Deep eutectic solvents (DESs) have been regarded as one of the most promising environmentally benign and cost-effective alternatives to conventional ionic liquids and volatile organic solvents. Aqueous mixtures of DESs have the potential to afford modified properties for specific applications. Densities and dynamic viscosities of a common and popular DES composed of choline chloride and urea in 1:2 molar ratio, named reline, and its aqueous mixtures in the temperature range 293.15 K to 363.15 K are reported. A decrease in density with increasing temperature is found to follow a quadratic expression. Excess molar volumes of the aqueous mixtures of reline are found to be negative at all temperatures and compositions. The absolute excess molar volume is found to decrease, in general, as the temperature is increased from 293.15 K to 323.15 K. For temperatures above 323.15 K, the excess molar volume does not change much with further increase in temperature to 363.15 K. The temperature dependence of dynamic viscosity of aqueous mixtures of reline in the temperature range 293.15 K to 363.15 K at all compositions is found to be better described by a Vogel−Fulcher−Tamman (VFT) model as opposed to an Arrhenius expression. Excess logarithmic viscosities for aqueous mixtures of reline are found to be negative at most temperatures and compositions; however, they become positive at 353.15 K and 363.15 K. The excess logarithmic viscosities of aqueous reline mixtures are in stark contrast to that reported for aqueous mixtures of DES glyceline, composed of choline chloride and glycerol in the same mole ratio, where the excess logarithmic viscosities are positive. Facile interstitial accommodation of water within H-bonded reline network as opposed to formation of extensive H-bonding is proposed to be the reason for this experimental observation. The important role of the H-bond donor as a constituent of DES is amply highlighted as it controls the interactions present in a DES and its aqueous mixtures. ■ INTRODUCTIONDuring the last couple of decades several greener alternatives to volatile organic compounds (VOCs) have been proposed by various research groups. Among the greener alternatives, water, supercritical CO 2 , ionic liquids, renewable solvents, liquid polymers, and so forth are becoming the obvious choices for researchers. 1−8 However, in many of these common environmentally benign alternate media, a lot of interest has been generated in ionic liquids due to their unique and tunable physicochemical properties. Among the key properties of ionic liquids, negligible vapor pressure, good thermal stability, high solubility, and nonflammability are the most noteworthy. 9−12 However, several disadvantages are also observed with common ionic liquids during their investigations, such as limited solute solubility, high viscosity, poor biodegradability, unfavorable toxicity, and high cost. 13−16 Recently, deep eutectic solvents (DESs) have emerged as attractive alternatives to ionic liquids showing several advantages over the latte...

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Deep eutectic solvents (DESs) have shown tremendous promise as green solvents with low toxicity and cost. Understanding molecular aggregation processes within DESs will not only enhance the application potential of these solvents but also help alleviate some of the limitations associated with them. Among DESs, those comprising choline chloride and appropriate hydrogen-bond donors are inexpensive and easy to prepare. On the basis of fluorescence probe, electrical conductivity, and surface tension experiments, we present the first clear lines of evidence for self-aggregation of an anionic surfactant within a DES containing a small fraction of water. Namely, well-defined assemblies of sodium dodecyl sulfate (SDS) apparently form in the archetype DES Reline comprising a 1:2 molar mixture of choline chloride and urea. Significant enhancement in the solubility of organic solvents that are otherwise not miscible in choline chloride-based DESs is achieved within Reline in the presence of SDS. The remarkably improved solubility of cyclohexane within SDS-added Reline is attributed to the presence of spontaneously formed cyclohexane-in-Reline microemulsions by SDS under ambient conditions. Surface tension, dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), density, and dynamic viscosity measurements along with responses from the fluorescence dipolarity and microfluidity probes of pyrene and 1,3-bis(1-pyrenyl)propane are employed to characterize these aggregates. Such water-free oil-in-DES microemulsions are appropriately sized to be considered as a new type of nanoreactor.

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.

Mixtures of ionic liquid (IL) with poly(ethylene glycol) (PEG) may afford media with favorable properties. Dynamic viscosities of mixtures of a common and popular IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) with PEGs of average molecular weight (MW) 200 (PEG200), average MW 400 (PEG400), number-average MW M(n) 570-630 (PEG600), and number-average MW M(n) 950-1050 (PEG1000) over a complete composition range at 10° intervals in the temperature range of 10-90 °C are measured. The temperature dependence of the dynamic viscosity shows ([bmim][PF(6)] + PEG) mixtures to behave as Newtonian fluids and is found to follow Arrhenius-type behavior. In the IL-rich region, excess logarithmic viscosities for the ([bmim][PF(6)] + PEG200) mixture are found to be negative and independent of the temperature. Mixtures of ([bmim][PF(6)] + PEG600) and ([bmim][PF(6)] + PEG1000) show rare and unusual viscosity "synergism" or "hyperviscosity" in the sense that the mixture viscosity is observed to be significantly higher than the viscosity of both the neat components forming the mixture, giving rise to large positive excess logarithmic viscosities. These positive excess logarithmic viscosities decrease with increasing temperature. Formation of extensive H-bonding between the IL and PEG more than compensates for the losses in Coulombic attractive and van der Waals interactions within [bmim][PF(6)] and PEG600/PEG1000, respectively, giving rise to viscosity synergism. This compensation is not enough for ([bmim][PF(6)] + PEG200) and ([bmim][PF(6)] + PEG400) mixtures. The evidence for H-bonding in the mixtures is provided by FTIR absorbance data. The product of the monomer-to-excimer emission intensity ratio and the lifetime of the intramolecular excimer fluorescence of a microfluidity probe, 1,3-bis(1-pyrenyl)propane (BPP), is used as a reflection of the microviscosity of the mixture at different temperatures. The microviscosity shows synergistic effects in all four ([bmim][PF(6)] + PEG) mixtures. The contribution of H-bonding to the microviscosity reported by BPP is observed to be more then that as compared to contributions of Coulombic and van der Waals interactions. Synergism in the dynamic viscosity and microviscosity of ([bmim][PF(6)] + PEG) mixtures is a complex interplay of inter- and intramolecular H-bonding as well as Coulombic and van der Waals type interactions.