Solute-induced dissolution of hydrophobic ionic liquids in water

Talanta 72 (2007) 315–320

Short communication

Solute-induced dissolution of hydrophobic ionic liquids in water夽 Paul G. Rickert a , Dominique C. Stepinski a , David J. Rausch a , Ruth M. Bergeron b , Sandrine Jakab a,c , Mark L. Dietz a,∗ a

Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, United States b North America-Analytical Sciences, BP, Naperville, IL 60563, United States c Ecole Nationale Superieure de Chimie de Paris, Paris, France

Received 11 August 2006; received in revised form 16 October 2006; accepted 19 October 2006 Available online 20 November 2006

Abstract Significant solubilization of ostensibly water-immiscible ionic liquids (ILs) in acidic aqueous phases is induced by the presence of any of a variety of neutral extractants, the apparent result of the formation of the protonated form of the extractant and its subsequent exchange for the cationic component of the IL. The extent of this solubilization is shown to diminish with increasing hydrophobicity of the IL cation and decreasing extractant basicity. These observations raise concerns as to the viability of ILs as “drop in replacements” for traditional organic solvents in the solvent extraction of metal ions. © 2006 Elsevier B.V. All rights reserved. Keywords: Ionic liquids; Crown ether; Neutral organophosphorus extractant; Solubility

1. Introduction In recent years, there has been increasing interest in the development of environmentally benign separation processes, both as an end in itself and in conjunction with the design of “green” methods for manufacturing, synthesis, and analysis. An important aspect of efforts to devise greener separations is the identification, evaluation, and application of novel solvent systems that exhibit few or none of the drawbacks associated with their conventional organic counterparts [1]. Of particular recent interest among alternative solvents have been ionic liquids (ILs), low-melting (≤100–150 ◦ C) ionic salts generally consisting of a bulky, asymmetric organic cation together with any of a wide variety of anions. These compounds exhibit a number of characteristics that make them attractive as the potential basis

夽

The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, non-exclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ∗ Corresponding author. Tel.: +1 630 252 3647; fax: +1 630 252 7501. E-mail address: [email protected] (M.L. Dietz). 0039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2006.10.033

for improved analytical- and process-scale separation methods, among them a near-absence of vapor pressure (and thus, negligible fugative emissions), the ability to solubilize a variety of compounds, high thermal stability, a wide liquid range, and an extraordinary degree of tunability [2]. Among separation methods, few could more clearly benefit from increased attention to environmental “friendliness” than liquid–liquid (l–l) extraction. Despite numerous advantages over competing techniques (e.g., flexibility, simplicity, the possibility of continuous operation), as traditionally practiced, l–l extraction suffers from a significant drawback: the need for waterimmiscible organic solvents that are often toxic, flammable, or volatile [3]. As a result, considerable attention has been devoted to the assessment of the utility of ILs as replacements for these conventional molecular solvents, and IL-based systems for the extraction of simple organic compounds [4–7], biomolecules (e.g., amino acids) [8,9], and metal ions [10–40] have been proposed. Among the many issues that will ultimately govern the viability of ILs as replacements for traditional organic solvents in these applications is their aqueous solubility. Although certain results indicate that the water solubility of the hydrophobic ionic liquids most often employed as extraction solvents (i.e., hexafluorophosphate (PF6 − ) and bis[(trifluoromethyl)sulfonyl]imide (Tf2 N− ) salts of N,N -dialkylimidazolium cations) is not appreciable [13,26], other studies employing more “process relevant”

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(i.e., real world) aqueous phases (e.g., mineral acid solutions, in the case of metal ion extraction) suggest that under certain conditions, their solubility may be significant [13,31]. Repeated contact of certain ostensibly water-immiscible ILs with aqueous phases containing high nitric acid concentrations, for example, has been reported to lead to degradation of the biphasic system [31]. In this report, we demonstrate that in contrast to the behavior of conventional organic solvents, the presence in a hydrophobic IL of certain solutes (in particular, neutral extractants/ligands) can significantly increase the solubility of the ionic liquid in acidic aqueous media. This observation has negative implications for the utility of ionic liquids as environmentally benign replacements for traditional organic solvents in certain separations applications. 2. Experimental 2.1. Materials The 1-alkyl-3-methylimidazolium ionic liquids were prepared and purified according to published methods [41]. The dicyclohexano-18-crown-6 (DCH18C6; Aldrich, Milwaukee, WI) was a mixture of the cis-syn-cis (A) and cis-anti-cis (B) isomers, consistent with prior work [11,13,31,38–40]. Tri-n-butyl phosphate (TBP; Aldrich) was distilled (T = 143 ◦ C) at reduced (20 mm) pressure prior to use, while bis-2-ethylhexylphosphoric acid (HDEHP) was purified by copper salt precipitation. Dibutyl butylphosphonate (DBBP; Aldrich), butyl dibutylphosphinate (BDBP; Organometallics Inc., Hampstead, NH), and tributylphosphine oxide (TBPO; Aldrich) were used as received. Aqueous acid solutions were prepared from Milli-Q2 water and Ultrex II nitric acid (J. T. Baker Chemical Co.). All other reagents were analytical grade and were used without further purification. 2.2. Methods 2.2.1. 1-Octanol solubilization The solubility of 1-octanol in water and in various nitric acid solutions was determined by gas chromatography. Briefly, a measured volume of 1-octanol (or a solution of DCH18C6 therein) was combined with an equal volume of the aqueous phase of interest in a constant temperature bath (T = 25 ◦ C) and gently stirred for at least 24 h. Following centrifugation, a measured portion of the aqueous phase (0.50 mL) was contacted three times with 6× its volume (3.00 mL) of methylene chloride, and the combined extracts diluted to a known volume with eicosane-spiked (100 ppm) CH2 Cl2 . Triplicate injections of each sample (typically 2–3 ␮L) were then made into an HP5890 gas chromatograph (initial temperature, 120 ◦ C; initial time, 3.00 min; ramp, 40 ◦ C/min; final temperature, 260 ◦ C; final time, 3.00 min; detector temperature, 300 ◦ C; injector temperature, 300 ◦ C) equipped with a flame ionization detector, along with injections of a series of standards comprising known masses of 1-octanol in eicosane-spiked (100 ppm) CH2 Cl2 . The octanol content of each sample was determined by comparison of the 1octanol/eicosane peak area ratio to that of the standards. Multiple

determinations of the solubility of 1-octanol in water indicate that the reported values are reproducible to within ±10–15%. 2.2.2. Ionic liquid solubilization The solubility of the ionic liquids, 1-pentyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (C5 mim+ Tf2 N− ) and its n-octyl-(C8 mim+ Tf2 N− ) and n-decyl-(C10 mim+ Tf2 N− ) analogs, in water and in various nitric acid solutions was determined by 1 H NMR. Briefly, a measured volume of the ionic liquid (or a solution of an extractant therein) was contacted with an equal volume of the aqueous phase of interest in a constant temperature bath (25 ◦ C) and gently stirred for at least 24 h. Following centrifugation, a known aliquot of the aqueous phase was taken to dryness on a rotary evaporator and the residue taken up in deuterated toluene for NMR analysis. The Cn mim+ content of each sample was determined by comparison of the area of the Cn -side chain methylene ((CH2 )2 ) or methyl (CH3 ) proton peak to that of a series of standards prepared by dissolution of the same IL in d-toluene. 3. Results and discussion 3.1. Crown ether in 1-octanol Prior work in this laboratory has shown that important insights into the behavior of IL-based solvent systems can often be obtained by direct comparison to analogous systems employing conventional organic solvents such as 1-octanol [31]. For this reason, our initial studies sought to determine the influence of changing aqueous acidity and organic phase solute (i.e., extractant) concentration on the extent of dissolution of 1-octanol in water. Fig. 1 depicts the effect of increasing concentrations of dicyclohexano-18-crown-6 (DCH18C6) (chosen for this investigation because it is among the most widely studied metal ion extractants in ionic liquid systems [11,13,14,31,32,36,38–40]) on this dissolution. As can be seen, the presence of increasing amounts of DCH18C6 is accompanied by a slight decrease in the solubility of the alcohol in water. Such a decrease is not unexpected, as for mixtures of sparingly soluble solutes in contact with water, the equilibrium aqueous solubility of each constituent is expected to be proportional to its respective mole fraction in the mixture [42]. It is worth noting that the water solubility of 1-octanol observed here in the absence of any crown ether, 4.82 × 10−3 M, is in excellent agreement with the previously reported value of 4.5 × 10−3 M [43]. Fig. 2 depicts the effect of increasing aqueous acidity (i.e., [HNO3 ]) on the solubility of 1-octanol in water. As shown, increasing acidity is accompanied by a slight increase in solubilization of the alcohol up to ca. 1 M HNO3 , above which further increases have no measurable effect. As is well known, the solubility of an organic solute (e.g., benzene) in aqueous solution can be altered by addition of inorganic salts (e.g., NaCl) [44]. Frequently, a decrease in the solubility of the organic solute is observed, which has been attributed to the binding of a portion of the water by the ions of the added salt and its consequent unavailability as a solvent [44]. In this system, however, an additional factor, interaction of 1-octanol and nitric acid, must be consid-

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Table 1 Effect of DCH18C6 concentration on the solubility of 1-octanol in aqueous (4 M) nitric acid

Fig. 1. The effect of dicyclohexano-18-crown-6 concentration on the solubility of 1-octanol in water (T = 25 ◦ C).

[DCH18C6]org (M)

[1-octanol]aq (M)

0 0.010 0.025 0.050 0.100 0.250 0.500

0.00694 0.00628 0.00561 0.00561 0.00656 0.00672 0.00594

ered. Previous work has demonstrated that 1-octanol can extract significant quantities of nitric acid (as much as 0.6 mol of acid for each mole of 1-octanol at sufficiently high acidities) [45]. Other work concerning the effect of mineral acids on the aqueous solubility of various aliphatic alcohols has shown that increasing acidity leads to protonation of the alcohol [46], which is accompanied by an increase in its water solubility [47]. Taken together, this suggests that the observed effect of aqueous acidity in this system (Fig. 2) represents a balance between the tendency of added nitric acid to decrease the solubility of the alcohol via “salting out” and to increase its solubility by protonation. Table 1 summarizes the results of measurements of the solubility of 1-octanol in 4 M HNO3 (i.e., on the plateau region of Fig. 2) in the presence of increasing quantities of DCH18C6. In this system, three effects might reasonably be anticipated as the extractant concentration is raised: a decrease in 1-octanol solubility in the aqueous phase arising from its lower mole fraction in the organic phase [42]; a decrease in its solubility in the aqueous phase arising from preferential interaction of the crown ether with nitric acid and the accompanying decrease in 1-octanol protonation [47,48]; and lastly, a diminution of the salting out effect of HNO3 (and thus, an increase in the solubility of 1-octanol in the aqueous phase) caused by extraction of HNO3 by the crown ether [49]. The net effect of these three factors is apparently such that no appreciable difference between the aqueous solubility of 1-octanol in the presence of 0.01 M and 0.50 M crown ether is observed. 3.2. Crown ether in ionic liquids

Fig. 2. The effect of aqueous nitric acid concentration on the solubility of 1octanol in water (T = 25 ◦ C).

In contrast, analogous experiments employing three ionic liquids, 1-pentyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide (C5 mim+ Tf2 N− ) and its n-octyl(C8 mim+ Tf2 N− ) and n-decyl-(C10 mim+ Tf2 N− ) analogs, containing increasing concentrations of the same extractant (DCH18C6), each contacted with an acidic (1 M HNO3 ) aqueous phase, yield markedly different results. That is, in each case, an increase in the initial IL phase concentration of the crown ether is accompanied by a corresponding increase in the solubility of the ionic liquid (as reflected in the concentration of the IL cation) in the aqueous phase. In fact, as shown in Fig. 3, a log–log plot of IL solubility versus DCH18C6 concentration yields a line of near-unit slope (0.85, 1.10, and 1.00 for the C5 mim+ Tf2 N− , C8 mim+ Tf2 N− , and C10 mim+ Tf2 N− , respectively) for each of the three systems. All of these solubilities, it must be noted, have

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Fig. 3. The effect of dicyclohexano-18-crown-6 concentration on the solubility of Cn mim+ Tf2 N− ILs in 1 M HNO3 (T = 25 ◦ C).

been corrected for that of the ILs in 1 M HNO3 in the absence of any crown ether (Fig. 4) (1.9 × 10−2 M, 1.4 × 10−3 M, and 2.6 × 10−4 M for the C5 mim+ Tf2 N− , C8 -, and C10 -compounds, respectively) and thus, represent only the additional solubility induced by the presence of the extractant. Such results are consistent with a mechanism for IL solubilization in which nitric acid forms a cationic 1:1 hydronium ion:DCH18C6 adduct in the aqueous phase, which is subsequently exchanged for the cationic constituent of the ionic liquid, resulting in loss of the IL (i.e., dissolution) to the aqueous phase: H3 Oaq + + DCH18C6aq ⇔ H3 O · DCH18C6aq +

(1)

H3 O · DCH18C6aq + + Cn mim+ Tf 2 Norg − ⇔ H3 O · DCH18C6+ Tf 2 Norg − + Cn mimaq +

(2)

This scheme is supported by examination of the infrared spectrum of the IL (C5 mim+ Tf2 N− ) phase before and after its equilibration with nitric acid (1 M), the latter of which exhibits absorbance bands characteristic of oxonium salts [50] (most importantly, a prominent band centered at ca. 2184 cm−1 ). It would be anticipated from Eq. (2) that as the hydrophobicity of the IL is increased (i.e., as n rises), this exchange process would become progressively more difficult, and the solubilizing effect of the extractant diminished. As can be seen from Fig. 3, this is indeed the case. That is, the effect of a given concentration of DCH18C6 in the IL phase is smaller for C10 mim+ Tf2 N− than for C8 mim+ Tf2 N− , which in turn, is smaller than for C5 mim+ Tf2 N− . It would also be expected that for an aqueous phase containing no acid, the addition of the

Fig. 4. The effect of aqueous nitric acid concentration on the solubility of Cn mim+ Tf2 N− ILs in water (T = 25 ◦ C).

crown ether would yield no systematic increase in the solubility of the ILs. This too is the case. 3.3. Organophosphorus extractants in ionic liquids To determine if the increased solubility of ionic liquids in acidic aqueous solution that accompanies addition of DCH18C6 is a peculiarity of this extractant or represents a general phenomenon whereby the protonated form of a neutral extractant will exchange with the cationic constituent of an ionic liquid, thus increasing the aqueous solubility of the IL, the effect of the addition of four neutral organophosphorus reagents (trin-butylphosphate (TBP), dibutyl butylphosphonate (DBBP), butyl dibutylphosphinate (BDBP), and tributyl phosphine oxide (TBPO)) upon the solubility of C5 mim+ Tf2 N− in an acidic aqueous phase was determined. For purposes of comparison, the effect of the addition of bis-(2-ethylhexyl)phosphoric acid (HDEHP), an acidic organophosphorus extractant, on the solubilization of C10 mim+ Tf2 N− was also examined. (The limited solubility of HDEHP in C5 mim+ Tf2 N− precluded the use of this IL for the comparison.) As shown in Fig. 5, as was the case for the crown ether, increasing neutral organophosphorus reagent concentrations are accompanied by a proportionate increase in the solubilization of the IL cation in the aqueous phase (here, 1 M HNO3 ). This observation is consistent with the work of Yakshin et al. [51], whose studies of three of these extractants indicated that they are readily protonated by strong inorganic acids. In addition, as would be expected, the variation in the sol-

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biological, or geological) samples for subsequent analysis are often highly acidic [52], this clearly poses a problem. Although our results for neutral organophosphorus extractants suggest that IL solubilization can be reduced by employing a weakly basic extractant, the solubilization is not entirely eliminated. These results suggest the need for both caution in attempts to employ ILs as “drop-in replacements” for conventional organic solvents in extraction systems involving neutral extractants and additional research to either identify ILs less prone to solubilization losses or to devise alternative configurations employing ILs in extractive separations (e.g., “task-specific” [53,54] or solidsupported [55,56] ILs). Work addressing these opportunities is now underway in this laboratory. Acknowledgement This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, United States Department of Energy under contract number W-31-109ENG-38. References

Fig. 5. The effect of neutral organophosphorus extractant concentration on the solubility of C5 mim+ Tf2 N− in 1 M HNO3 (T = 25 ◦ C).

ubilizing effect with extractant (TBP < DBBP < BDBP < TBPO) follows the same order as the extractant basicity (as reflected in the value of the protonation constant [51]). In contrast to the behavior of the neutral extractants, no systematic increase in the solubility of the C10 -IL in the aqueous phase (ca. 8 × 10−4 M) is observed as the concentration of the acidic reagent (HDEHP) is varied over the same range. Taken together with the results observed for DCH18C6, these data suggest that the increased solubilization of ostensibly hydrophobic ILs in acidic aqueous phases observed upon addition of a neutral extractant represents a general property of IL-neutral extractant combinations. 4. Conclusions The results presented here raise significant concerns regarding the viability of ionic liquids as replacements for conventional organic solvents in the extraction of metal ions from acidic media by neutral ligands/extractants. As we have demonstrated previously, high metal ion loading of a neutral extractant in an IL can lead to significant aqueous phase dissolution of the IL [31]. Unfortunately, the present results indicate that even in the absence of any metal ion extraction, appreciable loss of IL can occur as a result of the interaction of the extractant with matrix acid. Given that the aqueous solutions resulting from the digestion or leaching of many “real-world” (e.g., environmental,

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