Development of low-molecular-weight gelators and polymer-based gelators

In this review, the development of low-molecular-weight gelators and polymer-based gelators is described. The driving forces for physical gelation are non-covalent bonds, such as hydrogen bonds, electrostatic interactions, van der Waals interactions and π–π interactions. When gelation occurs, the gelator molecules self-assemble into macromolecule-like aggregates through non-covalent intermolecular interactions. The close relationship between crystallization and gelation is discussed. Crystallization is a phenomenon in which crystals are separated from solution by the formation of a three-dimensional arrangement of solute through intermolecular interactions. Conversely, physical gelation is caused by the trapping of solvent in fibrous networks that are formed by gelator molecules through intermolecular interactions. Amino acid derivatives and cyclic dipeptides are introduced as typical gelators. To develop gelators that form semipermanent stable gels, a new concept termed ‘gelation-driving segments’ is proposed. Polymer-based gelators that can form semipermanent stable gels are synthesized by connecting gelation-driving segments to polymers or oligomers. Low-molecular-weight compounds, which form physical gels, are called ‘gelators’ and have received a great amount of scientific and technological interest. The physical gelation by gelator results from non-covalent bonds, represented by hydrogen bond. Molecules of gelator are first self-assembled in cooling process, producing fibrous assemblies. Then, these fibrous assemblies form a three-dimensional network structure, and gelation occurs by trapping solvent in the networks. Fibrous assemblies can be observed by electron microscope. This is a transmission electron microscopy image of tetrachloromethane gel formed by N- octadecylamide of N -benzyloxycarbonyl- L -isoleucine.