Robust organogels from nitrogen-containing derivatives of (R)-12-hydroxystearic acid as gelators: comparisons with gels from stearic acid derivatives

Enantioseparation via Chiral Supramolecular Gels Comprising Ambidextrous Gelators Based on β-Peptide-type Primary Amines

While β-peptides have been paid attention due to their diverse secondary structures, their application to the design of low-molecular-weight gelators (LMWGs) is less explored. In this work, chiral cyclic β-amino acid-based β-peptides were developed as ambidextrous LMWGs, wherein multiple hydrogen bonds between the amide moieties led to high gelation ability. Their molecular assembly was elucidated using spectroscopies, microscopy, and X-ray analysis. Further, the supramolecular gel was used as a platform for the enantioselective extraction of (S)-naproxen from its racemate under optimized conditions. These findings have expanded the utility of β-peptides and shown the potential of supramolecular gels as a distinct dynamic medium for enantiomer separation.

Oleogels: Versatile Novel Semi-Solid System for Pharmaceuticals

Oleogels is a novel semi-solid system, focusing on its composition, formulation, characterization, and diverse pharmaceutical applications. Due to their stability, smoothness, and controlled release qualities, oleogels are frequently utilized in food, cosmetics, and medicinal products. Oleogels are meticulously formulated by combining oleogelators like waxes, fatty acids, ethyl cellulose, and phytosterols with edible oils, leading to a nuanced understanding of their impact on rheological characteristics. They can be characterized by methods like visual inspection, texture analysis, rheological measurements, gelation tests, and microscopy. The applications of oleogels are explored in diverse fields such as nutraceuticals, cosmetics, food, lubricants, and pharmaceutics. Oleogels have applications in topical, transdermal, and ocular drug delivery, showcasing their potential for revolutionizing drug administration. This review aims to enhance the understanding of oleogels, contributing to the evolving landscape of pharmaceutical formulations. Oleogels emerge as a versatile and promising solution, offering substantial potential for innovation in drug delivery and formulation practices.

Aluminum soap nanoparticles-lignin powder form phase-selective gelator as an efficient sorbent for oils/water separation

Rapid and efficient recovery of oil spill is the key link for oil spill remediation, and also a great challenge. Here, the organogelator-polymerized porous matrix composed of adsorbents and organogelators can provide a new strategy for solving this problem. The gelling mechanism of aluminum 12-hydroxystearate (Al HSA) to form spherical nano micelles in solvents was investigated via UV-vis, FT-IR, and XRD. A creative method for aluminum soap-lignin gelator (OTS-AL/Al HSA) syntheses was put forward through the saponification of 12-hydroxystearic acid (HSA) and lignin via epichlorohydrin (ECH) crosslinking. By adjusting the ECH content, the growth of Al HSA nanoparticles (15-40 nm) on lignin can be realized, and the accordingly increased roughness endowed gelator with better hydrophobicity (WCA of 134.6°) before octadecyltrichlorosilane (OTS) modification. Thanks to the porous structures, the gelator powder exhibited a high sorption capacity in the range of 3.5-5.2 g g-1 for oils and organic solvents. Rheological studies demonstrated high mechanical strength of gels (>1.6 × 105 pa) and the gelator still retained 70% sorption capacity after 6 gelation-distillation cycles. The gelation characteristics of OTS-AL/Al HSA were attributed to the rapid sorption of oils by lignin and the self-assembly of Al HSA nano micelles on lignin to form an aggregated network structure trapping oils, thus realizing the synergistic effect of oil sorption-gelation.

Trends in Fat Modifications Enabling Alternative Partially Hydrogenated Fat Products Proposed for Advanced Application

The natural properties of oils and fats do not always allow for their direct use in industry (e.g., for food, cosmetics, and pharmaceuticals). Furthermore, such raw materials are often too expensive. Nowadays, the requirements for the quality and safety of fat products are increasing. For this reason, oils and fats are subjected to various modifications that make it possible to obtain a product with the desired characteristics and good quality that meets the needs of product buyers and technologists. The modification techniques of oils and fats change their physical (e.g., raise the melting point) and chemical properties (e.g., fatty acid composition). Conventional fat modification methods (hydrogenation, fractionation, and chemical interesterification) do not always meet the expectations of consumers, nutritionists, and technologists. In particular, Hydrogenation, while it allows us to obtain delicious products from the point of view of technology, is criticised for nutritional reasons. During the partial hydrogenation process, trans-isomers (TFA), dangerous for health, are formed. One of the modifications that meets current environmental requirements and trends in product safety and sustainable production is the enzymatic interesterification of fats. The unquestionable advantages of this process are the wide spectrum of possibilities for designing the product and its functional properties. After the interesterification process, the biologically active fatty acids in the fatty raw materials remain intact. However, this method is associated with high production costs. Oleogelation is a novel method of structuring liquid oils with small oil-gelling substances (even 1%). Based on the type of oleogelator, the methods of preparation can differ. Most oleogels of low molecular weight (waxes, monoglycerides, and sterols) and ethyl cellulose are prepared by dispersion in heated oil, while oleogels of high molecular weight require dehydration of the emulsion system or solvent exchange. This technique does not change the chemical composition of the oils, which allows them to keep their nutritional value. The properties of oleogels can be designed according to technological needs. Therefore, oleogelation is a future-proof solution that can reduce the consumption of TFA and saturated fatty acids while enriching the diet with unsaturated fatty acids. Oleogels can be named "fats of the future" as a new and healthy alternative for partially hydrogenated fats in foods.

Self-Assembling Nanoarchitectonics of Twisted Nanofibers of Fluorescent Amphiphiles as Chemo-Resistive Sensor for Methanol Detection

The inhalation, ingestion, and body absorption of noxious gases lead to severe tissue damage, ophthalmological issues, and neurodegenerative disorders; death may even occur when recognized too late. In particular, methanol gas present in traces can cause blindness, non-reversible organ failure, and even death. Even though ample materials are available for the detection of methanol in other alcoholic analogs at ppm level, their scope is very limited because of the use of either toxic or expensive raw materials or tedious fabrication procedures. In this paper, we report on a simple synthesis of fluorescent amphiphiles achieved using a starting material derived from renewable resources, this material being methyl ricinoleate in good yields. The newly synthesized bio-based amphiphiles were prone to form a gel in a broad range of solvents. The morphology of the gel and the molecular-level interaction involved in the self-assembly process were thoroughly investigated. Rheological studies were carried out to probe the stability, thermal processability, and thixotropic behavior. In order to evaluate the potential application of the self-assembled gel in the field of sensors, we performed sensor measurements. Interestingly, the twisted fibers derived from the molecular assembly could be able to display a stable and selective response towards methanol. We believe that the bottom-up assembled system holds great promise in the environmental, healthcare, medicine, and biological fields.

Influence of solute association on the phase behavior of 12-hydroxystearic acid/n-alkane solutions

The phase behavior of 12-hydroxystearic acid (12-HSA) in even-numbered alkanes ranging from octane (C₈) to hexatriacontane (C₃₆) was measured by visual observation of liquid + solid to liquid and liquid-liquid to liquid cloud points and liquid + solid to liquid + liquid transitions. In general solid phases were stabilized to low concentration and higher temperature with increasing alkane length. Liquid-liquid immiscibility was observed in larger alkanes starting with octadecane. The liquidus lines of shorter alkanes (octane to hexadecane) showing only liquid to liquid + solid transitions were fit with an attenuated associated solution model based on the Flory-Huggins lattice model assuming that 12-HSA forms a carboxylic acid dimer over all concentrations investigated. The fit results show that 12-HSA forms associated structures with degrees of association ranging from 3.7-4.5 dimers in the neat 12-HSA. At low concentrations, the 12-HSA is dissociated into dimers, however the free energy cost of dissociation stabilizes the solid phase giving a sharp knee at low concentrations. The role of 12-HSA association in its phase behavior and gelation behavior are discussed. More broadly, the importance of solute association in small molecule organogelators and its potential as a molecular design parameter similar to other component thermodynamic parameters, such as melting temperature and heat of fusion, is discussed.

Multifunctional Self-Assembled Peptide Hydrogels for Biomedical Applications

Self-assembly is a growth mechanism in nature to apply local interactions forming a minimum energy structure. Currently, self-assembled materials are considered for biomedical applications due to their pleasant features, including scalability, versatility, simplicity, and inexpensiveness. Self-assembled peptides can be applied to design and fabricate different structures, such as micelles, hydrogels, and vesicles, by diverse physical interactions between specific building blocks. Among them, bioactivity, biocompatibility, and biodegradability of peptide hydrogels have introduced them as versatile platforms in biomedical applications, such as drug delivery, tissue engineering, biosensing, and treating different diseases. Moreover, peptides are capable of mimicking the microenvironment of natural tissues and responding to internal and external stimuli for triggered drug release. In the current review, the unique characteristics of peptide hydrogels and recent advances in their design, fabrication, as well as chemical, physical, and biological properties are presented. Additionally, recent developments of these biomaterials are discussed with a particular focus on their biomedical applications in targeted drug delivery and gene delivery, stem cell therapy, cancer therapy and immune regulation, bioimaging, and regenerative medicine.

Molecular and Aggregate Structural, Thermal, Mechanical and Photophysical Properties of Long-Chain Amide Gelators Containing an α-Diketo Group in the Presence or Absence of a Tertiary Amine Group

Three structurally related gelators, each containing octadecyl chains, an α-diketo group at the 9,10 positions, and each with a different N-amide group-isobutyl (DIBA), isopentyl (DIPA) or N-(2-(dimethylamino)ethyl) (DMEA)-have been synthesized. Their neat structures as well as the thermal mechanical, and photophysical properties in their gel states with various liquids have been investigated. The gelator networks of DIBA and DIPA in octane, hexylbenzene and silicone oil consist of bundles of fibers. These gels are partially thixotropic and mechanically, thermally (to above their melting or silicone oil gelation temperatures), and photophysically stable. They are mechanically and thermally stronger than the gels formed with DMEA, the gelator with a tertiary amine group. The lone pair of electrons of the tertiary amine group leads to an intra-molecular or inter-molecular charge-transfer interaction, depending on whether the sample is a solution, sol, or gel. Neat, solid DMEA does not undergo the charge-transfer process because its amino and diketo groups are separated spatially by a large distance in the crystalline state and cannot diffuse into proximity. However, the solution of DIPA upon the addition of triethylamine becomes unstable over time at room temperature in the dark or (more rapidly) when irradiated, which initiates the aforementioned charge-transfer processes. The eventual reaction of the gelators in the presence of a tertiary amine group is ascribed to electron transfer from the lone-pair on nitrogen to an α-diketo group, followed by proton transfer to an oxygen atom on the anion radical of the α-diketo group from a methyl or methylene group attached to the nitrogen atom of the cation radical. Finally, the formation of an α-diketyl radical leads to irreversible electronic and structural changes that are observed over time.

Regulatory perspectives of combination products

Combination products with a wide range of clinical applications represent a unique class of medical products that are composed of more than a singular medical device or drug/biological product. The product research and development, clinical translation as well as regulatory evaluation of combination products are complex and challenging. This review firstly introduced the origin, definition and designation of combination products. Key areas of systematic regulatory review on the safety and efficacy of device-led/supervised combination products were then presented. Preclinical and clinical evaluation of combination products was discussed. Lastly, the research prospect of regulatory science for combination products was described. New tools of computational modeling and simulation, novel technologies such as artificial intelligence, needs of developing new standards, evidence-based research methods, new approaches including the designation of innovative or breakthrough medical products have been developed and could be used to assess the safety, efficacy, quality and performance of combination products. Taken together, the fast development of combination products with great potentials in healthcare provides new opportunities for the advancement of regulatory review as well as regulatory science.

Uncovering potential interspecies signaling factors in plant-derived mixed microbial culture

Microbes use signaling factors for intraspecies and interspecies communications. While many intraspecies signaling factors have been found and characterized, discovery of factors for interspecies communication is lagging behind. To facilitate the discovery of such factors, we explored the potential of a mixed microbial culture (MMC) derived from wheatgrass, in which heterogeneity of this microbial community might elicit signaling factors for interspecies communication. The stability of Wheatgrass MMC in terms of community structure and metabolic output was first characterized by 16S ribosomal RNA amplicon sequencing and liquid chromatography/mass spectrometry (LC/MS), respectively. In addition, detailed MS analyses led to the identification of 12-hydroxystearic acid (12-HSA) as one of the major metabolites produced by Wheatgrass MMC. Stereochemical analysis revealed that Wheatgrass MMC produces mostly the (R)-isomer, although a small amount of the (S)-isomer was also observed. Furthermore, 12-HSA was found to modulate planktonic growth and biofilm formation of various marine bacterial strains. The current study suggests that naturally derived MMCs could serve as a simple and reproducible platform to discover potential signaling factors for interspecies communication. In addition, the study indicates that hydroxylated long-chain fatty acids, such as 12-HSA, may constitute a new class of interspecies signaling factors.

Computational Tools to Rationalize and Predict the Self-Assembly Behavior of Supramolecular Gels

Supramolecular gels form a class of soft materials that has been heavily explored by the chemical community in the past 20 years. While a multitude of experimental techniques has demonstrated its usefulness when characterizing these materials, the potential value of computational techniques has received much less attention. This review aims to provide a complete overview of studies that employ computational tools to obtain a better fundamental understanding of the self-assembly behavior of supramolecular gels or to accelerate their development by means of prediction. As such, we hope to stimulate researchers to consider using computational tools when investigating these intriguing materials. In the concluding remarks, we address future challenges faced by the field and formulate our vision on how computational methods could help overcoming them.

A tertiary amine group-based organogelator with pH-trigger recyclable property

The separation and recovery of oils and organogelators can be easily realized by tuning the pH value.

Synergistic Effect of Two Organogelators for the Creation of Bio-Based, Shape-Stable Phase-Change Materials

Two organogelators of different chemistry (a fatty acid derivative and a bis-urea derivative), as well as their blends, were used to impart shape stability to a bio-based phase-change material (PCM) bearing a near-ambient phase-transition temperature. Characterization of the individual gelators and their blends revealed their ability to immobilize the PCM by forming a continuous fibrillar network. The fibrils formed by the fatty acid derivative were helical, while the bis-urea derivative formed smooth fibrils. Also, the bis-urea derivative formed a continuous network at a lower critical concentration than the fatty acid derivative. At each fixed concentration, the bis-urea derivative yielded gels with higher thermal stability than the fatty acid derivative. The two gelators blended in certain ratios demonstrated a strong synergistic effect, providing gels with a significantly higher modulus (∼20-fold) and yield stress (∼1.5-fold) than each gelator individually. PCM gelation did not significantly affect its thermal behavior, however, affected its crystalline morphology. The gelled PCM displayed stacked structures, consisting of alternating pure PCM layers separated by layers formed by gelator fibrils. The phase diagram of the triple system comprising both gelators and PCM demonstrated either single or double gelation behavior depending on the composition. These findings may provide guidelines for the development of novel, shape-stable PCMs, which could be of potential use in various thermal energy storage applications.

An organogel library for solution NMR analysis of nanoparticle suspensions in non-aqueous samples

Surface contrast solution NMR methods (scNMR) are emerging as powerful tools to investigate the adsorption of small molecule ligands to the surface of nanoparticles (NP), returning fundamental insight into the kinetics and thermodynamics of sorption, as well as structural information on the adsorbed species. A prerequisite for the acquisition of high quality solution NMR data is the preparation of homogeneous and stable samples that return consistent NMR spectra and allow extensive signal averaging. Unfortunately, this condition does not apply to NMR samples containing NPs that often show a tendency to sediment and accumulate at the bottom of the NMR tube over the course of the experiment. We have recently shown that preparing NMR samples in an agarose gel matrix inhibits sedimentation and allows the characterization of small molecule-NP interactions by scNMR. Unfortunately, as the agarose gel only forms in aqueous solution, this sample preparation method cannot be used to stabilize NP suspensions in a non-aqueous environment. Here, we introduce a library of 48 organogels, based on low molecular-mass organic gelators (LMOGs), to prepare NMR samples of small molecule/NP systems in a wide range of organic solvents. In addition, we present a simple method that takes advantage of ¹H transverse relaxation (¹H-R₂) measurements to screen the library and identify the best gelator to characterize the small molecule-NP interaction of interest in the solvent of choice. We expect the results of this study will enable the preparation of homogeneous and stable samples of NPs in non-aqueous environments, therefore dramatically increasing the applicability of scNMR to the characterization of heterogeneous interactions and to the investigation of the role played by solvent molecules in regulating the kinetics and thermodynamics of sorption.

Counterion Effects on Aggregate Structure of 12-Hydroxystearate Salts in Hexane: A Quantum Mechanical and Molecular Dynamics Simulation Study

Salts of 12-hydroxystearate are important organogelators and grease thickeners, but a structural rationale for their rheological properties remains elusive. We use quantum mechanical calculations and molecular dynamics (MD) simulations to analyze aggregate structures for (1) ( R)-12-hydroxystearic acid (( R)-12HSA), (2) lithium ( R)-12-hydroxystearate (( R)-Li12HS), and (3) sodium ( R)-12-hydroxystearate (( R)-Na12HS). First, quantum mechanical calculations were used to establish the structure and complexation energies of dimers of acetic acid, lithium acetate, and sodium acetate. The expected acetic acid dimer is predicted, and both the lithium acetate and sodium acetate dimer formed a C_{2 h}-symmetric structure. All dimers were sufficiently stable to allow modeling them as pseudocovalent complexes in all-atom, explicit solvent MD. After microsecond-long MD, all systems produced strong ringlike ordered nuclei. The C_{2 h} lithium salt molecules produced aggregates that had the most efficient packing at the head group and a higher frequency of hydroxyl hydrogen bonding compared to the sodium salt. This ordering propensity explains the high melting temperature of ( R)-Li12HS. Also, the higher frequency of hydrogen bonding leads to fewer solvent-exposed hydrogen bond partners. This explains why lithium is a common counterion in high-temperature and water-resistant greases.

Halobenzyl alcohols as structurally simple organogelators

We report 11 simple halobenzyl alcohols, each comprising of only 16 atoms, as organogelators for aliphatic hydrocarbon solvents. Their self-assembly is similar in both gel and crystal states and involve OH⋯O, CH⋯O, CH⋯π, O⋯X, CH⋯X and X⋯X interactions.

Key characteristics and modelling of bigels systems: A review

Bigels are interesting semisolid formulations with better properties for different applications such as cosmetics and pharmaceutical systems. Due to the mixing of two phases of different nature (polar and apolar), bigels possess some interesting features like ability to deliver hydrophilic and hydrophobic drugs, better spreadability and water washability, improved permeability of drugs, enhanced hydration of stratum corneum and ability to manipulate the drug release rate. The main objective of this review article is to provide a thorough insight into the important characteristics of bigels together with the discussion on modelling of bigel systems to relate their properties with individual constituents and different parameters. Moreover, some important applications of bigels are also discussed by considering some examples from the literature.

Nitrogen-Doped Carbon Nanodots-Ionogels: Preparation, Characterization, and Radical Scavenging Activity

Hybrid diimidazolium-based ionogels were obtained by dispersing nitrogen-doped carbon nanodots (NCNDs) in ionic liquid (IL) solutions and by using dicationic organic salts as gelators. The properties of the NCND-ionogels were studied in terms of thermal stability, mechanical strength, morphology, rheological, and microscopic analyses. Insights into the formation of the hybrid soft material were attained from kinetics of sol-gel phase transition and from estimating the size of the aggregates, obtained from opacity and resonance light-scattering measurements. We demonstrate that, on one hand, NCNDs were able to favor the gel formation both in the presence of gelating and nongelating ILs. On the other hand, the gelatinous matrix retains and, in some cases, improves the properties of NCNDs. The NCND-ionogels showed the typical fluorescence emission of the carbon dots and a notable antiradical activity, with higher efficiency as compared to the single components. The presented hybrid materials hold great promise for topical applications in antioxidant fields.

Mechano-switchable, luminescent gels derived from salts of a long-chained, fatty-acid gelator

Stimulus-responsive molecular gel systems, based on metal salts of a luminescent gelator, 9,10-dioxooctadecanoic acid (DODA), are reported. These salts are structurally the simplest metallo-gelators of which we are aware that exhibit controllable mechano-responsive and luminescent properties. Aggregation is more favored by the metal salts than for DODA itself. However, gelation ability differs dramatically depending on the metal ion: whereas the salts with zinc(ii) and calcium(ii) are inefficient gelators, those with nickel(ii) and copper(ii) can gelate various aromatic liquids, alkanes, and long-chained alcohols. Unlike the DODA gels, no aggregation-induced shift in the positions of the emission spectra of the metal salts could be observed as the sols were transformed to their gel phases. Gels of both nickel(ii) and copper(ii) salts in benzonitrile are among the few known examples with crystalline networks and exhibiting thixotropic behavior. However, there are significant differences in their abilities to recover the initial viscoelastic properties. Structural data for the solid and gel states lead us to conclude that differences among the gelating abilities can be attributed principally to the specific nature of interactions of the salts at their head groups. They appear to control the mechanical and emissive properties of the gels as well as whether the initial aggregation of the salts in the sol phases will support the growth of 1D objects that are capable of maintaining strong contacts, leading to 3D networks and gel formation. Overall, the results provide a facile strategy for the design of luminescent materials with controllable mechano-responsiveness by modifying the metal ions within fibrillar assemblies.

Effects of Optical Purity and Finite System Size on Self-Assembly of 12-Hydroxystearic Acid in Hexane: Molecular Dynamics Simulations

12-Hydroxystearic acid (12HSA) and its derivatives are well-known organogelators, and they play critical roles in a variety of applications. The overall aggregate structure of 12HSA is sensitive to the chirality at the 12th carbon, but a fundamental understanding of this dependence is lacking. In this study, molecular dynamics simulations were conducted on microsecond long time scales for (1) (R)-12HSA, (2) (S)-12HSA, and (3) a 50/50 racemic mixture, each solvated at 12.5 wt % in explicit hexane. Self-assembly was accelerated by turning off alkyl chain dihedral gauche states and forcing the molecules to adopt an all-trans conformation. The stability of the resulting aggregates was tested by quenching them with access to gauche states restored. Ordered aggregates produced from optically pure (R)- and (S)-12HSA remained stable for at least 1 μs. The characteristic ordered structure observed is termed a "ring-of-rings" motif, and it contains two twisted six-membered ringlike bundles connected through acetic acid dimerization and surrounded by six satellite bundles. The chirality at the 12th carbon dictates the overall twist of the rings and thereby the handedness of the aggregates. Racemic mixtures did not produce stable ordered aggregates likely due to insufficient enantiomerically pure ring formation. The most prevalent finite-size effect observed was the stochastic formation of percolating aggregates, which were later avoided by using solvent-permeable, solute-impermeable, confining walls. The resulting ordered aggregates were in all important ways identical to those produced in unconfined systems. The combination of cycling off and on gauche states and the semipermeable walls may be an important new way to study the self-assembly underlying aggregation at industrially relevant concentrations of surfactants in organic solvents.

Amino Acids and Peptide-Based Supramolecular Hydrogels for Three-Dimensional Cell Culture

Supramolecular hydrogels assembled from amino acids and peptide-derived hydrogelators have shown great potential as biomimetic three-dimensional (3D) extracellular matrices because of their merits over conventional polymeric hydrogels, such as non-covalent or physical interactions, controllable self-assembly, and biocompatibility. These merits enable hydrogels to be made not only by using external stimuli, but also under physiological conditions by rationally designing gelator structures, as well as in situ encapsulation of cells into hydrogels for 3D culture. This review will assess current progress in the preparation of amino acids and peptide-based hydrogels under various kinds of external stimuli, and in situ encapsulation of cells into the hydrogels, with a focus on understanding the associations between their structures, properties, and functions during cell culture, and the remaining challenges in this field. The amino acids and peptide-based hydrogelators with rationally designed structures have promising applications in the fields of regenerative medicine, tissue engineering, and pre-clinical evaluation.

Kinetics of 12-Hydroxyoctadecanoic Acid SAFiN Crystallization Rationalized Using Hansen Solubility Parameters

Changes in solvent chemistry influenced kinetics of both nucleation and crystallization of 12-hydroxyoctadecenoic, as determined using differential scanning calorimetry and applying a modified Avrami model to the calorimetric data. Altering solvent properties influenced solvent-gelator compatibility, which in turn altered the chemical potential of the system at the onset of crystallization, the kinetics of gelation, and the resulting 12HOA crystal fiber length. The chemical potential at the onset of crystallization was linearly correlated to both the hydrogen-bonding Hansen solubility parameter and the solvent-gelator vectorial distance in Hansen space, R_a. Our work suggests that solvent properties can be modulated to affect the solubility of 12HOA, which in turn influences the kinetics of crystallization and the self-assembly of this organogelator into supramolecular crystalline structures. Therefore, modulation of solvent properties during organogelation can be used to control fiber length and thus engineer the physical properties of the gel.

Microcapsule-Type Organogel-Based Self-Healing System Having Secondary Damage Preventing Capability

We have developed a novel microcapsule-type organogel-based self-healing system in which secondary damage does not occur in the healed region. A mixture of an organogelator, poor and good solvents for the gelator is used as the healing agent; when the good solvent evaporates from this agent, a viscoelastic organogel forms. The healing agent is microencapsulated with urea-formaldehyde polymer, and the resultant microcapsules are integrated into a polymer coating to prepare self-healing coatings. When the coatings are scratched, they self-heal, as demonstrated by means of corrosion testing, electrochemical testing, optical microscopy, and scanning electron microscopy (SEM). After the healed coatings are subjected to vigorous vibration, it is demonstrated that no secondary damage occurs in the healed region. The secondary damage preventing capability of the self-healing coating is attributable to the viscoelasticity of the organogel. The result can give insight into the development of a "permanent" self-healing system.

Mechano-Responsive, Thermo-Reversible, Luminescent Organogels Derived from a Long-Chained, Naturally Occurring Fatty Acid

The gelating ability of an α-diketo derivative of oleic acid, 9,10-dioxooctadecanoic acid (DODA), is investigated. DODA can gelate aromatic liquids and many other organic liquids. By contrast, none of the liquids examined can be gelated by the methyl ester of DODA. DODA is a more efficient gelator than stearic acid and the monoketo derivative due to its more extensive intermolecular dipole-dipole interactions. Formation of organogels of DODA can be induced by both thermal and mechanical stimuli, during which the luminescent and mechanical properties can be modulated significantly. The emission from DODA in 1-octanol exhibits a large, reversible, hypsochromic shift (≈25 nm) between its thermally cycled gel and sol states. The emission changes have been exploited to probe the kinetics of the aggregation and deaggregation processes. DODA is the simplest gelator of which we are aware that exhibits a reversible shift in the emission. Although the self-assembled fibrillar networks of the DODA gels in 1-octanol, benzonitrile, or silicone oil are crystalline, isothermal mechanical cycling between the gel and the sol states is rapid and can be repeated several times (i.e., they are thixotropic). The single-crystal structure of DODA indicates that extended intermolecular dipole-dipole interactions are crucial to the thermal and mechanical formation of DODA gels and the consequential changes in emissive and mechanical properties. From analyses of structural information, gelator packing, and morphology differences, we hypothesize that the mechanical destruction and reformation of the gel networks involves interconversion between the 3D networks and 1D fiber bundles. The thermal processes allow the fibrillar 3D networks and their 0D components (i.e., isolated molecules or small aggregates of DODA) to be interconverted. These results describe a facile approach to the design of mechano-responsive, thermo-reversible gels with control over their emission wavelengths.

Correlations between thixotropic and structural properties of molecular gels with crystalline networks

This review focuses on correlations between the thixotropic and structural properties of molecular gels having crystalline fibrillar networks (SAFINs). Formation of thixotropic molecular gels and their recovery after the application of destructive strain depends on the strength and type of intermolecular interactions in the SAFINs of the gelator molecules. Here, we limit our discussion to gelator molecules with simple structures in order to dissect more easily the important contributors to the thixotropic behaviors. Possible mechanisms to explain the thixotropic phenomena, involving the transformation of the SAFINs into unattached objects, and their reassembly into 3-dimensional networks, are advanced. The data are analyzed to provide insights into the rational design of thixotropic molecular gelators.

Low-Molecular-Weight Gelators as Base Materials for Ointments

Ointments have been widely used as an efficient means of transdermal drug application for centuries. In order to create ointments suitable for various new medicinal drugs, the creation of ointment base materials, such as gels, has attracted much research attention in this decade. On the other hand, the chemical tuning of low-molecular-weight gelators (LMWGs) has been increasingly studied for two decades because LMWGs can be tailored for different purposes by molecular design and modification. In this review, several series of studies related to the creation of ointment base materials with enhanced properties using existing and newly-created LMWGs are summarized.

Selective aliphatic/aromatic organogelation controlled by the side chain of serine amphiphiles

Structural modifications in the side chain of N-Fmoc-l-serine amphiphiles induce the selective gelation of either aliphatic or aromatic hydrocarbon solvents.

Structural bases for mechano-responsive properties in molecular gels of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides. Rates of formation and responses to destructive strain

The self-assembly and gelation behavior of a series of (R)-12-hydroxy-N-(ω-hydroxyalkyl)octadecanamides (HS-n-OH, where n = 2, 3, 4 and 5 is the length of the alkyl chain on nitrogen), as well as those of two ‘model’ compounds, N-(3-hydroxypropyl)octadecanamide (S-3-OH) and (R)-12-hydroxy-N-propyloctadecanamide (HS-3), have been investigated in a wide range of liquids. A unique aspect of some of the HS-n-OH gels is the degree and velocity of their recovery of viscoelasticity after the cessation of destructive shear. The recovery times vary from less than one second to hundreds of seconds, depending on the length of the ω-hydroxyalkyl group on nitrogen. The data indicate that the modes and dynamics of aggregation of the gelator molecules from incubation of a sol phase below the gel melting temperature, as analyzed by Avrami and fractal equations, cannot be used to explain the degree and dynamics of the thixotropy: sol-to-gel transformations involve assembly of 0-dimensional objects (i.e., individual gelator molecules) into 1-dimensional fibrils and then into 3-dimensional networks; recovery after mechano-destruction of gels requires only 1-dimensional to 3-dimensional re-assembly or re-association of 3-dimensional spherulitic objects. A model to understand the extreme sensitivity of the thixotropy on the length of the ω-hydroxyalkyl group in the HS-n-OH (which is based upon detailed comparisons among the dynamic properties of the gels, the morphologies of the neat gelators, and the fibrillar networks of the gels) invokes the importance of the cleavage and reformation of H-bonds between fibers at ‘junction zones’ or between spherulitic objects.

To gel or not to gel: correlating molecular gelation with solvent parameters

Rational design of small molecular gelators is an elusive and herculean task, despite the rapidly growing body of literature devoted to such gels over the past decade. The process of self-assembly, in molecular gels, is intricate and must balance parameters influencing solubility and those contrasting forces that govern epitaxial growth into axially symmetric elongated aggregates. Although the gelator-gelator interactions are of paramount importance in understanding gelation, the solvent-gelator specific (i.e., H-bonding) and nonspecific (dipole-dipole, dipole-induced and instantaneous dipole induced forces) intermolecular interactions are equally important. Solvent properties mediate the self-assembly of molecular gelators into their self-assembled fibrillar networks. Herein, solubility parameters of solvents, ranging from partition coefficients (log P), to Henry's law constants (HLC), to solvatochromic parameters (ET(30)), and Kamlet-Taft parameters (β, α and π), and to Hansen solubility parameters (δp, δd, δh), are correlated with the gelation ability of numerous classes of molecular gelators. Advanced solvent clustering techniques have led to the development of a priori tools that can identify the solvents that will be gelled and not gelled by molecular gelators. These tools will greatly aid in the development of novel gelators without solely relying on serendipitous discoveries. These tools illustrate that the quest for the universal gelator should be left in the hands of Don Quixote and as researchers we must focus on identifying gelators capable of gelling classes of solvents as there is likely no one gelator capable of gelling all solvents.

Organogel formation rationalized by Hansen solubility parameters: influence of gelator structure

Some organic compounds form gels in liquids by forming a network of anisotropic fibres. Based on extensive solubility tests of four gelators of similar structures, and on Hansen solubility parameter formalism, we have probed the quantitative effect of a structural variation of the gelator structure on its gel formation ability. Increasing the length of an alkyl group of the gelator obviously reduces its polarity, which leads to a gradual shift of its solubility sphere towards lower δp and δh values. At the same time, its gelation sphere is shifted - to a much stronger extent - towards larger δp and δh values.