Low molecular-mass organic gelators (LMOGs) are the monomeric sub-unit which form self-assembled fibrillar networks (SAFINs) that entrap solvent between the strands. SAFINs arise from the formation of strong non-covalent interactions between LMOG monomeric sub-units. As SAFINs are forming, the long fibers become intertwined and trap solvent molecules. Once solvent molecules are entrapped within the network, they are immobilized by surface tension effects. The stability of a gel is dependent on the equilibrium between the assembled network and the dissolved gelators. One characteristic of an LMOG, that demonstrates its stability, is its ability to contain an organic solvent at the boiling point of that solvent due to extensive solvent-fibrillar interactions. Gels self-assemble through non-covalent interactions such as π-stacking, hydrogen-bonding, or Van der Waals interactions to form volume-filling 3D networks. Self-assembly is key to gel formation and dependent upon reversible bond formation. The propensity of a low molecular weight molecule to form LMOGs is classified by its Minimum Gelation Concentration (MGC). The MGC is the lowest possible gelator concentration needed to form a stable gel. A lower MGC is desired to minimize the amount of gelator material needed to form gels. Super gelators have a MGC of less than 1 wt%.
Background and significance LMOGs were first reported in the 1930s, but advances in the field were more often than not discoveries of chance; as there existed little theoretical understanding of gel formation. During this time LMOGs found applications in thickening lubricants, printing inks, and napalm. Interest in the field dwindled for several decades until the mid-1990s when Hanabusa, Shinkai, and Hamilton designed numerous LMOGs which form thermoreversible intermolecular amide-carbonyl hydrogen bonds. The LMOGs developed by Hanabusa et al. were suitable for forming hard gels, including gels with chloroform, which had been resistant to gelation prior to their discovery. These new LMOGs were rationally designed and represented the first time that scientists had been able to discover new LMOGs based on supramolecular principles. From these earliest studies and screening numerous compounds, it was determined that for thermoreversible gels based on the amide-carbonyl hydrogen bond, amino acid structure, enantiopurity, hydrophilic-lypophilic ratio, and increasing peptide substitution greatly affected the gelling ability of various new compounds. The aforementioned principles that developed in this field's infancy have proved successful in allowing researchers to tune LMOGs for different functions. Today, LMOGs have been extensively studied for their unique properties. This newfound functional diversity has led to a wide range of possible applications for LMOGs in agriculture, drug delivery, pollutant/heavy metal remediation, luminescent devices, and chemical sensing.
Gel formation and morphology
The majority of LMOGs can be triggered to form by manipulating the systems' properties, such as the pH, solvent, exposure to light, or by introducing oxidizing or reducing reagents. Researchers have proposed a set of guidelines for successful gel formation
1. It is necessary to have the presence of strong self-complementary and unidirectional inter-molecular interactions that can enforce 1D self-assembly. 2. The solvent-fiber interfacial energy should be manipulated to control solubility and prevent crystallization of the LMOG. 3. Some other factor must be present that can induce the fiber cross-linking network formation. Traditionally, gel phase transitions are strictly temperature dependent. However, it has recently been shown that non-liquid crystalline gelators, composed of (R)-18-(n-alkylamino)octadecan-7-ols (HSN-n), undergo first order gel-to-gel phase transitions; leading to different morphologies of the gel in carbon tetrachloride (CCl4). The uniqueness of this discovery stems from the idea that it is the solvent molecules entering and exiting the structure which leads to the different structural morphologies. All other previously known gel phase transitions have occurred as the result of temperature changes and only one previous case documents this type of solvent dependent morphological change. However, even in the case of N-isopropylacrylamide hydrogels that underwent conformational changes (folding and unfolding of their polymer chains); it occurred only via a temperature dependent process which resulted in water molecules, near the structure, entering or exiting the structure. The stability of a formed gelation matrix is dependent on the equilibrium between the assembled network and the dissolved gelator assemblies. LMOGs are functionally diverse and can be composed of both polar and non-polar regions (amphiphiles).
Scanning electron microscopy
Scanning Electron Microscopy is a useful means for researchers to determine the structural properties of a low molecular-mass weight gel. These gels exhibit a wide range of structures; from fibrous strands (of various lengths) to ribbons and tubes. The structure of these gels is a key factor in their ability to gel solvents or water. Their tertiary structure determines the critical gelation concentration of the gel.
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