Gels are generally classified into organogels, hydrogels, polymer gels and aerogels. In the present article, we will focus on the different aspects of organogels with an emphasis on the modulation of gelation properties and functions in the presence of a stimulus. Organogels are viscoelastic, thermoreversible materials consisting of low molecular-weight compounds . They form a continuous three-dimensional entangled network in the solvent, which in turn prevents the liquid from flowing. The self-assembly of these low molecular-weight organogelators (LMWG) into fibrous networks is driven by multiple, weak interactions such as dipole-dipole, van der Waals, p-stacking and hydrogen-bonding interactions. The self-assembly process from a single molecule to fibers and finally to entangled network structures is thus completely thermodynamically reversible. The network structure is constituted of well-ordered arrays of molecules, which can be of several micrometers in length and can have intriguing architectures like tapes, ribbons, rods, fibers, sheets, cylinders, etc. . Depending upon the operative driving forces for molecular aggregation the organogelators can be broadly classified into two groups: a) non-hydrogen-bond based gelators and b) hydrogen-bond based gelators. Cholesterol or steroid derivatives are examples of the former group while aliphatic amide, urea and saccharide containing gelators represent the latter group. Recently, a new designing approach has come into limelight which utilizes p-p stacking as one of the non-covalent forces to form 1-D organogels . When the gelator molecule shows very high gelation ability, i.e., a very small concentration of the compound (generally less than 0.1 wt%) is needed to gel the solvent, the compound is called a ‘supergelator’. On the other hand, when a gelator molecule can gel a wide range of solvents it is called a ‘versatile gelator’. One of the prominent examples for a versatile gelator is that of a cholesteryl appended pyridine ligand that gels a large spectrum of common organic solvents . With these lines of information in hand, organogels are now donated to the fields of supramolecular chemistry and materials science.
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