Mean Activity Coefficients of NaCl in the Mixture of ... - ACS Publications

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Mean Activity Coefficients of NaCl in the Mixture of 2‑Hydroxyethylammonium Butyrate + H2O at 298.15 K Eliseo Amado-González,* Irina Lupita González-Gutierrez, and Wilfred Goméz-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 a series of electrochemistry studies of mixtures of ionic liquids and water as a solvent to provide accurate data for future particular applications [Amado-Gonzalez et al. J. Chem. Eng. Data 2017, 62, 752]. The mean activity coefficients for NaCl in [2-hydroxyethylammonium butyrate (2-HEAB) + H2O] as a solvent mixture were determined by cell potential measurements: Na−ion selective electrode (ISE)|NaCl (m), 2-HEAB (w), H2O (1 − w)|Cl−ion selective electrode (ISE) at molalities from 0.10 to 3.20 mol·kg−1 at 298.15 K. Different weight fractions (w) of 2-HEAB with w = 0.01, 0.05, 0.1, 0.2, 0.3, and 0.4 were used. At higher concentrations of w = 0.40, NaCl is salting out by 2HEAB 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. The results may be interpreted by the clathrate-like formation of 2-HEAB + water. A qualitative description of the relation between water 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 as proposed by Lowe and Rendall.2 Due to the physicochemical properties of ILs,3−5 they may have many different industrial uses.6 Activity coefficients and osmotic coefficients of electrolyte solutions are extremely useful to test new electrolyte solution theories.7−15 Even though activity coefficients of ammonium ILs have been reported in aqueous solutions by the isopiestic method,16−23 to our knowledge, research papers about the activity coefficients of NaCl in ILs + water are still scarce, and studies of mean ionic activity coefficients of NaCl reported in the literature for ternary liquid mixtures are still limited.24−26 Because of the high costs of ILs and import restrictions, the synthesis of new ILs should be a goal. Protic ionic liquids offer a good possibility because they can be produced by a simple acid− base neutralization reaction.27 2-Hydroxyethylammonium butyrate (2-HEAB) is therefore evaluated for its potential abilities as a solvent. In this sense, our goal is to evaluate how the thermodynamic properties of salts like NaCl would be affected by the (2-HEAB + H2O) solvent mixture. In this work, we found the activity coefficients of NaCl in the mixture (2-HEAB + H2O) by cell potential measurements at 298.15 K. The results were fitted to the Pitzer model. Then, the osmotic coefficients, solvent mixture activities, and excess Gibbs free energies of these systems were calculated. A calculation of hydrogen interaction at w = 0.4 was completed using the general AMBER force field (GAFF). © 2017 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials, Synthesis, and Measurements. In Table 1 the chemicals, suppliers, and stated purity are shown. The IL, 2HEAB was synthesized by acid−base neutralization reactions between ethanolamine and the organic acid as previously described.26 The reagents monoethanolamine and butyric acid (Sigma-Aldrich, 99%) are analytical (AR grade) reagents. The synthesis of 2-HEAB was done as reported by literature.28−30 Strong agitation was performed at heating at 323 K. The IL was dried under a vacuum of 20 kPa during 8 h until ΔpH (pH at the equivalent point − pH experimental) < 0.07. An ATR-FTIR spectra for 2-HEAB was done. The ammonium structure was found at 3200−2400 cm−1. The OH stretching vibration was embedded in this band. The carbonyl stretching and N−H inplane bending vibrations were at 1600 cm−1. Water content was verified by a volumetric titroline KF (Schott instruments) as 6 × 10−4 mass fraction. No phase change of ILs was observed. The analysis of 1H NMR gave the next results for the 2-HEAB: (400 MHz, 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 a purity higher that 99% mass fraction (details available in the Supporting Information). Received: March 18, 2017 Accepted: July 10, 2017 Published: August 1, 2017 2384

DOI: 10.1021/acs.jced.7b00278 J. Chem. Eng. Data 2017, 62, 2384−2391

Journal of Chemical & Engineering Data

Article

Table 1. Chemicals, Suppliers, and Method To Check the Stated Puritya chemical name NaCl butyric acid ethanolamine 2-HEABb

source

purity

Sigma-Aldrich, Riedel-de Haën, p.a., Reag. ACS CAS no. 7647-14-5 Merck, CAS no.107-92-6 Merck, CAS no.141-43-5 synthesis

99.8%

no further purification

99.0% 99.5% 97.0%

no further purification no further purification Fischer-Karl titration 1H RMN, ATR-FTIR spectra double distillation

water a

purification method

mass water contenta

electrical conductivity (S·cm−1)

≤0.02% ≤0.09% ≤0.06% 2-HEAB/water 0.20 > 2-HEAB/water 0.30 > 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 of the variation in the dielectric constant of the (2-HEAB + H2O) mixture solvent from 78.08 to 72.69 where the addition of NaCl increases the electrostatic interactions. In (NaCl + 2-HEAB + H2O) mixtures, structures clathrate-like may be present for the minimum of the curves. Head-Gordon43 found the solvation of nonpolar 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 we found that the osmotic coefficient increased with the NaCl increment. One of the problems at higher w of IL is NaCl salting out. It may be possible that, after clathrate-like formation, this structure disappears as the NaCl concentration increase due to competition 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 is the 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 each weigh fraction as Ms = X1M1 + X2M2 and M1= 149.1855 g/mol and M2 = 18.0151 g/mol. In Table 5, GE and as are calculated by

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 molecular dynamic simulations for w = 0.4.

GE = RT[vAmA (1 − ϕ + ln γA )]

(6)

N ⎡ mi ⎤ ⎥ as = exp⎢ −ϕMs ∑ ⎢⎣ 1000 ⎥⎦ i=1

(7)

Figure 4 shows that solvent activity is reduced by increasing molalities of NaCl in the ternary system. Schröder et al.44 considered that the values of GE are produced by the electrostatic interaction among the IL, anions, cations, and water. In Figure 5, as shown on Table 5, the negative value to GE increases with water at w = 0.4; this may be produced by the interaction of the OH group in the anion. Figure 6 shows that increments of the negative values of GE depend on the increase of 2-HEAB content

In eq 2, the ion-interaction parameters were evaluated by using multiple linear regression technique. The calculations of the osmotic coefficient38 (ϕ) were given by ϕ − 1 = f ϕ + mBϕ + m2C γ

∑i = 1 xivP i i

(3)

The dielectric constant of (2-HEAB + H2O) solvent mixtures were calculated by Wang and Anderko procedure.39,40 Equation 2388



Article

Figure 7. Molecular dynamic snapshot along the NpT ensemble.

salting out by 2-HEAB in water. This suggests that 2-HEAB may have future uses in water desalting. The parameters of the Pitzer model β0, β1, and Cγ were calculated by the multiple linear regression method. The values of GE for NaCl in mixtures of (2HEAB + H2O) are negative. This suggests that the electrostatic interactions between the 2-HEAB and NaCl are dominant. The behavior of γ± (NaCl) and ϕ was discussed by the clathrate-like formation of (2-HEAB + H2O). At w = 0.4 the maximum number of hydrogen bonds calculated by molecular dynamic simulations is around 12 between the IL and water.

in the (2-HEAB + H2O) solvent mixture. In a previous paper, we found that ILs like 1-ethyl-3-methyl-imidazolium methanesulfonate, [Emim][MeSO3], or 1-ethyl-3-methyl-imidazolium ethyl sulfate, ([Emim][EtSO4] + H2O), have an decreasing effect of the NaCl activity coefficient.1 As proposed by the structural and electrostatic model,45 the values of GE of the system NaCl in the (2-HEAB + H2O) solvent mixture may be mainly controlled by electrostatic competitive interaction.46 3.4. Molecular Dynamics. All classical molecular dynamic simulations were performed using GROMACS 5.0.7 at w = 0.4 along 0.5 ns. Electrostatic interactions were calculated using the smooth particle mesh Ewald summation with a real space cutoff of 0.6 nm. Lennard−Jones interactions were calculated using a 12−6 potential function with a cutoff radii of 0.6 nm. Simulations were carried out with a single ionic liquid molecule surrounded by 127 TIP3 water molecules into a cubic box with an edge length of approximately 1.5 nm. For all cases, standard periodic boundary conditions in all directions were considered. At the beginning, 90 000 steps of steepest descent minimization were performed to get an energy minimum and elsewise to remove any bad intra−intermolecular bad contact. Molecular simulations were driven in three steps: (i) initial short 50 ps NVT ensemble simulation with a constant temperature of 298.15 K using the modified Berendsen thermostat with update frequencies of 0.4 ps; (ii) short 50 ps isobaric−isothermal (NpT) ensemble simulation at 298.15 K and 1 bar. The pressure control for NpT was achieved using a Parrinello−Rahman barostat with coupling time of 2 ps; (iii) long 500 ps NpT ensemble using the same above condition to final analysis. All bonds and angles were kept fixed using the LINCS algorithm.47−51 The variations of hydrogen bond numbers for IL + water suggest that the number of hydrogen bonds are higher in anion (butyrate) than in the cations 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 the structure of water as shown in Figure 7.

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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.7b00278. Data for calibration of Na+ and Cl− selective electrode pair at 298.15 K, ATR-FTIR spectra for 2-HEAB, and the 1H NMR for the 2-HEAB (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +51 7 3114621948. E-mail: [email protected]. ORCID

Eliseo Amado-González: 0000-0003-4523-1323 Wilfred Goméz-Jaramillo: 0000-0003-3645-4007 Funding

This project was supported by University of Pamplona. The first author is grateful to Escola d’Engyneria, University of Santiago de Compostela, for a scholarship and Dr. Miguel Angel Iglesias for acceptance in the research group. Financial support from University of Pamplona PR400-156.012-008 (GA313-BP2015) is acknowledged. Notes

The authors declare no competing financial interest.

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4. CONCLUSION The γ± of NaCl in mixtures of the (2-HEAB + H2O) solvent mixture was calculated by studied by the electrode potential method using Na-ISE and Cl-ISE at 298.15 K. The activity coefficients of NaCl in pure aqueous solution and in the (2HEAB + H2O) solvent mixture were measured by galvanic cells without liquid junction. The measurements were made at different weight fractions (w) of 2-HEAB. The addition of 2HEAB into the solvent (water/water + 2-HEAB) increases the electrostatic attraction between [2-HEA]+ and [B]− in the solvent mixture due to the decrease in the dielectric constant. The presence of NaCl has a competitive effect with 2-HEAB for water molecules. At the higher concentration of w = 0.40, NaCl is

LIST OF SYMBOLS E:cell potential GE:excess Gibbs free energy m:molality M:molecular weight Pm:polarization w:molar fraction xi:mole fraction of component

Greek Letters

αs:solvent activity ρ:density ϕ:osmotic coefficient γ:activity coefficient εr:dielectric constant

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vi:molar volume

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