Investigation On Formation Mechanism,Stimuli-response Properties And Applications Of Surfactant Hydrogels

Creation of novel objects with excellent performances is an important feature of nowadays science research. Besides chemical synthesis, molecular self-assembly, via non-covalent interactions, has been recognized as a fascinating, practical, bottom-up approach to achieve stable, structurally well-defined, and functional objects at the nano-and microscale in biological systems and materials synthesis. In recent years, hydrogels formed by functional amphiphiles have attracted much attention due to their potential applications in the fields of biology, material, environment and biomedicine science. In this thesis, we focused on the microstructures, properties self-assembled process, and applications of surfactant hydrogels. We designed and synthesized amphiphiles with special functional groups, and acquired their phased behavior. Based on the phase digrame, we constructed responsive hydrogels. Combined the microstructures observation and properties determination, we interpreted the formation theory and the self-assembled process of hydrogel, confirmed the controllable effects of various non-covalent forces, revealed the supermolecular structures of viscoelastic hydrogels and the theory of amphiphilic self-assembly, and explored the practical application of hydrogels. The outline and contents of this doctoral dissertation are as follows:Chapter I is a comprehensive introduction of the research background. The concepts of self-assembly, the hot knowledge of surfactant physical chemistry, and the characterization methods of hydrogels were introduced. The classification of gels and the role of various noncovalent interactions in the gel formation, and the application hydrogels in materials synthesis, catalysis, self-healing materials, and drug delivery were reviewed in detail. The objective and the scientific significance of this doctoral dissertation are also pointed out at the end of this part.In Chapter II, vesicle gels were formed and the phase behavior, microstructures, the rheological behavior were investigated in di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA)/tetradecyldimethylamine oxide (C14DMAO) and DEHPA/ L-Arginine (L-Arg) system. (1) The C14DMAO/DEHPA/H2O system exhibits rich phase behavior. At a fixed C14DMAO concentration, phase transition from L1 phase to Lav phase then to Lα1 occured with increasing amounts of DEHPA. The viscoelasticity of the La phase increases with DEHPA concentration at first and reaches the highest value at the molar ratio of DEHPA to C14DMAO being about 80:250, attributing to the increase of vesicle density, and then decreases remarkably with the transition from closed vesicle bilayers to unclosed planar bilayers. In the present system, besides the electrostatic interaction of ion pairs, hydrogen bonding between C14DMAOH+ and C14DMAO reduces the area of surfactant polar groups and changes the packing parameter, resulting in the formation of large aggregates with low curvatures. (2) In L-Arg/DEHPA/H2O system, at a fixed L-Arg concentration, a phase sequence of micellar phase (L1 phase), vesicle phase (Lαv phase), planar lamellar phase (Lα1 phase), and sponge phase (L3 phase) was obtained with increasing DEHPA concentration due to changes in the packing parameter. The viscosity of the La phase increased with increasing DEHPA concentration at first and reached a maximum value at L-Arg:DEHPA molar ratio of about 6:5, attributed to the increase in vesicle density, and then decreased gradually with a transition from closed vesicle bilayers to open planar bilayers and saddlelike bilayers. Porous CeO2 materials were produced by utilizing the L3 phase as template, and the porous CeO2 exhibited excellent catalytic oxidation activity toward CO due to its high surface area, which provides more active sites for CO conversion.In Chapter III, the gelation behavior, microstructure, and role of various noncovalent interactions in gel formation were studied in zwitterionic alkyldimethylamine oxide (CnDMAO) and bile acids mixtures. The application of hydrogels in dye wastewater purifying were explored. (1) In the first part, supramolecular hydrogels were prepared in the mixtures of a chiral amphiphilic lithocholic acid (LCA) and a nonionic surfactant, dodecyldimethylamine oxide (CNDMAO), in water. Compared with general hydrogels, gelation in the studied system was achieved at rather low temperatures (about 10-35 ℃) under sufficient mixing of gelators. The rheological measurements show a strong mechanical strength with an elastic modulus exceeding 5000 Pa and a yield stress exceeding 100 Pa. The dried gel was found to adsorb the dye in wastewater efficiently in an environmentally friendly way. The formation of the hydrogel fibrils was suspected to be driven by a delicate balance of multiple non-covalent interactions including hydrogen bonding, electrostatic interaction, hydrophobic interaction, and the steric effect of LCA molecules. It is considered that a subtle variation in the structure of the gelator molecule can result in a remarkable change in gelation behavior. Therefore, (2) in the second part, we investigated the influence of hydrophobic chain length on the gelation behavior in great detail. We focused on the gelation capability of LCA and alkyldimethylamine oxide (CnDMAO, n = 10,12,14,16, and 18) mixtures in water and the dependence of the self-assembly on temperature. Changing the hydrophobic chain length or temperature breaks the balance of the non-covalent interactions between LCA and the CnDMAO molecules, which leads to a re-distribution of various weak interactions, presenting different gelation behaviors and a transition between hydrogel fibrils and vesicle bilayers. The roles of the hydrophobic chain length, the chiral rigid steroid center, and the hydrogen bonds in the formation of hydrogels and the phase transition are clearly described, and this work could provide useful guidance in designing hydrogels with specific functions. (3)In the third part, the gelation behavior of the mixtures of cholic acid (CA) and CnDMAO was investigated. Hexagonal hollow microtubes were easily produced through a thermal annealing process from the mixtures in aqueous solution of CA and C14DMAO. The microtubes incorporated Bi2S3 quantum dots also exhibited excellent fluorescence property and catalytic performance for degradation of dye pollutants.In Chapter IV, the self-assembly and gelation capability of LCA with different hydramines were studied. The mixtures of DEA/LCA exhibits supergelation capability and the hydrogels were composed of plenty of network nanotubes with uniform diameters of about 60 nm. Moreover, the obtained hydrogels were excellent adsorbents to remove heavy-metal ions from aqueous solutions.The heavy-metal ions, Cu2+, Co2+, Ni2+, Pb2+, and Hg2+, can be efficiently absorbed by DEA/LCA hydrogels in an environmentally friendly way, owing to the bidentate chelation of the heavymetal ions to the hydrogels. Therefore, DEA/LCA hydrogels can be used as highly efficient adsorbents in practical applications to remove heavy-metal ions from industrial sewage.In Chapter V, terpyridine (TP) derivative, 4’-para-phenylcarboxyl-2,2’:6’,2"-terpyridine (PPCT), were synthesized, and the gelation behavior of the mixtures of PPCT and MOH (M+ = Li+, Na+, K+, Cs+, NH4+, (CH3)4N+, (CH3CH2)4N+, (CH3CH2CH2)4N+, (CH3CH2CH2CH2)4N+) in aqueous solutions were investigated. An important finding was that the hydration radius (Rh) of monovalent cations plays a vital role in gelation of PPCT/MOH systems. The capability of M+ for inducing PPCT to form hydrogels is K+> Na+> Li+, which is followed by the Hofmeister series. Due to the ultra-gelation capability of the PPCT/KOH system, the hydrogels with ultrahigh water content,～99 wt%, and highly mechanical strength can be facilely prepared by adding PPCT in KOH aqueous solution. The yield stress of the hydrogels can reach to 7000 Pa, presenting excellent mechanical strength. The hydrogels of PPCT and KOH mixtures are responsive to external stimuli including temperature and shearing force, and present gelation-induced enhanced fluorescence emission property. A typical function of the hydrogels is that formaldehyde (HCHO) can speedily be adsorbed via electrostatic interaction and converted into nontoxic salts (HCOOK and CH3OK), which is much hopeful to be an excellent candidate material for HCHO removal in home furnishings to reduce indoor environmental pollutants.