| Most nonionic surfactants possess polyethylene oxide chain or hydroxyl oxygen units as hydrophilic groups. Different from ionic surfactants, these surfactant molecules have no electrical charge in water. Thus, they have some unique characteristics of high stability, insensitivity to hard water and strong electrolyte inorganic salt, and good compatibility with other surfactants. Moreover, they can be assembled into various aggregates in different solvents, including micelle, liquid crystal, vesicle, and gel. In the past years, the aggregation behaviors of traditional nonionic surfactants in different solvents have been investigated in details. With the development of surfactant industry and the enhancement of environmental protection consciousness, sterol ethoxylate surfactants of good biocompatibility and low toxicity have attracted scientists’much attention to their physicochemical properties and applications. Due to the condensed sterol ring structure, they possess a more rigid character and a very stronger segregation tendency between the hydrophilic and hydrophobic parts compared to conventional alkyl ethoxylated nonionic surfactants. Thus, aggregates constructed by sterol ethoxylate surfactants could also show some characteristic of "rigidity". The regulation of structure and property is the characteristic of surfactant aggregates. Changes of environmental conditions can provide us more information on this kind of surfactant, such as aggregation behavior in the solvents and the regularity of molecular interaction. So the aggregates, including micelle, liquid crystal, and vesicle formed by sterol ethoxylate surfactants in different nonaqueous solvents have been investigated by small angle X-ray scattering (SAXS) and polarized optical microscopy(POM) in this dissertation. The studies can be divided into four parts.1. The aggregation behaviors of BPS-n in [Bmim]BF4. Aggregation behaviors of phyosterol ethoxylate surfactants (BPS-n) in apronic ionic liquids (AIL)[Bmim]BF4have been characterized by SAXS and POM. With the help of Fourier transform infrared spectra (FTIR) technique and the control experiment with traditional nonionic surfactants, the formation mechanism of ordered assemblies of BPS-n in [Bmim]BF4is recognized. Studies on aggregation behaviors of BPS-n in [Bmim]BF4and [Bmim]PF6have been done to explore the influence of anions of ionic liquids. Following results are obtained.(1) The BPS-n surfactants could form various lyotropic phases in [Bmim]BF4at room temperature. The stronger interactions between the steroid rings are proved to be the main driving force on the formation of the self-assemblies in ILs, which is not the same as the conventional nonionic surfactant, where the hydrogen bonding is demonstrated to be the dominating factor.(2) The stronger interactions between the steroid rings lead BPS-n a much closer packing, thus the LLC in BPS-10/[Bmim]BF4system show an obviously higher apparent viscosity.(3) The solvation of ILs also affects the phase behaviors of BPS-n surfactants. The LLC phases formed in [Bmim]BF4are few and the cubic phase appears only in systems with longer EO chains, which is different from those observed in water or [Bmim]PF6.Above results explore the differences between BPS-n and conventional nonionic surfactant on constructing the aggregates and enrich studies on BPS-n self-assembly in AIL. The influence of AIL on phase behaviors also provides useful information for further research on aggregation of BPS-n in AIL.2. Aggregation behaviors of BPS-n in two types of ionic liquids. Ionic liquids are classified as pronic ionic liquids (PILs) and apronic ionic liquids, depending whether or not they can form hydrogen-bonding networks. This work is focused on the aggregation behaviors of BPS-10in two different ionic liquids, EAN and [Bmim]BF4in order to explore the solvent effect. Following results can be obtained.(1) BPS-10can assemble into micelles and LLCs in EAN and [Bmim]BF4. Explicit phase behaviors differ in LLCs except for micelles. The H1phase, H1and La coexisting phase and La phase can be formed in BPS-10/EAN system while only La phase is formed in the BPS-10/[Bmim]BF4system.(2) Compared to [Bmim]BF4, EAN molecules form strong hydrogen-bonding network, which is similar to water. Hence, a stronger solvophobic interaction is found in the BPS-10/EAN system, resulting in lower CMC and smaller size of micelles. The formation of micelles is driven by entropy at low temperature and enthalpy at high temperature in the BPS-10/EAN system while only driven by enthalpy in the BPS-10/[Bmim]BF4system.(3) In the BPS-10/EAN system, the formation of LLCs is easier and the phase behaviors are richer for EAN’s due to hydrogen-bonding networks and higher Gordon parameter. The interaction becomes stronger between EAN and PEO segmentsSuch an investigation give a comparison betweeb aggregation behaviors of a surfactant in protic and aprotic ionic liquids directly, and help us better understand the influence of solvent structure on formation mechanism of ordered assemblies. Above results are a great supplement to previous reports on ionic liquids as solvent to construct aggregates.3. Organic solvents taken as solvents for self-assembly. Investigations have been done on the LLC formation in formamide (FA)、N-methylformamide (NMF), and N, N-dimethyl formamide (DMF). Following results can be obtained.(1) BPS-10can assemble into Hi and La phases in organic solvents. Compared with other systems, such H2CK EAN、and [Bmim]BF4, the LLCs formed in FA have high thermal stability.(2) With hydrogen replaced by methyl in formamide, organic solvents turn to aprotic solvent gradually. From FA to DMF, the ability of forming hydrogen-bonding become weaker and the structure is less ordered. Thus, solvophobic interaction of surfactants is not stronger and the LLCs is harder to form in aprotic solvent. According to the Gordon parameter, it is also proved that the driving force of solvent on aggregation is reduced, with the increase of methly replaced.(3) The stronger interactions between the steroid rings provide enough driving forces to form H1in NMF and DMF. These results present a new sequences (isotropic'H1'La) of ordered phases with increasing surfactant concentration.4. The effect of β-CD on the aggregation behavior of BPS-n in aqueous solution has been examined by surface tension、Cryo-TEM、SAXS and2D NOESY. Investigation on aggregates formation prove a different mechanism from other surfactant/β-CD systems. The following are obtained results. (1) When β-CD concentration is relatively low, there is no inclusion between β-CD and BPS-30for the weak inclusion interaction between them. When the BPS-30concentration is above CMC, micelles comprised of a core of sterol groups surrounded by a shell of ethoxy chains are formed to reduce the system energy. The weak inclusion interaction between BPS-30and β-CD do not remove the surfactant monomers in the micellization process and the ethoxy chains shell hamper the inclusion between the BPS-30hydrophobic groups and β-CD.(2) The addition of P-CD induce the micelle to vesicle or lamella phase transition. Due to the weak include interaction between β-CD and BPS-30, hydrophobic effect are proved to be the main driving force on aggregates formation.(3) The β-CD molecules have a tendency to disperse among the PEO blocks because of their hydrophilic shell. The hydrogen bondings make repulsion between PEO chains weaker, thus effective cross-sectional area per surfactant molecule decreases with the β-CD addition. Then the aggregates with bigger cpp value are easy to form.(4) From the perspective of solvent driving ability on the self-assembly, P-CD induces strong order on surrounding water. That is to say, the β-CD addition increases the Gordon parameter of solvent, and promote molecular self-assembly aggregation.Thanks for the supports from the National Natural Science Foundation of China(No.20973104,21033005)... |
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