures. Asshown in Fig. 5 the pH of DES1 did not change significantly for the tem-peratures of 298.15 to 318.15 K. Above a temperature of 50 °C the pH de-creased in the range of 6.0–6.6. The studied DESs have very similar pHvalues to DESs formed between [methyltriphenylphosphoniumbromide]and ethylene glycol in the ratio of 1:4. On the other hand, ILs havevarying pH values. Some ILs (such as [1-(3-methoxypropyl)-1-methylpiperidinium bis (trifluoromethylsulfonyl) imide], [n-butyl-3-methyl-pyridinium dicyanamide], [n -hexylpyridinium bis (trifluoro-methylsulfonyl) imide], and [1-hexyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl) imide]) [22] have very close pH valuesto the studied systems. It is clear that the pH of D-glucose basedDESs has low sensitivity to temperature. The neutral pH and lowacidity of D-glucose based DES make them good candidates forchemical, environmental and biological applications. The tempera-ture–pH relationship was modeled linearly. Table 8 shows the pa-rameters of this fit.

4. Conclusion

D-Glucose-based DES in different mole ratios has been successfullyprepared from choline chloride (2-hydroxyethyl-trimethylammonium

Table 8pH-temperature model parameters for the studied DESs.

a b.102

DES1 9.994 −1.061DES2 13.12 −2.018DES3 9.957 −0.993DES4 10.56 −1.157

chloride) and sugar (D-glucose anhydrous). The physical properties in-cluding density, viscosity, surface tension, refractive index, and pHweremeasured and reported in this study. It was found that the physicalproperties of different DESs depend on the salt to hydrogen bonddonor molar ratio. Physical properties of D-glucose based DES are simi-lar to common ionic liquids. The results revealed that the studied DESshave high viscosity, density and surface tension at room temperaturehence, industrially it is more recommended to heat up these types ofDESs before processing. The pH of these DESs was almost neutral withlow sensitivity to variation of temperatures which make them goodcandidates for chemical, environmental and biological applications.Refractive index values were in the range (1.6645–1.6657). The influ-ence of various temperatures on the measured physical propertieswas analyzed. Most of the DESs studied physical properties weremodeled linearly as function of temperature. On the other hand, an Ar-rhenius model type was used to fit the profiles of viscosity. Results ofthis fundamental study will be useful for process scale-up studies aswell as the development of sugar-based DES applications in a widerange of industrial applications.

Acknowledgments

The authors would like to express their thanks to Petroleum andChemical Engineering Department, Sultan Qaboos University, Sultanateof Oman (IG/ENG/PCED/11/04), the University of Malaya HIR-MOHE(D000003-16001), Centre for Ionic Liquids (UMCiL), the Bright SparksProgram at University of Malaya, and the National Plan for Science, Tech-nology, and Innovation at King Saud University (10-ENV1010-02), fortheir support to this research.

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