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Phase segregation in aqueous biphasic systems (ABS) composed of four hydrophilic ionic liquids (ILs): 1-butyl-3-methylimidazolium methylsulfate and 1-ethyl-3-methylimidazolium methylsulfate (C_{n}C_{1}im C_{1}SO_{4}, _{4441} C_{1}SO_{4}) and methylpyridinium methylsulfate (C_{1}Py C_{1}SO_{4}) and two high charge density potassium inorganic salts (K_{2}CO_{3} and K_{2}HPO_{4}) were determined by the cloud point method at 298.15 K. The influence of the addition of the selected inorganic salts to aqueous mixtures of ILs was discussed in the light of the Hofmeister series and in terms of molar Gibbs free energy of hydration. The effect of the alkyl chain length of the cation on the methylsulfate-based ILs has been investigated. All the solubility data were satisfactorily correlated to several empirical equations. A pesticide (pentachlorophenol, PCP) extraction process based on the inorganic salt providing a greater salting out effect was tackled. The viability of the proposed process was analyzed in terms of partition coefficients and extraction efficiencies.

Organochlorine micropollutants such as insecticides (lindane, heptachlor, DDT,

One of the viable alternatives for pesticides removal from aqueous solutions is the use of liquid-liquid extraction. In this sense, the strategies based on aqueous biphasic systems (ABS) where a complex competition between the polymers or salts for the water molecules [

Nowadays, greener alternatives have been developed, and ionic liquids (ILs)-based ABS have been reported since 2003, when the first work on this topic was published [

Up to date, PCP extraction was tackled only for hydrophobic ILs [

Methylsulfate-imidazolium-based ILs were chosen as models for this work since they belong to one of the most widely used families of ILs. Furthermore, they exhibit moderate viscosity, chemical stability, and low melting point temperature [_{2}HPO_{4}), and potassium carbonate (K_{2}CO_{3}). The inorganic salts were selected due to their different degrees of kosmotropicity, thus licensing to map the immiscibility region. The experimental solubility curves of the hydrophilic ILs were correlated through several empirical models and the Effective Excluded Volume theory (EEV) and the results were discussed in terms of standard deviations. The proposed systems were used to elucidate their potential to extract pentachlorophenol as model persistent organic pollutant.

The experimental binodal curves for the ternary mixtures composed of aqueous solutions of the ILs (C_{n}C_{1}im C_{1}SO_{4}, _{4441} C_{1}SO_{4} and C_{1}Py C_{1}SO_{4}) and inorganic salts (K_{2}CO_{3} and K_{2}HPO_{4}) are summarized in _{2}C_{1}im C_{1}SO_{4} and K_{2}HPO_{4} are available and a very good agreement is obtained.

A visual inspection of the experimental curves reveals that it is possible to analyze the role of the potassium salts as phase promoters in aqueous solutions of methylsulfate-based ILs from the point of view of the inorganic cation and the molar entropy of hydration. The size and type of the IL cation was also used to understand the observed phase segregation.

Two empirical three-parameter equations [

where _{1} and _{2} are the IL and potassium inorganic salt mass composition, respectively, and

The SOLVER function provided by Microsoft EXCEL was the tool used to fit the constants so that the objective function was minimized. The standard deviations were calculated by applying the following expression:

where the property values and the number of experimental and adjustable data are represented by _{DAT}, respectively.

The values of the coefficients obtained from the correlation of the experimental data along with the corresponding deviations are given in

A visual inspection of the standard deviations collected in _{2}HPO_{4}) by using the

The addition of an appropriate amount of the selected potassium based-inorganic salts allows triggering phase segregation due to the competition of the salt for the water molecules in the presence of the IL. This competition is won by the salt; and then the solubility of the IL in water subsequently decreases. The consequence associated to this phenomenon is the formation of a top phase mostly made up of IL and a bottom phase enriched in the potassium salt.

The experimental phase diagrams plotted in mol/Kg units and collected in _{2}HPO_{4} is the salt showing a stronger ability to form an immiscible area in the presence of the aqueous-IL mixtures. This salt-rank effect follows the Hofmeister series, which order ions according to their water structuring capacity. In this sense, it is possible to conclude that CO_{3}^{2−} is the ion with the weakest interactions with water, thus leading to a smaller biphasic region. In contrast, the most kosmotropic HPO_{4}^{2−} involves phase diagrams closer to the origin.

The molar entropy of hydration (Δ_{hyd}) is a novel tool to appropriately explain the observed patterns. These values were used in a previous paper [_{hyd} data published in this recent paper (HPO_{4}^{2−} = −272 J/molK and CO_{3}^{2−} = −245 J/molK) allows one to validate the tendency followed by the inorganic salts, thus confirming the correlation between the IL molality and the Δ_{hyd}.

The capacity of the selected ILs to be promoted from the aqueous mixture to a top phase can be compared attending to the IL family and cation size. It can be observed that the larger immiscible area is obtained for the quaternary phosphonium based-ionic liquids, followed by pyridinium and imidazolium families (P_{4441} > C_{4}Py > C_{n}C_{1}im). The reason for this phase segregation behavior may lie in the different charge dispersion among families. Thus, while the charge in the imidazolium family is dispersed along the aromatic moiety, the pyridinium and phosphonium cations possess the charges more concentrated on the heteroatom (nitrogen and phosphorous, respectively), as also concluded Freire and coworkers [

In general terms, the charge dispersion along the imidazolium moiety confers to these ILs the ability to form hydrogen bonds, thus they can be considered as “good water solvents”. Notwithstanding this statement, the ABS will be greatly influenced by the alkyl chain length of the imidazolium-based IL. In this case, ABS segregation capacity follows the order: C_{4}C_{1}im C_{1}SO_{4} > C_{2}C_{1}im C_{1}SO_{4} for both potassium-based inorganic salts. The explanation behind this trend is supported by the consideration that the solubility of ILs in water is strongly influenced by their molar volume. Larger cations have been considered to better segregate two phases than smaller cations, and then, the ABS formation depends on the size of the cation [

The phase behavior of each system can also be analyzed in terms of the EEV theory [

being
_{213} the volume fraction of unfilled effective available volume after tight packaging of salt molecules into the network of IL molecules in aqueous solutions, and _{1} and _{2}, the molar mass of IL and salt, respectively.

The values of the EEV and _{213} are listed in _{4}^{2−}) led to higher values of EEV than CO_{3}^{2−}.

Once the suitability of the proposed ILs to be salted out by the selected high charge density inorganic salts has been demonstrated, the systems allowing a greater immiscibility region (those with the K_{2}HPO_{4} salt) were chosen to extract PCP as model pesticide. One of the outstanding characteristics of this contaminant is its easy dissolution in water, so it is essential to reduce the concentration levels of this contaminant in wastewater. Therefore, the final step of this work consisted of analyzing the affinity of PCP for the IL-rich phase. To our knowledge, this is the first time that IL-based ABS have been used for PCP extraction. One of the useful parameters often employed for characterizing the viability of a given separation process is the partition coefficient (

where [PCP]_{IL} and [PCP]_{w} are the PCP concentration in the IL-rich phase and in the inorganic salt-rich phase, respectively.

In addition, the separation performance was analyzed in terms of extraction efficiency,

where
_{PCP} are the PCP mass content in the IL-rich phase and total PCP mass, respectively.

In this work, different cations paired to the same anion were investigated and the interaction with the contaminant PCP indicates significant effects on the partition coefficient into the IL phase. The values of the partition coefficients and extraction efficiencies obtained for each system are shown in

From the partition data obtained it is clear that systems proposed are all suitable for the extraction of PCP. More specifically, the extraction capacity follows the sequence: P_{4441} C_{1}SO_{4} > C_{1}Py C_{1}SO_{4} > C_{4}C_{1}im C_{1}SO_{4} > C_{2}C_{1}im C_{1}SO_{4}. These results match those previously obtained for the ILs ability to form greater immiscibility regions. Furthermore, the observed pattern is in agreement with the results of selectivity reported recently by Pilli

The effect of the pH in the partition coefficient of PCP has been investigated with the purpose to obtain a relationship with the chemical structures of the ILs. In this sense, pH could be considered as a preliminary data to predict the segregation capacity of an organic compound in the presence of aqueous solutions of ILs. The experimental data of pH from the IL and water -rich phases indicate that the mixtures are basic (varies from 8.5 to 9.6, data listed in _{1}SO_{4}^{−} anion govern the partition coefficients. This scenario would consequently influence the anionic form of PCP in the basic aqueous mixture, causing it to present a high hydrophobicity and also a strong electrostatic interaction with the cationic part of the phosphonium based-IL, leading to the pesticide being salted out to the charged IL-rich phase.

C_{4}C_{1}im C_{1}SO_{4}[_{1}Py C_{1}SO_{4}[_{2}C_{1}im C_{1}SO_{4} was purchased from Merck and P_{441} C_{1}SO_{4} was kindly donated by Cytec. All the ILs were characterized by its NMR spectra and positive FABMS (FISONS VG AUTOSPEC mass spectrometer) with purity better than 99%. The water content was reduced to values less than 0.02% by means of vacuum (0.2 Pa) and moderate temperature (333.15 K) during several days. 756 Karl Fisher coulometer was used to determine the IL-water content prior to their use. Dipotassium hydrogen phosphate (K_{2}HPO_{4}), and potassium carbonate (K_{2}CO_{3}) were supplied by Sigma-Aldrich (Madrid, Spain) with purity higher than 98%, and were used as received, without further purification. The information related to the selected ILs and the potassium-based inorganic salts is collected in

The phase diagrams of the ABS were carried out by means of the cloud point titration method [^{−4} g. The temperature was controlled with a F200 ASL digital thermometer with an uncertainty of ±0.01 K.

PCP extraction started with the addition of the pollutant (at a concentration lower than 15 mg/L) to a binary mixture (water and IL) within the miscibility region of known mass percentage and the salting out agent was added until reaching the phase segregation. The partition was carried out in graduated tubes at 298.15 K. The mixture was stirred vigorously and left to settle for 24 h to ensure a complete separation of the layers. The two phases were then carefully separated and the pesticide was quantified in both the top and bottom phases by liquid chromatography measurements. Possible interferences of the ILs were discarded by measuring each phase without PCP.

PCP concentrations in two phases were analyzed by reversed-phase high performance liquid chromatography (HPLC) equipped with a XDB-C8 reverse-phase column (150 × 4.6 mm i.d., 5 μm) with its corresponding guard column. The HPLC system was a HITACHI LaChrom Elite equipped with a quaternary pump (L2130) and photodiode array UV/Vis detector (280 nm). Prior to injection, the samples were filtered through a 0.45-μm Teflon filter. The injection volume was set at 0 μL, and the isocratic eluent (90:10 methanol/water) was pumped at a rate of 0.8 mL/min for 6 min.

The pH of the IL and water-rich phases was carried out at 298.15 K using a 2100 series pH meter (OAKTON instruments, Nijkerk, The Netherlands). The calibration of the pH meter was carried out with three buffers (pH values of 4.00, 7.00 and 9.00). The pH data listed in

In this work, the efficiency of several methylsulfate-based ILs as PCP extraction agents was investigated for the first time. It was demonstrated that the combination of K_{2}HPO_{4} with the selected ILs involved binodal curves closer to the origin, so it was selected for the evaluation of the separation of PCP in terms of partition coefficients and extraction efficiency. The analysis of the obtained data revealed a greater ability of phosphonium-based ILs for the separation of PCP, reaching extraction efficiencies of higher than 99% and partition coefficients higher than 1000. These data are promising for further implementation of the process on a larger scale.

Solubility curves of the aqueous biphasic systems (ABS) formed by ILs and inorganic salts: (Δ) K_{2}HPO_{4}; (□) K_{2}CO_{3}; (_{4441} C_{1}SO_{4}; (_{1}Py C_{1}SO_{4}; (_{4}C_{1}im C_{1}SO_{4}; (_{2}C_{1}im C_{1}SO_{4}. Solid lines represent the fitting to the best empirical equation.

Values of fitting parameters of correlation _{2}O at 298.15 K.

σ | ||||||
---|---|---|---|---|---|---|

C_{2}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
112.84 | −0.3184 | 3.0 × 10^{−5} |
0.499 | ||

111.63 | −0.4100 | 3.8 × 10^{−4} |
0.4 | 2.4 | 0.484 | |

| ||||||

C_{2}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
80.00 | −0.2495 | 2.1 × 10^{−5} |
0.315 | ||

80.01 | −0.2648 | 7.4 × 10^{−5} |
0.5 | 2.7 | 0.361 | |

| ||||||

C_{4}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
80.00 | −0.2969 | 5.0 × 10^{−5} |
0.464 | ||

80.00 | −0.3782 | 1.2 × 10^{−4} |
0.4 | 2.8 | 0.463 | |

| ||||||

C_{4}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
60.00 | −0.2156 | 5.3 × 10^{−5} |
0.152 | ||

30.61 | 0.0085 | 1.2 × 10^{−3} |
1.0 | 2.2 | 0.146 | |

| ||||||

P_{4441} C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
70.01 | −0.3191 | 2.7 × 10^{−4} |
0.616 | ||

71.28 | −0.1445 | 1.0 × 10^{−3} |
0.9 | 2.4 | 0.243 | |

| ||||||

P_{4441} C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
80.00 | −0.3584 | 1.4 × 10^{−4} |
0.335 | ||

33.54 | 0.1417 | 2.4 × 10^{−2} |
0.5 | 1.6 | 0.105 | |

| ||||||

C_{1}Py C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
100.01 | −0.2465 | 2.9 × 10^{−5} |
0.999 | ||

100.07 | −0.494 | 1.9 × 10^{−2} |
0.7 | 1.3 | 0.717 | |

| ||||||

C_{1}Py C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
70.00 | −0.1979 | 2.5 × 10^{−5} |
0.456 | ||

79.99 | −0.3226 | 1.5 × 10^{−3} |
0.3 | 1.9 | 0.327 |

Values of fitting parameters of correlation _{2}O at 298.15 K.

σ | ||||
---|---|---|---|---|

C_{2}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
−28.38 | 105.10 | −0.1494 | 0.988 |

C_{2}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
−22.17 | 81.12 | −4.9460 | 0.468 |

C_{4}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
−13.60 | 44.82 | −10.0118 | 0.510 |

C_{4}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
−21.04 | 74.61 | −3.1855 | 0.400 |

P_{4441} C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
−16.08 | 46.51 | −4.4159 | 0.556 |

P_{4441} C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
−17.72 | 54.93 | −3.8697 | 0.309 |

C_{1}Py C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
−21.82 | 78.61 | −8.7861 | 0.617 |

C_{1}Py C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
−31.56 | 122.26 | 2.8666 | 0.393 |

Values of parameters of Effective Excluded Volume (EEV) and _{213} for IL + potassium inorganic salt + H_{2}O at 298.15 K.

^{*}_{123}/(g/mol) |
_{213} |
σ | |
---|---|---|---|

C_{2}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
4.8 | 0.987 | 0.021 |

C_{2}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
7.3 | 0.984 | 0.014 |

C_{4}C_{1}im C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
8.0 | 0.983 | 0.046 |

C_{4}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
12.0 | 0.978 | 0.027 |

P_{4441} C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
20.0 | 0.973 | 0.028 |

P_{4441} C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
30.0 | 0.962 | 0.018 |

C_{1}Py C_{1}SO_{4} + K_{2}CO_{3} + H_{2}O |
12.0 | 0.967 | 0.015 |

C_{1}Py C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
20.0 | 0.954 | 0.011 |

Partition coefficients _{2}HPO_{4} + H_{2}O at 298.15 K.

System | ||
---|---|---|

C_{2}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
105 | 92 |

C_{4}C_{1}im C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
138 | 94 |

P_{4441} C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
1140 | 99 |

C_{1}Py C_{1}SO_{4} + K_{2}HPO_{4} + H_{2}O |
183 | 96 |

Materials provenance and purities.

Chemical name | Supplier | Mass fraction purity | Method of analysis |
---|---|---|---|

C_{4}C_{1}im C_{1}SO_{4} |
Synthesized | 0.99 | NMR and positive FAMBS |

C_{2}C_{1}im C_{1}SO_{4} |
Merck | 0.99 | None |

C_{1}Py C_{1}SO_{4} |
Synthesized | 0.99 | NMR and positive FAMBS |

P_{4441} C_{1}SO_{4} |
Cytec | 0.99 | None |

K_{2}CO_{3} |
Sigma-Aldrich | 0.98 | None |

K_{2}HPO_{4} |
Sigma-Aldrich | 0.98 | None |

F. Moscoso thanks FCT for a postdoctoral grant (code SFRH/BPD/86887/2012). F.J. Deive acknowledges Xunta de Galicia for funding through an Isidro Parga Pondal contract.

The authors declare no conflict of interest.

^{1}H NMR and molecular dynamics evidence for an unexpected interaction on the origin of salting-in/salting-out phenomena

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