Cholesterol-type Small Molecule Gelling Agent Gelling Behavior Of The Connection Arm Structure-dependent

Surpramolecular gels based on low molecular organic compounds, functioning as gelators(LMMGs), are a type of typical physical gels. Through hydrogen bonding,π-πstacking, van der Waals interactions, electrostatic interactions, coordination interactions, and dipole-dipole interactions, LMMGs form three dimensional networks structures, which are the bases of superamolecular gels. Unlike chemical gels, one of the advantages of the surpramolecular gels is their reversibility and sharp sol-gel phase transition as a result of external stimuli. The stimulus could be heat, ultrasound, light, pH, host-guest interaction, charge transfer, complexation, oxidation/reduction, and even a combination of them. Therefore, surpramolecular gels have promising potential applications in sensors and actuators, drug delivery, protection of historical relics, as templates for preparing micro/nano-materials, oil recovery, mild separation and purification, etc.Of all the LMMGs reported, cholesterol-based compounds have attracted more attention due to their rigid skeleton, several sterogenic centers, and a strong tendency to form aggregates via van der Waals interactions. Among the huge cholesterol derivatives, ALS and A(LS)2 type of LMMGs are the most important components, where A stands for an aromatic moiety, ligand and polymerizable unit, which is connected to a steroidal (S) group through a linker (L). A great deal of studies demonstrated that the aromatic unit A and L show large impact on gelation behaviors of LMMGs and it allows a fine tuning of the molecular organizations within the self-assembled superstructures and, in turn, the self-assembly properties of the gel phase materials. Therefore, in the design of cholesteryl-based LMMGs, through alteration of aromatic units and linker, it is possible to obtain many novel LMMGs with special properties and potential applications. On the basis of above considerations, a series of dimeric cholesteryl-based A(LS)2 gelators with a trans-lcis butene diacid as A unit are designed and prepared by varying the chirality of linker. These gelators display gelation properties in a variety of stereo-structure of gelators, and organic solvents that range from polar to nonpolar, protic to non-protic, and furthermore, large numbers of gelation systems formed at room temperature and some stimuli-responsive gels have been obtained. Many techniques, such as rheological methods,SEM,FT-IR,1HNMR,XRD etc., have been employed to investigate the properties of the gels and the gel formation mechanism. This thesis is mainly composed of the following two parts:In the first part, two cholesterol-containing LMOGs of A(LS)2 type with glycin in the linkers and butene diacid in the A part were prepared. Their gelation abilities were evaluated in 30 commonly found organic solvents. It was found that the change in the stereo-structure of the linker can produce a dramatic effect upon the gelation behaviors of the compounds. Interestingly, MA-C gels CC14 spontaneously at room temperature, and the gel as formed exhibits good thermal stability and thixotropic property as revealed by rheological studies. More interestingly, MA-C shows phase-selective gelation of CC14 from its mixture with water. SEM observation demonstrated that the morphologies of the aggregates of the gelators in the gels depend, but not only, on the structures of the gelators. The concentrations of the gelators, and even their interactions with the solvents also showed great effect upon the morphologies. Temperature and concentration dependent 1H NMR measurements revealed that inter-molecular hydrogen bonding andπ-πstacking among the molecules of the gelators are important driving forces for the formation of the gels. XRD study revealed that MA-C assembled into a layered structure in its CC14 gel.In the second part, four novel dimeric cholesterol-based A(LS)2 LMOGs with D(L)-phenylalanine as linker have been specially designed and synthesized by varying stereo-structure of A unit. The compounds are denoted as MaDC, FaDC, MaLC and FaLC, respectively. The gelation properties of these compounds were examined in the 27 organic solvents. Gelation test revealed that in all 47 gel systems,14 gels are formed spontaneously at room temperature, in which 4 gels are very transparent. In addition, FaDC and FaLC, those that contain trans moiety are more efficient gelators than the corresponding compounds MaDC and MaLC. FaDC and FaLC show phase-selective gelation of the solvents from their mixtures with water at room temperature. Purification of water contaminated by toxic dyes by selective gelation has been conceptually demonstrated. SEM and CD spectroscopy studies revealed that the stereo-structure and the amino chirality of gelators affect the aggregation mode, the morphology and the chirality of the network structures of the gels. In addition, the different aggregation behaviors also affect mechanical properties of the gels, which were consistent with rheological studies. Furthermore, temperature and concentration dependent'H NMR and FTIR measurements demonstrated that inter-molecular hydrogen bonding andπ-πstacking are important driving forces for the formation of the gels.