Tetrabutylammonium Chloride Based Ionic Liquid Analogues and

Article pubs.acs.org/jced

Tetrabutylammonium Chloride Based Ionic Liquid Analogues and Their Physical Properties Farouq S. Mjalli,* Jamil Naser, Baba Jibril, Vahid Alizadeh, and Zaharaddeen Gano Department of Petroleum and Chemical Engineering, Sultan Qaboos University, Muscat 123, Sultanate of Oman S Supporting Information *

ABSTRACT: During the past few years, there has been a surge in interest and research in the arena of utilizing deep eutectic solvents (DESs) as green solvents. This manifested in applying DESs in a variety of industrial applications. Most of the reported work in this field was directed toward the choline chloride-based DES. Recently, the area of DES synthesis was widened by considering other quaternary ammonium and phosphonium salts. In this work tetrabutylammonium chloride (TBAC) is used as a salt for the synthesis of three different DES systems based on three different hydrogen bond donors (HBDs), namely, glycerol, ethylene glycol, and triethylene glycol. Screening tests for each DES system was performed to identify salt:HBD ratios that exhibit a minimum freezing point, and at least three such ratios were selected for each system. Physical properties including melting point, density, viscosity, surface tension, refractive index, conductivity, and pH were measured for the three DES systems at different temperatures ranging from (293.15 to 353.15 K). It is worth mentioning that this class of DES exhibits a wide range of properties that can be tailored toward specific chemical and other engineering applications.

1. INTRODUCTION Deep eutectic solvents (DESs) are evolving as viable alternatives for ionic liquids.1 In this case the eutectic phenomenon is utilized to convert a mixture of two components (not necessarily in the liquid form) to a liquid at room temperature. In its simplest form, a DES is a mixture of two or more components involving a salt and one or more components able to donate hydrogen bond (HBD) and complex with the salt ions. These solvents possess many superior properties over conventional ionic liquids. One of the most important features of these solvents is that they can be custom-built to suit certain application by the proper selection of their startup ingredients. They can be prepared at high purity and with low synthesis cost. Their constituting ingredients can be selected from biodegradable and low-toxicity species, and they are usually not reactive with water. The development of these ionic liquid analogues have seen continuous growth during the past decade. So far, four different types of DESs have been reported in the literature.2 In the first three types of these DESs, a quaternary ammonium or phosphonium salt is utilized. The other component can be a metal salt of the form MClx, where M = Zn, Sn, Fe, Al, Ga (type I DES); a hydrated metal halide of the form MClx·yH2O, where M = Cr, Co, Cu, Ni, Fe (type II DES); or an organic component of the form R5Z, ZCONH2, COOH, or OH (type III DES). The last type of these solvents (type IV DES) is identified as a mixture of a metal halide of the form MClx and different HBDs such as urea, glycols, amides, or alcohols. Choline chloride (ChCl) was one of the first reported quaternary ammonium salts used in this area. This salt has a melting point of 575 K; when it is mixed with another salt or © XXXX American Chemical Society

HBD, it forms an eutectic of melting points below 373 K. For example, the ChCl:urea DES attains an eutectic mixture melt as low as 285 K.3 Other ChCl-based DESs attain subzero melting points; examples are ChCl:ethylene glycol (233 K), ChCl:ethylene glycol (207 K), and ChCl:2,2,2-trifluoroacetamide (228 K). Other salts have been reported as possible ingredients for DESs. Examples are ethylammonium chloride (EtNH3Cl), trimethylammonium chloride (TMAC), tributylammonium chloride (TBAC), methylphosphoniumtriphenyl bromide (MeP(Ph)3Br), diethylethanolammonium chloride (Et2(EtOH)ACl), and tributylammonium bromide (TBAB).4 DESs have been used in many successful applications, and some were already commercialized. In the area of CO2 capture, the ChCl:urea and ChCl:ethylene glycol DESs have been tested. These studies revealed that the amount of CO2 absorbed in the DES is directly proportional to pressure and the DES molar ratio.5 On the other hand, the increase in mixture temperature reduces the solubility of CO2. DESs were also successfully used as solvents for metal oxides.6 For this purpose ChCl was combined with urea, malonic, propionic, or phenylpropionic acid to form DESs capable of dissolving a variety of metal oxides including (ZnO, CuO, CaO, MnO2, NiO, PbO2, and Fe3O4). Their application for the electropolishing of stainless steel was also studied whereby a DES composed of choline chloride and ethylene glycol showed three main advantages over the commercial alternative: (i) high current efficiencies were Received: March 3, 2014 Accepted: June 19, 2014

A

dx.doi.org/10.1021/je5002126 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

obtained; (ii) the gas evolution at the anode/solution interface during polishing is negligible; and (iii) the liquid used is comparatively benign and noncorrosive compared to the current aqueous acid solutions.7 Other applications involved zinc electroplating.8 DESs have been also tested as reaction media for the synthesis of lanthanide−organic frameworks. It was found that DESs can act as unusual reaction media by serving as template-delivery agents in a controlled manner and can as well be used for the synthesis of materials that cannot be prepared using other standard techniques.9 Furthermore, DESs were used for the removal of residual palm oil based biodiesel catalyst.10 TBAC salt has been used in many applications. It is used as cool energy storage media for air-conditioning systems since their dissociation heats of phase transitions are as large as (200 to 500) kJ/kg and they form at (278 to 293) K under atmospheric pressure.11 Additionally, it is used as a medium in the CrCl2catalyzed conversion of glucose to hydroxymethylfurfural (HMF).12 TBAC has also been used as a hydrophobic charged additive of the mobile phase in the chromatographic system involving D,L-dansyl amino acids as the test solute enantiomers and immobilized human serum albumin as the chiral stationary phase.13 In the polymerization field, TBAC is used to enhance the kinetics and molecular weight distribution in the polymerization of styrene initiated by L-phenylethyl chloride/SnC4 in CH2−Cl2.14 TBAC has been used as a salt in the synthesis of a variety of DESs. It has been used in combination with glycerol (TBAC:G) for performing a variety of enzymatic reactions. Examples of such reactions include the following: transesterification, aminolysis, hydrolysis, perhydrolysis, alcohol dehydrogenation, oxidation−reduction, or dehydrogenation.15 In most of the previously mentioned applications, ChCl was the dominant salt used in the DES synthesis. Other salts need to be thoroughly explored and their DESs characterized. A careful look into the DES literature reveals that TBAC-based DESs were not investigated. In this work, three commonly used HBDs (namely, glycerol, ethylene glycol, and triethylene glycol) were used with TBAC and screened for possible DES formation. Three DES systems of different salt:HBD molar ratios were synthesized, and their basic physical properties of melting point, viscosity, density, surface tension, refractive index, conductivity, and pH were experimentally determined. The effects of DES molar ratios and temperature on the studied properties were studied, and relevant explanations were postulated. The data obtained were modeled and validated with simple correlations.

Scheme 1. Chemical Structures of the DES Components

more molar ratios of each DES were synthesized as shown in Table 1 with their abbreviated names used in this work. Table 1. Composition, Abbreviations, Glass Transition, and Melting Points for the Studied DESs molar ratio

abbreviation

Tg/K

Tm/K

1 TBAC:3 glycerol 1 TBAC:4 glycerol 1 TBAC:5 glycerol 1 TBAC:2 ethylene glycol 1 TBAC:3 ethylene glycol 1 TBAC:4 ethylene glycol 1 TBAC: 1 triethylene glycol 2 TBAC: 1triethylene glycol 3 TBAC: 1 triethylene glycol 4 TBAC: 1 triethylene glycol

TBAC-G3 TBAC-G4 TBAC-G5 TBAC−EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

195.94 194.49 194.18 212.37 196.24

231.51 230.54 230.37 243.02 242.27 256.31 290.28 275.57 260.46 263.54

252.52 254.71 236.54

For each of the DES ratios, an incubator shaker (Brunswick Scientific Model INNOVA 40R) was used to mix the TBAC and the relevant hydrogen bond donor. The salt:HBD mixtures were mixed at 400 rpm and 353.15 K for 2 h until homogeneous transparent colorless liquids were formed. DES samples were synthesized at atmospheric pressure and under tight control of moisture content. Unsuccessful combinations of the screened DES ingredients resulted in a white highly viscous liquid or a liquid with suspended particles. However, successful combinations initially formed a white viscous gel within the first 30 min and then converted to a homogeneous liquid phase. A total mixing time of 120 min was used in all experiments in order to get a homogeneous liquid phase DES. 2.3. Physical Properties Measurement. Samples of 100 mL for each of the 10 successful DES ratios were prepared and stored in tight-sealed glass vials and were used in sequence for characterization. Fresh samples were used for analysis to avoid any structural change during storage and to avoid humidity

2. EXPERIMENTAL METHODOLOGY 2.1. Chemicals Used. Tetrabutylammonium chloride, glycerol, ethylene glycol, and triethylene glycol (>98%), were supplied by Merck Chemicals (Darmstadt, Germany). The chemical structures of these compounds are shown in Scheme 1. Prior to being used, these chemicals were treated by drying in a vacuum oven to ensure a low moisture content of less than 200 ppm. 2.2. Preparation of TBAC Based DES. For each combination of TBAC with one of the three HBDs, an initial screening was performed to determine the molar ratios suitable for producing successful DESs, that is, DESs with low freezing points. The TBAC-glycerol (TBAC-G) system started forming DES at a 1:3 molar ratio and TBAC-ethylene glycol (TBAC-EG) at 1:2 molar ratio, while the first successful molar ratio of the TBAC-triethylene glycol (TBAC-TEG) DES was 1:1. Hence, B



Article

compared to pure glycerol (Tm = 290 K). The other two systems attained less depression in system melting point compared to the melting point of pure HBD. For ethylene glycol (Tm = 260 K) the range of depression is (3.69 to 16.98) K and for TEG (Tm = 266 K) the range is (−2.46 to 24.28) K. Glycerol-based DESs had the least melting point values which were in the vicinity of 230 K. Comparing the glycerol-based DES system studied here with a recently reported potassium carbonate16 reveals that the glass transition points have a similar trend with respect to the amount of glycerol in the DES structure. However, no melting points were observed in the potassium carbonate system compared to the current one. In general, increasing the hydrogen bond donor shifts the melting point away from zero in this system despite the fact that these melting points are very close to each other. However, this is not the case for the ethylene glycol and the triethylene glycol systems. As the amount of hydrogen bond donor increases, the melting point goes into an eutectic point. These points are located at the ratio of 1:3 of salt:HBD for the two systems. 3.2. Density. Density is a very important physical property that affects other properties and has an important role in defining the nature of substances. It is used during the early design stage as well as during operation. The density of the three TBAC-based DESs was measured as a function of temperature and DES molar composition. The TBAC-TEG system showed the lowest density among the three DESs with a maximum of 1.027 g·cm−3at a molar ratio of (1:1) at 293.15 K and a minimum of 0.955 g·cm−3 at a molar ratio of (4:1) at 353.15 K. The density of the TBACEG system was in the middle range with a maximum of 1.04 g· cm−3 at a molar ratio of (1:4) at 293.15 K and a minimum of 0.976 g·cm−3 at a molar ratio of (1:2) at 353.15 K. The TBAC-G system attained the highest densities with a maximum of 1.145 g· cm−3 at a molar ratio of (1:3) at 293.15 K and a minimum of 1.067 g·cm−3 at a molar ratio of (1:5) at 353.15 K. These data are displayed graphically in Figure 1. The inverse variation of density as a function of temperature was typical, since the increase in temperature results in an increase in the mobility of the DES molecules and consequently the thermal expansion of the DES volume will increase. Generally, it can be inferred that the glycerol-based DESs have the highest densities among the studied DESs in this work. This is also observed in other glycerol-based DES systems.17 Increasing the amount of salt in the DES molar ratio reduces the density significantly. This is also attributed to the mobility hindrance due to the increase in the overall mass of the two complexing species. Moreover, the fact that the ρTEG > ρEG > ρG indicates that DESs with bigger HBD molecules have lower density due to the same effect. It is interesting to see that the TBAC-TEG DES density is close to 1. This density being similar to that of water makes this particular DES a good substitute for anhydrous applications involving mixing, fluidization, and reactions. The experimental densities as a function of temperature data were regressed linearly in the following form:

effects from the environment which may affect the physical properties of DES. The temperature range considered for all measured physical properties was (293.15 to 353.15) K. Each DES sample was pretreated by drying under vacuum overnight in order to eliminate any possible contamination with moisture before measuring the physical property under consideration. The densities of all samples of DESs were measured using Anton Paar DMA4500 M while the viscosities of the DESs were measured using Anton PaarRheolabQc. The temperature was controlled using an external water-circulatortype Techne-Tempette TE-8A instrument. The surface tensions of DES samples were measured using an automated tensiometer Krüss K10ST classification B with the Du Noüy ring method. Refractive indices were measured using a Bellingham and Stanley Abbe refractometer (model 60/ED) with a sodium D1 line. Deionized water was used for calibration before each experiment. Conductivity of the DES samples was measured using a Jenway conductivity meter (model 4520). pHs of synthesized DESs were measured using a Thermo Scientific 3 star pH bench top meter. The pH meter was calibrated using standard pH buffers. The temperature of each sample was controlled using a water circulator (JulaboLabortechnik). The melting and glass transition points of all DESs were measured using a TA-Q20 differential scanning calorimeter (DSC) with an autosampler. The DSC was combined with a refrigerated cooling system RSC90 in case of characterizing the glycerol-based samples, while a liquid nitrogen quench cooling system was used for the case of ethylene glycol and TEG melting point measurements. To ensure the measurement’s accuracy and repeatability, the DSC was calibrated for its baseline as well as the temperature calibration using an indium post-transition-metal standard. Table 2 shows the estimated uncertainties for the experimental measurement of each physical property. Table 2. Standard Uncertainties in Measurements measurement

estimated uncertainty

melting and glass transition points density viscosity(relative) surface tension refractive index pH conductivity

0.01 K 0.0001 g·cm−3 5 % of measured value 0.1 mN·m−1 0.001 0.05 5 μS·cm−1

3. RESULTS AND DISCUSSION 3.1. Melting Point. Their existence as liquids under nearroom-temperature conditions is a fundamental concept behind using DESs in different industrial applications. The phase change of the 10 different DESs was characterized thermally by determining their melting points utilizing a high-sensitivity DSC. Results of the DSC analysis are given in Table 1. The glass transition (Tg) was observed in most tested samples; the glycerol-based system showed very close Tg, while those for the other two systems were more scattered. The values of the Tg temperatures for the different HBDs used in the DES systems in ascending order are glycerol < ethylene glycol < triethylene glycol based DESs. Melting points for the three studied DES systems follow a similar trend of the glass transition points in terms of order and scatter. The extent of melting point depression is mostly clear in the glycerol-based DESs in which it was around 60 K as

ρ = a + bT

(1)

where the density is in g·cm−3, T is the temperature in kelvin, and a and b are constants for the DES molar ratio under consideration. The values of a and b and regression coefficients for the studied DESs are presented in Table 3. The relationship between the density and temperature is highly linear with an R2 value of more than 0.995 for all studied DESs. C



Article

Figure 1. Density, ρ, ofTBAC-based DESs: A, with glycerol as HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

temperature profiles similar to other reported ionic liquids (ILs)19 and DES systems.20 TBAC-TEG attained high-viscosity values followed by the TBAC-G and the TBAC-EG DESs. It is also observed that bigger HBD molecules result in higher viscosity of the DES. This is due to the larger molecular complexes having slower overall mobility. It is also clear that increasing amount of salt in the DES structure increases its viscosity. This is due to the fact that the fractional molar free volume in the DES is decreased as more salt is added. This effect is clearer in the TBAC-EG and TBAC-TEG DESs than in the TBAC-G DES. In the same figure (Figure 2), the viscosity variations with temperature for pure glycerol,21 ethylene glycol,22 and TEG23 are also shown. It is noticed that the viscosity of DESs based on glycerol is within the vicinity of that of pure glycerol. However, DESs based on ethylene glycol and TEG have much higher viscosities than the pure HBD. This can be explained using the hole theory.3 Based on this theory, the viscosity of fluids is limited by the availability of holes large enough for the mobile species to be able to move into. Having bigger HBD molecules increases the ionic radius of the DES complex which reduces the free volume and consequently increases viscosity. Using an Arrhenius viscosity model as in eq 2, the relationship of velocity with temperature can be expressed.

Table 3. Density−Temperature Model Parameter DES TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

a 1.2924 1.3203 1.3353 1.2014 1.2167 1.2361 1.2029 1.1884 1.1719 1.1636

R2

b −4

−6.393 × 10 −6.500 × 10−4 −6.500 × 10−4 −6.393 × 10−4 −6.393 × 10−4 −6.679 × 10−4 −6.000 × 10−4 −6.178 × 10−4 −6.000 × 10−4 −5.892 × 10−4

0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.995 0.999 0.997

3.3. Viscosity. Viscosity data are of considerable importance for equipment design and fluid flow calculations. Using fluids as reaction mediums or solvents as well as for transportation depends to a large extent on the viscosity of these fluids. Additionally, studying the variation of viscosity with temperature is important to reduce energy requirements for processing these fluids.18 Figure 2 gives the dynamic viscosity, μ, of TBAC-based DESs with glycerol, ethylene glycol, and triethylene glycol as HBDs. In general, these DESs have exponentially decaying viscosity− D



Article

Figure 2. Dynamic viscosity, μ, of TBAC-based DESs: A, with glycerol as a HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

μ = μo e[Eμ / RT ]

3.4. Surface Tension. The strength of cohesive forces between liquid molecules at the surface is measured by the surface tension. Knowing the surface tension of liquids is crucial for many industries including oil production, dyes, and adhesives and food and pharmaceutical industries. This property is very sensitive to the composition of the liquid as well as to temperature and pressure. From Figure 3, it can be generalized that the temperature has a negative effect on the surface temperature variation. Higher temperatures result in increasing the internal energy of the liquid molecules resulting in vigorous molecular vibrations that reduce the effect of cohesion forces between the surface molecules. It is further noticed that the higher the salt content (and, consequently, the lower the HBD content) in the DES, the greater is the surface tension. This is evident in all tested DESs. More HBD in the structure of the DES results in increasing the HBD molecules on the surface which consequently brings the DES surface tension close to its corresponding HBD values at the same temperature. The values for glycerol-based DES are slightly higher than the corresponding ethylene glycol and TEG values. Surface tension is governed by London cohesion forces. These forces are insensitive to the permanent dipole moment of the molecule

(2)

where μ is the viscosity, μo is a preexponential constant, Eμ is the activation energy of viscosity, R is the gas constant, and T is the temperature in kelvin. Values of μo and Eμ and regression coefficients for the studied systems are shown in Table 4. Higher viscosity activation energy indicates a higher energy barrier to be overcome by mass transport and hence higher viscosity values. Table 4. Viscosity−Temperature Model Parameters Eμ.R−1

μo DES TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

mPa·s −7

5.829 × 10 2.372 × 10−7 1.764 × 10−7 3.121 × 10−4 1.574 × 10−4 3.695 × 10−4 3.383 × 10−5 5.233 × 10−5 5.987 × 10−24 2.411 × 10−6

K

R2

6279.3 6482.1 6620.8 4615.1 3935.7 3617.2 5006.8 5194.1 19518.5 6380.3

0.999 0.997 0.998 0.998 0.997 0.997 0.998 0.989 0.997 0.991 E



Article

Figure 3. Surface tension, γ, of TBAC-based DESs: A, with glycerol as a HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

and depend on the potential of ionization and diameter of the molecule.24 Comparing the surface tension values to that of the K2CO3 system reported recently,16 the TBAC-glycerol DES attained lower values while the ethylene glycol based DES exhibited slightly higher surface tension values. Surface tension behavior was fitted linearly for each DES according to the following relationship: γ = a + bT

Table 5. Surface Tension−Temperature Model Parameters DES TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

(3)

where γ is the surface tension, T is the temperature, and a and b are constants for the molar ratio of DES under consideration. The values of a and b for the studied DESs are shown in Table 5. 3.5. Conductivity. For electrochemical applications, the conductivity of the electrolyte plays an important role to determine the performance and lifetime of the electrochemical device. The is very important in applications such as batteries, electropolishing, electroplating, and deposition of metals and alloys. The experimental measurement of conductivity for the three DESs studied here showed that TBAC-TEG attained the least conductivities since the structure of this DES contains less salt and hence ionic species than TBAC-EG and the TBAC-G DESs. The highest conductivity was that of TBAC-EG (1:4) DES with a

a 69.89 69.98 68.22 65.50 64.85 64.49 71.01 65.50 65.53 61.39

R2

b −1

−0.794 × 10 −0.771 × 10−1 −0.700 × 10−1 −0.869 × 10−1 −0.817 × 10−1 −0.786 × 10−1 −1.034 × 10−1 −0.869 × 10−1 −0.849 × 10−1 −0.694 × 10−1

0.993 0.991 0.995 0.994 0.994 0.999 0.986 0.994 0.981 0.984

value of 7220 μS/cm at 353.15 K, while the lowest was that of TBAC-TEG (4:1) DES with a value of 18 μS/cm at 293.15 K. The variation of conductivity is related to that of viscosity.25 More viscous fluids will retard the motion of the ionic species and hence have a negative effect on conductivity. As the temperature is increased, viscosity decreased and consequently conductivity increased. This is apparent in the conductivity−temperature trends given in Figure 4. The trends grow exponentially similar to that of viscosity, however, with opposite direction. It is also apparent from Figure 5 that as more salt is added in the structure of the DES, more charge carrying species is being added to the system and hence conductivity increases. F



Article

Figure 4. Conductivities of TBAC-based DESs: A, with glycerol as a HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

molar ratios. On the other hand, the molar ratios of the TBACTEG DESs attained the highest RI values of (1.4786, 1.4836, 1.4847, and 1.4856), respectively. In general, it can be seen that the values of refractive indices decrease with the increase in mole fraction of the HBD. One reason for this could be that the values of the refractive indices of all used HBDs for synthesizing the DESs are less than that of the corresponding DESs. At the lowest temperature of 393.15 K, the maximum RI was observed for the three different DESs. These RI values were as follows: 1.4802 for TBAC-G at a molar ratio of (1:3), 1.4703 for TBAC-EG at a molar ratio of (1:2), and 1.4871 for TBAC-TEG at a molar ratio of (4:1). Because of the reduction attained in the density of a liquid with temperature, the speed of light in a liquid will normally increase as the temperature increases. Consequently, the normal behavior of the refractive index for a liquid is to decrease as the temperature increases. The three TBAC-based DESs attained the expected RI behavior with temperature. For the three DES systems under study, the variations of RI with temperature were regressed using a linear model of the following form:

The variation of conductivity with temperature was captured by regressing the data to an Arrhenius-like equation of the following form:

κ = κoe[−Eκ / RT ]

(4)

where κ is the conductivity in mS·cm−1, κo is a constant, Eκ is the activation energy of conductivity, and R is the gas constant. Regression correlation coefficients for the glycerol and ethylene glycol based DESs were of high values (>0.98), whereas that of the TEG DES was less. Values of κo, Eκ, and regression coefficients are shown in Table 6. 3.6. Refractive Index. The speed of light in a substance is slower than in a vacuum due to the fact that the light is being absorbed and reemitted by the atoms in the sample. Refractive index (RI) is a measure of the speed of light in the sample. Materials have different electrical permittivities and magnetic permeabilities, and hence, RI is usually used to estimate sample purity in a given matrix. The measured values of RI for the three systems are shown in Figure 5. At room temperature, the TBAC-EG system showed the lowest RI values among the three DESs. The RI for the three different molar ratios are (1.4688, 1.4636, and 1.4600), respectively. The RI of the TBAC-G system was in the middle range with values of (1.4788, 1.4762 and 1.4768) at the three

RI = a + bT

(5)

where RI is refractive index, T is temperature in kelvin, and a and b are constants that vary according to the type of DES. Because G



Article

Figure 5. Refractive indices of TBAC-based DESs: A, with glycerol as a HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

Table 6. Conductivity−Temperature Model Parameters −EκR−1

κo DES TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

μS·cm

Table 7. Refractive Index−Temperature Model Parameters

−1

5.660 × 108 5.301 × 108 2.601 × 108 3.821 × 103 3.489 × 103 2.439 × 103 7.324 × 107 1.031 × 107 5.816 × 105 4.642 × 106

2

K

R

4376 4312 4129 2356 2219 2051 3814 3261 2657 3359

0.992 0.999 0.991 0.984 0.987 0.994 0.998 0.947 0.823 0.962

DES

a

b·104

R2

TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

1.5694 1.5635 1.5633 1.5646 1.5553 1.5556 1.5718 1.5722 1.5714 1.5744

−3.035 −2.932 −2.900 −3.210 −3.078 −3.203 −3121 −2.975 −2.904 −2.979

0.998 0.999 0.999 0.998 0.997 0.999 0.999 0.999 0.998 0.999

three DESs systems under investigation were measured under the effect of varying their molar ratios and temperature. Figure 6 shows the variation of pH for the studied systems under the effect of molar ratio variation and temperature. Increasing temperature has the expected effect of reducing DES pH. However, the rate of change depends on the pH range. HighpH DESs have the highest pH reduction with temperature (TBAC-G4, TBAC-EG2, TBAC-EG3, TBAC-EG4, and TBACTEG4 having pH reduction rates of 0.024, 0.027, 0.024, 0.019, and 0.017 pH units/K). On the other hand, DES systems with relatively low pH values, have low pH reduction rates with

refractive index is a dimensionless property, a and b are dimensionless parameters. Table 7 shows values of refractive index a and b for eq 5. 3.7. pH. Many important industrial applications require accurate knowledge of liquid acidity or alkalinity defined by its pH. Examples include areas such as pharmaceutical reactions, bioreactions, catalyst preparation, nanoparticles shape control processes, and the corrosivity of these liquids. The pH of the H



Article

Figure 6. pH of TBAC-based DESs: A, with glycerol as a HBD; B, with ethylene glycol as a HBD; C, with triethylene glycol as a HBD. Predicted values are indicated with lines. Data are reported at pressure p = 0.1 MPa.

temperature (TBAC-G3, TBAC-G5, and TBAC-TEG2 having pH reduction rates of 0.007, 0.006, and 0.008 pH unit/K). DES molar ratio effect was also noticed. Glycerol-based DESs exhibited their highest pH values at the 1:4 molar ratio with a basic range of pH (7.501 to 8.945), while the other two molar ratios were slightly acidic in nature (pH (6.114 to 6.806)). EGand TEG-based DESs showed similar trends in terms of HBD effect on the pH. Adding more HBD increased the pH range. An interesting feature of the TEG-based DESs is that the relative change in pH due to a change in molar ratio is consistent for all tested ratios. The pH−temperature trend lines look almost parallel. This property makes it easy to control the acidity of the DES system by controlling its molar ratio. A linear model was sufficient to fit the pH−temperature behavior for the studied DESs according to the following relationship:

pH = a + b(T )

Table 8. pH−Temperature Model Parameters DES TBAC-G3 TBAC-G4 TBAC-G5 TBAC-EG2 TBAC-EG3 TBAC-EG4 TBAC-TEG1 TBAC-TEG2 TBAC-TEG3 TBAC-TEG4

a 8.437 16.067 8.769 17.200 16.330 15.150 13.158 12.495 10.719 8.738

R2

b −3

−6.57 × 10 −24.30 × 10−3 −6.70 × 10−3 −2.74 × 10−2 −2.43 × 10−2 −1.97 × 10−2 −1.74 × 10−2 −1.64 × 10−2 −1.28 × 10−2 −7.99 × 10−3

0.999 0.998 0.996 0.986 0.996 0.988 0.999 0.998 0.999 0.997

ethylene glycol, or triethylene glycol as hydrogen bond donors. A total of 10 different successful DESs were synthesized by varying the salt to HBD molar ratios. The physical properties of these DESs including density, viscosity, surface tension, refractive index, conductivity, and pH were measured at different temperatures in the range of (293.15 to 353.15) K. The glycerol-based DESs had the highest densities and conductivities among other tested DESs. On the other hand, TEG-based DESs attained the highest viscosities and refractive indices. Varying the amount of TBAC or the HBD associated with it had a clear effect on the physical properties. Sometimes

(6)

Table 8 shows values of the dimensionless pH parameters a and b for eq 6.

4. CONCLUSION The quaternary ammonium salt TBAC was successfully used in this work to prepare three different DES systems using glycerol, I



Article

[Bmim]Cl or tetrabutylammonium chloride (TBAC)/CrCl2. Tetrahedron Lett. 2012, 53, 983−985. (13) Peyrin, E.; Guillaume, Y. Effect of tetrabutylammonium chloride as eluent modifier on the retention and enantioselectivity of d,l-dansyl amino acids using immobilized human serum albumin. Talanta 1999, 49 (2), 415−423. (14) Lin, C. H.; Xiang, J. S.; Matyjaszewski, K. “Living” Cationic Polymerization of Styrene in the Presence of Tetrabutyleammonium Salts. Macromolecules 1993, 26, 2785−2790. (15) Gorke J. T.; Kazlauskas R. J.SriencF. Enzymatic Processing in Deep Eutectic Solvents. U. S. Pat. US 20090117628 A1. (16) Mjalli, F. S.; Baba, J. N.; Al-Hatmi, S.; Gano, Z. S. Ionic liquids analogues based on potassium carbonate. Thermochim. Acta 2014, 575, 135−143. (17) Shahbaz, K.; Mjalli, F. S.; Hashim, M. A.; Alnashef, I. M. Prediction of deep eutectic solvents densities at different temperatures. Thermochim. Acta 2011, 515, 67−72. (18) Hayyan, A.; Mjalli, F. S.; AlNashef, I. M.; Al-Wahaibi, T.; AlWahaibi, Y.; Hashim, M. A. Fruit sugar-based deep eutectic solvents and their physical properties. Thermochim. Acta 2012, 541, 70−75. (19) Ghatee, M. H.; Zare, M.; Moo, F. Temperature-Dependent Density and Viscosity of the Ionic Liquids 1-Alkyl-3-methylimidazolium Iodides: Experiment and Molecular Dynamics Simulation. J. Chem. Eng. Data 2010, 55, 3084−3088. (20) Kareem, M. A.; Mjalli, F. S.; Hashim, M. A.; AlNashef, I. M. Phosphonium-Based Ionic Liquids Analogues and Their Physical Properties. J. Chem. Eng. Data 2010, 55, 4632−4637. (21) Segur, J. B.; Oberstar, H. E. Viscosity of glycerol and its aqueous solutions. Ind. Eng. Chem. 1951, 43, 2117−2120. (22) Rebsdat, S., Mayer, D., Ethylene Glycol. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, Germany, 2000. (23) Gupta, S. B. Natural GasExtraction to End Use. InTech Press: New York, NY, USA, 2012. (24) Erbil, H. Y. Surface Chemistry of Solid and Liquid Interfaces; Blackwell: Oxford, U. K., 2006. (25) Harris, R. C.. Physical Properties of Alcohol Based Deep Eutectic Solvents; University of Leicester: Leicester, U. K., 2008.

this effect is so pronounced such as in the case of viscosity and conductivity variations in the TBAC-TEG system, while it was not as very clear in the case of the TBAC-G system. The effects of temperature and composition of these DESs were correlated for all of the studied properties using simple models of the linear or Arrhenius forms. The current investigation opens the door to more thorough investigation of the chemical and mechanical applications of these new fluids. Further studies are needed to cover possible applications of these new DESs as solvents, reaction media, catalysts, geometry directing agents, or electrolytes.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables listing all experimental data along with their average uncertainties. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +968-24142558. Fax: +968-24141354. Funding

We appreciate the financial support of the College of Engineering, Sultan Qaboos University, Muscat, Oman. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Hou, Y.; Gu, Y.; Zhang, S.; Yang, F.; Ding, H.; Shan, Y. Novel binary eutectic mixtures based on imidazole. J. Mol. Liq. 2008, 143, 154− 159. (2) Abbott, A. P.; Barron, J. C.; Ryder, K. S.; Wilson, D. Chem.Eur. J. 2007, 13, 6495−6501. (3) Abbott, A. P. Application of Hole Theory to the Viscosity of Ionic and Molecular Liquids. ChemPhysChem. 2004, 5, 1242−1246. (4) Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jerome, F. Deep eutectic solvents: Syntheses, properties and applications. Chem. Soc. Rev. 2012, 41, 7108−7146. (5) Leron, R. B.; Caparanga, A.; Li, M.-H. Carbon dioxide solubility in a deep eutectic solvent based on choline chloride and urea at T=303.15− 343.15K and moderate pressures. J. Taiwan Inst. Chem. Eng. 2013, 44, 879−885. (6) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Shikotra, P. Selective Extraction of Metals from Mixed Oxide Matrixes Using Choline-Based Ionic Liquids. Inorg. Chem. 2005, 44, 6497−6499. (7) Abbott, A. P.; Capper, G.; Glidle, A.; Ryder, K. S. Electropolishing of stainless steels in a choline chloride based ionic liquid: an electrochemical study with surface characterisation using SEM and atomic force microscopy. Phys. Chem. Chem. Phys. 2006, 8, 4214−4221. (8) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel ambient temperature ionic liquids for zinc and zinc alloy electrodeposition. Trans. Inst. Met. Finish. 2001, 79, 204−206. (9) Himeur, F.; Stein, I.; Wragg, D. S.; Slawin, A. M. Z.; Lightfoot, P.; Morris, R. E. The ionothermal synthesis of metal organic frameworks, Ln(C9O6H3) ((CH3NH)2CO)2 using deep eutectic solvents. Solid State Sci. 2010, 12, 418−421. (10) Shahbaz, K.; Mjalli, F. S.; Hashim, M. A.; AlNashef, I. M. Eutectic solvents for the removal of residual palm oil-based biodiesel catalyst. Sep. Purif. Technol. 2011, 81, 216−222. (11) Sato, K.; Tokutomi, H.; Ohmura, R. Phase equilibrium of ionic semiclathrate hydrates formed with tetrabutylammonium bromide and tetrabutylammonium chloride. Fluid Phase Equilib. 2013, 337, 115−118. (12) Jadhav, H.; Taarning, E.; Pedersen, C. M.; Bols, M. Conversion of D-glucose into 5-hydroxymethylfurfural (HMF) using zeolite in J


Tetrabutylammonium Chloride Based Ionic Liquid Analogues and

Recommend Documents