Photo-Responsive Ordered Molecular Aggregates Constructed By Surface Active Ionic Liquids

Stimuli-responsive colloid materials and soft matters have attracted considerable attention of the global academics in the past decade. Their microstructure and macroscopic properties could change dramatically with minor variation in the external environment stimulus (such as pH, temperature, light, electricity, and magnetism). Studies on the photo-responsive ordered molecular aggregates have become one of hot topics due to their advantages such as without additives, simple operation and extensive application range. As novel and green solvent, designability is one of important properties of ionic liquids (ILs). ILs bearing long alkyl chain can be named as surface active ionic liquids (SAILs). Photo-responsive ordered molecular aggregates constructed by SAILs can enrich the properties of molecular aggregates formed by SAILs and also exploit research category of photo-responsive systems. Based on above, in this dissertation, we designed a series of photo-responsive ordered molecular aggregates which were constructed by SAILs and comprehensively characterized their microstructures and macroscopic physicochemical properties before and after UV light irradiation by cryogenic transmission electron microscopy (Cryo-TEM), polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. At the same time, we also explained the formation mechanism of the photo-responsive ordered molecular aggregates through experimental techniques combing with theoretical calculation. There are four main parts in this dissertation as follows.Chapter 1 is a brief introduction of basic knowledge, recent achievements closely related to the aggregation behavior of SAILs and stimuli-responsive ordered aggregates constructed by surfactants, and the research ideas for this dissertation.In Chapter 2, we investigated the UV light-stimulated self-assembly behavior of a SAIL, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), with an azobenzene derivative, sodium azobenzene 4-carboxylate (AzoCOONa) in aqueous solution. The properties and structures of the aggregates, formed at a concentration ratio of 2:1 ([Ci6mimBr]:[AzoCOONa]), were comprehensively characterized by rheometer and Cryo-TEM. Initially, viscoelastic wormlike micelles with a viscosity of 0.65 Pa·s were constructed in the C16mimBr/AzoCOONa system. Upon irradiation by UV light (365 ran), particularly fascinating is that the wormlike micelles became much longer and more entangled, exhibiting a high viscosity of 6.9 Pa · s. This can be attributed to a photoisomerization of AzoCOONa molecule from trans to cis forms. The cation-π interaction prevailing over the hydrophobic interaction and electrostatic interaction between C16mimBr and AzoCOONa molecules is supposed to be responsible for this peculiar phase behavior. The wormlike micelles constructed with SAIL and photo-sensitive additive exhibit controllable viscoelastic behavior in the photo-responsive process. It is the first time that, with exposure to UV or visible light, the aggregate type of the photoresponsive system has remained unchanged, with only a change of internal property parameters.In Chapter 3, we synthesized two imidazolium-based SAILs with photoresponsive cinnamate aromatic counterions, viz. 1-dodecyl-3-methylimidazolium cinnamate ([C12mim][CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C12mim][PCA]) and systematically explored their self-assembly behaviors in aqueous solutions. Results of surface tension and conductivity measurements show that both [C12mim][CA] and [C12mim][PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecyl-3-methylimidazolium bromide (C12mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C12mim][CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C12mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H1) and cubic liquid-crystalline phase (V2) were constructed in the [C12mim][CA] aqueous solutions. In contrast, the [C12mim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by POM, SAXS, and rheometer. Under UV irradiation, trans-cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases.In Chapter 4, we investigated a novel UV light-stimulated fluid composed of N-hexadecyl-N’-methylpyrrolidinium bromide (C16MPB) and photosensitive compound, sodium cinnamate (CAS), in aqueous solution. When a certain amount of trans-CAS (60-140mM) was added to 100 mM C16MPB, viscoelastic wormlike micelles was constructed. The concentration of trans-CAS has a significant influence on the viscoelastic proeprties and the average contour length of wormlike micelles, which was characterized by rheological measurements. However, after the UV irradiation, the long wormlike micelles transformed into spherical micelles. This can be attributed to a photoisomerization of CAS molecule from trans to cis forms, which results in the decrease of hydrophobicity and increase of steric hindrance. As a consequnce, the molecular critical packing decreases and wormlike micelles transform to spherical micelles, with a sharp drop in the viscosity value on a macro-scale.The research work mentioned above gives a guideline for the construction of stimuli-responsive orderer molecular aggregates. The photoresponsive wormlike micelles constructed with SAILs exhibit controllable viscoelastic behavior and the photoresponsive LLCs formed by SAILs show adjustable structural tightness in the photoresponsive process. They are expected to have broad applications in various fields, such as drug delivery, biochemistry, and materials science.