page_title: Organogels
section_title: Factors affecting gelation
hierarchical_section_title: Organogels / Factors affecting gelation
caption_attribution_description: Oxidation of dihydropyridine
context_page_description: An organogel is a class of gel composed of a liquid organic phase within a three-dimensional, cross-linked network. Organogel networks can form in two ways. The first is classic gel network formation via polymerization. This mechanism converts a precursor solution of monomers with various reactive sites into polymeric chains that grow into a single covalently-linked network. At a critical concentration, the polymeric network becomes large enough so that on the macroscopic scale, the solution starts to exhibit gel-like physical properties: an extensive continuous solid network, no steady-state flow, and solid-like rheological properties. However, organogels that are “low molecular weight gelators” can also be designed to form gels via self-assembly. Secondary forces, such as van der Waals or hydrogen bonding, cause monomers to cluster into a non-covalently bonded network that retains organic solvent, and as the network grows, it exhibits gel-like physical properties. Both gelation mechanisms lead to gels characterized as organogels.
  Gelation mechanism greatly influences the typical organogel properties.
context_section_description: Typically, gels are synthesized via sol-gel processing, a wet-chemical technique involving a colloidal solution (sol) that acts as the precursor for an integrated network (gel). There are two possible mechanisms whereby organogels form depending on the physical intermolecular inter-actions, namely the fluid-filled fiber and the solid fiber mechanism. The main difference is in the starting materials, i.e. surfactant in apolar solvent versus solid organogelator in apolar solvent. Surfactant or surfactant mixture forms reverse micelles when mixed with an apolar solvent. The fluid-fiber matrix forms when a polar solvent (e.g. water) is added to the reverse micelles to encourage the formation of tubular reverse micelle structures. As more polar solvent is added, the reverse micelles elongate and entangle to form organogel. Gel formation via solid-fiber matrix, on the other hand, forms when the mixture of organogelators in apolar solvent is heated to give apolar solution of organogelator and then cooled down below the solubility limit of the organogelators. The organogelators precipitate out as fibers, forming a 3-dimensional network which then immobilizes the apolar solvent to produce organogels. Table 1 lists the type of organogelators and the properties of the organogels synthesized.
  Table 1. Types of Organogelators and the Characteristics of their Organogels
  Gelation times vary depending on the organogelators and medium. One can promote or delay gelation by influencing the molecular self-assembly of organogelators in a system.
  Molecular self-assembly is a process by which molecules adopt a defined arrangement without guidance or management from an external source. The organogelators may undergo physical or chemical interactions so as to form self-assembled fibrous structures in which they become entangled with each other, resulting in the formation of a three-dimensional network structure. It is believed that the self-assembly is governed by non-covalent interactions, such as hydrogen bonding, hydrophobic forces, van der Waals forces, π-π interactions etc. Although molecular self-assembly is not fully understood so far, researchers have demonstrated by adjusting certain aspects of the system, one is able to promote or inhibit self-assembly in organogelator molecules.
  Factors affecting gelation include but are not limited to:
  Molecular structures of organogelators – e.g. chirality, functional groups
  Properties of medium – pH, solvent-molecule interaction or solubility, temperature and solvent chain length.
  Organogelators can be divided into two groups based on whether or not they form hydrogen bonds. Hydrogen bond forming organogelators include, amino acids/amides/urea moieties and carbohydrates whereas non-hydrogen bond forming organogelators (e.g. π-π stacking) include anthracene-, anthraquinone- and steroid-based molecules.
  Solubility and/or solvent-molecule interactions play an important role in promoting organogelator self-assembly. Hirst et al. showed that the solubility of the gelators in media can be modified by tuning the peripheral protecting groups of the gelators, which in turn controls the gel point and the concentrations at which crosslinking takes place (See Table 2 for data). Gelators that have higher solubility in medium show less preference for crosslinking. These gelators (Figure 1) are less effective and require higher total concentrations to initiate the process. In addition, solvent-molecule interactions also modulate the level of self-assembly. This was shown by Hirst et al. in the NMR binding model as well as in SAXS/SANS results.
  Garner et al. explored the importance of organogelator structures using 4-tertbutyl-1-aryl cy-clohexanol derivatives showing that a phenyl group in an axial configuration induces gelation, unlike derivatives with the phenyl group in equatorial configuration. Polymeric organogelators can induce gelation even at very low concentrations (less than 20 g/L) and the self-assembly capability could be customized by modifying the chemical structure of the polymer backbone.
  Table 2. 
caption: Figure 3. Oxidation of dihydropyridine. The product formed was opaque and gel-like. Adapted from Chen et al.[19]
exif: {"Image XResolution": "72", "Image YResolution": "72", "Image ResolutionUnit": "Pixels/Inch", "Image ExifOffset": "78", "EXIF ExifImageWidth": "287", "EXIF ExifImageLength": "138"}