Oliver Zech

Microemulsions with an ionic liquid surfactant and room temperature ionic liquids as polar pseudo-phase.

In this investigation we present for the first time microemulsions comprising an ionic liquid as surfactant and a room-temperature ionic liquid as polar pseudo-phase. Microemulsions containing the long- chain ionic liquid1-hexadecyl-3-methyl-imidazolium chloride ([C16mim][Cl]) as surfactant, decanol as cosurfactant, dodecaneas continuous phase and room temperature ionic liquids (ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]][BF4]), respectively) as polar microenvironment have been formulated. The phase diagrams of both systems were determined at a constant surfactant/cosurfactant molar ratio. EAN microregions in oil have been confirmed with conductivity measurements. In presence of EAN a model of dynamic percolation could be applied. Dynamic light scattering (DLS) measurements indicated a swelling of the formed nano-structures with increasing amount of EAN, a linear dependence of the hydrodynamic radii on the EAN weight fraction was observed. Both systems exhibited a single broad peakin SAXS and follow a characteristic q-4 dependence of the scattering intensity at large q values. The Teubner-Strey model was successfully used to fit the spectra giving fa, the amphiphilic factor, and the two characteristic length scales of microemulsions, namely the periodicity, d, and the correlation length, zeta. Furthermore, the specific area of the interface could be determined from the Porod limit and the experimental invariant. The amphiphilic factor clearly demonstrated structural differences between the two systems confirming that the nature of the polar ionic liquid plays an important role on the rigidity of the interfacial film. The adaptability of three different models ranging from spherical ionic liquid in oil over repulsive spheres to bicontinuous structures has been checked.

Ionic Liquids in Microemulsions-A Concept To Extend the Conventional Thermal Stability Range of Microemulsions

Ionic liquids (ILs), which are defined as salts with a melting point below 100°C are often considered as future solvents for catalysis, chemical reactions, extractions and electrochemical purposes. Apart from these classical applications, ILs have also gained interest in classical colloid and surface chemistry. The formation of amphiphilic association structures in and with ionic liquids, such as micelles, vesicles, microemulsions and liquid crystalline phases have been described in literature. The thesis can be subdivided into three main parts: Conductivity studies of the anion effect on imidazolium based ionic liquids over a wide temperature range (-25-195)°C, formulation and characterization of nonaqueous, high temperature stable microemulsions with room temperature ionic liquids as polar phase and the synthesis and characterization of new ionic liquids based on alkali cations. In the first part conductivities of four different highly pure imidazolium based room temperature ionic liquids (RTILs) have been studied within a temperature range between (-25 to 195)°C. Thereby, the cationic scaffold, the 1-butyl-3-methylimidazolium cation ([bmim+]), was kept constant while the anions were varied. The investigated anions were dicyanamide ([DCA-]), hexafluorophosphate ([PF6-]), trifluoroacetate ([TA-]) and trifluoromethanesulfonate ([TfO-]). It is quite surprising that studies of important physicochemical transport properties are still scarce in the field of ionic liquids. At a given temperature the conductivity decreased in the order [bmim][DCA] > [bmim][TA] > [bmim][TfO] > [bmim][PF6]. Temperature dependence of the conductivity could be well described by the empirical Vogel-Fulcher-Tammann equation. Whilst our data compare favorably with some literature results, significant deviations from others were noted. To calculate the molar conductivity of the RTILs densities were measured between (5 and 65)°C. Walden plots of the molar conductance, available for [bmim][PF6], [bmim][TfO] and [bmim][TA] in the limited temperature range of (5 to 65)°C, suggest that these RTILs can be classified as high-ionicity ionic liquids. All studies concerning ILs in microemulsions described in literature have been performed below the boiling point of water. In the present work, we were interested in microemulsions that are stable over a wide temperature range at ambient pressure. For this purpose, water must be replaced by a RTIL. Two different RTILs were used to replace water in microemulsions, namely ethyl ammonium nitrate (EAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]][BF4]). Furthermore, the microemulsions contained the long- chain ionic liquid 1-hexadecyl-3-methyl-imidazolium chloride ([C16mim][Cl]) as surfactant, decanol as cosurfactant, and dodecane as oil phase. Both systems were studied in a temperature range between (30 and 150)°C. The most promising microemulsions were obtained with EAN. The microemulsions were characterized by means of conductivity, rheology, dynamic light scattering (DLS), small angel X-Ray (SAXS) and small angle neutron scattering (SANS). In the case of EAN reverse microemulsions with EAN cores were obtained. The EAN systems exhibited a typical percolation behavior over the whole investigated temperature range, the corresponding percolation threshold volume fractions were significantly influenced by temperature. All scattering experiments were in agreement with EAN droplets stabilized by surfactants in a continuous oil matrix. The temperature dependent SANS experiments confirmed the existence of microemulsions up to 150°C. The results obtained for the [bmim]][BF4] system demonstrate the high thermal stability of these microemulsions as well, whereby the structures are less defined and can be assumed to be more a bicontinuous one than a reverse microemulsion. Conventional ILs typically contain bulky organic cations with a low degree of symmetry such as imidazolium, pyrrolidinium, tetraalkylphosphonium, trialkylsulfonium or quaternary ammonium. These cations hinder the regular packing in a crystal lattice. Consequently, the solid crystalline state becomes energetically less favorable, leading to low melting points. This effect can be enhanced further by the implementation of an anion with a delocalized charge, resulting in decreased interionic interactions. To date, little attention has been paid to systems of ionic liquids involving small inorganic cations. In this work, ionic liquids based on small inorganic cations and oligoether carboxylate anions were successfully synthesized. A new family of ILs comprising alkali cations and 2,5,8,11-tetraoxatridecan-13-oate (TOTO) as anion and alkali cations have been developed. These substances are promising materials due to their pronounced electrochemical and thermal stability. The concept of the ionicity plot was successfully applied to the sodium salt for which strong ion pairing was observed. Further, it was shown that the cytotoxicity of such “simple” alkali carboxylate ionic liquids compared to conventional imidazolium based ILs is very low.

Conditions for and characteristics of nonaqueous micellar solutions and microemulsions with ionic liquids

Research on nonaqueous microemulsions containing ionic liquids as polar and/or apolar phase, respectively, is growing at a fast rate. One key property of ionic liquids that highlights their potential and their diversification compared to water is their wide liquid temperature range. In this emerging-area review article we survey recent developments in the field of nonaqueous micellar solutions and microemulsions containing ionic liquids in general with a strong emphasis on the effect of temperature in particular. Various systems are discussed and compared to their aqueous counterparts.

Alkali Metal Oligoether Carboxylates—A New Class of Ionic Liquids

On the way to greener ILs: The combination of a short oligoether carboxylate (CH3O-(CH2CH2O)3-CH2COO-) with small alkali metal cations leads to the formation of a new class of ionic liqs. that exhibit high thermal and electrochem. stability as well as low cytotoxicity.

[emim][etSO4] as the Polar Phase in Low-Temperature-Stable Microemulsions

We demonstrate here that microemulsions with an IL as the continuous phase can be formed so that they are stable over a wide temperature range and have intermediary properties between flexible and stiff microemulsions. Three components (1-ethyl-3-methylimidazolium ethylsulfate ([emim][etSO(4)]), limonene, and octylphenol ethoxylate (Triton X 100, abbreviated as TX-100)) were used. This ternary system has been characterized from ambient temperature down to -10 °C by means of conductivity, viscosity, and small-angle X-ray scattering (SAXS) measurements. The SAXS data exhibit a characteristic single, broad scattering peak in conjunction with a typical q(-4) decay at large q values. The SAXS data have also been interpreted in terms of a dimensionless dilution plot, demonstrating that microstructures are neither isolated droplets nor a random flexible film structure but resemble molten liquid crystals (i.e., they are formed from locally cylindrical or planar structures). This semirigidity is attributed to a good match between the surfactant and the ionic liquid; this holds in a temperature range well below 0 °C.

Oligoether Carboxylates: Task-Specific Room-Temperature Ionic Liquids

Recently, a new family of ionic liquids based on oligoether carboxylates was introduced. 2,5,8,11-Tetraoxatridecan-13-oate (TOTO) was shown to form room-temperature ionic liquids (RTILs) even with small alkali ions such as lithium and sodium. However, the alkali TOTO salts suffer from their extremely high viscosities and relatively low conductivities. Therefore, we replaced the alkali cations by tetraalkylammonium (TAA) ions and studied the TOTO salts of tetraethyl- (TEA), tetrapropyl- (TPA), and tetrabutylammonium (TBA). In addition, the environmentally benign quaternary ammonium ion choline (Ch) was included in the series. All salts were found to be ionic liquids at ambient temperatures with a glass transition typically at around -60 °C. Viscosities, conductivities, solvent polarities, and Kamlet-Taft parameters were determined as a function of temperature. When using quaternary ammonium ions, the viscosities of the resulting TOTO ionic liquids are >600 times lower, whereas conductivities increase by a factor of up to 1000 compared with their alkali counterparts. Solvent polarities further reveal that choline and TAA cations yield TOTO ionic liquids that are more polar than those obtained with the, per se, highly polar sodium ion. Results are discussed in terms of ion-pairing and structure-breaking concepts with regard to a possible complexation ability of the TOTO anion.

Biodiesel, a sustainable oil, in high temperature stable microemulsions containing a room temperature ionic liquid as polar phase

Biodiesel has gained more and more attention in recent years resulting from the fact that it is made of renewable resources. Parallel to its environmental compatibility, biodiesel also exhibits a high thermal stability. We demonstrate here that biodiesel can replace conventional oils as apolar phase in nonaqueous microemulsions containing the room temperature ionic liquid ethylammonium nitrate as polar phase. In addition to the phase diagram and the viscosity of the microemulsions, we study the thermal stability of these systems. Along an experimental path in the phase diagram, no phase change could be observed between 30 °C and 150 °C. Conductivity measurements confirm the high thermal stability of these systems. The microemulsion exhibits a percolative behavior between 30 °C and 150 °C. Small angle X-ray scattering spectra show a single broad scattering peak similar to aqueous microemulsions. The spectra could well be described by the Teubner–Strey model. Furthermore, the adaptability of different models ranging from bicontinuous structures to ionic liquid in oil spheres as well as disordered open connected cylinders has been checked. These high temperature stable, nonaqueous, free of crude oil based organic solvent microemulsions highlight an efficient way towards the formulation of environmentally compatible microemulsions and open a wide field of potential applications.

Influence of surfactant amphiphilicity on the phase behavior of IL-based microemulsions.

In this work, we report on the phase behavior of 1-ethyl-3-methyl-imidazolium-ethylsulfate ([emim][etSO(4)])/limonene/polyethylene glycol tert-octylphenyl ether (Triton X-114 or TX-114) microemulsions as a function of ionic liquid (IL) content and temperature. Phase diagrams, conductivity measurements, and small angle X-ray scattering (SAXS) experiments will be presented. A hydrophilic IL, instead of water is used with the goal to enlarge the temperature range on which stable microemulsions can be formed. Indeed, the system shows remarkably large temperature stability, in particular down to -35 °C. We will emphasize on a comparison with a recently published work about microemulsions composed of [emim][etSO(4)], limonene, and Triton X-100 that to some extent are stable at temperatures well below the freezing point of water. The key parameter responsible for the difference in phase behavior, microstructure, and temperature stability is the average repeating number of ethylene oxide units in the surfactant head group, which is smaller for Triton X-114 compared to Triton X-100. Among the fundamental interest, how the amphiphilicity of the surfactant influences the phase diagram and phase behavior of IL-based microemulsions, the exchange of Triton X-100 by Triton X-114 results in one main advantage: along the experimental path the temperature where phase segregation occurs is significantly lowered leading to single phase microemulsions that exist at temperatures beneath 0 °C.

Low Toxic Ionic Liquids, Liquid Catanionics, and Ionic Liquid Microemulsions

In the future the demand of sustainable and low toxic surfactants and solvents will constantly increase. In this article, we present some new approaches to meet these requirements. Whereas ionic liquids are often based on imidazolium ions, we will show that there are also much less toxic ones, especially with choline as cation. Choline salts, even if solid at room temperature, can advantageously be mixed with other sustainable solids to form deep eutectic solvents, that is, “green” liquids at room temperature. Further, choline can be used to dissolve long-chain carboxylates in water thus maybe permitting new applications of soaps. Alternatively, choline and other natural cations can be part of promising low toxic cationic surfactants. By combining them with ethoxylated carboxylates, interesting charged room temperature liquid surfactant combinations can be obtained. Finally, we shortly discuss the potential benefits of ionic liquids in microemulsions.

Nonaqueous Microemulsions Containing Ionic Liquids – Properties and Applications

There is a still growing interest in ionic liquids (ILs) in general and room temperature ionic liquids (RTILs) in particular resulting from their fascinating and outstanding properties and wide range of potential applications. The research field of ILs was almost entirely related to imidazolium, pyridinium and pyrrolidinium based substances in the last decade (Welton 1999; Earle & Seddon 2000; Wasserscheid & Keim 2000). Beside these classical aprotic ionic liquids, attention has been paid to protic (Greaves & Drummond 2007; Greaves et al., 2008 b) and to the development of less toxic ILs (Tao et al., 2006; Fukaya et al., 2007; Pernak et al., 2007; Zech et al., 2009 a). ILs are often considered as future solvents for catalysis (Welton 1999; Wasserscheid & Keim 2000; Pârvulescu & Hardacre 2007; van Rantwijk & Sheldon 2007), chemical reactions (Haumann & Riisager 2008; Martins et al., 2008), extractions (Blanchard et al., 1999), and electrochemical purposes (Hapiot & Lagrost 2008). Apart from these applications, ILs also stimulated research in classical colloid and surface chemistry. The formation of amphiphilic association structures in and with ionic liquids, such as micelles, vesicles, microemulsions and liquid crystalline phases has been reviewed three times between 2007 (Hao & Zemb 2007) and 2008 (Qiu & Texter 2008; Greaves & Drummond 2008 a), reflecting the growing interest and progress in this field. In this review we focus on ILs in nonaqueous microemulsions, because significant new work has been reported in this field since these earlier reviews have been published. Microemulsions are thermodynamically stable, isotropic transparent mixtures of at least a hydrophilic, a hydrophobic and an amphiphilic component. The first microemulsion structures termed at that time “oleophatic hydro-micelle” were discovered in 1943 by Hoar and Schulman (Hoar & Schulman 1943). The term microemulsion was introduced by Schulman and coworkers in 1959 describing optically isotropic transparent solutions consisting of water, oil, surfactant and alcohol (Schulman et al., 1959). A more recent definition was given by Danielsson and Lindman: “A microemulsion is a system of water, oil and an amphiphile which is a single optically isotropic and thermodynamically stable liquid solution” (Danielsson & Lindman 1981). Herein, “water” corresponds to a polar phase that is classically an aqueous solution that can contain electrolytes and other