Ordered Molecular Aggregates Constructed By Surface Active Ionic Liquids

Ionic liquids (ILs) have attracted much attention for their specific physicochemical properties. Designability is one of important properties of ILs. Their physicochemical properties can be controlled by a reasonable selection of cations, anions, and substituents. There has emerged a novel kind of amphiphiles and known as surface active ionic liquids (SAILs). This combination of ILs and surfactant brings the unique properties of ILs into colloid field, which is quite interesting and significant. In this dissertation, we synthesized new series of SAILs and investigated their various aggregation behavior in water and mixed solvents. There are four main parts in this dissertation as follows:Chapter 1 is a brief introduction of basic knowledge and recent achievements? related to the ordered aggregates constructed by SAILs, and the ideas for research were present.Chapter 2. Halogen-free alkylcarboxylate-based anionic SAILs, namely 1-butyl-3-methylimidazolium alkylcarboxylates ([C4mim][CnH2n-1O2], n= 8,10,12), were synthesized through the simple neutralization of imidazolium hydroxide by alkylcarboxylic acids. A systematic study of their self-aggregation behavior in water was investigated by surface tension, electrical conductivity, steady-state fluorescence quenching and 1H NMR. The micellization properties of this series of SAILs in ethylammonium nitrate (EAN) were also studied by surface tensiometry for comparison. The results shows that they exhibit a higher ability to aggregate in water than the traditional anionic surfactants, sodium alkylcarboxylates (SAC). It can be attributed to the more effective screening for intramicellar electrostatic repulsions offered by [C4mim]+, which decreases the charge repulsion between polar head groups then facilitates the formation of micelles. Moreover, the higher hydrophobicity of [C4mim]+ is considered to result in the lower cmc values of these SAILs. The surface activity of [C4mim][CnH2n-1O2] is also superior to another anionic SAILs, 1-butyl-3-methylimidazolium alkylsulfates ([C4mim][CnH2n+1SO4]) with the same hydrocarbon chain length. Density functional theory (DFT) calculations manifest that the higher surface activity of [C4mim][CnH2n-1O2] mainly originates from the lower electronegativity of its anion by comparison to [CnH2n+1SO4]-. Surface tension results show that the surface activity of [C4mim][CnH2n-1O2] in EAN is inferior to that in water. DFT calculations represent that the interaction energy of binary combination SAILs-EAN is larger than that of SAILs-H2O due to the formation of hydrogen-bonding, resulting in a higher cmc value in EAN.Chart 3. Aggregation behaviour of 1-butyl-3-methylimidazolium dodecylsulfate ([C4mim][C12H25SO4]) was investigated in water-ethylammonium nitrate (EAN) mixed solvents. It is particular interesting that this simple surfactant could not only form lyotropic liquid crystals (LLC) with multi-mesophases, i.e. normal hexagonal (H1), lamellar liquid crystalline (Lα), and reverse bicontinuous cubic phase (V2) in the water-rich environment, but also act as an efficient low molecular-weight gelator (LMWG) which gelated EAN-abundant binary media in a broad concentration range. The constructions of ordered aggregate can be readily manipulated by adjusting [C4mim][C12H25SO4] concentration and solvent composition. The peculiar nanodisk cluster morphology of gels composed of similar bilayer units was observed by TEM and SEM observations. FT-IR spectra and density functional theory (DFT) calculations reveal that strong H-bonding and electrostatic interactions between EAN and the head group of [C4mim][C12H25SO4] are primarily responsible for gelation. The self-assembled gels displayed thermoreversible sol-gel transition and excellent mechanical strength comparable to polymer gels. This environmentally friendly system is expected to have broad applications in various fields, such as materials science, drug delivery systems, and supramolecular chemistry.Chart 4. A supramolecular gel was constructed by using a COOH-functionalized imidazolium-based SAIL, N-cetyl-N’-carboxymethyl imidazolium bromide ([N-C16, N’-CO2H-Im]Br), in the DMSO/H2O binary solvent mixtures. With increasing DMSO contents in the mixed solvents, critical gelation concentration (CGC) shows a trend of increase, while gel-sol phase-transition temperature (Tgei) decreases. The self-assembled gel displays a morphology of microplatelet stacked by bilayer units with interdigitated hydrocarbon tails, and the structure remains unchanged below the sol-gel transition temperature. Few LMWGs present such a type of self-assembly process:0 D (gelator)â†'2D (nanothin layer)â†'3D (platelets). The gel also exhibits strong birefringence property and excellent mechanical strength. In particular, the gels show superior performance in removal of the anionic dye molecules, indicative of their prospect in water treatment.