Low molecular-mass organic gelators (LMOGs) are the monomeric sub-unit which form self-assembled fibrillar networks (SAFINs) that entrap solvent between the strands.Frkanec, L.; Zinic, M. Chiral bis(amino acid)- and bis(amino alcohol)-oxalamide Gelators. Gelation Properties, Self-Assembly Motifs and Chirality Effects” 2010 Chem. Commun. 46, 522-537. 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.Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. 1999 J. Org. Chem. 64, 412. 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.Esch, J. H. We Can Design Molecular Gelators, But Do We Understand Them? 2009 Langmuir 25(15), 8392-8394. 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.Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H.Small Molecular Gelling Agents to Harden Organic Liquids: Alkylamide of N-Benzyloxycarbonyl-L-valyl-L-valine 1993 J. Chem. Soc. Commun., 390. 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 designedSupramolecular gelling agents: can they be designed? P Dastidar, Chem. Soc. Rev., 2008,37, 2699-2715 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.Bai, H.; Li, C.; Wang, X.; Shi, G. A pH-Sensitive Graphene Oxide Composite Hydrogel 2010 Chem. Commun. 46, 2376-2378. 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).Mallia, V.A.; Butler, P. D.; Sarkar, B.; Holman, K. T.; Weiss, R. G. "Reversible Phase Transitions Within Self-Assembled Fibrillar Networks of (R)-18-(n-alkylamino)octadecan-7-ols in Their Carbon Tetrachloride Gels" 2011 J. Am. Chem. Soc. 133, 15045-15054. 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.Schild, H. G. 1992 Prog. Polym. Sci. 17, 163–249.Hirokawa, Y.; Tanaka, T. J. 1984 Chem. Phys. 81, 6379–6380.Zhang, X.-Z.; Xu, X.-D.; Cheng, S.-X.; Zhuo, R.-X. 2008 Soft Matter 4, 385–391.Qiu, Y.; Park, K. 2001 Adv. Drug Delivery Rev. 53, 321–339. 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.

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 possess solid-like traits when characterized by rheological measurements. Rheological characterization, tests materials by applying stress to measure the material's 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 time.Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels "Chem. Rev." 1997, 97, 3133-3159. Furthermore, rheological properties of different gels can occasionally be used to compare naturally occurring biopolymer gels with synthetic LMOGs.Burchard, W.; Ross-Murphy, S. B. '"Physical Networks, Polymers and Gels'" Elsevier: London, 1990

Interactions of gel and solvent

Researchers have not been able to reliably predict novel LMOGs. A key aspect in predicting new gelator materials is understanding the interaction between the gel molecules and the solvent. The most common solvents for LMOGs are organic in nature and result in organogels. Much rarer are hydrogels, or gels that form with water as the solvent.Tritt-Goc, J.; Bielejewski, M.; Luboradzki, R. 2011 Tetrahedron, 67, 8170-8176 Several attempts have been made to quantify the gel and solvent interaction using a variety of parameters: The single or multi-component solubility parameter (δ) of a solvent can give insight on how well-suited the solvent will be for gelation. Depending on the gelator/solvent system, a high solubility parameter can indicate high or low thermal stability of the gel.Hanabasu, K.; Matsumoto, M.; Kimura, M.; Kakehi, A.; Shirai, H. 1999 Colloid Interface Science, 224, 231-244 The dielectric constant (ε) reflects the bulk polarity of the solvent.Bielejewski, M.; Lapinski, A.; Luboradzki, R.; Tritt-Goc, J. 2009 Langmuir, 25, 8274-8279 The Dimroth-Reichardt parameter (ET(30)) is a measure of ionizing power of a solvent.Muller P. "Glossary of Terms Used in Physical Organic Chemistry" 1994 Pure and Applied Chemistry 66(5), 1077-1184 The Kamlet-Taft solvent parameters establish solvatochromic relationships which measure separately the hydrogen bond donor (α), hydrogen bond acceptor (β), and polarizability (π*) of solvents. The Hildebrand parameter measures the energy it takes to create a cavity within a solvent.

Applications

Agricultural industry

Pheromone release devicesJadhav, S. R.; Chiou, B.; Wood, D. F.; DeGrande-Hoffman, G.; Glenn, G. M.; John, G. "Molecular Gels-Based Controlled Release Devices for Pheromones" 2011 Soft Matter 7, 864-867. 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.Shorey, H. H.; Sisk, C. B.; Gerber, R. G. 1996 Environ. Entomol. 25, 446.Glenn, G. M.; Klamczynski, A. P.; Shey, J.; Chiou, B. S.; Holtmann, K. M.; Wood, D. F.; Ludvik, C.; DeGrandi-Hoffman, G.; Orts, W.; Imam, S. 2007 Polym. Adv. Technol. 18, 636.Yosha, I.; Shani, A.; Magdassi, S. 2006 J. Agric. Food Chem. 56, 8045. There are several draw-backs associated with current CRDs because they involve multi-step preparation protocols, 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.Sammataro, D.; Finley, J.; LeBlanc, B.; Wardell, G.; Ahumada-Segura, F.; Carroll, J. M. 2009 J. Agric. Res. 48, 256.Gashout, H. A.; Guzman-Novoa, E. 2009 J. Agric. Res. 48, 263. 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.