Navigating the Gel–Crystal Landscape of Low-Molecular-Weight Gelators

Gelation and crystallization are two distinct yet interrelated molecular self-assembly manifestations. Despite both originating from the same molecular building blocks and relying upon weak noncovalent interactions, they represent orthogonal outcomes because the conditions that promote one pathway may not necessarily favor the other. This divergence in pathways stems from the manner in which the molecular interactions are expressed in space and time, the differences in molecular packing, and the relative influence of kinetic versus thermodynamic control of self-assembly. Physicochemical environment and stimuli triggers, such as temperature change, variation in solvent polarity, pH, mechanical agitation, and concentration, play vital roles in dictating which self-assembly pathway dominates and whether one pathway can be controlled or selectively manipulated with respect to the other. While orthogonality is evident, there is also a degree of interdependence and mutual influence of these pathways on one another. Gels can act as scaffolds or templates for crystallization by providing a uniform and ambient nucleation environment, thus giving rise to crystal outputs with desirable properties. In the case of molecules that are practically important but inherently challenging to crystallize via conventional methods, formation of a metastable gel state can even act as a precursor for the crystallization process. Thus, understanding the interplay between gelation and crystallization is not only limited to laboratory research but extends beyond it for applications ranging from the design of soft materials to pharmaceutical formulations and polymorph control of drug molecules. This review aims to bridge the long-standing gap between the realms of crystallization and gelation, highlighting the significance of studying the gel–crystal interface.