Applications of ionic liquids in biphasic separation: Aqueous biphasic systems and liquid-liquid equilibria.

Ionic liquids (ILs) have been receiving much attention in many fields of analytical chemistry because of their various interesting properties which distinguish them from volatile organic compounds. They offer both directional and non-directional forces towards a solute molecule and therefore act as excellent solvents for a wide range of polar and non-polar compounds. Because of the presence of various possible interactions, ILs easily undergo biphasic separation with water and other less polar/non-polar organic solvents. Their ability to create biphasic splitting makes them a promising candidate for liquid-liquid separation processes, such as aqueous biphasic systems and liquid-liquid equilibria. Various aspects of ILs in these separation methods are discussed in view of the origin of physical forces responsible for the biphasic interactions, the effect of structural components, temperature, pressure, pH and additives. The specific advantages of using ILs in aqueous biphasic systems and liquid-liquid equilibria in binary and ternary systems are discussed with a view to defining their future role in separation processes by giving major emphasis on developing non-toxic ILs with physical and solution properties tailored to the needs of specific sample preparation techniques.

Ionic Liquid-Based Optical and Electrochemical Carbon Dioxide Sensors.

Due to their unusual physicochemical properties (e.g., high thermal stability, low volatility, high intrinsic conductivity, wide electrochemical windows and good solvating ability), ionic liquids have shown immense application potential in many research areas. Applications of ionic liquid in developing various sensors, especially for the sensing of biomolecules, such as nucleic acids, proteins and enzymes, gas sensing and sensing of various important ions, among other chemosensing platforms, are currently being explored by researchers worldwide. The use of ionic liquids for the detection of carbon dioxide (CO2) gas is currently a major topic of research due to the associated importance of this gas with daily human life. This review focuses on the application of ionic liquids in optical and electrochemical CO2 sensors. The design, mechanism, sensitivity and detection limit of each type of sensor are highlighted in this review.

Fluorescein prototropism within poly(ethylene glycol)s and their aqueous mixtures.

Depending on the solubilizing milieu and conditions, fluorescein may exist in one or more of its many prototropic forms [cationic, neutral (zwitterionic, quinoid, and lactone), monoanionic (phenolate and carboxylate), and dianionic]. Fluorescein prototropism is investigated in liquid poly(ethylene glycol)s (PEGs) of different average molecular weight (MW) and their aqueous mixtures using UV-vis absorbance along with static and time-resolved fluorescence spectroscopic techniques. Information regarding various prototropic forms of fluorescein in up to 30 wt % different average MW PEG-added aqueous buffers at varying pH reveals that addition of PEG causes lactonization of fluorescein in the milieu; higher the average MW of PEG, the more the lactonization is. Neat PEG200, PEG400, and PEG600 are found to support the dianionic form of fluorescein, while PEG1000 supports the neutral lactonized form. It is demonstrated that various prototropic forms of fluorescein may be generated within PEGs by addition of adequate amounts of acidic aqueous buffer. Significant bathochromic shift in absorbance and fluorescence band maxima of dianionic fluorescein as concentration of PEG200 is increased correlates well with hydrogen bond accepting basicity, hydrogen bond donating acidity, and dipolarity/polarizability of the aqueous PEG200 mixture. Interestingly, fluorescence emission from the cationic form of fluorescein is observed from dilute aqueous acidic media in the presence of high concentration of PEG200, whereas the fluorescence emission from cation in the absence of PEG200 is observed only from aqueous solutions of very high acidity (>5 M [H(+)]). Excited-state intensity decay is also used to support this outcome. It is proposed that, in the presence of a small amount of acid in PEG200, a highly acidic water-rich solvation microenvironment is formed around fluorescein, which converts its dianionic form to cationic form and considerably hinders the rapid deprotonation of the excited state of the cationic form.

Optically responsive switchable ionic liquid for internally-referenced fluorescence monitoring and visual determination of carbon dioxide.

Modulation in the local viscosity and polarity within a reversible carbamate ionic liquid system forms the basis for the fluorescence excimer-based estimation of CO(2). Inherently self-referencing, the photonic response to CO(2) recognition shows excellent sensitivity and complete reversibility, making possible a striking visual display discernible to the naked eye.

Fluorescent probe studies of polarity and solvation within room temperature ionic liquids: a review.

Ionic liquids display an array of useful and sometimes unconventional, solvent features and have attracted considerable interest in the field of green chemistry for the potential they hold to significantly reduce environmental emissions. Some of these points have a bearing on the chemical reactivity of these systems and have also generated interest in the physical and theoretical aspects of solvation in ionic liquids. This review presents an introduction to the field of ionic liquids, followed by discussion of investigations into the solvation properties of neat ionic liquids or mixed systems including ionic liquids as a major or minor component. The ionic liquid based multicomponent systems discussed are composed of other solvents, other ionic liquids, carbon dioxide, surfactants or surfactant solutions. Although we clearly focus on fluorescence spectroscopy as a tool to illuminate ionic liquid systems, the issues discussed herein are of general relevance to discussions of polarity and solvent effects in ionic liquids. Transient solvation measurements carried out by means of time-resolved fluorescence measurements are particularly powerful for their ability to parameterize the kinetics of the solvation process in ionic liquids and are discussed as well.

Binding of the ionic liquid cation 1-alkyl-3-methylimidazolium to p-tetranitrocalix[4]arene probed by fluorescent indicator displacement.

Acridine orange (AO) was used as a fluorescent probe molecule to study the encapsulation of an alkylimidazolium cation from a water-soluble ionic liquid (IL) within two cavitand species, p-tetranitrocalix[4]arene (1) and calix[4]resorcinarene (2), both in alkaline aqueous media. The addition of IL to the preformed [1·AO] adduct resulted in significantly increased fluorescence due to the expulsion of AO from the inclusion complex to the aqueous phase by competitive recognition of the 1-alkyl-3-methylimidazolium cation ([C(n)mim](+), n = 4 and 6) by 1. Conversely, the fluorescence signal dropped upon the addition of IL to the [2·AO] host-guest complex due to unfavorable binding between [C(n)mim](+) and 2. The formation of these postulated adducts is corroborated using ab initio calculations, which also provide evidence for the location of [bmim](+) at the lower external rim of [2·AO], providing an explanation for the observed luminescence quenching in the latter case. These results point to a number of different paths for exploration, ranging from the fluorescence monitoring of IL contamination in groundwater to the "daisy chaining" of macrocyles toward supramolecular ionic networks. They also broadly encourage the exploration of ILs in host-guest-based optical and mass spectrometric sensory systems.

Pronounced hydrogen bonding giving rise to apparent probe hyperpolarity in ionic liquid mixtures with 2,2,2-trifluoroethanol.

The fascinating and attractive features of ionic liquids (ILs) can be considerably expanded by mixing with suitable cosolvents, opening their versatility beyond the pure materials. We show here that mixtures of the IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 2,2,2-trifluoroethanol (TFE) display the intriguing phenomenon of hyperpolarity, examples of which are notably sparse in the literature. From the perspective of the E(T)(N) polarity scale and Kamlet-Taft parameters for hydrogen bond acidity (α) and basicity (ß), the polarity of this mixture exceeds that of either neat component. Fluorescent molecular probes capable of engaging in hydrogen bonds (e.g., 2-(p-toluidino)naphthalene-6-sulfonate, TNS; 6-propionyl-2-(dimethylamino)naphthalene, PRODAN) also exhibit this curious behavior. The choice of IL anion appears to be essential as hyperpolarity is not observed for mixtures of TFE with ILs containing anions other than hexafluorophosphate. The complex solute-solvent and solvent-solvent interactions present in the [bmim][PF6] + TFE mixture were further elucidated using infrared absorbance, dynamic viscometry, and density measurements. These results are discussed in terms of Coulombic interactions, disruption of TFE multimers, formation of hyperanion preference aggregates, and "free" [bmim]⁺. It is our intent that these results open the door for computational exploration of related solvent mixtures while inspiring practical questions, such as whether such systems might offer the potential for stabilization of highly charged transition states or ionic clusters during (nano)synthesis.

Contrasting behavior of classical salts versus ionic liquids toward aqueous phase J-aggregate dissociation of a cyanine dye.

The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH ≥ 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (<5 wt %), the added IL acts similarly to the inorganic salts, reducing the overall absorbance of TDBC in the solution most likely due to cation exchange causing TDBC precipitation. Addition of a molecular solvent, ethanol, at 15 wt % results in an initial increase in monomer absorbance, albeit to a much lesser extent than for the corresponding molar fraction of IL, which then decreases over time with recovery of J-aggregate absorbance--quite opposite the time-dependent behavior seen for TDBC in PB at pH 12.0 with >5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated.

Novel dansyl-appended calix[4]arene frameworks: fluorescence properties and mercury sensing.

Covalently-attached fluorophores may impart enhanced chemosensing capabilities to calixarene frameworks. Synthesis and characterization of six novel dansyl-appended calix[4]arenes, namely, H/Dan4, NO2/Dan4, H/(OH)2Dan2, H/(Ester)2(Dan)2, t-Bu/(OH)2Dan2, and t-Bu/(Ester)2Dan2, containing two or four dansyl moieties are reported. Among these, fluorescence intensity of NO2/Dan4 is observed to decrease significantly in the presence Hg2+ in the solution. Based on the decrease in fluorescence, a limit of detection for Hg2+ of 20 ppb is obtained. NO2/Dan4 as a chemosensing agent for Hg2+ shows excellent selectivity and adequate reversibility. Complexation of NO2/Dan4 with Hg2+ is investigated using fluorescence spectroscopy and is observed to be 2:1. The formation constant of (NO2/Dan4)2Hg2+ is estimated to be 5.2(+/- 0.8) x 10(10) M(-2) at ambient conditions. These observations are traced to the fact that while all other dansyl-appended calix[4]arenes show cone conformation in the solution, NO2/Dan4 is in the 1,3-alternate conformation. Stokes shift versus solvent orientational polarizability for NO2/Dan4 also indicates the difference in the ground- to excited-state dipole moment of this compound to be the maximum among all six, rendering it most sensitive to its environment. Fluorescence emission of NO2/Dan4 in nonpolar chloroform, polar-aprotic acetonitrile, and polar-protic ethanol is observed to be different than that of the rest of the dansyl-appended compounds as well.

Quenching of pyrene fluorescence by calix[4]arene and calix[4]resorcinarenes.

Interactions involving calixarene and its derivatives are of major importance due to their widespread applications as unique hosts. Fluorescence from a common probe pyrene is used to study interactions involving calix[4]resorcinarene [1a] and its tetra-morpholine derivative [1b] in 1 M aqueous NaOH. These compounds efficiently quench the pyrene fluorescence. A comparison with the fluorescence quenching behavior of N-methylmorpholine clearly indicates the presence of long-range interactions involving 1a and 1b; the interactions are specific to the calixarene molecular framework. This is not the case for a tetra-nitro-substituted calix[4]arene [2b], an electron/charge acceptor quencher, as p-nitrophenol also shows similar interactions with pyrene. Effectiveness of cesium as the quencher of pyrene fluorescence is reduced in the presence of electron/charge donating 1b; fluorescence enhancement is observed upon addition of cesium as the concentration of 1b is increased in the solution. The role of calixarene framework in interactions involving such compounds is established.

A new class of cationic surfactants inspired by N-alkyl-N-methyl pyrrolidinium ionic liquids.

A series of N-alkyl-N-methylpyrrolidinium halide salts have been synthesized and investigated as potentially useful and tunable detergents for a variety of applications.