Article pubs.acs.org/jced
Density and Surface Tension of Ionic Liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6] Kiki A. Kurnia,* M. I. Abdul Mutalib, Zakaria Man, and M. Azmi Bustam PETRONAS Ionic Liquids Centre, Department of Chemical Engineering, Universiti Teknologi PETRONAS, Tronoh 31750, Malaysia ABSTRACT: Two amino functionalized ionic liquids 1-(2-aminoethyl)3-methylimidazolium hexafluorophosphate, [H2N−C2mim][PF6], and 1(3-aminopropyl)-3-methylimidazolium hexafluorophosphate, [H2N− C3mim][PF6], were synthesized and characterized. The density and surface tension of these ionic liquids were measured from (293.15 to 343.15) K. Their values decreased with increasing temperature. The physical properties such as coefficient of thermal expansion, molecular volume, standard molar entropy, lattice energy, and molar enthalpy of vaporization were estimated using experimental data. The critical temperature of the ionic liquids was estimated using Eötvos equations. The values were then used to estimate the boiling temperature of the ionic liquids according to methods of Rebelo. The interstice model was used to predict the thermal expansion coefficient of the ionic liquids, α, and the result was in very good agreement with the experimental value. In addition, the parachor method was used to predict the physical properties of the ionic liquids [H2N−Cnmim][PF6] (n = 4, 5, 6).
1. INTRODUCTION Ionic liquids are composed solely by cation and anion and have melting points below 373.15 K.1 Due to their low vapor pressure and high thermal stability, ionic liquids have been widely considered as a greener alternative to current volatile organic solvent used in many chemical processes. Currently, there is a developing trend on preparing functionalized ionic liquids for specific application.2−9 The ionic liquids with an amino functional group have been reported in the literature as a medium to absorb CO2.4,10 It has showed that CO2 absorption in amino functionalized ionic liquids was higher than that in corresponding ionic liquids without a functional group. To design these ionic liquids as solvents for CO2 absorption, it is necessary to know a range of physical properties including density and surface tension. As a continuation of our previous study,11 we have synthesized ionic liquids with an amino functional group, namely, 1-(2-aminoethyl)-3-methylimidazolium hexafluorophosphate, [H2N−C2mim][PF6], and 1-(3aminopropyl)-3-methylimidazolium hexafluorophosphate, [H2N−C3mim][PF6], and measured their density and surface tension in the temperature range (293.15 to 343.15) K. The physical properties12 such as coefficient of thermal expansion, molecular volume, standard entropy, lattice energy, and molar enthalpy vaporization13 were estimated using experimental density data. Meanwhile, the surface tension data were used to estimate the surface entropy and enthalpy. The boiling and critical temperature were also estimated according to the Eötvos14 and Rebelo15 methods. In addition, the interstice model was used to predict the coefficient of thermal expansion, and meanwhile, the parachor method was used to predict the © 2012 American Chemical Society
physical properties of the ionic liquids [H2N−Cnmim][PF6] (n = 4, 5, 6).
2. EXPERIMENTAL SECTION 2.1. Chemicals. The purity of the chemicals is reported in mass fraction as follows. 1-Methylimidazole (for synthesis, ≥ 0.99, CAS number 616-47-7) was purchased from Merck. 2Bromoethylamine hydrobromide (0.99, CAS number 2576-478), 3-bromopropylamine hydrobromide (0.98, CAS number 5003-71-4), and potassium hexafluorophosphate (0.96, CAS number 17084-13-8) were purchased from Aldrich. Acetonitrile (anhydrous, 99.8 %, CAS number 75-05-8), methanol (anhydrous, 99.8 %, CAS number 67-56-1), and ethylacetate (anhydrous, 99.8 %, CAS number 141-78-6) were purchased from Sigma-Aldrich. All chemicals were used without further purification. 2.2. Preparation of the Ionic Liquids. The ionic liquids were synthesized according to the standard methods developed and reported in the literature.4,9 A similar method was used to synthesize two studied ionic liquids in this work. During anion metathesis, KPF6 was used instead of KBF4.11 The synthesized ionic liquids were dried in a vacuum oven for 24 h and kept in a sealed bottle with a PTFE septum. The characterizations of ionic liquids were conducted using 1H and 13C NMR spectroscopy, CHNS analysis, and water content analysis. Received: February 28, 2012 Accepted: October 16, 2012 Published: October 25, 2012 2923
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Table 1. Experimental Values of Density, ρ, and Surface Tension, γ, of the Ionic Liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6] from (293.15 to 353.15) K and at a Pressure of 0.1 MPaa
The details of the characterization procedure were reported in our previous work.11 The NMR, elemental analysis, water, and bromide content of the ionic liquids are as follows: [H2N−C2mim][PF6]: 1H NMR (400 MHz MeOD): δ 9.13 (s, 1H ring, im), 7.45 (d, 1H, ring H), 7.17 (d, 1H, ring H), 3.74 (s, 3H, H3C−Nring), 3.44 (t, 2H, H2C−Nring), 2.74 (m, 2H, H2C−Namine), 2.09 (t, 2H, H− Namine). Analysis % found (% calculated): C, 26.60 (26.58); H, 4.45 (4.46); N, 15.51 (15.50); water 142 ppm, bromide 98 ppm. [H2N−C3mim][PF6]: 1H NMR (400 MHz MeOD): δ 9.11 (s, 1H ring, im), 7.45 (d, 1H, ring H), 7.18 (d, 1H, ring H), 3.73 (s, 3H, H3C−Nring), 3.64 (t, 2H, H2C−Nring), 2.94 (m, 2H, CH2), 2.43 (m, 2H, H2C−Namine), 1.99 (t, 2H, H−Namine). Analysis % found (% calculated): C, 29.49 (29.48); H, 4.93 (4.95); N, 14.73 (14.74); water 197 ppm, bromide 84 ppm. The structure of the synthesized ionic liquids is shown in Figure 1.
[H2N−C2mim][PF6]
[H2N−C3mim][PF6]
T
ρ
γ
ρ
γ
K
kg·m−3
mN·m−1
kg·m−3
mN·m−1
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
1565.7 1561.9 1558.1 1553.9 1550.5 1546.2 1542.3 1538.5 1534.7 1530.9 1526.2 1523.0 1518.7
55.86 55.42 55.04 54.59 54.29 53.80 53.63 53.23 52.94 52.57 52.35 52.07 51.86
1499.4 1496.0 1492.6 1488.5 1484.7 1480.9 1477.5 1473.4 1468.9 1465.8 1462.1 1458.4 1454.7
51.06 50.84 50.61 50.25 50.02 49.94 49.70 49.29 49.26 49.04 48.79 48.61 48.49
a
The uncertainties of density, surface tension, and temperature were found to be ± 0.3 kg·m−3, ± 0.4 mN·m−1, and ± 0.1 K, respectively.
3.1. Estimation of Volumetric Properties. From experimental density data, we can estimate other properties such as coefficient of thermal expansion, molecular volume, standard molar entropy, lattice energy, molar enthalpy of vaporization, interstice volume, interstice fraction, and so forth.21−37 The coefficient of thermal expansion can be estimated by plotting the ln ρ against (T − 298.15) K (see Figure 2). A
Figure 1. Structure of the studied ionic liquids: (a) 1-(2-aminoethyl)3-methylimidazolium hexafluorophosphate, [H2N−C2mim][PF6], and (b) 1-(3-aminopropyl)-3-methylimidazolium hexafluorophosphate, [H2N−C3mim][PF6].
2.3. Measurement of Density and Surface Tension. The density and surface tension of two studied ionic liquids were measured using a digital vibrating glass U-tube densimeter (DMA 5000, Anton-Paar) and the pendant drop method, respectively. The details of the measurement procedure can be found elsewhere.11,16−18
3. RESULTS AND DISCUSSION Table 1 gives the experimental values of the density and surface tension of the studied ionic liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6] within the temperature range (293.15 to 353.15) K. The uncertainty of density and surface tension were found to be ± 0.3 kg·m−3 and ± 0.4 mN·m−1, respectively. The reported density and surface tension of the liquid [H2N− C2mim][PF6] at 298.15 K were 1560 kg·m−3 and 55.42 mN·m−1, respectively.9 Our values for density and surface tension for the ionic liquid [H2N−C2mim][PF6] at 298.15 K were 1561.9 kg·m−3 and 55.39 mN·m−1, respectively. The deviations were found to be less than 0.5 %. Compared to our previous report, the densities and surface tension of amino functionalized ionic liquids with hexafluorophosphate anion (M PF6 = 144.96 g·mol−3) were found to be higher than tetrafluoroborate (M BF4 = 86.81 g·mol−3). It indicates that the molecular weight of the anion significantly determines the density and surface tension of the ionic liquids. This is in good agreement with investigations of ionic liquids containing 1alkyl-3-methylimidazolium cations, where increasing densities with increasing molecular weight of anion were observed.19,20
Figure 2. Plot of ln ρ vs temperature T of the ionic liquids: ■, [H2N− C2mim][PF6]; ◆, [H2N−C3mim][PF6].
straight line was obtained and correlated with the following empirical equation ln[ρ /cm−3] = c − α(T − 298.15 K)
(1)
where c is an empirical constant and the negative value of the slope, α = −(∂ ln ρ/∂(T − 298.15))P, is the coefficient of thermal expansion. Then, α = [(5.09·10−4) ± (2.87 ± 10−6)] K−1 and [(5.13·10−4) ± (3.01 ± 10−6)] K−1 for [H2N− C2mim][PF6] and [H2N−C3mim][PF6], respectively. The molecular volumes, Vm, standard molar entropy for ionic liquids, S0,12 and lattice energy, UPOT,12 of the ionic liquids can be estimated using eqs 2, 3, and 4, respectively. 2924
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M (NA ·ρ)
where P is the parachor, M is the molar mass, ρ, density, and γ, surface tension. The estimated parachor values are listed in Table 2. The parachor values of [H2N−C2mim][PF6] and [H2N−C3mim][PF6] show a difference of 35.5. As [H2N− C3mim][PF6] has an extra methylene compared to [H2N− C2mim][PF6], it indicates that the contribution per methylene (−CH2−) to the parachor values of these ionic liquids is 35.5. This indicates that the contribution per methylene (−CH2−) to parachor is 35.5. The value was used to predict the parachor of the ionic [H2N−Cnmim][PF6]. Further, the surface tension of [H2N−Cnmim][PF6] (n = 4, 5, 6) was predicted using eq 5. The values are listed in Table 2. From the experimental density and surface tension, the molar enthalpy of vaporization can be estimated using the equation proposed by Kabo et al.13
(2)
S 0(298.15)/J ·K·mol−1 = 1246.5(Vm/nm 3) + 29.5
(3)
UPOT/kJ·mol−1 = 1981.2(ρ /M )1/3 + 103.8
(4)
where M is the molecular weight of the ionic liquids, NA is Avogadro’s constant, and ρ is the density of ionic liquids. The values are given in Table 2. The molecular volume of [H2N− Table 2. Estimated and Predicted Values of Physicochemical Properties of Ionic Liquids [H2N−Cnmim][PF6] (n = 2, 3, 4, 5, 6) at 298.15 K property M (g·mol−1) Vm (nm3) ρ (kg·m−3) S0 (J·K−1·mol−1) UPOT (kJ·mol−1) V (cm−3·mol−1) P γ (mN·m−1) Δgl H0m (kJ·mol−1) 1024 v (cm3) ∑v (cm3) 102 ∑v (V) 104 α (K−1) 104 α (K−1)
[H2N− C2mim] [PF6]a
[H2N− C3mim] [PF6]a
[H2N− C4mim] [PF6]b
[H2N− C5mim] [PF6]b
[H2N− C6mim] [PF6]b
271.14 0.2884 1561.9c 388.9
285.27 0.3168 1496.0c 424.3
299.4 0.3452 1440.9 459.7
313.53 0.3736 1394.2 495.1
327.66 0.4020 1354.1 530.6
459
448
438
430
422
173.6
190.7
207.8
224.9
242.0
473.6 55.42c 165.6
509.2 50.84c 161.8
544.7 47.21 159.2
580.2 44.29 157.4
615.7 41.90 156.4
13.75 16.55 9.5 5.09c 4.80
15.64 18.83 9.9 5.13c 4.97
17.47 21.04 10.1
19.23 23.15 10.3
20.90 25.16 10.4
5.09
5.18
5.23
a
Data in column were estimated value. predicted value. cExperimental data.
b
Δgl Hm0(298.15 K) = 0.01121(γV 2/3NA1/3) + 2.4
The molar enthalpies of vaporization for the ionic liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6] are listed in Table 2, together with the Δgl H0m(298.15 K) for [H2N− Cnmim][PF6] (n = 4, 5, and 6). From Table 2, the enthalpy of vaporization for [H2N−Cnmim][PF6] (n = 2 to 6) at 298.15 K decreases with increasing number of alkyl chains. This is due to electrostatic interaction between cation and anion in ionic liquids, where increasing number of alkyl chains attached in the imidazolium cation decreased its electrostatic interaction with the [PF6] anion. In addition, compared to our previous publication,11 amino functionalized ionic liquids with [PF6] anion show higher Δgl H0m v value than [BF4] anion. The greater the molecular weight of the anion, the greater is its molar standard vaporization enthalpy. This fact shows that, in contradiction to the effect of the cation on the molar standard vaporization enthalpy, the nonelectrostatic force for the anion is primary.21−36 3.2. Estimation of Surface Properties. From the temperature dependence of surface tension, the surface enthalpy and entropy can be calculated. By plotting the experimental values of surface tension against temperature (see Figure 3), a straight line is obtained and correlated using the empirical equation:
Data in column were
C2mim][PF6] and [H2N−C3mim][PF6] shows a difference of 0.0284 nm3. As [H2N−C3mim][PF6] have an extra methylene compared to [H 2 N−C 2 mim][PF 6 ], it indicates that a contribution per methylene (−CH2−) to the molecular volume of these ionic liquids is 0.0284 nm3. This value is comparable to the reported methylene contribution of 0.0280 nm3, 0.0272 nm3, and 0.0267 nm3 for n-alcohols, n-amines, and n-paraffins, respectively. Assuming the contribution per −CH2− to the molecular volume for a series of ionic liquids [H2N− Cnmim][PF6] (n = 2 to 6) is similar, the physicochemical properties of [H2N−Cnmim][PF6] (n = 4, 5, 6) can be predicted. All of the predicted data are listed in Table 2. As can be seen from Table 2, the estimated lattice energies for [H2N−C2mim][PF6] and [H2N−C3mim][PF6] are 459 and 448 kJ·mol−1, respectively. The value is found to be lower than fused salt. As an example, the lattice energy for fused salt CsI is 613 kJ·mol−1.38 The lower lattice energy of the studied ionic liquids compared to fused salt indicates weaker interaction between ions in ionic liquids than ions in fused salt. The low lattice energy may explain why ionic liquids are liquids at room temperature.21−36 The parachor, P, has been used to predict the surface tension of organic compounds from their density data and vice versa. The parachor is defined using the following equation P(298.15 K) = (Mγ 1/4)/ρ
(6)
γ = a − bT
(7)
where γ is the surface tension, a and b are fitting coefficients, and T is the absolute temperature. In eq 7, the fitting coefficients a and b are equal to a = γ − (∂γ/∂T)P = Ea and b =
Figure 3. Plot of γ vs temperature (T) of the ionic liquids: ■, [H2N− C2mim][PF6]; ◆, [H2N−C3mim][PF6].
(5) 2925
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−(∂γ/∂T)P = Sa, where Ea and Sa are the surface enthalpy and entropy, respectively. Table 3 gives the estimated values of
v = 0.6791(k bT /γ )3/2
Table 3. Surface Entropy, Sa, and Surface Enthalpy, Ea, of the Ionic Liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6]
[H2N−C2mim][PF6] [H2N−C3mim][PF6]
103·Sa
Ea
mJ·K−1·m−2
mJ·m−2
66.9 ± 1.8 43.5 ± 2.2
75.3 ± 0.6 63.8 ± 0.7
Table 4. Estimated Polarity, K, Critical Temperatures, Tc, and Normal Boiling Temperature, Tb, of the Ionic Liquids [H2N−C2mim][PF6] and [H2N−C3mim][PF6] ionic liquid [H2N−C2mim][PF6] [H2N−C3mim][PF6]
J·K
1.4·10−7 0.9·10−7
Tc
Tb
K
K
1425 2188
855 1313
Σv /V
(11)
V = Vi + 2NAv
(12)
α = (1/V )(∂V /∂T )P = 3NAv /VT
(13)
4. CONCLUSIONS This work presents an experimental study of the density and surface tension of two amino functionalized ionic liquids 1-(2aminoethyl)-3-methylimidazolium hexafluorophosphate, [H2N−C2mim][PF6], and 1-(3-aminopropyl)-3-methylimidazolium hexafluorophosphate, [H2N−C3mim][PF6], as a function of temperature at atmospheric pressure. In terms of the Glasser theory of ionic liquids and parachor, a series of physicochemical propertiesmolecular volume, standard entropy, surface tension, and molar enthalpy of vaporization were predicted. The thermal expansion coefficients of the ionic liquids were calculated according to the interstice model, and in comparison with experimental values, they were found to be in good agreement.
where γ is the surface tension, Tc is the critical temperature, K is an empirical constant, M is the molecular weight, and ρ is the density of the ionic liquid. The values are given in Table 4.
−1
(10)
The predicted coefficient of thermal expansion using eq 13 for [H2N−Cnmim][PF6] (n = 2 to 6) is given in Table 2. Compared to α (estimated) using eq 1 for ionic liquids [H2N− C2mim][PF6] and [H2N−C3mim][PF6], the deviations were found to be 5.3 and 4.5 %, respectively. It indicates that the interstice model can be used to predict the coefficient of thermal expansion of the ionic liquids.
(8)
K
Σv = 2NAv
where kb is Boltzmann’s constant, T is the thermodynamic temperature, and γ is the surface tension of the ionic liquid. The interstice model for [H2N−Cnmim][PF6] (n = 4, 5, 6) is shown in Table 2. The interstice model can also be used to predict the coefficient of thermal expansion. By assuming the volume of the ionic liquid, V, consists of the inherent volume, Vi, and the total volume of all interstices, Σv = 2NAv, and expansion of the ionic liquid volume only results from the expansion of the interstices when temperature increases, then the calculation expression of α was derived from the interstice model:
surface enthalpy and entropy for the studied ionic liquids. The values are within the reported result for the other ionic liquids in the literature.39−43 It is interesting to notice that the studied ionic liquids present lower surface entropy as compared to a fused salt, such as NaNO3 (Sa = 146 mJ·m−2).44 The surface entropy is much closer to that of an organic compound such as benzene (Sa = 67 mJ·m−2) and octane (Sa = 51.1 mJ·m−2).44 This fact shows that the interaction energy among ions in the studied ionic liquids is less than that in inorganic fused salts because the surface excess energy is dependent on the interaction energy between ions; that is, this means the crystal energy of ionic liquids is much less than that of inorganic fused salts.21−36 Another physical property that can be estimated from density and surface tension is critical temperature, Tc, using well-known Eötvos14 empirical equations: ⎛ M ⎞2/3 γ ⎜ ⎟ = K (TC − T ) ⎝ρ⎠
(9)
■
Once critical temperature is estimated, the boiling temperature can be estimated according to an equation developed by Rebelo et al.,15 in which Tb ≈ 0.6 Tc for an ionic liquid. The estimated boiling temperature for the studied ionic liquids is also presented in Table 4. From eq 8, K can represent the polarity of an ionic liquid. The polarity of a fused salt, such as NaCl, is 0.4·10−7 J·K−1; meanwhile, for organic liquids, it is about 2.1·10−7 J·K−1.38 Therefore, as can be seen from Table 4, the value of K implies that [H2N−C2mim][PF6] and [H2N− C3mim][PF6] have medium polarity between organic liquid and fused salt. 3.3. Interstice Model for the Ionic Liquids. Recently, there is a developing trend on using the interstice model for the ionic liquids. The theoretical details of the interstice model for the ionic liquids can be found elsewhere.21−36 Using the same procedure of the interstice model, the interstice volume, v, molar volume of the interstice, Σv, and the volume fractions of the interstice, Σv/V, can be calculated using eqs 9, 10, and 11, respectively.
AUTHOR INFORMATION
Corresponding Author
́ *Present address: CICECO, Departamento de Quimica, Universidade de Aveiro, 3810-193 Aveiro, Portugal. E-mail:
[email protected]. Phone: +351969438025. Notes
The authors declare no competing financial interest.
■
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dx.doi.org/10.1021/je300288a | J. Chem. Eng. Data 2012, 57, 2923−2927
