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Mean Activity Coefficients of NaCl in the Mixture of2‑Hydroxyethylammonium Butyrate + H2O at 298.15 KEliseo Amado-Gonzalez,* Irina Lupita Gonzalez-Gutierrez, and Wilfred Gomez-Jaramillo
Department of Chemistry, University of Pamplona, (57 + 7) 5685303−5685304, IBEAR FJ-207 Biofuels Laboratory, Pamplona,Colombia
*S Supporting Information
ABSTRACT: The main objective of this work is to continue with aseries of electrochemistry studies of mixtures of ionic liquids and wateras a solvent to provide accurate data for future particular applications[Amado-Gonzalez et al. J. Chem. Eng. Data 2017, 62, 752]. The meanactivity coefficients for NaCl in [2-hydroxyethylammonium butyrate(2-HEAB) + H2O] as a solvent mixture were determined by cellpotential measurements: Na−ion selective electrode (ISE)|NaCl (m),2-HEAB (w), H2O (1 − w)|Cl−ion selective electrode (ISE) atmolalities from 0.10 to 3.20 mol·kg−1 at 298.15 K. Different weightfractions (w) of 2-HEAB with w = 0.01, 0.05, 0.1, 0.2, 0.3, and 0.4 wereused. At higher concentrations of w = 0.40, NaCl is salting out by 2-HEAB in water. The Pitzer ion interaction parameters β0, β1, and Cγ
were used to find the values of osmotic coefficients, solvent activity,and the excess Gibbs free energy for the mixed electrolyte system. Theresults may be interpreted by the clathrate-like formation of 2-HEAB + water. A qualitative description of the relation betweenwater and the 2-HEAB was done using the general AMBER force field (GAFF).
1. INTRODUCTION
Thermodynamic properties of mixtures of [ionic liquids (ILs) +H2O] are important to evaluate future applications.1 Ionic liquids(ILs) or the future new solvents can be structurally designed asproposed by Lowe and Rendall.2 Due to the physicochemicalproperties of ILs,3−5 they may have many different industrialuses.6 Activity coefficients and osmotic coefficients of electrolytesolutions are extremely useful to test new electrolyte solutiontheories.7−15 Even though activity coefficients of ammonium ILshave been reported in aqueous solutions by the isopiesticmethod,16−23 to our knowledge, research papers about theactivity coefficients of NaCl in ILs + water are still scarce, andstudies of mean ionic activity coefficients of NaCl reported in theliterature for ternary liquid mixtures are still limited.24−26
Because of the high costs of ILs and import restrictions, thesynthesis of new ILs should be a goal. Protic ionic liquids offer agood possibility because they can be produced by a simple acid−base neutralization reaction.27 2-Hydroxyethylammonium buty-rate (2-HEAB) is therefore evaluated for its potential abilities as asolvent. In this sense, our goal is to evaluate how thethermodynamic properties of salts like NaCl would be affectedby the (2-HEAB + H2O) solvent mixture. In this work, we foundthe activity coefficients of NaCl in the mixture (2-HEAB + H2O)by cell potential measurements at 298.15 K. The results werefitted to the Pitzer model. Then, the osmotic coefficients, solventmixture activities, and excess Gibbs free energies of these systemswere calculated. A calculation of hydrogen interaction at w = 0.4was completed using the general AMBER force field (GAFF).
2. EXPERIMENTAL SECTION
2.1. Materials, Synthesis, andMeasurements. In Table 1the chemicals, suppliers, and stated purity are shown. The IL, 2-HEAB was synthesized by acid−base neutralization reactionsbetween ethanolamine and the organic acid as previouslydescribed.26 The reagents monoethanolamine and butyric acid(Sigma-Aldrich, 99%) are analytical (AR grade) reagents. Thesynthesis of 2-HEAB was done as reported by literature.28−30
Strong agitation was performed at heating at 323 K. The IL wasdried under a vacuum of 20 kPa during 8 h untilΔpH (pH at theequivalent point − pH experimental) < 0.07. An ATR-FTIRspectra for 2-HEAB was done. The ammonium structure wasfound at 3200−2400 cm−1. The OH stretching vibration wasembedded in this band. The carbonyl stretching and N−H in-plane bending vibrations were at 1600 cm−1. Water content wasverified by a volumetric titroline KF (Schott instruments) as 6 ×10−4 mass fraction. No phase change of ILs was observed. Theanalysis of 1H NMR gave the next results for the 2-HEAB: (400MHz, D2O) δ 4.72 (s, 4H, H3b y H4b), 3.75−3.66 (m, 2H,H2b), 3.06−2.97 (m, 2H, H1b), 2.05 (t, J = 7.3 Hz, 2H, H1a),1.52−1.40 (m, 2H, H2a), 0.79 (t, J = 7.4 Hz, 3H, H3a) and apurity higher that 99% mass fraction (details available in theSupporting Information).
Received: March 18, 2017Accepted: July 10, 2017
Article
pubs.acs.org/jced
© XXXX American Chemical Society A DOI: 10.1021/acs.jced.7b00278J. Chem. Eng. Data XXXX, XXX, XXX−XXX
2.2. Density Measurements. The density of the 2-HEABwas measured by an oscillating U-tube densimeter (RudolphResearch Analytical DDM 2911 Plus) with a resolution of 1 ×10−5 g cm−3, an accuracy of 1 × 10−5 g cm−3 for the IL at 298.15K, and 101 kPa. The temperature inside the U-tube is controlledby a Peltier system with a precision of 0.01° with the accuracy of0.03°. The calibration of the densimeter was checked from timeto time with ultrapure water. The values of density were betterthan 2 × 10−4 g cm−3.2.3. Refractive Index Measurements. Refractive index
values for the D-line, n, were measured with an Abberefractometer with a precision of ±0.0001, with a thermostatmaintained at ±0.05 K. The properties of 2-HEAB are density
(ρ) = 1.07512 g·cm−3, nD = 1.4659, and dielectric constant (εr) =65.10 ± 2.5 at 298.15 K and 101 kPa.
2.4. Dielectric Constant Calculation. The εr of the 2-HEABwas calculated by a square minimum fitted from the valuesof ILs with OH-functionalized cations for the series 2-HEAF, 2-HEAL, and 2-HEAA.31 The uncertainty was estimated accordingto literature.32
2.5. Potential Measurements of NaCl. NaCl reactive(Merck, pro analysis), was dried in a vacuum at 373 K for 72 h.NaCl reactive was stored over silica gel in a desiccator. Solutionswere prepared by adding weighted NaCl to a solution previouslyprepared of ILs. Conductivity water (k < 5.10−7 S·cm−1) wasu s e d . N a - I S E (mo d . 8 6 1 1 BNWP) a n d C l - I S E(mod.9617BNWP) were obtained from Thermo Scientific Co.An Orion Versa Star 40 Dual channel benchtop meter was used(model VSTAR40B) to EMF measurements with a 0.02 mVaccuracy. The solutions and ISE electrodes were kept in a glasscell with a double wall. The temperature of the solutions wascontrolled with a thermostat−cryostat Polyscience ScientificThermostat model 1156 and checked by calibrated type Kthermocouple with a precision of 0.02 K. The potentiometricmeasurements were performed by an electrochemical cellcontaining ion selective electrodes (ISE) as previouslydescribed.33
− | − − |
−
m w wNa ISE NaCl( ); 2 HEAB( ); water(1 ) Cl
ISE
2.6. Force Field Parameters. A qualitative description ofthe relation between water and the 2-HEABwas done. Anion andcation geometries of the ionic liquid were initially optimized atthe HF/6-31G* level using the Gaussian 09 package. In addition,vibrational mode analysis was performed to ensure the absence ofnegative frequencies and verify the existence of a true minimum.After that, force field parameters selected for the ionic liquid isthe general AMBER force field (GAFF) generated by using theAntechamber module of the AMBER17 suite of programs andAM1-BCC partial charges.34,35 The AMBER topology andstructure file of the ionic liquid were converted into a format file,recognized by the GROMACS 5.0.7 program, using anAmb2gmx.pl script.36
3.1. Mean Ionic Activity Coefficients. It is important toestablish that, for NaCl in the (H2O + 2-HEAB) solvent mixture,the reference state is (2-HEAB + H2O) solvent mixture and,therefore, γ± (NaCl) = 1 when mNaCl = 0. The type Nernstequation is used to fit the cell potential measurements (E):
γ= − ±E E k m2 ln0 (1)
where k = RT/F is the Nernst slope. R and F are the universal gasconstant and Faraday constant, respectively, and the othersymbols have their usual meaning.
Table 1. Chemicals, Suppliers, and Method To Check the Stated Puritya
chemical name source purity purification methodmass watercontenta electrical conductivity (S·cm−1)
NaCl Sigma-Aldrich, Riedel-de Haen, p.a., Reag. ACSCAS no. 7647-14-5
99.8% no further purification
butyric acid Merck, CAS no.107-92-6 99.0% no further purification ≤0.02%ethanolamine Merck, CAS no.141-43-5 99.5% no further purification ≤0.09%2-HEABb synthesis 97.0% Fischer-Karl titration 1H RMN,
ATR-FTIR spectra≤0.06%
water double distillation <5.00 × 10−7
aDetermined by the Karl Fisher method. b2-Hydroxyethylammonium butyrate.
Table 2. Values of the Average Mass Fraction (w), AverageMolecular Mass (M), Relative Permittivity (εr), Density (ρ),andDebye−Hu ckel Parameter (Aϕ) for (2-HEAB +H2O) atT= 298.15 K and p = 101 kPaa
IL w M (g mol−1) εr ρ (g cm−3) Aϕ (kg1/2 mol−1/2)
0.0115 18.1986 78.08 0.9994 0.39410.0502 18.8414 77.55 1.0076 0.39930.1010 19.7727 76.87 1.0163 0.40620.2011 21.8845 75.48 1.0301 0.42050.3051 24.4817 74.08 1.0407 0.42430.3993 27.7609 72.69 1.0481 0.4508
aStandard uncertainties are ur(w) = 0.03, u(εr) = 2.5, u(ρ) = 0.0001 gcm−3, u(T) = 0.01 K, u(p) = 10 kPa.
Figure 1. Experimental densities obtained by this work and Rocha-Pinto,30 for comparison. In the cells,m is the molality of NaCl in the (2-HEAB + H2O) solvent mixture, and w is the mass fraction of 2-HEAB atw = 0.01, 0.05, 0.10, 0.20, 0.30, and 0.40. Experimental density valuescompared with literature data30 agreed to 0.2%.
Journal of Chemical & Engineering Data Article
DOI: 10.1021/acs.jced.7b00278J. Chem. Eng. Data XXXX, XXX, XXX−XXX
B
The mean activity coefficients37 of pure NaCl at different mvalues in water were used to calculate the values of k and E0 at298.15 K. The calibration of the electrode were done bytriplicate, and the mean values of E0 = 153.35 ± 0.06 and k =
25.63 ± 0.05 mV with a regression coefficient (R2 = 0.999997)were calculated. The potential values (E), molar weight (M),dielectric constants (εr), density (ρ), and Debye−Huckelparameters (Aϕ) are listed in Table 2 for each weight fraction
Table 3. Experimental E Values, Osmotic Coefficients (ϕ), and Mean Activity Coefficients (γ±) of NaCl in the (2-HEAB + H2O)Mixture at Different Mass Fractions (w) at 298.15 K and p = 101 kPaa
m (mol kg−1) E (mV) γ±NaCl ϕ m (mol kg−1) E (mV) γ±NaCl ϕ
w = 0.0115 (2-HEAB)0.1387 45.06 0.7598 0.9311 1.8629 168.77 0.6436 0.94580.3534 88.04 0.7046 0.9239 2.0781 174.65 0.6455 0.95240.5671 110.78 0.6794 0.9232 2.2923 180.12 0.6487 0.95900.7793 126.33 0.6644 0.9245 2.5082 185.27 0.6533 0.96770.9928 138.25 0.6547 0.9269 2.7317 190.16 0.6593 0.97671.2189 148.24 0.6482 0.9305 2.9551 194.84 0.6666 0.98641.4339 156.27 0.6447 0.9348 3.1692 199.12 0.6747 0.99621.6458 163.41 0.6433 0.9399
w = 0.0502 (2-HEAB)0.1205 30.64 0.7410 0.9167 1.4991 151.90 0.6363 0.98260.2918 70.76 0.6758 0.9051 1.6691 157.74 0.6416 0.99640.4651 92.74 0.6481 0.9082 1.8431 163.26 0.6477 1.01050.6365 107.80 0.6348 0.9167 2.0186 168.36 0.6541 1.02440.8060 119.54 0.6289 0.9278 2.1868 173.04 0.6605 1.03740.9816 129.56 0.6273 0.9408 2.3569 177.50 0.6669 1.05021.1559 138.16 0.6286 0.9546 2.5276 181.74 0.6733 1.06261.3308 145.46 0.6319 0.9688
w = 0.1010 (2-HEAB)0.1074 29.30 0.7479 0.9182 1.8382 165.16 0.6257 0.98880.3236 78.76 0.6648 0.9020 2.0582 171.40 0.6323 1.00460.5405 102.62 0.6340 0.9055 2.2680 177.10 0.6391 1.01940.7550 119.42 0.6203 0.9153 2.4839 182.12 0.6465 1.03430.9738 132.30 0.6146 0.9283 2.7003 187.34 0.6541 1.04891.1876 142.78 0.6138 0.9425 2.9150 192.12 0.6616 1.06291.4040 150.86 0.6160 0.9576 3.1297 196.28 0.6690 1.07641.6223 158.52 0.6202 0.9733
w = 0.2011 (2-HEAB)0.1101 51.06 0.7250 0.9079 1.8256 185.18 0.5965 0.99510.3242 100.38 0.6357 0.8890 2.0424 191.48 0.6030 1.01320.5387 123.14 0.6030 0.8937 2.2584 196.90 0.6097 1.03060.7534 139.00 0.5888 0.9062 2.4742 202.68 0.6163 1.04730.9685 151.54 0.5835 0.9223 2.6875 207.84 0.6225 1.06301.1821 161.78 0.5833 0.9399 2.9018 211.58 0.6283 1.07791.3977 170.76 0.5861 0.9584 3.1478 216.04 0.6342 1.09381.6120 178.34 0.5907 0.9769
w = 0.3051 (2-HEAB)0.1228 66.74 0.7082 0.9022 1.6248 192.36 0.6306 1.03910.3404 112.32 0.6284 0.8925 1.8397 199.98 0.6408 1.06390.5532 135.80 0.6051 0.9079 2.0551 206.70 0.6502 1.08700.7659 152.26 0.5998 0.9313 2.2706 212.80 0.6580 1.10820.9793 164.96 0.6029 0.9578 2.4854 217.86 0.6639 1.12721.1958 175.88 0.6105 0.9856 2.7011 222.10 0.6675 1.14421.4106 184.92 0.6202 1.0129 2.9154 225.71 0.6686 1.1587
w = 0.3993 (2-HEAB)0.1245 81.92 0.6354 0.8601 1.8441 207.30 0.5043 1.07400.3408 122.66 0.5271 0.8352 2.0597 213.94 0.5058 1.10250.5554 145.92 0.4933 0.8540 2.2733 219.22 0.5039 1.12630.7694 161.88 0.4830 0.8876 2.4894 223.74 0.4981 1.14550.9833 174.08 0.4831 0.9264 2.7031 226.38 0.4884 1.15971.1989 184.20 0.4878 0.9666 2.9177 228.54 0.4746 1.16901.4145 192.64 0.4947 1.0055 3.1326 229.36 0.4568 1.17321.6303 200.40 0.5001 1.0415
aStandard uncertainties are u(m) = 0.0001 mol kg−1, ur(w) = 0.03, u(T) = 0.03 K, u(p) = 10 kPa, u(E) = 0.2 mV, u(γ±NaCl) = 0.05.
Journal of Chemical & Engineering Data Article
DOI: 10.1021/acs.jced.7b00278J. Chem. Eng. Data XXXX, XXX, XXX−XXX
C
(w). A densimeter (Rudolph Research Analytical DDM 2911)was used (0.00001 g cm−3). Figure 1 shows density valuesobtained by this work and Rocha-Pinto,30 with a differencearound 0.003. The difference may be addressed to slightdifferences in equipment precision, Besides, in the paper ofRocha-Pinto,30 it is not possible to establish the exact value ofpurity.3.2. Correlation Data. The Pitzer equations for the mean
activity coefficient are well-known38 (γ±) as
γ = + +γ γ γ± f mB m Cln 2
(2)
Figure 2. Activity coefficients of NaCl in the (2-HEAB + H2O) solventmixture at 298.15 K.
Figure 3. Osmotic oefficients of NaCl in the (2-HEAB + H2O) solventmixture at 298.15 K.
Table 4. Pitzer Parameters for the NaCl in the (2-HEAB +H2O) Solvent Mixture at 298.15 K and p = 101 kPaa
IL w β0 (kg mol−1) β1 (kg mol−1) Cγ (kg2 mol−2) σ
0.0000 0.07650 −0.05736 −0.01642 0.006150.0115 0.04989 0.04347 0.00440 0.022840.0502 0.14008 −0.05736 −0.01642 0.028460.1010 0.12091 −0.01335 −0.01142 0.008480.2011 0.14855 −0.15002 −0.01843 0.052150.3051 0.22569 −0.34356 −0.04063 0.010370.3993 0.34501 −1.14258 −0.08381 0.01707
aStandard uncertainties are ur(w) = 0.03, u(T) = 0.03 K, u(p) = 10kPa.
Table 5. Solvent Activity as and Excess Gibbs Free EnergiesGE
in the (2-HEAB +H2O)Mixture at 298.15 K and p = 101 kPaa
m(mol kg−1) as
GE
(kJ mol−1)m
(mol kg−1) asGE
(kJ mol−1)
w = 0.0115 (2-HEAB)0.1387 0.9975 −0.1415 1.8629 0.9658 −3.56930.3534 0.9936 −0.4799 2.0781 0.9616 −4.01970.5671 0.9897 −0.8709 2.2923 0.9574 −4.45990.7793 0.9859 −1.2881 2.5082 0.9531 −4.89220.9928 0.9820 −1.7256 2.7317 0.9486 −5.32541.2189 0.9778 −2.2004 2.9551 0.9440 −5.74141.4339 0.9738 −2.6575 3.1692 0.9395 −6.12181.6458 0.9699 −3.1091
w = 0.0502 (2-HEAB)0.1218 0.9980 −0.1293 1.5160 0.9736 −3.23000.2951 0.9952 −0.4296 1.6880 0.9702 −3.64240.4704 0.9923 −0.7884 1.8639 0.9667 −4.06490.6436 0.9894 −1.1709 2.0414 0.9631 −4.49170.8151 0.9865 −1.5645 2.2114 0.9596 −4.90210.9926 0.9833 −1.9812 2.3835 0.9560 −5.31941.1689 0.9801 −2.3998 2.5561 0.9523 −5.74091.3458 0.9768 −2.8223
w = 0.1010 (2-HEAB)0.1074 0.9981 −0.1111 1.8382 0.9663 −4.17100.3236 0.9945 −0.4978 2.0582 0.9618 −4.72390.5405 0.9908 −0.9679 2.2680 0.9574 −5.25080.7550 0.9871 −1.4707 2.4839 0.9527 −5.79300.9738 0.9831 −2.0041 2.7003 0.9480 −6.33691.1876 0.9791 −2.5354 2.9150 0.9433 −6.87861.4040 0.9750 −3.0781 3.1297 0.9385 −7.42341.6223 0.9707 −3.6274
w = 0.2011 (2-HEAB)0.1101 0.9978 −0.1253 1.8256 0.9610 −4.63220.3242 0.9937 −0.5498 2.0424 0.9557 −5.25610.5387 0.9895 −1.0670 2.2584 0.9503 −5.88330.7534 0.9852 −1.6279 2.4742 0.9449 −6.51730.9685 0.9806 −2.2133 2.6875 0.9394 −7.15361.1821 0.9760 −2.8071 2.9018 0.9338 −7.80521.3977 0.9711 −3.4141 3.1478 0.9274 −8.57201.6120 0.9661 −4.0220
w = 0.3051 (2-HEAB)0.1228 0.9976 −0.1505 1.6248 0.9635 −4.02890.3404 0.9933 −0.6026 1.8397 0.9578 −4.64160.5532 0.9890 −1.1251 2.0551 0.9520 −5.27340.7659 0.9844 −1.6797 2.2706 0.9460 −5.92980.9793 0.9795 −2.2514 2.4854 0.9401 −6.61531.1958 0.9744 −2.8401 2.7011 0.9341 −7.34261.4106 0.9690 −3.4308 2.9154 0.9282 −8.1114
w = 0.3993 (2-HEAB)0.1245 0.9970 −0.1936 1.8441 0.9465 −6.93600.3408 0.9921 −0.8034 2.0597 0.9389 −8.00760.5554 0.9869 −1.5442 2.2733 0.9314 −9.14790.7694 0.9812 −2.3471 2.4894 0.9239 −10.39690.9833 0.9750 −3.1887 2.7031 0.9167 −11.74381.1989 0.9683 −4.0684 2.9177 0.9097 −13.22551.4145 0.9613 −4.9821 3.1326 0.9030 −14.85591.6303 0.9539 −5.9379
aThe standard uncertainties for u(m) = 0.0001 mol kg−1, ur(w) = 0.03,u(T) = 0.03 K, u(p) = 10 kPa.
Journal of Chemical & Engineering Data Article
DOI: 10.1021/acs.jced.7b00278J. Chem. Eng. Data XXXX, XXX, XXX−XXX
D
In eq 2, the ion-interaction parameters were evaluated by usingmultiple linear regression technique. The calculations of theosmotic coefficient38 (ϕ) were given by
ϕ − = + +ϕ ϕ γf mB m C1 2(3)
The dielectric constant of (2-HEAB + H2O) solvent mixtureswere calculated by Wang and Anderko procedure.39,40 Equation
4 relates the polarization per unit volume of the fluid (P) to thedielectric constant (εr).
ε εε
=− +
P( 1)(2 1)
9r r
r (4)
Then, the Oster rule41 is used to calculate the polarization of themixture (Pm) in function of vi and Pi, the molar volume andpolarization of pure component i, respectively, in eq 5.
=∑
∑=
=
Px vP
x viN
i i i
iN
i im
1
1 (5)
In eq 5 supposes a zero volume change upon mixing. Therefore,the calculation of the dielectric constant by interactionmethod ofthe 2-HEAB+ H2O solvent mixture becomes equivalent to thecalculation of polarization, Pm and Pi.
42
The cell potential data (E), molalities (m) for each system,activity coefficients of NaCl in the solvent mixture, and theosmotic coefficients are listed in Table 3.Figure 2 shows that the values of γ± are affected by the increase
of 2-HEAB content in the solvent mixture. At 0.7700 mol·kg−1,the γ± for the mixed 2-HEAB/water systems has a negative trend:2-HEAB/water 0.40 > 2-HEAB/water 0.20 > 2-HEAB/water0.30 > 2-HEAB/water 0.05 > 2-HEAB/water 0.01 > 2-HEAB/water 0.10. The decrease of the γ± (NaCl) may be the result ofthe variation in the dielectric constant of the (2-HEAB + H2O)mixture solvent from 78.08 to 72.69 where the addition of NaClincreases the electrostatic interactions. In (NaCl + 2-HEAB +H2O) mixtures, structures clathrate-like may be present for theminimum of the curves. Head-Gordon43 found the solvation ofnonpolar groups by hydrogen-bonded.In Figure 3 the osmotic coefficients show a minimum of the
curves varying between 0.32 and 0.34 m at w = 0.4. Then wefound that the osmotic coefficient increased with the NaClincrement. One of the problems at higher w of IL is NaCl saltingout. It may be possible that, after clathrate-like formation, thisstructure disappears as the NaCl concentration increase due tocompetition between the ions of NaCl and 2-HEAB.In Table 4, β0, β1, and Cγ with the standard deviation (σ) are
listed. β0 and β1 are nearly dependent on w. However, Cγ is nearly
independent of w.3.3. Activity of Solvent (as) and Excess Gibbs Free
Energy (GE) by the Pitzer Model. In this work, the solvent isthe mixture of (2-HEAB + H2O) in weight fraction (w = 0.01,0.05, 0.10, 0.20, 0.30, and 0.40). Following the methodology,25
the molar weight of (2-HEAB + H2O) was calculated for eachweigh fraction as Ms = X1M1 + X2M2 and M1= 149.1855 g/molandM2 = 18.0151 g/mol. In Table 5, GE and as are calculated by
ϕ γ= − +G RT v m[ (1 ln )]EA A A (6)
∑ϕ= −=
⎡⎣⎢⎢
⎤⎦⎥⎥a M
mexp
1000i
Ni
s s1 (7)
Figure 4 shows that solvent activity is reduced by increasingmolalities of NaCl in the ternary system. Schroder et al.44
considered that the values ofGE are produced by the electrostaticinteraction among the IL, anions, cations, and water. In Figure 5,as shown on Table 5, the negative value to GE increases withwater at w = 0.4; this may be produced by the interaction of theOH group in the anion. Figure 6 shows that increments of thenegative values ofGE depend on the increase of 2-HEAB content
Figure 4. as for the (2-HEAB + H2O) solvent mixture at T = 298.15 K.
Figure 5. Excess free Gibbs energy GE of NaCl in the (2-HEAB + H2O)solvent mixture at T = 298.15 K.
Figure 6. Hydrogen bond number along 0.5 ns of classical moleculardynamic simulations for w = 0.4.
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DOI: 10.1021/acs.jced.7b00278J. Chem. Eng. Data XXXX, XXX, XXX−XXX
E
in the (2-HEAB + H2O) solvent mixture. In a previous paper, wefound that ILs like 1-ethyl-3-methyl-imidazolium methanesulfo-nate, [Emim][MeSO3], or 1-ethyl-3-methyl-imidazolium ethylsulfate, ([Emim][EtSO4] + H2O), have an decreasing effect ofthe NaCl activity coefficient.1
As proposed by the structural and electrostatic model,45 thevalues of GE of the system NaCl in the (2-HEAB + H2O) solventmixture may be mainly controlled by electrostatic competitiveinteraction.46
3.4. Molecular Dynamics. All classical molecular dynamicsimulations were performed using GROMACS 5.0.7 at w = 0.4along 0.5 ns. Electrostatic interactions were calculated using thesmooth particle mesh Ewald summation with a real space cutoffof 0.6 nm. Lennard−Jones interactions were calculated using a12−6 potential function with a cutoff radii of 0.6 nm. Simulationswere carried out with a single ionic liquid molecule surroundedby 127 TIP3 water molecules into a cubic box with an edge lengthof approximately 1.5 nm. For all cases, standard periodicboundary conditions in all directions were considered. At thebeginning, 90 000 steps of steepest descent minimization wereperformed to get an energyminimum and elsewise to remove anybad intra−intermolecular bad contact. Molecular simulationswere driven in three steps: (i) initial short 50 ps NVT ensemblesimulation with a constant temperature of 298.15 K using themodified Berendsen thermostat with update frequencies of 0.4ps; (ii) short 50 ps isobaric−isothermal (NpT) ensemblesimulation at 298.15 K and 1 bar. The pressure control forNpT was achieved using a Parrinello−Rahman barostat withcoupling time of 2 ps; (iii) long 500 ps NpT ensemble using thesame above condition to final analysis. All bonds and angles werekept fixed using the LINCS algorithm.47−51 The variations ofhydrogen bond numbers for IL + water suggest that the numberof hydrogen bonds are higher in anion (butyrate) than in thecations as shown in Figure 6 for w = 0.40.A molecular dynamic snapshop at 125, 250, 375, and 500 ps
along NpT ensemble that the IL (2-HEAB) has an effect over thestructure of water as shown in Figure 7.
4. CONCLUSIONThe γ± of NaCl in mixtures of the (2-HEAB + H2O) solventmixture was calculated by studied by the electrode potentialmethod using Na-ISE and Cl-ISE at 298.15 K. The activitycoefficients of NaCl in pure aqueous solution and in the (2-HEAB + H2O) solvent mixture were measured by galvanic cellswithout liquid junction. The measurements were made atdifferent weight fractions (w) of 2-HEAB. The addition of 2-HEAB into the solvent (water/water + 2-HEAB) increases theelectrostatic attraction between [2-HEA]+ and [B]− in thesolvent mixture due to the decrease in the dielectric constant.The presence of NaCl has a competitive effect with 2-HEAB forwater molecules. At the higher concentration of w = 0.40, NaCl is
salting out by 2-HEAB in water. This suggests that 2-HEAB mayhave future uses in water desalting. The parameters of the Pitzermodel β0, β1, and Cγ were calculated by the multiple linearregression method. The values of GE for NaCl in mixtures of (2-HEAB + H2O) are negative. This suggests that the electrostaticinteractions between the 2-HEAB and NaCl are dominant. Thebehavior of γ± (NaCl) and ϕ was discussed by the clathrate-likeformation of (2-HEAB +H2O). Atw = 0.4 the maximum numberof hydrogen bonds calculated by molecular dynamic simulationsis around 12 between the IL and water.
■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jced.7b00278.
Data for calibration of Na+ and Cl− selective electrode pairat 298.15 K, ATR-FTIR spectra for 2-HEAB, and the 1HNMR for the 2-HEAB (PDF)
■ AUTHOR INFORMATIONCorresponding Author*Tel.: +51 7 3114621948. E-mail: [email protected] Amado-Gonzalez: 0000-0003-4523-1323Wilfred Gomez-Jaramillo: 0000-0003-3645-4007FundingThis project was supported by University of Pamplona. The firstauthor is grateful to Escola d’Engyneria, University of Santiago deCompostela, for a scholarship and Dr. Miguel Angel Iglesias foracceptance in the research group. Financial support fromUniversity of Pamplona PR400-156.012-008 (GA313-BP-2015) is acknowledged.NotesThe authors declare no competing financial interest.
■ LIST OF SYMBOLSE:cell potentialGE:excess Gibbs free energym:molalityM:molecular weightPm:polarizationw:molar fractionxi:mole fraction of component
Greek Lettersαs:solvent activityρ:densityϕ:osmotic coefficientγ:activity coefficientεr:dielectric constant
Figure 7. Molecular dynamic snapshot along the NpT ensemble.
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vi:molar volume
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