Ionic liquids in microemulsions – a concept to extend the conventional thermal stability range of microemulsions

摘要

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.udThe 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.udIn 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.udAll 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. udConventional 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.ud