Performance of Sodium Chloride versus Commercial Ionic Liquid as

Article pubs.acs.org/IECR

Performance of Sodium Chloride versus Commercial Ionic Liquid as Salting-Out Media for the Separation of Nicotine from Its Aqueous Solutions Joana M. Lopes,† Ana V. M. Nunes,† Manuel Nunes da Ponte,† Zoran P. Visak,*,‡ and Vesna Najdanovic-Visak*,§ †

REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal ‡ Centro Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal § Energy Lancaster, Engineering Department, Faculty of Science and Technology, Lancaster University, LA1 4YW Lancaster, United Kingdom S Supporting Information *

ABSTRACT: In this work, the performance of inorganic salt sodium chloride and ionic liquid (molten salt) ECOENG212 as salting-out media for the separation of nicotine from its aqueous solutions was examined. In this respect, liquid−liquid equilibria of the ternary solutions nicotine + water + NaCl and nicotine + water + ECOENG212 were determined at ambient pressure, 0.1 MPa, and humidity at three temperatures. The related phase diagrams were constructed by means of both cloud points and chemical analysis of phases in equilibrium (tie-line data). The latter were used to calculate two important separation parameters: partition coefficients of nicotine and separation factors. The effects of the initial compositions of the solutions and of the temperature on the phase behavior and partition coefficients were analyzed and discussed. The results obtained, particularly in the case of sodium chloride as a salt, clearly showed that both investigated salts provided good salting-out media for the efficient and sustainable separation of nicotine from solutions with water.

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INTRODUCTION Nicotine is a widely known and extremely toxic alkaloid (lethal dosage 30−60 mg1). It can enter the environment through aqueous2 and solid3 wastes from the tobacco industry as well as from some pharmacological products.4 Because it is completely soluble in water at temperatures lower than 61 °C,5 nicotine is easily transported from the dust to groundwater6−8 and from pharmacological products to urban wastewater.9,10 Therefore, the need for nicotine separation/extraction is emerging, and for this purpose, liquid−liquid extraction using organic solvents, such as toluene, kerosene, and hexane, has been widely applied.6 Meanwhile, studies of alternative approaches that avoid the use of those volatile organic compounds have been gaining importance: microbiological degradation,11,12 absorption,13,14 application of aqueous biphasic systems based on hydrophilic ionic liquids and inorganic salts,15 and extraction by hydrophobic ionic liquids.16 In our previous study,17 we examined the possibility of more sustainable treatment of nicotine solutions by applying ionic liquids as environmentally friendly solvents. Later18,19 we focused on phase separations−salting-out effectsin nicotine aqueous solutions due to the addition of inorganic salts or ionic liquids. As a continuation, in this work, the separation of nicotine from water by the direct addition of inorganic salts or ionic liquids as salting-out media was investigated. Thus, liquid−liquid equilibria, at 0.1 MPa pressure and at three temperatures, of the ternary solutions (nicotine + water + salt) were determined, where salt refers to inorganic salt sodium © 2014 American Chemical Society

chloride (NaCl), ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate, or ECOENG212 [C2mim][EtSO4] (IUPAC name 1-ethyl-3-methyl-1H-imidazol-3-ium ethylsulfate). For these systems, ternary liquid−liquid equilibrium data are reported and the related phase diagrams constructed. The partition coefficients of nicotine and the separation factors of nicotine from water were calculated from the obtained tie-line data. The results show a very good performance of both salts with obtained separation factors ranging from 16.2 to 224.3 for NaCl and from 7.2 to 111.6 for ECOENG212.

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EXPERIMENTAL SECTION Materials. Table 1 shows names, abbreviations, molecular structures, sources, and estimated purities of all the compounds used in this work. Nicotine was used without further purification. Karl-Fischer coulometric titration (Metrohm 831 KF coulometer) revealed 150 ppm of water. Purification of 1-ethyl-3-methylimidazolium ethylsulfate (ECOENG212) was carried out by the usual procedure (see Table 1), applied as well in the previous studies.17−19 KarlFischer coulometric titration registered 50 ppm of water in ECOENG212. Thus, the water content, either in nicotine or in ECOENG212, was not accounted for in the subsequent Received: Revised: Accepted: Published: 9883

February 5, 2014 May 8, 2014 May 9, 2014 May 9, 2014 dx.doi.org/10.1021/ie500514y | Ind. Eng. Chem. Res. 2014, 53, 9883−9888

Industrial & Engineering Chemistry Research

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Table 1. Chemicals Used in This Work

Figure 1. Cloud-point data for the ternary solutions nicotine + water + NaCl (a) and nicotine + water + ECOENG212 (b) and at 0.1 MPa pressure and at temperatures 298.2 K (filled circles), 323.2 K (filled triangles), and 343.2 K (filled diamonds). The lines given in both graphs are only a guide to the eye.

Methods. All of the liquid mixtures were prepared gravimetrically using a Kern 770 balance (Kern & Sohn GmbH, Balingen, Germany) with a precision of ±0.0001 g. Ternary solutions containing water, nicotine, and NaCl (or ECOENG212) were placed in septum-sealed conical glass vessels, with continuous stirring enabled. Cloud points were determined by the well-known synthetic method, based on titration at a constant temperature, as was already described in the literature.23,24 Briefly, a binary mixture of known composition was titrated with the third component at a constant temperature. The cloud point is taken as the appearance/disappearance of turbidity in the sample. After the turbidity was observed, final mixtures were weighed in order

preparation of the aqueous solutions. The stated chloride content in the ionic liquid was 404 ppm. As in our previous studies17,19 and according to recommendations from lthe iterature,20 the stated chloride contents were taken as acceptable and no further reduction in this respect was attempted. Considering the possibility of anion hydrolysis,21 we performed 1H NMR analysis (Bruker ARX 400 NMR spectrometer) of the used ECOENG212. The spectra showed no changes in the anion structure compared to the literature.22 NaCl with a stated purity of 99.5 mass % was purchased from Panreac and used without further purification. Distilled water, deionized using a Milli-Q water filtration system from Millipore, was used for preparation of the solutions. 9884

dx.doi.org/10.1021/ie500514y | Ind. Eng. Chem. Res. 2014, 53, 9883−9888


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to calculate the composition corresponding to the cloud-point composition. The tie-lines were determined according to the procedure also explained in detail previously;25 herein only a concise description follows. Biphasic ternary [water + nicotine + salt (NaCl or ECOENG212)] mixtures of known composition were agitated for at least 3 h, at constant, controlled temperatures of 298.2, 313.2, and 343 K. Next, the mixtures were left still for at least 12 h at a given temperature to allow complete phase separation. Using a syringe (Hamilton, Bonaduz, Switzerland), samples from both phases were taken into the vials and prepared for further analysis in order to determine their composition. A DU 800 spectrophotometer (Beckman Coulter, Carnaxide, Portugal) was used to determine the nicotine concentrations in both phases, using the maximum absorption peak at 259 nm. Nicotine concentrations were determined according to the method of Willits et al.26 A calibration curve was generated, and all of the experiments were performed at least twice; the average of the obtained values is given. The amount of water was measured using a Karl-Fischer model 831 coulometer and model 728 stirrer (both from Metrohm, Herisau, Switzerland). The solutions were prepared with 1% sample in acetonitrile. The apparatus gives the result of the amount of water in milligrams per gram. In order to quantify potential mutual contamination of the phases during sampling from the bottom phase, four random experiments were selected and repeated at least 10 times. The average reproducibility in the compositions (mole fractions of nicotine and water) was 0.002. In all of the experiments, a water bath with a built-in temperature controller was used to keep the temperature constant, and the temperature was monitored with a mercury thermometer, with an accuracy of ±0.1 K. Calculations. The tie-line compositions were used to calculate the partition coefficients (Ki) according to the relationship Ki =

xitop phase xibottom phase

phase xtop i

(1) phase xbottom i

where and are the mole fractions of component i (nicotine or water) in the top (nicotine-rich) and bottom (water-rich) phases, respectively. The separation factor (α) of nicotine and water was calculated as the ratio of their distribution coefficients: α=

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K nicotine K water

(2)

RESULTS AND DISCUSSION Tables S1 and S2 (Supporting Information, SI) present cloudpoint and equilibrium tie-line data for the ternary solutions [nicotine + water + salt (NaCl or ECOENG212)] at 0.1 MPa pressure and three temperatures. Experimental cloud-point and tie-line data are presented in Figures 1−3. Figure 2 gives the ternary phase diagrams with the tie-line data included, at three temperatures, for the solution nicotine + water + ECOENG212. It can be observed from the presented figures that the mutual solubility in the solution with NaCl as a salt is moderately enhanced by a temperature decrease. This was expected because the binary system nicotine + water exhibits a lower-

Figure 2. Phase diagrams for the ternary mixture nicotine + water + ECOENG212 at different temperatures in mole fraction: (a) 298.2 K; (b) 323.2 K; (c) 343.2 K. Filled and empty circles represent cloudpoint and tie-line compositions, respectively. The lines are only given as a guide to the eye.

critical-solution-temperature phase behavior, as a part of its well-known epitome closed-loop immiscibility.2,27−30 This, in other words, denotes that the solutions approach phase demixing and finally separate with a temperature increase. Therefore, a smaller quantity of NaCl salt is needed to induce phase splitting at higher temperatures (see Figure 1a). However, in the case of solutions with the ionic liquid, the 9885



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Figure 3. Comparison of the ternary phase diagrams at 343.2 K. Full and dashed lines represent guides to the eye for nicotine + water + NaCl and nicotine + water + ECOENG212, respectively. Filled crosses stand for intersections of the tie lines of the two systems.

In the case of the solutions with NaCl as a salt, the maximum values of Knic and α were observed for the composition xnic = 0.155 and xNaCl = 0.035 at 323.2 K; see Figure 4a. These parameters increase with increasing content of nicotine and NaCl at 298.2 K. Nevertheless, at the other two isotherms, this dependence is 2-fold: the initial increase is followed by a decline. Separation factors reaching to 224 indicate a very efficient separation capability of NaCl. In general, ionic liquids are both kosmotropic and chaotropic salts.31 This dual behavior is a reflection of another duality that is in their structure: the existence of polar (hydrophilic) and nonpolar (hydrophobic) domains.32 Therefore, the impact of the addition of ECOENG212 to nicotine aqueous solutions is more complex than that in the case of NaCl:19 initially, at lower concentrations, the ionic liquid provokes a salting-in effect, promoting solubility and raising the temperature of phase demixing; it is only after a certain concentration of the ionic liquid is reached (approximately 6 mol %) that the salting-out effect starts to occur. However, ECOENG212 has one favorable feature: it is completely miscible with water, thus overcoming the problem of a limited aqueous solubility of inorganic salts (approximately 0.1 mole fraction of NaCl in water at room temperature) and avoiding precipitation. The aforementioned duality is most probably the reason for much lower partition coefficients of nicotine in the case of solutions with ECOENG212 compared to those with NaCl. These coefficients depend as well on both the temperature and the total composition of the solutions. Figure 4b shows that the temperature dependence of Knic is nearly linear. Thus, at lower nicotine/ECOENG212 compositions, the values of Knic increase with the temperature (Knic = 0.1497T − 38.991, with R2 = 0.9934), following the trend of solubility in the studied ternary solutions (see above). However, the linear trend is the opposite (decreasing; Knic = −0.0597T + 28.253, with R2 = 0.9927) at higher compositions. With respect to the

aforesaid temperature-solubility effect stands only for the ECOENG212-rich solutions (see Figure 1b). This behavior is due to the facts that we revealed in the previous study:17 (i) ECOENG212 has upper-critical-solution-temperature phase behavior with nicotine phase demixing as the temperature is lowered and (ii) the solubility is much higher in the ECOENGrich solutions.17 The partition coefficients of nicotine, Knic, and the separation factors, α, for the studied solutions are calculated according to eqs 1 and 2, included in Tables S1 and S2 in the SI. The partition coefficients are presented graphically in Figure 4a,b as functions of the total composition and temperature for solutions containing NaCl and ECOENG212, respectively. One can see from the figures that both of these parameters are greatly influenced by the temperature and total composition of the solutions, despite the aforementioned slight influence of the temperature on the mutual solubility. For the sake of comparison, Figure 3 was constructed including ternary phase diagrams for nicotine + water + NaCl and nicotine + water + ECOENG212 at 343.2 K. This direct comparison is not possible to perform at the other two temperatures (298.2 and 323.2 K) because two-phase regions with liquid−liquid equilibria of the two systems do not overlap. Two interception points are recognized from Figure 3: first with the total composition xwater = 0.810, xnic = 0.125, and xsalt = 0.065 and second with xwater = 0.650, xnic = 0.320, and xsalt = 0.030. In the case of the system involving NaCl, both mixtures are placed in the single tie line, reaching values of the partition coefficient of nicotine and separation factors of 53.4 and 93.3, respectively. Significantly smaller partition coefficients of nicotine and separation factors were obtained for the two mixtures: Knic = 8.5 and α = 17.8 for the first total composition xwater = 0.810, xnic = 0.125, and xECOENG = 0.065 and Knic = 7.2 and α = 13.3 for the second total composition xwater = 0.810, xnic = 0.125, and xsalt = 0.065. 9886



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lowest temperature of 298.2 K, at the same time being added in very low quantities. Ionic liquid ECOENG212 provided much lower, but yet very good partition coefficients, as well as high separation factors. Also, being a molten salt, completely soluble in water, ECEOENG212 has an advantage that it can be added to nicotine aqueous solutions without any limits. Finally, both salts studied herein are not expensive compounds. Thus, the sustainability of the application of these two salts is not only in their positive environmental impact but in their economical aspect as well. Ultimately, the results presented in this paper are encouraging further investigation of the selectivity of ionic liquids for the recovery of important bioactive (pharmaceutical) and polymer carrier molecules.

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ASSOCIATED CONTENT

S Supporting Information *

Results of cloud points, tie-line equilibrium data, partition coefficients, and separation factors and obtained parameters of eq 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Fundaçaõ para a Ciência e a Tecnologia (FCT; Lisboa, Portugal) through Project PTDC/ EQU-EPR/103505/2008. A.V.M.N. is thankful to the FCT for the postdoctoral fellowship SFRH/BPD/74994/2010.

Figure 4. Partition coefficient of nicotine, Knic, in the ternary solutions nicotine + water + NaCl (a) and nicotine + water + ECOENG212 (b) as a function of the total composition at 0.1 MPa pressure and three temperatures: 298.2 K (solid fill), 323.2 K (pattern fill), and 343.2 K (white fill).

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REFERENCES

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composition, higher nicotine/ECOENG212 compositions almost doubled Knic at 298.2 K, while at 343 K, the increase in these compositions lowered Knic by nearly 40%. The behavior at the lower temperature seems quite reasonable: more ECOENG212 provides better water structuring and more efficient separation of nicotine from water. However, the behavior at 343.2 K is far more intriguing. An explanation might be found in the closed-loop phase diagram of the binary solution nicotine + water.5 Namely, the point related to the solution at 343.2 K, with a concentration of nicotine of 0.542 by mole fraction, is further away from the binodal curve than the point referring to the solution having 0.347 mole fractions of nicotine, at the same temperature. Thus, the latter solution is more favorable for the separation of nicotine.

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CONCLUSION The results obtained herein clearly point out ionic liquid ECOENG212 and, particularly, NaCl as good and efficient salting-out media for the sustainable separation of nicotine from its aqueous solutions. The inorganic salt provided extremely high partition coefficients and very favorable separation factors, even at the 9887



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Performance of Sodium Chloride versus Commercial Ionic Liquid as

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