Synthesis, Gelling And Gel Emulsion Behavior And Template Amphiphilic Cholesterol Derivatives

It is well known that amphiphilic molecules possess the ability to self-assemble into various inerratic structures, which may possess very different properties from the original components. As a result, self-assembly has been well recognized as a powerful method to create materials with superior properties. The main driving force for the self-assembly of the amphiphilic compounds in an aqueous phase is the hydrophobic interaction between them. In recent years, considerable attention has been paid to design novel amphiphilic molecules with additional week interactions, such as hydrogen bonding, π-π stacking, electrostatic interactions, which would assist the formation of advanced supramolecular structures or materials, such as supramolecular gels. This effort is important because it not only extends the utility of amphiphilic compounds but also provides the opportunity to turn the idea of creating novel materials via "synthesis+self-assembly" into reality.Cholesteryl derivatives are widely used in creating liquid crystals and as low-molecular mass gelators (LMMGs) due to their rigid skeleton, multiple chiral centers, and the strong aggregation tendency of the cholesteryl unit. Up to now, a large number of cholesterol-based LMMGs have been reported, among these LMMGs, only a few contain polar residues. Recently, some cholesterol-based ambidextrous gelators with glucose as polar component were reported, and it was found that the gels formed by these materials possess both super-gelation and gel-film formation properties. However, too many hydroxyl groups in the gelators inhibited the dissolution of the compounds in less polar solvents, resulting in decreased gelation ability of the cholesteryl derivatives to them. Therefore, it should be worthwhile to design and prepare cholesteryl derivatives with less hydroxyl groups and interrogate their self-assembly behavior in different solvents. Therefore, on the basis of above considerations, our group designed and synthesized four cholesteryl derivatives, of which each contains two hydroxyl groups. Their gelation behaviors in a variety of solvents were also tested.In the first part, the gelation behaviors of four amphiphilic cholesteryl derivatives (1,2,3and4) were evaluated. It was found that the gel formation process can be controlled by introduction of water at room temperature. Addition of water into acetone solution of4made the transparent solution become turbid immediately, and it then became a gel within minutes. Interestingly,4is a super-gelator for mixed solvent of acetone and water at room temperature when their volume ratio is close to1:1, at which the critical gelation concentration (CGC) is0.06%(w/v). It was revealed that introduction of water favors the formation of gel networks, and the gel possesses smart and reversible thixotropic property. FT-IR,1H NMR studies confirmed that hydrogen bonding is one of the main driving forces for the gelation of mixed solvents. XRD revealed a possible layered structure of4in its acetone-water mixed solvent gel. Furthermore, compounds1and2can be used as excellent stabilizers for creating gel-emulsions of alkanes and water, and their water content can reach97%(v/v).In the second part, ultra-low density porous polystyrene monoliths have been prepared via using water/styrene gel-emulsions, which were stabilized by stabilizer1as templates. It was demonstrated that the porous materials as prepared possess a number of superiors such as highly adjustable internal structure and density, of which the lowest density could be lower than0.01g/cm3, as well as ideal mechanical strength and toughness. Moreover, such materials can be used as excellent absorbents for many organic liquids including viscous oils like mineral oil and high-density oils such as dichloromethane. In particular, the absorption process is fast, selective and efficient when they are used in the purification of oil contaminated water. Importantly, the oils absorbed in the absorbent can be easily squeezed out, and furthermore the wet absorbent as obtained can be reused by suitable washing and natural drying. More importantly, the LMMGs-based gel-emulsions employed as templates are simple in preparation, only mixing and agitation at ambient temperature are required. Moreover, further treatment of the wet porous materials as prepared from the templates is just washing and natural drying, which is completely different from those such as freeze-drying or supercritical drying widely adopted by others in the preparation of low density materials via routine approaches. It is believed that the LMMGs-based gel-emulsion template method developed in the present work may have created a novel and facile way for preparing low density materials with ideal properties.