Volumetric Properties of Protic Ionic Liquids Based on Alkylammonium

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Volumetric Properties of Protic Ionic Liquids Based on Alkylammonium Cations at T = (293.15−353.15) K and Atmospheric Pressure Dmitriy M. Makarov* and Lyubov P. Safonova G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Ivanovo, 153045, Russia

J. Chem. Eng. Data Downloaded from pubs.acs.org by TULANE UNIV on 12/16/18. For personal use only.

S Supporting Information *

ABSTRACT: In this work, we measured the densities of 20 protic ionic liquids (PILs) based on four different cations: diethylammonium, triethylammonium, dimethylethylammonium, and triethanolammonium, and 14 anions of the following acids: phosphoric, acetic, trifluoroacetic, methanesulfonic, trifluoromethanesulfonic, benzoic, salicylic, propanoic, sulfuric, nitrobenzenesulfonic, sulfobenzoic, salicylic, sulfosalicylic, and bis(trifluoromethylsulfonyl)imide ones. The values of density were measured over the temperature range from 293.15 K to 353.15 K by densimeter based on the vibrating tube principle and they were approximated by a polynomial as a function of temperature. The thermal expansion coefficients were calculated; the fractional free volume in the molar volume of the studied PILs was estimated, and the influence of the ion structure on their volumetric properties was analyzed. Two different group contribution methods were analyzed to determine their applicability to predicting the density of the studied PILs.

1. INTRODUCTION Ionic liquids (ILs) are salts with the melting temperature below 373 K possessing unique thermophysical properties thanks to the asymmetry and charge dispersion of their organic and inorganic ions.1 These properties can be regulated by combining different cations and anions, which makes it possible to adjust the required properties to specific applications. ILs now represent an alternative to conventional solvents with a wide variety of applications in extraction,2 electrochemistry,3 catalysis,4 food industry,5 solar, and fuel cells,6,7 etc. Among the whole range of ILs, protic ionic liquids (PILs) are of special importance. PILs are formed via proton transfer from Brønsted acid to Brønsted base and have protons capable of taking part in the strong hydrogen bond between the protonated base (cation) and the acid anion. 8 The attractiveness of PILs consists in their low cost, simplicity of synthesis, low toxicity,9 and a great application potential of this type of ILs.10−16 For example, PILs synthesized based on alkyl ammonium are studied as electrolytes for fuel cells, batteries, and capacitors.17,18 Besides, ionic liquids are used for absorbing contaminating gases (CO2, SO2) from the air,19,20 in ionconducting polymer membranes,21 and as solvents of biological media (cellulose and proteins).22 The development of industrial processes based on new materials depends on thermophysical properties with a big role played by density and volumetric properties obtained on its basis. For example, accurate values of volumetric characteristics depending on external state parameters are necessary for designing industrial hubs and equipment. Density values are used for finding quantitative structure−activity relationship © XXXX American Chemical Society

models, empirical equations, and for liquid state modeling. Besides, density is required for calculating molar conductivity that is used for constructing the Walden plot. That is why accurately determined volumetric characteristics of PILs are of both fundamental and practical interest. In this work, we have studied the volumetric properties of 20 PILs in a wide temperature range. Seven of them contained a triethylammonium (TEA) cation, three−a diethylmmonium (DEA) cation, one−a dimethylethylammonium (DMEA) cation, and nine−a triethanolammonium (TEOA) cation. The following compounds were used as the anions: dihydrogen phosphate (H2PO4); acetate (Ac); trifluoroacetate (TFA); methanesulfonate (MsO); trifluoromethanesulfonate (TfO); benzoate (BA); salicylate (SA); propionate (PA); hydrogen sulfate (HSO4); metanilate (MTA); 3-nitrobenzenesulfonate (NBSu); 2-sulfobenzoate (SBA); sulfosalicylate (SSA); bis(trifluoromethylsulfonyl)imide (TFSI). The obtained volumetric properties were analyzed in order to determine the temperature effect and the role of anion and cation nature.

2. EXPERIMENTAL SECTION The PILs samples were obtained by the method described in detail in ref 23. All the synthesized salts have been characterized by the 1H NMR, 13C NMR, 1H/15N NMR, and FT-IR spectroscopic methods. All the data on the Received: August 16, 2018 Accepted: December 4, 2018

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DOI: 10.1021/acs.jced.8b00725 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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identification of the substances and their purity was presented in the works published earlier.23−27 Their name, abbreviated notation, CAS number, molecular structure, purity, and water content are given in Table 1. The PILs under study are hygroscopic, and traces of water impurities may have a critical effect on their volumetric properties; that is why the PILs were dried and degassed in high vacuum at the temperature of 323.15 K for 8−12 h. The water concentration in each of the PILs was determined by coulometric titration by K. Fischer’s method (V30, Mettler Toledo). The density was measured with an Anton Paar DMA 5000 M oscillating tube density meter at the atmospheric pressure of 0.10 ± 0.005 MPa in the temperature range from 293.15 K to 333.15 K. For some of the PILs the measurement temperature range was limited because of their melting point or high viscosity. The instrument calibration was conducted according to the producer recommendations and the density measurements of dry air and bidistilled water. The temperature in the DMA 5000 M cell was controlled by several Peltier devices, and its uncertainty was equal to 0.01 K. The samples under study were loaded to the density meter cell with an Anton Paar sample changer Xsample 352 H with a heating attachment. This device simplifies loading high-viscosity samples and prevents the appearance of air bubbles in the cell. The reproducibility of density measurements was higher than 7·10−6 g·cm−3, and expanded uncertainty (at the 95% confidence level, k = 2) of the density was 1.5·10−3 g·cm−3. All the density measurements were corrected for the influence of PILs viscosity by using density meter selfcalibration.

Table 1. Ionic Structures, Names, Abbreviations, CAS Numbers, Purity, and Water Content of the Studied PILs

3. RESULTS AND DISCUSSION 3.1. Density. The density values of the studied PILs in the temperature range of 293.15−353.15 K at the atmospheric pressure are given in Table 2. The temperature dependence of density was described by the following polynomial second-order equation: ρ = a0 + a1T + a 2T 2

(1)

where a0, a1 and a2 are fitting parameters. T is the temperature in K. The root-mean-square deviation (RMSD) was calculated as ÄÅ ÉÑ1/2 RMSD = ÅÅÅÅ∑ (ρexp − ρcal )2 /(N − 2)ÑÑÑÑ (2) ÅÇ ÑÖ where ρexp and ρcal are the densities experimentally obtained and calculated, respectively; N is the total number of experimental points. The values of parameters obtained by eqs 1 and 2 for all the PILs are given in Table 3. The obtained standard deviation of the correlation was below 1·10−5 g·cm−3. We have found literature data only for the densities of TEA/ Ac,28−30 TEA/MsO,31,32 TEA/TfO,32 TEA/H2PO433,34 and DEA/HSO4.29,35 The comparison of our data with those given in the literature at different temperatures is shown in Figure 1. For TEA/Ac, there is a good agreement between the obtained values with the work by Venkatesu et al.,28−30 with the relative deviations from −0.1 to −0.6%. The biggest difference of 17.8% between our data and the data represented in the work Kavitha et al.29 is observed for DEA/HSO4.We also obtain a large deviation (16.5%) for TEA/H2PO4.33 In this case, the

density values at the temperatures 298.15, 303.15, and 308.15 K were extrapolated from eq 1. At the same time, it should be noted that the observed deviations can be caused, to a great extent, by the influence of traces of water in the composition of the PILs synthesized in laboratory conditions. B



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Table 2. Density, ρ, of the PILs Studied at Different Temperatures, T, and p = 0.1 MPaa ρ/g cm−3 T/K

TEA/Ac

TEA/TFA

TEA/MsO

TEA/TfO

TEA/BA

TEA/SA

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1.01329 1.00963 1.00587 1.00210 0.99833 0.99455 0.99075 0.98696 0.98314 0.97932 0.97548 0.97162 0.96774 DMEA/TFA

1.16569 1.16157 1.15744 1.15332 1.14921 1.14510 1.14100 1.13691 1.13282 1.12874 1.12467 1.12060 1.11654 TEOA/PA

1.12047 1.11711 1.11377 1.11044 1.10713 1.10383 1.10054 1.09726 1.09399 1.09072 1.08747 1.08422 1.08098 TEOA/HSO4

1.26863 1.26456 1.26051 1.25647 1.25245 1.24845 1.24446 1.24048 1.23653 1.23258 1.22865 1.22473 1.22084 TEOA/TfO

1.10147 1.09802 1.09465 1.09127 1.08787 1.08447 1.08107 1.07768 1.07430 1.07092 1.06755 1.06418 1.06082 TEOA/MTA

1.09907 1.09579 1.09257 1.08933 1.08608 1.08283 1.07958 1.07635 1.07313 1.06993 1.06673 1.06354 1.06036 TEOA/NBSu

293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15

1.21241 1.20811 1.20382 1.19956 1.19528 1.19103 1.18678 1.18254 1.17833 1.17409 1.16990 1.16569 1.16151

1.13953 1.13651 1.13347 1.13043 1.12731 1.12425 1.12115 1.11801 1.11484 1.11168 1.10850 1.10529 1.10206

1.42130 1.41880 1.41633 1.41385 1.41131 1.40887 1.40637

1.46495 1.46134 1.45780 1.45425 1.45066 1.44723 1.44382 1.44030 1.43696 1.43353 1.43019 1.42688 1.42360

1.33507 1.33243 1.32979 1.32717 1.32457

TEA/H2PO4

1.38646 1.38315 1.37988 1.37681 1.37371

DEA/HSO4

DEA/TFA

DEA/MsO

1.38276 1.37953 1.37633 1.37315 1.36999 1.36685 1.36373 1.36062 1.35753 1.35445 1.35138 1.34831 1.34525 TEOA/SA

1.15882 1.15474 1.15069 1.14665 1.14262 1.13855 1.13450 TEOA/SSA

1.11750 1.11427 1.11106 1.10786 1.10468 TEOA/TFSI

1.38214 1.37910 1.37603 1.37288 1.36983

1.42264 1.41867 1.41468 1.41069 1.40674 1.40275 1.39881 1.39488 1.39089 1.38692 1.38299 1.37905 1.37511

1.32948 1.32708 1.32469 1.32231 TEOA/SBA

1.37312 1.37019 1.36725 1.36428 1.36132

1.24843 1.24542 1.24240 1.23942 1.23644 1.23342

Standard uncertainties u(T) = 0.01 K, u(p) = 10 kPa and the combined expanded uncertainty U is U(ρ) = 0.0015 g·cm−3 (0.95 level of confidence). a

the TEOA cation and the anion is stabilized by numerous Hbonds between the cation protons and the anion oxygen atoms. The high density group also includes the PILs with the studied inorganic anions. The density values for the salts with anions of different acids rise in the following order: for the TEA cation: A < SA < BA < MsO < TFA < TfO < H2PO4; for the DEA cation: MsO < HSO4 < TFA; for the TEOA cation: PA < SA < MTA < SBA < SSA < NBSu < TFSI < HSO4 < TfO. The density decreases as the temperature grows because of the thermal expansion and weakening of the intermolecular interactions between the anions. 3.2. Free Volume. According to the free volume theory, the molar volume of a liquid consists of the molecule occupied volume and free volume. Free volume correlates well with electric conductivity and viscosity37,38 and is the main property that can be used to control the solubility of a low molecular gas and its selectivity.39 The fractional free volume occupied by one mole of a substance molecule can be calculated from39,40

Figure 1. Comparison between experimental data given in this work and data available in the literature for TEA/MsO (■),32 (□);31 TEA/ Ac (●),30 (○);29 TEA/TfO (▲),32 TEA/H2PO4 (◀),33 DEA/HSO4 (▼).29

The studied PILs depending on the density value can be divided into three groups for convenience: low density ρ < 1.1 g·cm3; medium density 1.1< ρ < 1.3 g·cm3; and high density ρ > 1.3 g·cm3. Low density is only characteristic of PILs based on a TEA cation with Ac, BA, and SA anions. The fact that the TEOA cation has three hydroxyl groups capable of forming interion hydrogen bonds with the anions leads to the formation of the densest liquids. The quantum-chemical calculations36 conducted have shown that the ion pair between

FFV =

(Vm − 1.3NAVW ) Vm

(3)

where Vm is the molar volume, VW is the van der Waals volume of the ith molecule, NA is the Avogadro’s number. The van der Waals volumes were calculated as the additive sum of atomic C



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Table 3. Adjustable Parameters, ai, and Root-Mean-Square Deviation, RMSD, in eq 1 a0/g·cm−3 a1·104/g·cm−3·K−1 a2·107/g·cm−3·K−2 RMSD105/ g·cm−3

−3

a0/g·cm a1·104/g·cm−3K−1 a2·107/g·cm−3K−2 RMSD 105/g·cm−3

TEA/Ac

TEA/TFA

TEA/MsO

TEA/TfO

TEA/BA

TEA/SA

TEA/H2PO4

DEA/HSO4

DEA/TFA

DEA/MsO

1.20969 −5.95 −2.54 5.4 DMEA/ TFA

1.41924 −9.03 1.29 1.1

1.53327 −9.91 3.01 1.1 TEOA/TfO

1.30675 −7.19 0.64 4.1 TEOA/ MTA

1.30171 −7.29 1.29 5.6 TEOA/ NBSu

1.50660 −5.68 1.30 4.6 TEOA/ SBA

1.59762 −8.23 3.08 4.8

TEOA/PA

1.33438 −7.89 2.04 2.4 TEOA/ HSO4

TEOA/SA

1.42630 −8.44 5.00 2.6 TEOA/ SSA

1.36765 −8.55 3.11 2.5 TEOA/ TFSI

1.48126 −9.74 1.95 7.9

1.28355 −3.81 −3.77 4.1

1.58293 −5.03 0.07 3.9

1.72510 −10.05 5.61 9.8

1.45881 −6.79 1.16 2.6

1.56079 −4.60 −2.29 5.1

1.66307 −8.43 0.79 4.8

1.54109 −7.06 2.63 0.6

1.53813 −4.06 −2.69 0.5

Table 4. Molar Volume, Vm, van der Waals Volume, VW, Free Volume, Vfree, and Fractional Free Volume, FFV, of the Studied PILs at T = 298.15 K

and group increments with the help of the MarvinSketch 18.11 program produced by ChemAxon Ltd. The values of the PILs molar volume, Vm, at T = 298.15 K, the van der Waals volume of the molecules, VW, the fractional free volume, FFV, and the free volume, Vfree, (calculated as the difference between the molar and the van der Waals volumes) are given in Table 4. Khan et al.41 studied four PILs based on 1-propanenitrileimidazolium cation and noticed that the free volume increased with the anion size growth. Table 4 and Figure S1 in Supporting Information show that for the PILs that we have studied there is no clear dependence of the free volume or its fractional volume on the anion volume. However, we can observe some local regularities. For example, for triethylammonium salts, the replacement of the methyl group protons with fluorine in the anion (during the transition from acetate to trifluoroacetate and from methanesulfonate to trifluoromethanesulfonate) leads to an increase in the liquid density, which is associated with a higher electric negativity of the fluorine atoms, with the simultaneous growth in the fractional free volume. The replacement of a hydrogen atom with the hydroxyl group during the transition from the benzoic acid anion to the salicylic acid anion results in insignificant growth in the fractional free volume and a density decrease. However, during the transition from TEA/TfO to TEOA/TfO, the fractional free volume becomes smaller, while the density grows. 3.3. Thermal Expansion. Figures 2−4 show the coefficients of isobaric thermal expansion, α, of the studied PILs that were calculated by eq 4: a1 + 2a 2T 1 i ∂ρ y αP = − jjj zzz = − ρ k ∂T { P a0 + a1T + a 2T 2

1.80937 −1.87 17.9 5.0

PILs

Vm/cm3·mol−1

VW/Å3

Vfree/cm3·mol−1

FFV

TEA/Ac TEA/TFA TEA/MsO TEA/TfO TEA/BA TEA/SA DEA/HSO4 DMEA/TFA TEOA/PA TEOA/TfO

159.71 185.28 176.61 198.69 203.38 218.40 124.10 154.92 196.45 204.78

182 197 199 214 236 245 156 163 225 240

16.8 30.7 20.6 31.0 18.4 26.7 2.1 27.1 20.0 16.9

0.11 0.17 0.12 0.16 0.09 0.12 0.02 0.17 0.10 0.08

Figure 2. Isobaric thermal expansion as a function of temperature for PILs based on TEA cation: ■, TEA/H2PO4; ●, TEA/Ac; ▲, TEA/ TFA; ▼, TEA/MsO; ⧫, TEA/TfO; ◀, TEA/BA; ▶, TEA/SA.

(4)

where a0, a1, and a2 are the coefficients from Table 3. For the studied PILs, values α vary from 3·10−4 to 8·10−4 K−1 and lie within the same range as the values reported earlier for other types of PILs.41,42 The salts based on a TEOA cation and with inorganic anions have the lowest α values, that is, low α values are characteristic of PILs with a strong bond between the cation and the anion. The temperature dependence of the thermal expansion of the studied PILs is close to a linear one and compared to hydrocarbons and alcohols,43 the value α has a weak dependence on temperature. For the salts with inorganic anions and an MsO anion, value α is practically independent of temperature (Figures 2−4). In several cases (for PILs with TfO anion and TEOA/NBSu), the derivative ∂α/∂T has a negative sign; such dependences are not typical of conventional solvents but can be observed for some ionic liquids.44−46

3.4. Density Prediction. In this work, we have considered two models of density prediction based on the method of additive group contributions for calculating ion exchange. The models use the Jenkins hypothesis47 suggesting that salt molecular volume (V0) is equal to the sum of cation (V+) and anion (V−) volumes. According to these models: ρ=

M NAV0

(5)

where M is the molar mass and V0 is the molecular volume. Since the value of V0 calculated by the additive group contribution method corresponds to the temperature of 298.15 K and atmospheric pressure, for prediction of PILs density at other temperature and pressure values, Gardas and Coutinho48 D



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where VW+ and VW− are the van der Waals volumes of the cation and anion, respectively; P is the packing factor. The values of P change insignificantly for different classes of ILs50 but the average value is 0.6875 ± 0.0055. That is why the molecular volume value can be obtained from the relation: V0 =

VW 0.6875

(8)

51

Marcus in his work had earlier reported on his successful application of this model for analyzing 174 (aprotic) ILs as an example. Figure 5 shows a comparison of the calculated density with the experimental value. The average absolute relative deviation

Figure 3. Isobaric thermal expansion as a function of temperature for PILs based on DEA and DMEA cations: ■, DEA/HSO4; ●, DEA/ MsO; ▲, DEA/TFA; ▼, DMEA/TFA.

Figure 5. Comparison between experimental and calculated densities by Marcus’ model.51 ■, TEA; ●, DEA and DMEA; ▲, TEOA

AARD% = 100/N∑(|ρcal − ρexp |/ ρexp) between the experimental and predicted densities for the 20 PILs studied is equal to 5.7% (RMSD = 0.071 g·cm3). The standard deviation for the 162 ILs analyzed earlier51 is a little lower and equals 3.8%. It means that the accuracy of such a method in predicting density is not satisfactory and it cannot be used for the PILs studied by us. The second model applied by us, the one developed by Paduszyński and Domańska,52 considers V0 as a sum of empirically determined contributions of functional groups. It is now the most developed model. It was obtained based on approximately 20000 experimental density values for over 1000 ILs. This model did not allow us to predict the density for several PILs: TEOA/NBSu, TEOA/SSA, and TEOA/TFSI as there was no data about the volumes for the functional groups present in these liquids. The value AARD% obtained by this method for 17 liquids studied by us is equal to 3.4% (RMSD = 0.052 g·cm3). This result is better than the one obtained using the first model but much worse than the one obtained by the authors of the model0.45% (RMSD = 0.006 g·cm3)52who predicted the density of 200 liquids that were not included, at first, in the set of experimental data for calculating the volume of the functional groups. As Figure 6 shows, for all TEA cationbased salts, the deviation of the calculated data from the experimental ones is the same and equals 0.076 g·cm−3, which may urge us to check and correct the volumes of the increments constituting TEA represented in the work done by the model authors.

Figure 4. Isobaric thermal expansion as a function of temperature for PILs based on TEOA cation: ■, TEOA/PA; ●, TEOA/HSO4; ▲, TEOA/TfO; ▼, TEOA/MTA; ⧫, TEOA/NBSu; ◀, TEOA/SBA; ▶, TEOA/SA; □, TEOA/SSA; ○, TEOA/TFSI.

proposed an equation based on a modification of Ye and Shreeve’s49 model: ρ=

M NAV0(a + bT + cp)

(6)

where T is the temperature in K and p is the pressure in MPa. Gardas and Coutinho48 analyzed the experimental data on a large number of ionic liquids and they obtained the values of coefficients a, b, and c, equal to 0.8005 ± 0.0002, 6.652 × 10−4 ± 0.007 × 10−4 K−1 and −5.919 × 10−4 ± 0.024 × 10−4 MPa−1, respectively, at the 95% confidence level. Such an equation can predict the density of ILs in the range of temperatures of 273.15 K−393.15 K and pressures of 0.10− 100 MPa. This equation was used by us to analyze the obtained experimental data. The first model considered by us suggests that molecular volume can be determined by using the volumes of the ions (V+ and V−) obtained from the crystal structure. At the same time, values V+ and V− have the following relation with the van der Waals volume:50 VW = VW + + VW − = (V+ + V −)P

(7) E



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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The density measurements were carried out on the equipment of the Interlaboratory Scientific Center “The Upper Volga Region Center of Physicochemical Research”.

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Figure 6. Comparison between experimental and calculated densities by Paduszyński and Domańska’s model.52 ■, TEA; ●, DEA and DMEA; ▲, TEOA.

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CONCLUSION In this work, we measured densities of 20 protic ionic liquids containing cations diethylammonium, triethylammonium,dimethylethylammonium and triethanolammonium in combination with 14 different organic and inorganic anions within a wide temperature range from 293.15 K to 353.15 K. We compared the density values with the literature data and found the obtained deviations to be satisfactory. We analyzed the dependence of volumetric properties on the PILs structure and temperature. The densest and least expandable liquids are those based on triethanolammonium cation, which is explained by the formation of a larger number of hydrogen bonds compared to alkyl ammonium salts. The analysis of fractional free volume values of PILs has shown that there is no connection between them and the anion volume. The dependences of density and thermal expansion of the PILs on temperature are almost linear. A temperature increase makes the density value higher in all the PILs. Following a temperature increase, the thermal expansion coefficients grow for some of the PILs and decrease for the others. The calculation of density by using two different approaches based on the method of group contributions has shown satisfactory agreement between the calculated and experimental values.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00725. Plot of the fractional free volume versus the volume of anion (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dmitriy M. Makarov: 0000-0001-5923-5662 Funding

This work was supported by the Russian Science Fund Grant No. 16-13-10371 F



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