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Low Molecular-Mass Organic Gelators (LMOGs) are a relatively new and dynamic soft material capable of a numerous possible applications; LMOGs form by self-assembled fibrillar networks (SAFINs) that entrap solvent between the strands. Once solvent molecules are entrapped within the network, they are immobilized by surface tension effects. The stability of these gels is dependent on the equilibrium between the assembled network and the dissolved gelators. One characteristic of an LMOG that demonstrates its stability is an LMOGs 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 critical gelation concentration (cgc). The cgc is the lowest possible gelator concentration needed to form a stable gel. A lower cgc is desired to minimize the amount of gelator material needed to form gels. Super gelators have a cgc of less than 1 wt%.



History
LMOGs were first reported in the 1930's, but advances in the field were more often than not discoveries of chance as there existed little theoretical understanding of the new technology. 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 chloroform which had been resistant to gelation prior to their discovery. These new LMOGs were rationally designed and represented the first time that scientist had been able to discover new LMOGs based on supramolecular principles. From these earliest studies and screening numerous compounds, it was determined by Hanabusa "et. al" that for thermoreversible gels based on the amide-carbonyl hydrogen bond that amino acid structure, enantio-purity, hydrophilic-lypophilic ratio, and increasing peptide substitution greatly effected 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

Optical Electron Microscopy

X-Ray Diffraction (XRD):

Rheological Measurements
Generally, rheology is used to study the flow of matter within a substance. In order for a substance to be considered a gel it must posses solid-like traits when characterized by rheological measurements. Rheological characterization tests materials by applying stress to measure the materials resistance to deformation. From rheological measurements, a gel can be classified as either a "strong" or "weak" gel. This classification emphasizes the strength of the interactions between gelator molecules in a particular gel. A "weak" gel is often not considered a true gel because it does not conform to a purely solid-like material's rheological traits. Instead, "weak" gels are generally better classified as viscoelastic liquids.

As a result of this distinction, these classes of gels demonstrate different elasticity as calculated by the elastic modulus, a mathematical model for predicting the elasticity of different materials under different stressors. The shear modulus (G) of a "strong" gel exhibits a smaller dissipation of energy than "weak" gels, and the "strong" gel's G values plateau for longer periods of times. Furthermore, rheological properties of different gels can occasionally be used to compare naturally occurring biopolymer gels with synthetic LMOGs.

Agricultural Industry
Pheromone Release Devices Multiple reservoir-type controlled release devices (CRDs) have been developed to achieve the controlled release of highly volatile pheromones into an agricultural setting; whereby they could act as pesticides throughout the growing season. There are several draw-backs associated with current CRDs because they involve multi-step preparation protocals, exhibit low pheromone-holding capacities, are not biodegradable, and exhibit leaking of the pheromones when compressed or broken. To address these functional issues a sugar alcohol-based amphiphilic super-gelator, mannitol dioctanoate (M8), has been developed that efficiently gelled the pheromones, 2-heptanone and lauryl acetate. The miticide, 2-heptanone controls the parasitic mite, varroa (Varroa destructor), that are responsible for honey bee (Apis mellifera L) colony destruction. The researchers further developed the application of this supergelator by developing a reservoir-type CRD that consisted of the 2-heptanone gel in a vapor-barrier-film sealed pouch which was then activated by boring a small hole through the vapor barrier. The CRD had a high loading capacity of 92% wt/wt allowing for the construction of small devices with a high biocompatibility and because, M8 is composed of mannitol and fatty acids it is also biodegradable.

Pollutant/Heavy Metal Remediation
In 2010 researchers developed phase-selective gelators toward the containment and treatment of oil spills. They developed a class of LMOGs that were capable of gelling diesel, gasoline, pump, mineral, and silocone oils. These LMOGs were composed of dialkanoate derivatives of the sugar alcohols, mannitol and sorbital. These sugar alcohol derivatives were ideal as they are biodegradable, inexpensive, and non-toxic. Once the oil was taken up by the gel fibers; it could then be separated from the gel by utilizing vacuum distillation and furthermore the gelator could be recycled.

Chemical Sensing
Molecular gels can be sensitized toward an external stimuli aka light, heat, or chemicals. Also, LMOG's can be sensitized by the incorporation of a receptor unit or a spectroscopically active unit into the gelator molecule.