Tejwant Singh

Static dielectric constant of room temperature ionic liquids: internal pressure and cohesive energy density approach.

Measurements of the static dielectric constant (epsilon) of ionic liquids (ILs) are very difficult because of the decay of field by the ionic conductivity of ILs. Herein, we describe an easy method for the prediction of epsilon of various imidazolium-based ILs [C_n mim] from n, i.e. the ratio of internal pressure (P_i) and cohesive energy density (ced). A calibration curve of n vs epsilon for conventional organic solvents (mainly the linear alcohols) has been used to estimate the epsilon of the ILs. Estimated epsilon values for ILs having the anions [Cl]-, [BF 4]-, [PF 6]-, [TfO]-, and [Tf 2N]- showed a very good comparison with the literature results, whereas ILs having the anions [C_n OSO3]- tend to deviate from such correlation. Also, for a series of ILs having a common anion, the epsilon is shown to follow a very good correlation with the molecular volumes. Predicted values show that both the nature of the anion and alkyl chain length of the cation contribute significantly to the epsilon of the ILs. The method developed makes use of properties which can be either experimentally determined or estimated with good accuracy and can be extended to the other categories of ILs with ease and reasonable accuracy.

Aggregation behavior of amino acid ionic liquid surfactants in aqueous media.

Self-aggregation of amino acid ionic liquid surfactants (AAILSs) in aqueous solution has been investigated through surface tension, conductivity, steady-state fluorescence, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The critical aggregation concentration (cac) of AAILSs obtained from different techniques showed fairly good agreement. Surface tension measurements have been used to derive surface adsorption properties such as adsorption efficiency (pC(20), effectiveness of surface tension reduction (Π(cac)), and minimum surface area per molecule (A(min)) at the air-water interface. Temperature-dependent conductivity measurements have been used to obtain the degree of counterion binding (β), and the thermodynamic parameters such as standard free energy (ΔG(agg)(0)), enthalpy (ΔH(agg)(0)), and entropy (ΔS(agg)(0)) of aggregation. The aggregation number (N(agg)) for various AAILSs has been derived by using the fluorescence quenching technique. Size of the aggregates has been obtained from DLS and TEM measurements. The aggregation properties of AAILSs have been analyzed as a function of structure of amino acids and compared with those of analogous ionic liquids (ILs) and conventional ionic surfactants. Surface activity of the AAILSs has been found superior to that of analogous ILs and conventional ionic surfactants of the same alkyl chain length.

Fluorescence behavior and specific interactions of an ionic liquid in ethylene glycol derivatives.

The excitation-wavelength- and concentration-dependent fluorescence response of an ionic liquid (IL), 1-octyl-3-methylimidazolium tetrafluoroborate [Omim][BF(4)], in the ethylene glycol derivatives, [CH(3)(OCH(2)CH(2))n-OH, n = 1-3], has been critically examined in the entire composition range. The pure IL exhibited a large red shift beyond an excitation wavelength of 390 nm, showing the heterogeneous nature of the liquid. Concentration dependence of the fluorescence spectra in the organic solvent-rich region favors the association of IL molecules into the aggregated structures. The maximum of the fluorescence spectra shifted toward blue with the increase of organic component concentration in all of the mixtures, reflecting an appreciable solute-solvent interaction. Very high concentrations of the organic liquid in the mixtures resulted in the inversion of the spectral shift toward red, indicating the dominance of pi-pi* transitions over the n-pi* transitions as a consequence of imidazolium ring stacking. (1)H NMR and FT-IR investigations over the whole composition range of the mixtures showed multiple hydrogen-bonded interactions of varying strengths between the unlike molecules and the existence of associated species of the IL in the dilute region. Both the specific interactions between unlike molecules and the tendency of aggregation of IL molecules in the dilute region reduced with the introduction of the -OC(2)H(4) group in the glycol derivative. A comparison of specific interactions with the volumetric properties of the similar mixtures shows that the packing efficiency depending on differences in the shape and size of the molecules mainly decides the overall magnitude of deviations from ideality.

Dissolution, regeneration and ion-gel formation of agarose in room-temperature ionic liquids

The suitability of several ionic liquids, containing imidazolium or pyridinium cations with different alkyl chains and anions ranging from small hydrogen-bond acceptors to those of a large and non-coordinating nature, has been tested for solubilization of a widely used biopolymer, agarose. The solubility of agarose was found to depend on both the nature of anion and amphiphilicity of the cation. Dissolved agarose was regenerated using methanol, and ionic liquids were recovered and recycled for different experiments. Regenerated agarose largely maintained the features of native agarose in terms of molecular weight, polydispersity, thermal stability and crystallinity but varied slightly in conformation preferences. Subsequently, agarose-based highly conducting soft ion-gels having small thermal hysteresis were prepared and characterized. Such ion-gels have possible applications as electrochemical devices.

Task‐Specific, Biodegradable Amino Acid Ionic Liquid Surfactants

Besides several conventional uses, such as in cleaning products, food, beverages, dairy processing, water treatment, healthcare, fuel and lubricant additives, and emulsifiers or stabilizers in paints or cosmetics, there is an increasing interest in the synthesis of new surfactants for high-end applications such as gene transfection agents, the denaturation/encapsulation of proteins or drugs, or templates for shapeor size-selective and highly ordered nanomaterials. Because of their modular nature and unique physicochemical properties, ionic liquids (ILs) have found widespread application and are used in many areas of chemistry. The inherent amphiphilic character of some ILs has yielded surface-active properties that are different (and better) than those of conventional surfactants, and thus they have emerged as a superior class of surfactants. Also, conventional ionic surfactants suffer from phase separation, owing to solubility limitations, and the fact that the creation of hierarchical micellar systems is usually thermodynamically unfavorable. This restricts their use in many applications, for example, as templates for a desired nanomaterial architectures. In contrast, owing to their strong and directional polarizability and excellent water solubility ILs have been shown to self-assemble into highly structured forms, useful for the preparation of a variety of nanomaterials (e.g. , metals, metal oxides, zeolites). Although IL surfactants used so far are “green” in terms of their negligible vapor pressure, they generally contain synthetic quaternary nitrogen cations (such as alkylammonium, dialkylimidazolium, or pyridinium) with halogen atoms as anions (such as Cl or F). They can release HCl or HF by hydrolysis under certain conditions, which may pose a hazard when they are released into the environment through wastewater effluents. Therefore, toxicity and biodegradation are vital issues when dealing with ILs. In this context, the green credentials of ILs have been tremendously improved by the development of biobased ILs. Herein, we choose natural amino acids and sodium lauryl sulphate (SLS) as precursors for amino acid ionic liquid surfactant (AAILS) architectures. Amino acid-based surfactants, featuring amino acids modified with long aliphatic chains to generate linear, dimeric, or glycerolipid-like structures, have been reported extensively, however, ionic liquid surfactants based on amino acids having a superior surface activity and solvent miscibility are reported here for the first time. From the natural amino acids, l-glycine, l-alanine, l-valine, l-glutamic acid, and l-proline were chosen allow simple variations in the side chain through branching, the addition of another COOH group, or cyclization. Because the incorporation of an ester group into an amino acid has been shown decrease the melting point and significantly increase biodegradability, esterification of the amino acids was carried out by using either isopropyl or isobutyl alcohol. For esterification, thionyl chloride was slowly added to isopropyl or isobutyl alcohol at 0 8C. Amino acids were slowly added to the reaction mixtures, which were then refluxed for 4 h. The reaction mixtures were concentrated in a rotary evaporator, and crude amino acid ester hydrochlorides were titurated with hexane at 0 8C. Pure crystals of amino acid ester hydrochlorides (AAECls) were obtained by recrystallization with methanol/hexane. Equimolar amounts of the AAECls and SLS were then dissolved in hot water. After the completion of reaction, water was removed under vacuum and AAILSs were extracted by the addition of dichloromethane. All of the AAILSs except the one obtained from glutamic butyl ester hydrochloride (white crystalline solid at room temperature) were clear but slightly viscous liquids at room temperature. The reaction sequence is shown in Scheme 1. Detailed preparation, washing, and drying procedures can be found in the Supporting Information. The structures of the AAILSs were confirmed by

Interaction of Gelatin with Room Temperature Ionic Liquids: A Detailed Physicochemical Study

Interaction of gelatin (G) with room temperature ionic liquids (ILs), 3-methyl-1-octylimidazolium chloride [C(8)mim][Cl] and 1-butyl-3-methylimidazolium octylsulfate [C(4)mim][C(8)OSO(3)], have been investigated through tensiometry, conductivity, steady-state fluorescence, turbidity, and dynamic light scattering (DLS). We have observed that the nature of interactions in G-[C(8)mim][Cl] system are remarkably different as compared to G-[C(4)mim][C(8)OSO(3)] system. At low concentrations, much below the critical micelle concentration (cmc) of IL, the IL monomers are adsorbed at the native G at the interface forming G-IL (monomer) complex, whereas both the monomers and lower IL aggregates are interacted with G in bulk leading to G-IL (aggregate) complex. The increased hydrophobic character of the G-IL complexes is evidenced from pyrene fluorescence. Turbidity measurements showed interestingly distinguished coacervation characteristics in the investigated systems. In case of G-[C(4)mim][C(8)OSO(3)] system, the coacervates dissolve in the free micellar solution whereas G-[C(8)mim][Cl] coacervates remain stable up to very high concentration. DLS provided useful information about the changes in size of gelatin and the nature of interactions between gelatin and ILs. Thermodynamic parameters of micellization with and without gelatin have been derived and compared.

Aqueous-Mixed Ionic Liquid System: Phase Transitions and Synthesis of Gold Nanocrystals

Micelle-vesicle-micelle (MVM) transitions are observed in the aqueous-mixed ionic liquid (1-butyl-3-methylimidazolium octyl sulfate and 3-methyl-1-octylimidazolium chloride) system. The surface activity of mixed ILs, phase behavior, and solution structures in the system have been thoroughly characterized using conductometry, tensiometry, fluorimetry, dynamic light scattering (DLS), viscometry, turbidity, atomic force microscopy (AFM), transmission electron microscopy (TEM), and (1)H NMR techniques. Synergetic interactions between the two ILs in monolayers at the air/water interface and in micelles/vesicles have been determined using the regular solution approach, and the origins of spontaneous vesicle formation in this novel system are discussed. Using a photoreduction method, the formation of stable gold nanoparticles (GNPs) and microscale nanosheets of different shapes and sizes in the micellar and vesicle solutions has been reported. The studies show the potential of a mixed IL system in constructing stable micelles/supramolecular assemblies, such as bilayer vesicles, which are effective in the preparation of the desired nanomaterials.

Effect of ethylene glycol and its derivatives on the aggregation behavior of an ionic liquid 1-butyl-3-methyl imidazolium octylsulfate in aqueous medium.

The effect of ethylene glycol (EG) and its derivatives, ethylene glycol monomethyl ether (EGMME), or ethylene glycol dimethyl ether (EGDME), on the aggregation behavior of a surfactant-like ionic liquid (IL), 1-butyl-3-methyl imidazolium octylsulfate, [C(4)mim][C(8)OSO(3)], in aqueous solutions is investigated using conductivity, surface tension, fluorescence, (1)H NMR, and dynamic light scattering (DLS) measurements. Thermodynamic parameters such as Gibbs free energy (ΔG(m)°), standard enthalpy (ΔH(m)°), and standard entropy (ΔS(m)°) of aggregation are determined from the temperature dependence of conductivity. The interfacial properties of IL at the air/water interface in various mixed solvents are evaluated from surface tension measurements. Information about the local microenvironment and size of the aggregates is obtained from steady-state fluorescence using pyrene as a polarity probe and DLS measurements, respectively. (1)H NMR data has been employed to get detailed insight into the effect of organic additives on the IL aggregate structure and aggregation number. It has been observed that the addition of organic additives to water decreases the spontaneity of aggregation of IL.

Polarity Behaviour and Specific Interactions of Imidazolium-Based Ionic Liquids in Ethylene Glycol

The molecular interactions of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate [C(4)mim][BF(4) ], 3-methyl-1-octylimidazolium tetrafluoroborate [C(8)mim][BF(4)] and 1-butyl-3-methylimidazolium octylsulfate [C(4)mim][C(8)OSO(3)] are investigated in ethylene glycol (EG) over the whole mole fraction range using fluorescence (steady-state and time-resolved), Fourier transform infrared and nuclear magnetic resonance (NMR) spectroscopy. The cybotactic region surrounding the pyrene fluorescent probe exhibits peculiar characteristics for different ILs in the EG-rich region. The extent of solute-solvent interactions is assessed by determining the deviations of experimentally observed vibronic band intensity ratios of peak 1 to peak 3 of pyrene fluorescence (I(1)/I(3)) from a composite I(1)/I(3) value obtained using a preferential solvation model. A distinct vibrational frequency shift for various stretching modes of EG (O−H) or ILs (C−H of ring protons, B−F and S=O of anions) indicates specific interactional preferences of EG toward the IL protons/anion. Splitting of the O−H vibration band of EG at 3000-3700 cm(-1) into three separate bands, and analysis of the changes in location and area of these bands as a function of concentration enable precise determination of the effect of ILs on hydrogen bridges of EG. NMR chemical shifts and their deviations from ideality show multiple hydrogen-bonding interactions of varying strengths between unlike molecules in the mixtures. A comparison of spectroscopic results with thermodynamic properties shows that the mixing microscopic behaviour of the investigated systems is completely different from the macroscopic behaviour, which is primarily governed by the difference in shape, size and nature of the molecules.

Effect of Sodium Sulfate on the Gelling Behavior of Agarose and Water Structure Inside the Gel Networks

Agarose hydrogels which constitute a special class of soft matter are undoubtedly one of the most studied biopolymer gels. However, certain issues such as why the sulfate salts and sulfate content in the agarose molecules reduce the gel strength are still not very clear. The present work provides a detailed analysis of structural changes with respect to coil-helix transition or aggregation of helices in the aqueous agarose solutions and hydrogels that accompanied the systematic addition of sodium sulfate. A combined approach which includes the differential scanning calorimetry and temperature-dependent vacuum-ultraviolet circular dichroism measurements permitted the accurate estimates of the energy changes for coil-helix transition and helix-helix interactions. The hydration process of agarose molecule investigated from differential scanning calorimetry and concentration-dependent ultrasonic measurements indicated the loss of both the freeze bound and nonfreezable water molecules with the increase of sulfate content in the solution. Temperature-dependent fluorescence measurements using pyrene as a probe indicated polarity changes when the gel network is created in waters of different salt concentration. Changes in the hydrogen bonding of the water molecules confined in the gel network have been monitored from the intensity ratios of ice-like and liquid-like -OH stretching band of water Fourier transform infrared (FTIR) spectra. Analysis of the -OH stretching band showed the strengthening of hydrogen bonding of water molecules in the gel which tend to weaken with the subsequent addition of sodium sulfate.