The gelation behaviors of four recently reported amphiphilic cholesteryl derivatives (1, 2, 3 and 4) have been evaluated. It was found that the gel formation process can be controlled by introduction of water at room temperature. Addition of water to an acetone solution of 4 immediately results in the system becoming turbid, and a gel subsequently forms within a few minutes. Interestingly, 4 is a super-gelator for a mixed solvent of acetone and water at room temperature, in particular when their volume ratio is close to 1:1 at which the critical gelation concentration (CGC) is 0.06% (w/v). It was found that the introduction of water favors the formation of gel networks, and the gel possesses smart and reversible thixotropic properties. FTIR and 1H NMR spectroscopy confirmed that hydrogen bonding is one of the main driving forces for the gelation of the solvents. XRD demonstrated that 4 self-assembled into a layered structure within the acetone-water mixed solvent gel. Furthermore, 1 and 2 can be used as excellent stabilizers for gel emulsions of alkanes and water. The maximum of the dispersed phase, water, in one of the gel-emulsions can be as high as 97% (v/v).
To investigate the effect of spatial configuration on the gelation properties of low molecular mass gelators(LMMGs),four novel di-cholesteryl derivatives have been specially designed and synthesized by introducing the cis-/trans-isomers of butene diacid and the optical isomers of D/L-phenylalanine into the linker between two cholesteryl moieties.These isomers have been denoted as MaDC,FaDC,MaLC and FaLC,respectively.The gelation properties of the compounds were examined in 26 organic solvents,and it was found that the trans-configuration is more favorable for the gelation,but the chirality of the linker shows little effect to the gelation.FaDC has the strongest gelation ability among the four isomers.Interestingly,FaDC and FaLC display phase-selective gelation of benzene,toluene and xylene from their mixtures with water at room temperature,which establishes a foundation for the purification of water contaminated by oil or aromatic solvents.SEM and CD spectroscopy studies revealed that the spatial configuration of the linkers of the gelators affects significantly the aggregation mode,the morphologies and the chirality of the network of the gels.Moreover,the different aggregation behaviors also have an impact on mechanical properties of the gels,which are consistent with the results from rheological studies.Furthermore,temperature-and concentration-dependent 1 H NMR and FTIR measurements demonstrated that intermolecular hydrogen bonding and-stacking are the main driving forces for the formation of the gels.
A scorpion-shaped di-NBD(4-substituted-7-nitrobenzoxadiazole) derivative of cholesterol(Chol-2NBD) was designed and synthesized. The gelation behaviors of the compound in a series of single and mixed liquids were tested. It was shown that the compound is an effective gelator for mixture liquids of THF and benzene at room temperature. Furthermore, FT-IR and temperature-/concentration-dependent 1H NMR spectroscopy studies revealed that hydrogen bonding and π-π stacking among the molecules of Chol-2NBD are two main driving forces for the physical gelation of the mixture liquids. Interestingly, as observed in the gelation test and confirmed by rheological studies, the Chol-2NBD-THF/benzene gel systems, at least the one with 2:8 of the volume ratio of THF to benzene, are mechanically stable, but very sensitive to the stimulus of shear stress, which means that the gel changes into a liquid upon shaking. More interestingly, the liquid returns to gel instantly once the shear stress is removed. This phase transition process could be repeated for many times at room temperature. In addition, primary tests demonstrated that the fluorescence emission of Chol-2NBD is significantly quenched by the presence of water, ammonia water, or ammonia gas, but the emission recovers after evaporation of them. Further detailed investigation is under progress.
This article is intended to provide an overview of recent progress in the studies of cholesterol-based low-molecular mass gelators (LMMGs) with unusual properties,in particular,gelation and selective gelation at room temperature,gel emulsions and gel film formation,etc.Potential applications of the LMMGs in spilled oil collection and water purification are briefly presented.
LIU KaiQiang,HE PanLi & FANG Yu Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University),Ministry of Education
