Characterization of a solid-supported aqueous biphasic system for the sorption of dyes from aqueous solution.

An aqueous biphasic system (ABS) comprising solutions of ammonium sulfate and polyethylene glycol-2000 (PEG-2000) in which the latter is supported on a porous polymeric substrate (Amberlite XAD-16) has been characterized by examining its performance in the removal of dyes from aqueous solution. Comparison of its behavior to that of a commercial sorbent (ABEC) consisting of a polyethylene glycol covalently bound to a polymer support indicates that the capacity, uptake kinetics, and efficiency of the supported ABS are comparable or superior to that of the ABEC resin. In addition, results obtained at various PEG-2000 concentrations and with PEGs of a range of molecular weights demonstrate that in contrast to ABEC resin, the behavior of a supported ABS can be readily "tuned" to provide the desired dye retention. The relative retention of various dyes is not generally predictable from their behavior in an analogous liquid-liquid system, however, the apparent result of synergistic effects between the XAD-16 support and the PEG phase.

Systematic evaluation of hydrophobic deep eutectic solvents as alternative media for the extraction of metal ions from aqueous solution.

The recent development of hydrophobic deep eutectic solvents (HDESs) has led to growing interest in these reagents as possible environmentally benign replacements for conventional organic media in a host of applications, among them metal ion separations by liquid-liquid extraction. To evaluate the potential utility of these novel solvents in this application, a systematic examination of the facilitated transfer of selected alkali and alkaline earth cations into representative HDESs from aqueous solution in the presence of a macrocyclic polyether (i.e., a crown ether) has been undertaken. Comparison of the results to those obtained for a series of oxygenated, aliphatic solvents (n-alcohols) and for several 1-alkyl-3-methylimidazolium-based ionic liquids (ILs) under the same conditions indicate that despite frequent suggestions that some HDESs resemble ILs, metal ion distribution in HDES-aqueous systems more closely mimics that seen for the alcohols. Metal ion partitioning in these systems appears less dependent on the water content of the organic phase and on structural variations in the solvent than is the case for either alcohols or ionic liquids, however. The implications of these results for the design and application of HDES-based extraction systems for metal ions are described.

Support loading effects on the performance of an extraction chromatographic resin: Toward improved separation of trivalent lanthanides.

Changes in the level of impregnation of an Amberchrom CG-71m support with bis(2-ethylhexyl)phosphoric acid (HDEHP) are shown to alter the column efficiency, peak tailing, and metal ion uptake capacity associated with the resulting extraction chromatographic resins. Optimum efficiency and minimum peak tailing are observed at intermediate levels (ca. 20% (w/w)) of support loading. Metal ion uptake capacity is reduced relative to a commercial (loaded to 40% (w/w)) resin under the same conditions, however. The utility of the improved efficiency arising from reduced support loading is illustrated in the separation of selected trivalent lanthanide ions, including Gd(III) and Eu(III), whose resolution is unsatisfactory using commercial extraction chromatographic materials.

High-capacity extraction chromatographic materials based on polysulfone microcapsules for the separation and preconcentration of lanthanides from aqueous solution.

Extractant-loaded polysulfone (PS) capsules suitable for the separation and preconcentration of rare earth ions from acidic media have been prepared and characterized. Specifically, PS macro- and microcapsules have been impregnated with bis(2-ethylhexyl)phosphoric acid (HDEHP), and their performance in the extraction of europium (Eu3+) from nitric acid solution evaluated. The HDEHP-loaded microcapsules were found to exhibit sorption efficiency superior to those of analogous macrocapsules. Comparison to a conventional (i.e., commercial) extraction chromatographic (EXC) material (Ln•Resin), comprising HDEHP-loaded beads of poly(methyl methacrylate) (PMMA) of comparable size (50-100 µm), showed that the capacity of the HDEHP-loaded microcapsules for europium is ca. 2.5-fold greater than that of the conventional material. This larger capacity is the apparent result of both the higher extractant loading achievable with the microcapsules and an unexpected change in the complexation stoichiometry of europium by HDEHP upon extractant encapsulation. The microcapsule-based sorbent equaled or exceeded the performance of the commercial EXC material in other respects as well, most notably uptake kinetics and column efficiency, making it a promising alternative to established EXC resins.

Systematic evaluation of hydrophobic deep-melting eutectics as alternative solvents for the extraction of organic solutes from aqueous solution.

The partitioning of a number of organic compounds, including a series of n-alkanols and various simple, substituted benzene derivatives, between several hydrophobic (i.e., water-immiscible) deep eutectic solvents (HDESs) and water has been examined. The extent of extraction is shown to vary with the charge state of the molecule and the composition of the eutectic. In addition, the HDES-water distribution of a given solute is found to be directly proportional to (but typically less than) its partitioning in the octanol-water system, consistent with a significant role for solute hydrophobicity in the observed extraction behavior. Comparison of solute extraction into an HDES to that observed for other "unconventional" solvents (e.g., room-temperature ionic liquids and a soybean-derived oil) shows that hydrophobic deep eutectic solvents provide comparable or superior extraction efficiency.

Rapid quantification of imidazolium-based ionic liquids by hydrophilic interaction liquid chromatography: Methodology and an investigation of the retention mechanisms.

The separation of nine N,N'-dialkylimidazolium-based ionic liquids (ILs) by an isocratic hydrophilic interaction high-performance liquid chromatographic method using an unmodified silica column was investigated. The chosen analytical conditions using a 90:10 acetonitrile-ammonium formate buffer mobile phase on a high-purity, unmodified silica column were found to be efficient, robust, and sensitive for the determination of ILs in a variety of solutions. The retention window (k' = 2-11) was narrower than that of previous methods, resulting in a 7-min runtime for the nine IL homologues. The lower limit of quantification of the method, 2-3 µmol L(-1), was significantly lower than those reported previously for HPLC-UV methods. The effects of systematically modifying the IL cation alkyl chain length, column temperature, and mobile-phase water and buffer content on solute retention were examined. Cation exchange was identified as the dominant retention mechanism for most of the solutes, with a distinct (single methylene group) transition to a dominant partitioning mode at the highest solute polarity.

Evaluation of solid-supported room-temperature ionic liquids containing crown ethers as media for metal ion separation and preconcentration.

Extraction chromatographic (EXC) resins incorporating an appropriate crown ether in an oxygenated organic solvent such as 1-octanol are well established as sorbents for the analytical-scale separation and preconcentration of radiostrontium from a variety of sample types. Recent solvent extraction studies employing crown ethers in various 1-alkyl-3-methylimidazolium-based (CnC1im(+)) room-temperature ionic liquids (RTILs) indicate that under certain conditions, distribution ratios (DSr) for strontium far in excess of those observed with conventional organic solvents are observed. To determine if this increase in liquid-liquid extraction efficiency will lead to improved strontium sorbents, several EXC resins and sol-gel glasses incorporating di-tert-butylcyclohexano-18-crown-6 (DtBuCH18C6) in either 1-decyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (C10C1imTf2N) or the related hydroxyalkyl-functionalized IL 1-(12-hydroxydodecyl)-3-butylimidazolium bis[(trifluoromethyl)sulfonyl]imide (C12OHC4im Tf2N) were prepared and characterized. Unexpectedly the performance of these materials was not uniformly better than that of a conventional EXC resin, an apparent result of the greater viscosity of the ionic liquids and the lower solubility of the crown ether in ILs versus conventional organic solvents.

Novel tandem column method for the rapid isolation of radiostrontium from human urine.

A method has been developed for the isolation of strontium from human urine for subsequent determination in sample volumes as low as 5-20 mL. This method involves the acidification of the sample using methanesulfonic acid and its decolorization using charcoal, treatment of the filtrate with Diphonix(®) resin, and subsequent concentration of strontium on Sr resin. Data from retention model simulations provided the initial conditions which were then optimized by actual column separations. Diphonix(®) resin was shown to be effective at removing alkali metal ions from the urine matrix under conditions that retain higher valence ions. The suggested processing method provides 99% recovery of Sr(2+), a concentration factor of 50, and an expected per sample processing time of less than 1 h.

Effect of aqueous phase anion on the mode of facilitated ion transfer into room-temperature ionic liquids.

Measurements of the partitioning of various alkali and alkaline earth cations between solutions of hydrochloric acid and a series of 1,3-dialkylimidazolium-based ionic liquids (ILs) to which a crown ether has been added have revealed substantial differences in extraction behavior versus both conventional molecular solvents (e.g., 1-octanol) under the same conditions and the same ILs when nitric acid solutions are employed as the aqueous phase. These results can be rationalized by application of a three-path model for metal ion partitioning into ILs in the presence of a neutral extractant. Additionally, the results point to a significant role for anion hydration energy in determining the balance amongst the possible modes of partitioning and strongly suggest that ion exchange involving the cationic metal complex and the cationic constituent of the ionic liquid constitutes the "default" route for metal ion extraction in IL systems incorporating a neutral extractant.

On the radiation stability of crown ethers in ionic liquids.

Crown ethers (CEs) are macrocyclic ionophores used for the separation of strontium-90 from acidic nuclear waste streams. Room temperature ionic liquids (ILs) are presently being considered as replacements for traditional molecular solvents employed in such separations. It is desirable that the extraction efficacy obtained with such solvents should not deteriorate in the strong radiation fields generated by decaying radionuclides. This deterioration will depend on the extent of radiation damage to both the IL solvent and the CE solute. While radiation damage to ILs has been extensively studied, the issue of the radiation stability of crown ethers, particularly in an IL matrix, has not been adequately addressed. With this in mind, we have employed electron paramagnetic resonance (EPR) spectroscopy to study the formation of CE-related radicals in the radiolysis of selected CEs in ILs incorporating aromatic (imidazolium and pyridinium) cations. The crown ethers have been found to yield primarily hydrogen loss radicals, H atoms, and the formyl radical. In the low-dose regime, the relative yield of these radicals increases linearly with the mole fraction of the solute, suggesting negligible transfer of the excitation energy from the solvent to the solute; that is, the solvent has a "radioprotective" effect. The damage to the CE in the loading region of practical interest is relatively low. Under such conditions, the main chemical pathway leading to decreased extraction performance is protonation of the macrocycle. At high radiation doses, sufficient to increase the acidity of the IL solvent significantly, such proton complexes compete with the solvent cations as electron traps. In this regime, the CEs will rapidly degrade as the result of H abstraction from the CE ring by the released H atoms. Thus, the radiation dose to which a CE/IL system is exposed must be maintained at a level sufficiently low to avoid this regime.

Hydrogen-bonding interactions and protic equilibria in room-temperature ionic liquids containing crown ethers.

Nuclear magnetic resonance (NMR) spectroscopy has been used to study hydrogen-bonding interactions between water, associated and dissociated acids (i.e., nitric and methanesulfonic acids), and the constituent ions of several water-immiscible room-temperature ionic liquids (ILs). In chloroform solutions also containing a crown ether (CE), water molecules strongly associate with the IL ions, and there is rapid proton exchange between these bound water molecules and hydronium associated with the CE. In neat ILs, the acids form clusters differing in their degree of association and ionization, and their interactions with the CEs are weak. The CE can either promote proton exchange between different clusters in IL solution when their association is weak or inhibit such exchange when the association is strong. Even strongly hydrophobic ILs are shown to readily extract nitric acid from aqueous solution, typically via the formation of a 1:1:1 {H(3)O(+)•CE}NO(3)(-) complex. In contrast, the extraction of methanesulfonic acid is less extensive and proceeds mainly by IL cation-hydronium ion exchange. The relationship of these protic equilibria to the practical application of hydrophobic ILs (e.g., in spent nuclear fuel reprocessing) is discussed.

Anion concentration-dependent partitioning mechanism in the extraction of uranium into room-temperature ionic liquids.

The mode of partitioning of uranyl ion between nitrate-containing aqueous phases and various N,N'-dialkylimidazolium-based room-temperature ionic liquids (RTILs) in the presence of tri-n-butyl phosphate (TBP) is shown to change from an ion-exchange process to one involving extraction of a neutral uranyl-TBP-nitrato complex as the aqueous nitrate concentration is increased. Increasing the hydrophobicity of the RTIL cation eventually leads to nitrato complex extraction as the predominant mode of partitioning, regardless of nitrate concentration.

Tetraalkylphosphonium polyoxometalate ionic liquids: novel, organic-inorganic hybrid materials.

Pairing of a Keggin or Lindqvist polyoxometalate (POM) anion with an appropriate tetraalkylphosphonium cation is shown to yield the first members of a new family of ionic liquids (ILs). Detailed characterization of one of them, an ambient-temperature "liquid POM" comprising the Lindqvist salt of the trihexyl(tetradecyl) phosphonium cation, by voltammetry, viscometry, conductimetry, and thermal analysis indicates that it exhibits conductivity and viscosity comparable to those of the one previously described inorganic-organic POM-IL hybrid but with substantially improved thermal stability.

Tetraalkylphosphonium polyoxometalates: electroactive, "task-specific" ionic liquids.

The pairing of selected polyoxometalate (POM) anions with appropriate tetraalkylphosphonium cations is shown to yield an original family of ionic liquids, among them an ambient-temperature "liquid POM".

Solute-induced dissolution of hydrophobic ionic liquids in water.

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.

Fluorous ionic liquids as solvents for the liquid-liquid extraction of metal ions by macrocyclic polyethers.

The predominant mode of strontium ion transfer from aqueous nitrate media into a series of 1-fluoroalkyl-3-methylimidazolium bis[(trifluoromethylsulfonyl)]imides containing dicyclohexano-18-crown-6 (DCH18C6) is shown to shift from cation exchange to strontium nitrato-crown ether complex partitioning as the length of the fluoroalkyl substituent is increased. Fluoroalkyl substituents are shown to be only slightly more effective than their non-fluorous analogs at inducing this shift. At the same time, the fluorinated ionic liquids (ILs) yield strontium distribution ratios as much as an order of magnitude lower than the corresponding 1-alkyl-3-methylimidazolium (C(n)mim(+)) salts. Fluorous ILs thus appear to offer no compelling advantages over C(n)mim(+) ionic liquids as extraction solvents.

Ionogel-templated synthesis and organization of anisotropic gold nanoparticles.

Photochemical reduction of tetrachloroaurate (AuCl4-) ions in the highly constrained aqueous domains of a nanostructured ionogel template, formed via self-assembly of the ionic liquid 1-decyl-3-methylimidazolium chloride (C10mim+Cl-) in water, results in the formation of anisotropic gold nanoparticles with a variety of sizes and morphologies, which include previously unattainable trigonal prismatic nanorods. Unexpectedly, small-angle X-ray scattering studies of the Au-ionogel composite reveal that the in situ formation of the nanoparticles increases the mesoscopic order of the ionogel, which results in its conversion to a near-monodomain structure. The findings demonstrate that nanostructured, ionic liquid-based gels can be used to template the formation of new nanoparticle morphologies with technologically important optical, electronic, and catalytic properties. It may also be possible to design soft templates that permit the fabrication of highly ordered nanoparticle array-hydrogel composites, thereby enabling control and tuning of the collective properties of the encapsulated nanoparticles.

EXAFS investigations of strontium complexation by a polymer-supported crown ether.

Extended X-ray absorption fine-structure (EXAFS) measurements have been used to probe the coordination environment of strontium ion sorbed from aqueous nitric acid solutions on an extraction chromatographic resin comprising a macrocyclic polyether dispersed on a polymeric support. The strontium EXAFS of the metal ions sorbed onto the resin are consistent with a 1:1:2 strontium:crown ether:nitrate stoichiometry in which the strontium is enveloped in the crown ether ring and both nitrate anions are coordinated to the strontium as bidentate ligands. This is the same structure and stoichiometry observed for complexes in liquid-liquid extraction when the macrocyclic polyether is dissolved in a diluent with low water miscibility like 1-octanol.

SYNTHESIS OF SYMMETRICAL METHYLENEBIS(ALKYL HYDROGEN PHOSPHONATES) BY SELECTIVE CLEAVAGE OF METHYLENEBIS(DIALKYL PHOSPHONATES) WITH MORPHOLINE.

The preparation of partial esters of methylenebisphosphonic acids has been of recent interest due to their potential therapeutic applications. This paper describes a convenient method to prepare symmetrical methylenebis(alkyl hydrogen phosphonates) by the selective cleavage of the corresponding methylenebis(dialkyl phosphonate) with refluxing morpholine. The effects of structural variations on the amine as well as the substrate have been investigated to understand the scope and limitations of this reaction. A superior approach to hindered bisphosphonic acid esters involves the cleavage of their dimethyl esters, 4, using morpholine. This method is also useful to access a number of C-alkyl dialkyl methylenebisphosphonic acids such as 6. This study clearly shows that cleavage with morpholine is convenient, inexpensive, and allows for the preparation of a variety of P,P'-disubstituted partial esters in good yields and high purity.

EXAFS investigations of the mechanism of facilitated ion transfer into a room-temperature ionic liquid.

The Sr(II)-crown ether complexes formed in a room-temperature ionic liquid (RTIL), 1-methyl-3-pentylimidazolium bis[(trifluoromethyl)sulfonyl]amide, have been studied by X-ray absorption fine structure measurements at the Sr K-edge. When a Sr(NO(3))(2)-crown ether complex is directly dissolved in a water-saturated RTIL, both nitrate ligands and the crown ether coordinate the Sr, as observed in a conventional two-phase water-octanol system. When the cationic Sr-crown ether complex is created in a two-phase water-RTIL system, however, only cationic Sr-crown ether complexes are observed in the RTIL phase. This difference in the coordination complexes arises from differences in the mechanism of cation extraction between the RTIL and conventional molecular organic solvents, a finding with important implications for synthesis, catalysis, and ion separations using two-phase water-RTIL systems.