| The self-assembly of surfactant molecules is of fundamental interest and is important in many applications such as nanomaterial synthesis, drug delivery, separation, pharmaceutical formulation, and other dispersant technologies. Recently, the mixed systems of surfactants with salt or other surfactants have received many attentions due to their higher surface activity than individual surfactant. The cationic and anionic surfactants mixed system, generally called catanionic system, is taken as a typical example, which has unique characteristics. Many catanionic systems have their own specialty in forming aggregates with various morphologies. The mixed surfactant systems can also transform from one phase to another phase under external stimuli, such as light, redox reaction, pH. and temperature. Therefore, the study of mixed surfactant systems is valuable in the theoretical research.As a novel kind of green solvents, ionic liquids (ILs) have attracted a lot of attentions due to their unique properties. Their physicochemical properties can be easily modulated by suitable selection of cations and anions. An interesting characteristic of ILs is that the cations possess an inherent amphiphilicity, such as1-alkyl-3-methylimidazolium ([C17mim]+),. Therefore, these ILs can exhibit as novel surfactants to self-assemble to form aggregates with specific structure, shape, and properties. The investigations of surface active ILs can enrich the species of ILs and also establish the basis for their applications in different fields. There are four main parts in this dissertation.Chapter one is a brief introduction of the research background of this work, in which the basic knowledge of surfactant solution and aggregates of surface active ILs are introduced. Especially, the properties of wormlike micelles and the typical system of formed wormlike micelles are focused on.In Chapter two, a photo-responsive system composed of an imidazolium-type surfactant1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br) and a commonly photosensitive aromatic compound cinnamic acid (CA) is investigated. There are several isoforms and analogs of CA in nature. The trans-and cis-forms are two configurational stereoisomers of CA, and trans-CA is the predominate form of natural CA. So in this work, the mixed system of [C16mim]Br and trans-CA is studied.. Under the UV irradiation, trans-CA can transform to cis-CA. UV spectra proves that the structural transformation of CA, and the conversion ratio calculated by the1H NMR spectrum is60%. In aqueous solution, the compounds [C16mim]Br and trans-CA can form three phase regions:homogeneous phase (spherical micelles), heterogeneous phase (precipitation), and homogeneous phase (wormlike micelles). When the concentration of [C16mim]Br and trans-CA reaches a certain value, the mixed system shows highly viscous, viscoelastic, and flow-birefringence phenomenon. All these phenomenon and rheological results indicate the presence of wormlike micelles. After the UV irradiation, the flow-birefringence phenomenon disappears, meanwhile the viscosity drops. The rheological and visual observations indicate a phase transition from long, wormlike micelles to much shorter micelles upon UV irradiation. Cryo-TEM pictures further proves the phase transition intuitively. This transition is due to the molecular photoisomerization of CA molecules from trans-to cis-, which results in the increase of effective cross-sectional. Then the molecular critical packing decreases and wormlike micelles transform to spherical micelles.In Chapter three, the formation of wormlike micelle of surface active ILs induced by organic salts in water is investigated. Rheological methods and cryo-TEM measurement were employed to study the viscoelastic properties and formation of wormlike micelles, which are formed by N-hexadecyl-N-methylpyrrolidinium bromide (C16MPB) in aqueous solution in the presence of sodium tosylate (NaTos). The NaTos concentration is varied while the C16MPB concentration is fixed.The surface active ILs can form wormlike micelles in the presence of organic salts, which is similar to the conventional ionic surfactants (e.g., alkyltrimethylammonium bromide) in aqueous solution. At low shear rates, the relationship between stress and shear rate is linear. There is almost no stress yield value for the C16MPB/NaTos wormlike micelles, exhibiting pseudoplastic fluids. The modulus G’ is initially smaller than G", indicating that the system is more viscous than elastic. Then, as frequency increases, G’> G", showing the elastic behavior. The steady-shear viscosity (η) and the absolute value of the complex viscosity (η*) are superimposed at equivalent values of shear rate(γ) and frequency (ω), suggesting that C16MPB/NaTos wormlike micelles obeys the Cox-Merz rule. The C16MPB/NaTos wormlike micelles also follow the Maxwell behavior. The Cryo-TEM images confirms the formation of wormlike micelles in aqueous solution. The effect of hydrocarbon chain length of ILs on the wormlike micelles formation is also studied.In Chapter four, the mixed system of cationic and anionic surfactants is studied. The mixtures of cationic and anionic surfactant generally have great industrial significance. They exhibit a wide range of unique properties, aggregate morphologies, and interesting phase behaviors. However, except for few systems, most of the mixtures of catanionic surfactant form precipitates or become turbid, which limits their research and application. In this work, we investigated the mixed system of C16MPB and sodium dodecylbenzenes sulfonate (SDBS). The work can be divided into two parts:the homogeneous phase and aqueous two-phase system (ATPS) at different mixture molar ratios. The results are listed below:(1) C16MPB and SDBS can form homogeneous phase at high molar ratio. When the molar ratio of C16MPB:SDBS is greater than10, the homogeneous phase is ordinary solutions without polarized light and viscoelasticity. When the ratio of C16MPB: SDBS is between7.3and5. the solution shows high viscoelasticity. Typical rheological results indicate that the viscoelasticity of system follows the Maxwell model. This is due to the strong electrostatic interactions between the oppositely charged headgroups.(2) The system of C16MPB/SDBS forms ATPS within a narrow range near equimolar ratio. Time of ATPS formation is related to total concentration of surfactant. The higher the concentrations are, the longer the formation time becomes. The longest formation time of ATPS in experiment range is about two week. The formed ATPS is very stable and can be maintained for more than one year at25℃. All the POM, DLS, TEM, and surface tension results confirm that the aggregates are in the upper phase, and lower phase of ATPS contains vesicles. The possible formation mechanism of ASTP can be explained as follows. When C16MPB and SDBS are mixed at equimolar ratio, vesicles are formed. More and more vesicles aggregate together. Then the aggregated vesicles form a type of network structure containing water and the network structure separates from the original phase. Compared with the other dispersed vesicles, the surface charge of aggregated vesicles is closer to1:1which led to lower density. As a result, the network structure composed of aggregated vesicles formed the upper part of ATPS, while those dispersed vesicles formed the lower part. |
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