Formation, Properties, And Applications Of Hydrogels By Lithocholate Salts

As a kind of soft matter, the surfactant gels attract broad attention because of their potential applications. Surfactant gels show solid-like properties and are constructed by the three-dimensional network, which is formed by the gelator molecules to immobilize solvent molecules. In the process of gel formation, non-covalent interactions such as hydrogen bonding, electrostatic interaction,Ï€-Ï€ stacking, and van der Waals interactions play important roles. Gelator molecules usually contain functional groups such as carboxyl, hydroxyl, and amino, therefore they are easy to interact with other ones.The complex three-dimensional networks are formed by self-assembly of individual amphiphilic molecules and are mainly driven by hydrogen bonding and other non-covalent interactions, which are significantly affected by external environment. In most gel systems external stimuli can trigger the transition in microstructures and macroscopic properties, bringing more possibility in applicative aspects of analysis, cell culture, drug delivery and controlled release, nano-material preparation, etc.In this dissertation lithocholic acid is chosen as the main building block of the hydrogel. By changing the salts, hydrogels driven by different non-covalent interactions are compared in the microstructures, formation mechanism, rheological behavior and other properties. Their responsiveness to temperature and pH, and applications in dye adsorption, nano-material preparation and detection of biomolecules are investigated.Chapter I is a brief introduction of the colloid science, surfactants and characterization methods. The emphasis of this chapter is the research progress of surfactant gels, including the fabrication, driving force, response to external stimuli and the application of gels. The basis and significance of the dissertation are discussed at the end of this chapter.In Chapter II, lithocholate (LC-) with different monovalent cations, M+(M+= Li+, Na+, K+, Rb+, Cs+, NH44+). Among the studied monovalent cations, unlike the transparent solutions of cholate salts and the weak gels formed by deoxycholate with only Na+, we obtained hydrogels with high gelation capability using lithocholate with all above cations. The hydrogels were formed only by introducing M+to sodium lithocholate (SLC) solutions at room or even lower temperatures. The tubular microstructures were observed in all hydrogel systems. Compared with fibrous and helical structures, tubular structures were relatively rarely reported in hydrogel systems. The main driving force was considered to be the electrostatic interaction, which was affected by the ionic radii of the cations, leading to different gelation capability. It was notable that increasing temperature induced the enhancement of mechanical strength of the hydrogels instead of the general gel-sol transition, accompanying with the increase in the size of the tubular structures. The gels showed excellent adsorption capability for positively charged dyes, such as methylene blue and rhodamine 6G, with the maximum adsorption efficiency and adsorption capacity among reported results, and was hopeful to act as water-purifying agents in an environmentally friendly way.In Chapter III, we investigated the gelation behavior of lithocholate (LC-) with different metal ions. Characterization of microstructures revealed that hydrogels were constructed by fibrils with inhomogeneous sizes. The gelation was mainly driven by the coordination between carboxylate and metal ions to form the continuous lamellar structures, which were proven by FT-IR and small angle XRD. The hydrogels showed stability to high temperature, and can maintain the mechanical strength after heating. The increase in acidity induced the enhancement of gelation capability and transition in fibrous sizes. The CuS nanoparticle networks were prepared from hydrogels of LC-and Cu2+ mixtures which served as the precursors. The as-prepared CuS nanomaterials performed high fluorescence quenching ability and were used as an effective fluorescent sensing platform for ssDNA detection.