A Sugar-Based Gelator for Marine Oil-Spill Recovery

Understanding the gelation properties of the fluorophenyl glycosides of arabinoside gelators: experimental and theoretical studies

In supramolecular gelation, fluorinated gelators are important due to the unique properties displayed by these compounds that arise out of the presence of fluorine atoms. Generally, incorporation of fluorine leads to higher mechanical strength of the gels compared to their non-fluorinated counterparts and this property is enhanced with increasing the number of fluorine atoms. Herein, we show that the incorporation of fluorine into the phenyl ring of phenyl arabinoside allows the molecule to act as a gelator, unlike the non-fluorinated compound. We also show that the mechanical strength and stiffness of the gels is not only dependent on the positions of the fluorine atoms but also guided by their number. Detailed experimental studies, supported by computational studies, allowed us to rationalize the observed supramolecular interactions and propose reasons based on the conformational preferences of these compounds that allow additional hydrogen bonds and π-π interactions which guide the self-assembly, in addition to the primary H-bonding interactions. This, in turn, affects the mechanical behavior of these gels.

An Encyclopedic Compendium on Chemosensing Supramolecular Metal-Organic Gels

Chemosensing, an interdisciplinary scientific domain, plays a pivotal role ranging from environmental monitoring to healthcare diagnostics and (inter)national security. Metal-organic gels (MOGs) are recognized for their stability, selectivity, and responsiveness, making them valuable for chemosensing applications. Researchers have explored the development of MOGs based on different metal ions and ligands, allowing for tailored properties and sensitivities, and have even demonstrated their applications as portable sensors such as paper-based test strips for practical use. Herein, several studies related to MOGs development and their applications in the chemosensing field via UV-visible or luminance along with electrochemical approach are presented. These papers explored MOGs as versatile materials with their use in sensing bio or environmental analytes. This review provides a foundational understanding of key concepts, methodologies, and recent advancements in this field, fostering the scientific community.

Ultralight aerogels via supramolecular polymerization of a new chiral perfluoropyridine-based sulfonimidamide organogelator

Chiral and enantiopure perfluorinated sulfonimidamides act as low-molecular weight gelators at low critical gelation concentration (<1 mg mL-1) via supramolecular polymerization in nonpolar organic solvents and more heterogenic mixtures, such as biodiesel and oil. Freeze-drying of the organogel leads to ultralight aerogel with extremely low density (1 mg mL-1). The gelation is driven by hydrogen bonding resulting in a helical molecular ordering and unique fibre assemblies as confirmed by scanning electron microscopy, CD spectroscopy, and computational modeling of the supramolecular structure.

Reusable Solid-form Phase-Selective Organogelators for Rapid and Efficient Remediation of Crude Oil Spill

Phase-selective organogelators (PSOGs) are considered as a prospective tool for their application in oil spill remediation. However, the number of reports on the PSOGs that can be used in powder form for prompt phase-selective gelation of crude oils is still limited. In this study, a series of compounds with l-mandelic acid as the scaffold bearing different amino acid fragments have been prepared. Also, the gelation behaviors and properties of these derivatives toward organic liquids, product oils, and a type of Chinese crude oil were investigated via heating-and-cooling process, stirring, or resting operation. Besides, the micromorphologies of the resulting gels and the driving forces for the gel formation have been studied by scanning electron microscopy, Fourier transform infrared, UV spectroscopy, concentration-dependent 1H NMR, and X-ray diffraction. Particularly, gelator C15-Phe-Mac-Nap was shown to have the capability of congealing the Chinese crude oil selectively from water in powder form with a relatively lower gelator dosage, as compared with the other gelators we reported in the current and previous works. Moreover, gelator C15-Phe-Mac-Nap displayed some advantageous behaviors such as the reusability of gelator, excellent mechanical and chemical stability of the crude oil gels, and nontoxicity of the gelator in the aquatic environment, indicating its great potential application value for marine oil spill remediation.

Sodium Alginate Aerogel as a Carrier of Organogelators for Effective Oil Spill Solidification and Recovery

Marine oil spills pose a serious threat to the marine ecological environment. Phase-selective organogelators (PSOGs) are ideal candidates for oil spill gelation when used in combination with a mechanical recovery method. However, the toxicity of an organic solvent carrier has become a key problem when it is applied in the remediation of marine oil pollution. In this study, through an inexpensive and nontoxic ionic cross-linking and freeze-drying method, we successfully developed composite oil gelling agents that used a biomass sodium alginate aerogel as the carrier of 12-hydroxystearic acid (12-HSA). Simultaneously, carboxylated cellulose nanofibers (CNF-C) with large specific surface area and graphene oxide (GO) with excellent mechanical properties as reinforcing fillers were combined with an alginate matrix. 12-HSA, as a green and inexpensive organic gelator, was uniformly loaded on the aerogels by vacuum impregnation. The sodium alginate aerogel was capable of absorbing and storing oil due to its three-dimensional network skeleton and high porosity. Rheological studies have demonstrated that the organic gelator 12-HSA can be released from the aerogel substrate and self-assemble to form an oleogel with the absorbed oil quickly. The synergistic effect between absorption and congelation endows the composite oil gelling agent with efficient oil spill recovery capability. Based on eco-friendly, biodegradable, and simple synthesis methods, this composite oil gelling agent shows great potential for application in marine oil spill recovery.

Sustainable approach to oil recovery from oil spills through superhydrophobic jute fabric

Ecosystems suffer from increased oil exploitation and frequent oil spills, which calls for effective, environment-friendly, and economically viable solutions. To address this, abandoned gunny sacks as the concerned jute fabric were superhydrophobically (water contact angle ∼159°) modified, incorporating titanium dioxide (TiO2) nanoparticles and hexadecyltrimethoxysilane (HDTMS), rendering a facile drop casting procedure. The modified superhydrophobic-superoleophilic jute fabric has been identified as a high-performance filter with superior reusability that can separate oil-water mixtures in challenging environmental conditions (including potent acidic, alkaline, highly saline, aqueous, frigid, and blistering water environments) while maintaining high separation efficiency. In continuation, static conditions indulging a batch and continuous oil separation performance and dynamic conditions stimulating turbulence in the oil-water mixture were proficiently carried out, mimicking real-world circumstances. As a result, the modified jute fabric has the advantages of high separation efficiency, stable recyclable properties, and outstanding durability, highlighting its enormous potential for use in practical applications.

(N-Alkyloxalamido)-Amino Acid Amides as the Superior Thixotropic Phase Selective Gelators of Petrol and Diesel Fuels

(N-Alkyloxalamido)-amino acid amides 9-12 exhibit excellent gelation capacities toward some lipophilic solvents as well as toward the commercial fuels, petrol and diesel. Gelator 10 exhibits an excellent phase-selective gelation (PSG) ability and also possesses the highest gelation capacity toward petrol and diesel known to date, with minimum gelation concentration (MGC) values (%, w/v) as low as 0.012 and 0.015, respectively. The self-assembly motif of 10 in petrol and toluene gel fibres is determined from xerogel X-ray powder diffraction (XRPD) data via the simulated annealing procedure (SA) implemented in the EXPO2014 program and refined using the Rietveld method. The elucidated motif is strongly supported by the NMR (NOE and variable temperature) study of 10 toluene-d8 gel. It is shown that the triple unidirectional hydrogen bonding between gelator molecules involving oxalamide and carboxamide groups, together with their very low solubility, results in the formation of gel fibres of a very high aspect ratio (d = 10-30 nm, l = 0.6-1.3 μm), resulting in the as-yet unprecedented capacity of gelling commercial fuels. Rheological measurements performed at low concentrations of 10 confirmed the strength of the self-assembled network with the desired thixotropic properties that are advantageous for multiple applications. Instantaneous phase-selective gelation was obtained at room temperature through the addition of the 10 solution to the biphasic mixture of diesel and water in which the carrier solvent was congealed along with the diesel phase. The superior gelling properties and PSG ability of 10 may be used for the development of more efficient marine and surface oil spill recovery and waste water treatment technologies as well as the development of safer fuel storage and transport technologies.

Synthesis of a Series of Trimeric Branched Glycoconjugates and Their Applications for Supramolecular Gels and Catalysis

Carbohydrate-derived molecular gelators have found many practical applications as soft materials. To better understand the structure and molecular gelation relationship and further explore the applications of sugar-based gelators, we designed and synthesized eight trimeric branched sugar triazole derivatives and studied their self-assembling properties. These included glucose, glucosamine, galactose, and maltose derivatives. Interestingly, the gelation properties of these compounds exhibited correlations with the peripheral sugar structures. The maltose derivative did not form gels in the tested solvents, but all other compounds exhibited gelation properties in at least one of the solvents. Glucose derivatives showed superior performance, followed by glucosamine derivatives. They typically formed gels in toluene and alcohols; some formed gels in ethanol-water mixtures or DMSO water mixtures. The glycoclusters 9 and 10 demonstrated rate acceleration for the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reactions. These were further studied for their metallogels formation properties, and the copper metallogels from compound 9 were successfully utilized to catalyze click reactions. These metallogels were able to form a gel column, which was effective in converting the reactants into the triazole products in multiple cycles. Moreover, the same gel column was used to transform a second click reaction using different reactants. The synthesis and characterization of these compounds and their applications for catalytic reactions were discussed.

Para-Methoxybenzylidene Acetal-Protected D-Glucosamine Derivatives as pH-Responsive Gelators and Their Applications for Drug Delivery

Carbohydrate-based low molecular weight gelators (LMWGs) are compounds with the capability to self-assemble into complex molecular networks within a solvent, leading to solvent immobilization. This process of gel formation depends on noncovalent interactions, including Van der Waals, hydrogen bonding, and π-π stacking. Due to their potential applications in environmental remediation, drug delivery, and tissue engineering, these molecules have emerged as an important area of research. In particular, various 4,6-O-benzylidene acetal-protected D-glucosamine derivatives have shown promising gelation abilities. In this study, a series of C-2-carbamate derivatives containing a para-methoxy benzylidene acetal functional group were synthesized and characterized. These compounds exhibited good gelation properties in several organic solvents and aqueous mixtures. Upon removal of the acetal functional group under acidic conditions, a number of deprotected free sugar derivatives were also synthesized. Analysis of these free sugar derivatives revealed two compounds were hydrogelators while their precursors did not form hydrogels. For those protected carbamates that are hydrogelators, removal of the 4,6-protection will result in a more water-soluble compound that produces a transition from gel to solution. Given the ability of these compounds to form gels from solution or solution from gels in situ in response to acidic environments, these compounds may have practical applications as stimuli-responsive gelators in an aqueous medium. In turn, one hydrogelator was studied for the encapsulation and release of naproxen and chloroquine. The hydrogel exhibited sustained drug release over a period of several days, with the release of chloroquine being faster at lower pH due to the acid lability of the gelator molecule. The synthesis, characterization, gelation properties, and studies on drug diffusion are discussed.

Biomimetic cellulose-nanocrystalline-based composite membrane with high flux for efficient purification of oil-in-water emulsions

The massive discharge of oily wastewater and oil spills are causing serious pollution to water resources. It is urgent to require clean and efficient method of purifying oily emulsions. Although the separation membranes with selective wettability have been widely used in the efficient purification of oil/water emulsions. It is still very challenging to develop functional films that are environmentally friendly, fouling resistant, inexpensive, easy to prepare, easy to scale, and highly efficient. Cellulose nanocrystalline-based composite membranes (CCM) were prepared by surface-initiated atom transfer radical polymerization (SATRP) and vacuum self-assembly. The prepared CCM is superhydrophilic and superoleophobic underwater due to the hydrophilic nature of the modified cellulose-nanocrystalline and the micro-nano surface structure. The CCM shows high separation efficiency (> 99.9 %), high flux (16,692 L^-1·m^-2·h^-1·bar^-1) for surfactant-stabilized oil-in-water emulsions, good cycle stability and anti-fouling performance. This biomass-derived membrane is green, cheap, easy to manufacture, scalable, super-wettability, and durability, it promises to be an alternative to separation membranes in today's market.

Novel supramolecular organo-oil gelators for fast and effective oil trapping: Mechanism and applications

In this paper, for the sorption of oil from oil polluted soil/water systems, nine new supramolecular organo-oil gelators were synthesized using three distinct diisocyanates and alcohols. The manufactured gelators were characterized using various techniques. The Fourier transform infrared (FTIR) and mass spectra confirmed the successful formation of the oil gelators. The synthesis of the proposed gelators was confirmed by the ¹H NMR, which exhibited three singlets that were attributed to an aliphatic side chain containing 29 protons. The scanning electron microscopy (SEM) analysis exhibited porous, sheets, prisms, and fibrous structures for the supramolecular oil gelators. The oil uptake data analysis was subjected to the Langmuir and Freundlich isotherm models which showed the R² value of 0.99 and a maximum adsorption capacity (q_max) of 45 mLg^-1. From the mechanistic point of view, it was proposed that the organo-oil gel initially leads to self-assembly and further entanglements forming the fibers, which finally make a trap for the oil molecules. Among all the nine gelators and different combinations used, the combination of ditetradecyl (TDI 14: DMI14: HMI 14) gelators in the ratio of 1:2:1 exhibited maximum oil uptake of ∼58% initially which further boosted to ∼99% using gasoline as the co-congealed solvent. Interestingly, the complete gelation of the oil from the oil-water mixture was achieved within 30 min of application with high oil recovery. The presented study confirmed that the oil removal by organo-oil gelator is a simple, novel, and facile technique, which could be employed for treating oil-contaminated soil/water mixture.

Smart supramolecular photoresponsive gelator with long-alkyl chain azobenzene incorporated sugar derivatives for recycling aromatic solvents and sequestration of cationic dyes

Phase-selective gelation of low molecular-weight photoresponsive organogelator possessing long aliphatic chain azobenzene sugar derivatives and its applications in the recycling of aromatic solvents and also the removal of cationic dyes is reported. Very low critical gelation concentration (CGC) in aromatic solvents implies that it acts as a very good gelator. The photoinduced gel-to-sol transition was attained by irradiation with UV light at 350 nm. These organogels work as a selective adsorbent for efficiently removing cationic dyes from individual aqueous dye solutions and in a mixture of cationic and anionic dye solutions show more than 95% removal within 12 h. These insights indicate that these sugar derivatives could be exploited in implementing smart materials for environmental remediation.

Adjusting the water-sensitivity of sugar/boronate-based organogels

During the investigation of the water-sensitivity of (arylboronate alkylglucoside)-based organogels, we evaluated a series of twelve potential organogelators. They were synthesised in a single step from the corresponding arylboronic acids and alkylglucosides. Eight of them showed organogelation abilities in three solvents (toluene, cyclohexane, and ethyl myristate). Conformational minimisations of the potential organogelators permitted a clear relationship between the arylboronate orientation and the gelation effectiveness to be established. These gels were characterised by rheometry and SEM which revealed a gel-state originating from the self-assembly of the organogelators into long entangled fibres. SAXS confirmed the mode of packing in a hexagonal phase. Gels in toluene were found to be water-sensitive both after addition of a small amount of water and immersion into water. This study demonstrated that the main parameter impacting the water-sensitivity was the length of the alkyl chain at the anomeric position of the glucoside unit, much more than the functionalisation of an arylboronate moiety.

Facile and Efficient Phase-Selective Powder Polymer Organogelator for Oil Spill Remediation

Phase-selective organogelators that gel oils from oil/water mixtures are useful to remediate oil spills on water. We designed and synthesized polymer organogelators, poly(styrene-co-10-undecenoic acid) with five different 10-undecenoic acid contents that could be added as powders at room temperature to gel oils with different viscosities. The morphologies and mechanical strengths of the gels were investigated using field-emission electron microscopy and rheological measurements, respectively. The gels formed porous fibrillar structures and had high stiffness. Fourier transformm infrared (FTIR) spectroscopy studies of these gels showed that hydrogen bonding and van der Waals forces helped create three-dimensional networks. The straightforward synthesis procedure, room-temperature conditions, and easy powder delivery make poly(styrene-co-10-undecenoic acid) an attractive alternative to existing oil spill response methods.

Methionine-Derived Organogels as Lubricant Additives Enhance the Continuity of the Oil Film through Dynamic Self-Healing Assembly

(S)-2-((1-(Hexadecylamino)-4-(methylthio)-1-oxobutan-2-yl)carbamoyl)benzoic acid (HMTA) was efficiently synthesized and successfully applied as an additive to several types of blank lubricant oils. Initially, HMTA self-assembles to fibrous structures and traps blank lubricant oils to form gel lubricants. The prepared gel lubricants show thermo-reversible properties and enhanced lubricating performance by 3∼5-fold. X-ray photoelectron spectrometry of the metal surface and the quartz crystal microbalance illustrated that there are no obvious interactions between HMTA and the metal surface. The results of Fourier transform infrared spectroscopy and X-ray diffraction further confirm that inter/intro-molecular H-bonding interactions are the main driving force for the self-healing of HMTA. Finally, molecular dynamics (MD) simulations show that the number of noncovalent H-bonding interactions fluctuates with time, and this highly dynamic H-bonding network could regulate the self-assembly process and result in the self-healing property of the HMTA organogel, which is consistent with the results of the step-strain tests. Especially, the Hirshfeld independent gradient model method at the quantum level demonstrated that C8/C9 aromatics of 500SN have strong π-π stacking interactions with the aromatic heads of HMTA and van der Waals interactions with the hydrophobic tails of HMTA, which disrupt the self-assembly behavior of the 500SN model. Therefore, the calculation studies offer a rational explanation for the superior lubricant property of the PAO10 gel as compared to that for 500SN.

Hydrogel Adsorbents for the Removal of Hazardous Pollutants—Requirements and Available Functions as Adsorbent

Over the last few decades, various adsorption functions of polymer hydrogels for the removal of hazardous pollutants have been developed. The performance of hydrogel adsorbents depends on the constituents of the gels and the functions produced by the polymer networks of the gels. Research on hydrogels utilizing the characteristic functions of polymer networks has increased over the last decade. The functions of polymer networks are key to the development of advanced adsorbents for the removal of various pollutants. No review has discussed hydrogel adsorbents from the perspective of the roles and functions of polymer networks in hydrogels. This paper briefly reviews the basic requirements of adsorbents and the general characteristics of hydrogels as adsorbents. Thereafter, hydrogels are reviewed on the basis of the roles and functions of the polymer networks in them for the removal of hazardous pollutants by introducing studies published over the last decade. The application of hydrogels as adsorbents for the removal of hazardous pollutants is discussed as well.

In Situ Supramolecular Gel Formed by Cyclohexane Diamine with Aldehyde Derivative

Low-molecular-weight gels have great potential for use in a variety of fields, including petrochemicals, healthcare, and tissue engineering. These supramolecular gels are frequently metastable, implying that their properties are kinetically controlled to some extent. Here, we report on the in situ supramolecular gel formation by mixing 1,3-cyclohexane diamine (1) and isocyanate derivative (2) without any catalysis at room temperature in various organic solvents. A mixture of building blocks 1 and 2 in various organic solvents, dichloromethane, tetrahydrofuran, chloroform, toluene, and 1,4-dioxane, resulted in the stable formation of supramolecular gel at room temperature within 60–100 s. This gel formation was caused by the generation of urea moieties, which allows for the formation of intermolecular hydrogen-bonding interactions via reactions 1 and 2. In situ supramolecular gels demonstrated a typical entangled fiber structure with a width of 600 nm and a length of several hundred μm. In addition, the supramolecular gels were thermally reversible by heating and cooling. The viscoelastic properties of supramolecular gels in strain and frequency sweets were enhanced by increasing the concentration of a mixed 1 and 2. Furthermore, the supramolecular gels displayed a thixotropic effect, indicating a thermally reversible gel.

Rapid and Scalable Synthesis of a Vanillin-Based Organogelator and Its Durable Composite for a Comprehensive Remediation of Crude-Oil Spillages

Phase-selective organogelators (PSOGs) that have immense potential in effective oil/water separation, antifouling coating, ice-repellent coating, and so on are often synthesized by following complex and multistep synthesis procedures that involve additional and tedious purification steps. On the other side, a comprehensive, selective, environmentally friendly, and energy-efficient separation of different and complex forms of oil spillages (e.g., floating oil or oil-in-water emulsions) from contaminated aqueous phase is challenging to achieve based on earlier-reported PSOGs and their composites. Here, vanillin, a naturally abundant molecule, is unprecedentedly exploited to synthesize a purified PSOG (with a yield of 97%) by adopting a catalyst-free, single-step, and rapid (<2 min) synthesis process under ambient conditions. The Schiff's base reaction between the aldehyde group of vanillin and the primary amine group of octadecylamine provided the desired and purified PSOG-without demanding any additional purification processes (e.g., column chromatography). The appropriate coexistence of the imine linkage, hydrocarbon tail, and hydroxyl group in the vanillin-derived organogelator (VDOG) played an important role in achieving a self-standing organogel that sustained ∼60 times the external load of its weight-without having any noticeable physical deformation. Further, an appropriate and facile integration of the synthesized VDOG with a commercially available biodegradable porous and spongy matrix (i.e., polyurethane sponge) allowed us to develop an oil-selective absorbent with (1) enhanced water repellency (140°) and (2) superior oil-absorption capacity (i.e., 55.2 times its own weight). Such composite material remained durable for repetitive (at least for 50 cycles) and distillation-free separation/recovery of crude oil at practically relevant severe and diverse settings. Thereafter, the synthesized VDOG was successfully and unprecedentedly extended to demonstrate rapid, facile, and efficient separation of surfactant-stabilized oil-in-water emulsions.

Fine-Tuning of Molecular Structures to Generate Carbohydrate Based Super Gelators and Their Applications for Drug Delivery and Dye Absorption

Carbohydrate-based low molecular weight gelators (LMWGs) exhibit many desirable properties making them useful in various fields including applications as drug delivery carriers. In order to further understand the structural connection to gelation properties, especially the influence of halide substitutions, we have designed and synthesized a series of para-chlorobenzylidene acetal protected D-glucosamine amide derivatives. Fifteen different amides were synthesized, and their self-assembling properties were assessed in multiple organic solvents, as well as mixtures of organic solvents with water. All derivatives were found to be gelators for at least one solvent and majority formed gels in multiple solvents at concentrations lower than 2 wt%. A few derivatives rendered remarkably stable gels in aqueous solutions at concentrations below 0.1 wt%. The benzamide 13 formed gels in water and in EtOH/H2O (v/v 1:2) at 0.36 mg/mL. The gels were characterized using optical microscopy and atomic force microscopy, and the self-assembly mechanism was probed using variable temperature 1H-NMR spectroscopy. Gel extrusion studies using H2O/DMSO gels successfully printed lines of gels on glass slides, which retained viscoelasticity based on rheology. Gels formed by the benzamide 13 were used for encapsulation and the controlled release of chloramphenicol and naproxen, as well as for dye removal for toluidine blue aqueous solutions.

Phase-Selective Gelation of the Water Phase in an Oil-Water Mixture: An Approach Based on Oil-Activated Nanoparticle Assembly in Water

Phase-selective gelation refers to the selective gelation of one phase in an immiscible mixture. Thus far, all such examples have involved a molecular gelator forming nanofibers in (and thus gelling) the oil phase in an oil/water mixture. Here, for the first time, we report the counterpart to the above phenomenon, i.e., selective gelation of the water phase in an oil/water mixture (while leaving the oil undisturbed). This has been a challenging problem because moieties that gel water tend to be either amphiphilic or oil-soluble; thus, if combined with an oil/water mixture, they invariably form an emulsion. Our approach solves this problem by exploiting the tunable self-assembly of laponite (LAP) nanoparticles. Initially, LAP nanoparticles (25 nm disks) are dispersed in water, where they remain unaggregated due to the steric stabilization provided by a triblock copolymer (Pluronic P123) adsorbed on their surface. Thus, the dispersion is initially a low-viscosity sol. When an immiscible oil such as hexadecane is introduced above the sol, the mixture remains biphasic, and both phases remain unaffected. Next, an organic acid such as butanoic acid (BA) is added to the oil. The BA is oil-soluble but also has limited solubility in the water. Over about 30 min, some of the BA enters the water, whereupon it "activates" the self-assembly of LAP particles into a three-dimensional "house-of-cards" network. Ultimately, the water phase is converted into a homogeneous gel with a sufficient yield stress: the aqueous gel holds its weight in the inverted vial while the oil phase remains a thin liquid that can be poured out of the vial. On the whole, the concept advanced here is about activating nanoparticle assembly in water through an adjacent, immiscible phase. This concept could prove useful in conducting certain separations or reactions in the laboratory as well as in enhanced oil recovery.

Supramolecular gel formation regulated by water content in organic solvents: self-assembly mechanism and biomedical applications

As one of the most important and fruitful methods, supramolecular self-assembly has a significant advantage in designing and fabricating functional soft materials with various nanostructures. In this research, a low-molecular-weight gelator, N,N'-di(pyridin-4-yl)-pyridine-3,5-dicarboxamide (PDA-N4), was synthesized and used to construct self-assembled gels via a solvent-mediated strategy. It was found that PDA-N4 could form supramolecular gels in mixed solvents of water and DMSO (or DMF) at high water fraction (greater than or equal to 50%). By decreasing the water fraction from 50% to 30%, the gel, suspension and solution phases appeared successively, indicating that self-assembled aggregates could be efficiently modulated via water content in organic solvents. Moreover, the as-prepared PDA-N4 supramolecular gels not only displayed solid-like behavior, and pH- and thermo-reversible characteristics, but also showed a solution-gel-crystal transition with the extension of aging time. Further analyses suggested that both the crystal and gel had similar assembled structures. The intermolecular hydrogen bonding between amide groups and the π-π stacking interactions between pyridine groups played key roles in gel formation. Additionally, the release behavior of vitamin B12 (VB12) from PDA-N4 gel (H2O/DMSO, v/v = 90/10) was evaluated, and the drug controlled release process was consistent with a first-order release mechanism. The human umbilical venous endothelial cell culture results showed that the PDA-N4 xerogel has good cytocompatibility, which implied that the gels have potential biological application in tissue engineering and controlled drug release.

Recently Developed Carbohydrate Based Gelators and Their Applications

Carbohydrate based low molecular weight gelators have been an intense subject of study over the past decade. The self-assembling systems built from natural products have high significance as biocompatible materials and renewable resources. The versatile structures available from naturally existing monosaccharides have enriched the molecular libraries that can be used for the construction of gelators. The bottom-up strategy in designing low molecular weight gelators (LMWGs) for a variety of applications has been adopted by many researchers. Rational design, along with some serendipitous discoveries, has resulted in multiple classes of molecular gelators. This review covers the literature from 2017-2020 on monosaccharide based gelators, including common hexoses, pentoses, along with some disaccharides and their derivatives. The structure-based design and structure to gelation property relationships are reviewed first, followed by stimuli-responsive gelators. The last section focuses on the applications of the sugar based gelators, including their utilization in environmental remediation, ion sensing, catalysis, drug delivery and 3D-printing. We will also review the available LMWGs and their structure correlations to the desired properties for different applications. This review aims at elucidating the design principles and structural features that are pertinent to various applications and hope to provide certain guidelines for researchers that are working at the interface of chemistry, biochemistry, and materials science.

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.

Demonstrating Aqueous-Phase Low-Molecular-Weight-Gel Wicking of Oil for the Remediation of Oil Spilled into Surface Water

Oils spilled into surface water require effective and timely treatment. In this paper, we report on a low-molecular-weight gelator that can form gels in organic and aqueous phases. The aqueous gel was observed to absorb oils, which is proposed as a new class of materials for remediating oil spilled into surface water. The gels and the low-molecular-weight gelator have both fundamental and applied significance. Fundamentally, identifying the mechanisms that govern the formation of these gels and their resultant mechanical properties is of interest. Subsequently, these fundamental insights aid in the optimization of these gels for addressing spilled oil. First, we briefly compare the organic and aqueous gels qualitatively before focusing on the aqueous gel. Second, we demonstrate the ability of the aqueous gel to wick oils through experiments in a Hele-Shaw cell and compare our results to the Washburn equation for porous media. The Washburn equation is not entirely adequate in describing our results due to the change in volume of the porous media during the wicking process. Finally, we investigate mechanisms proposed to govern the formation of low-molecular-weight gels in the literature through rheological shear measurements during gel formation. Our experiments suggest that the proposed mechanisms are applicable to our aqueous gels, growing as anisotropic crystal networks with fractal dimensions between one and two dimensions from temporally sporadic nucleation sites.

How Far Are We in Combating Marine Oil Spills by Using Phase-Selective Organogelators?

Marine oil spills is one of the frequent natural disasters that adversely affect the economy and ecosystem. A variety of methods have been developed to combat marine oil spills. However, none of these methods is ideal and universal for tackling different kinds of oil spills. In addition, most of these methods do not offer the possibility for recovering the spilt oil. There is great interest in developing novel and better methods for combating marine oil spills that allow recovery of the spilt oil. The use of low molecular weight organogelators that can selectively congeal oil from oil-water mixtures have been proposed to be useful for oil spill recovery. From this initial proposal, the area has progressed gradually towards their practical implementation. The advancements and novel concepts in this area are reviewed.

One-step-synthesized d-gluconic acetal-based supramolecular organogelators with effective phase-selective gelation

Two effective and one-step-synthesized organogelators based on d-gluconic acetal derivatives have been developed to show phase-selective gelation behaviours towards aromatic solvents from their biphasic mixtures with water. The dominant factors that drive gelation have been studied using FT-IR and temperature-dependent ¹H NMR spectroscopy. Particularly, gelator GAA-2 in powder form could selectively congeal toluene, benzene and o-xylene at room temperature under mild stirring. Additionally, GAA-2 could gelate the aromatic solvents within 10 min and the recovery rate of the aromatic solvents could reach about 82% under a certain condition. The benefits of wide source availability, being easy to synthesize, and recyclable performance of the gelator make GAA-2 ideal for real-world remediation of aromatic solvents.

Two effective and one-step-synthesized organogelators based on d-gluconic acetal derivatives have been developed to show phase-selective gelation behaviours towards aromatic solvents from their biphasic mixtures with water.

Boron Effect on Sugar-Based Organogelators

The reaction of several alkylglucosides with phenyl boronic acid permitted easy access to a series of alkylglucoside phenyl boronate derivatives. This type of compound has structures similar to those of known benzylidene glucoside organogelators except for the presence of a boronate function in place of the acetal one. Low to very low concentrations of these amphiphilic molecules produced gelation of several organic solvents. The rheological properties of the corresponding soft materials characterized them as elastic solids. They were further characterized by SEM to obtain more information on their morphologies and by SAXS to determine the type of self-assembly involved within the gels. The sensitivity of the boronate function towards hydrolysis was also investigated. We demonstrated that a small amount of water (5 % v/v) was sufficient to disrupt the organogels leading to the original alkylglucoside and phenyl boronic acid; an important difference with the stable benzylidene-based organogelators. Such water-sensitive boronated organogelators could be suitable substances for the preparation of smart soft material for topical drug delivery.

Automated access to well-defined ionic oligosaccharides

Ionic polysaccharides are part of many biological events, but lack structural characterisation due to challenging purifications and complex synthesis. Four monosaccharides bearing modifications not found in nature are used for the automated synthesis of a collection of ionic oligosaccharides. Structural analysis reveals how the charge pattern affects glycan conformation.

Monopeptide-Based Powder Gelators for Instant Phase-Selective Gelation of Aprotic Aromatics and for Toxic Dye Removal

Through a combinatorial screening of 35 possible phase-selective monopeptide-based organogelators readily made at low cost, we identified five of them with high gelling ability toward aprotic aromatic solvents in the powder form. The best of them (Fmoc-V-6) is able to instantly and phase-selectively gel benzene, toluene, and xylenes in the presence of water at room temperature at a gelator loading of 6% w/v. This enables the gelled aromatics to be separated by filtration and both aromatics and the gelling material to be recycled by distillation. We also identified Fmoc-I-16 as the best gelator for benzyl alcohol, and the corresponding organogel efficiently removes toxic dye molecules by 82-99% from their highly concentrated aqueous solutions. These efficient removals of toxic organic solvents and dyes from water suggest their promising applications in remediating contaminated water resources.

Supramolecular organogels fabricated with dicarboxylic acids and primary alkyl amines: controllable self-assembled structures

Supramolecular organogels are soft materials comprised of low-molecular-mass organic gelators (LMOGs) and organic liquids. Owning to their unique supramolecular structures and potential applications, LMOGs have attracted wide attention from chemists and biochemists. A new "superorganogel" system based on dicarboxylic acids and primary alkyl amines (R-NH2) from the formation of organogels is achieved in various organic media including strong and weak polar solvents. The gelation properties of these gelators strongly rely on the molecular structure. Their aggregation morphology in the as-obtained organogels can be controlled by the solvent polarity and the tail chain length of R-NH2. Interestingly, flower-like self-assemblies can be obtained in organic solvents with medium polarity, such as tetrahydrofuran, pyridine and dichloromethane, when the gelators possess a suitable length of carbon chain. Moreover, further analyses of Fourier transformation infrared spectroscopy and 1H nuclear magnetic resonance spectroscopy reveal that the intermolecular acid-base interaction and van der Waals interaction are critical driving forces in the process of organogelation. In addition, this kind of organogel system displays excellent mechanical properties and thermo-reversibility, and its forming mechanism is also proposed.

Amino Acid-Containing Phase-Selective Organogelators: A Water-Based Delivery System for Oil Spill Treatment

The simple structural modification of replacing a terminal carboxylic acid with a primary amide group was found to lower the minimum gelation concentration (MGC), by at least an order of magnitude, for a series of N-lauroyl-l-amino acid phase-selective organogelators in decane. The amide-functionalized analogue N-lauroyl-l-alanine-CONH₂ was demonstrated to gel a broad range of solvents from diesel to THF at MGCs of 2.5% w/v or less, as well as to produce gels with a higher thermal stability (ca. 30 °C) and enhanced mechanical properties (5 times increase in complex modulus), compared to the carboxylic acid analogue, N-lauroyl-l-alanine-COOH. These improved properties may be due to the additional hydrogen bonding in the primary amide analogue as revealed by SCXRD. Most significantly for this study, the introduction of the primary amide functionality enabled N-lauroyl-l-alanine-CONH₂ to form a self-assembled fibrillar network in water. The aqueous network could then actively uptake and rapidly gel decane, diesel, and diluted bitumen ("dilbit") with MGCs of 2.5% w/v or less. This aqueous delivery method is advantageous for oil-remediation applications as no harmful carrier solvents are required and the gel can be easily separated from the water, allowing the oil to be recovered and the gelator recycled.

Novel Class of Isoxazole-Based Gelators for the Separation of Bisphenol A from Water and Cleanup of Oil Spills

A series of low-molecular-weight gelators based on an isoxazole backbone were synthesized, which showed robust and phase-selective gelation of a series of oils. Due to their excellent phase-selective and cogelation properties, they were employed for the separation of bisphenol and the recovery of oil spills from water. The driving force and morphology of these gels were characterized by spectroscopic and microscopic studies.

In situ formation of low molecular weight organogelators for slick solidification

We have investigated the in situ formation of Low Molecular Weight Organogelator (LMWO) molecules in oil-on-water slicks through dual reactive precursor injection. This method alleviates the need for any carrier solvent or prior heating, therefore reducing the environmental impact of LMWOs, giving instantaneous gelation, even at low temperatures (−5 °C). We show minimal leaching from our gels into the water layer.

Instantaneous gelation: a reactive precursors approach for the near-instant gelation of oil-on-water slicks down to −5 °C.

Porous amorphous powder form phase-selective organogelator for rapid recovery of leaked aromatics and spilled oils

Phase-selective organogelators (PSOGs) have drawn wide attention due to their potential applications in recovery of leaked aromatics and spilled oils. However, powder form PSOGs with fast gelling abilities and broad applicabilities are still limited. Herein, we developed three D-gluconic acetal-based gelators with different alkyl chains, all of which show excellent gel properties for hydrocarbon solvents. The spectroscopic and X-ray results revealed that the gel formation was the synergy of hydrogen bonding, π-π stacking and van der Waals forces. Surprisingly, the powder form gelator A with a cis double bond in the alkyl chain could instantly and selectively gel aromatic hydrocarbons, and also rapidly solidify crude oils with widely ranging viscosities from seawater at room temperature within minutes. Further research revealed that A powder exhibited porous amorphous morphology because the cis double bonds broke the crystalline chain-chain interdigitation between the assemblies. Therefore, the fast dispersion and recombination of fibers under the action of oil molecules lead to the fast room temperature gel process. Overall, a non-toxic and low-cost powder form PSOG with rapid room temperature phase selective gelation ability for a wide range of oils makes it promising for the emergency treatment of oil spill and aromatics leakage.

A gelator–starch blend for dry powder based instant solidification of crude oil at room temperature

A sugar based organogelator and natural polymer blend has been found to be useful for the ultrafast, room temperature solidification of crude oil in a dry powder form.

Triazole Linked N-Acetylglucosamine Based Gelators for Crude Oil Separation and Dye Removal

Marine oil-spills have a long-lasting impact on the environment; therefore, it is a major concern in the scientific community to find a solution for remediation. Recently, phase selective organo-gelators emerged as potential materials for removal of oil from water through selective gelation. Herein, we report synthesis of a series of C-6 triazole linked N-acetylglucosamine derivatives, among which three have shown excellent selective gelation of organic solvents, diesel, petrol, and crude oils in water and seawater. We have studied phase selective gelation against different API grade crude oils (from light to heavy), and the gelation was achieved using nontoxic carrier solvent at room temperature in less than 15 min, and gelators were found useful for recovering crude oils. Critical gel concentration (CGC) of crude oil gelators was found to be 2.3-12% (w/v). The variable temperature NMR and FTIR experiments reveal that intermolecular hydrogen bonding was responsible for gel formation. Furthermore, a gelator was utilized for selective dye removal from water.

Synthesis and Characterization of Hybrid Glycolipids as Functional Organogelators and Hydrogelators

Carbohydrate-based low-molecular-weight gelators are useful and versatile compounds for the preparation of soft materials. Using N-acetyl-d-glucosamine as the starting material, we synthesized and characterized 15 glycolipids containing an amide with different ester functional groups. These include aliphatic derivatives with varying chain lengths and aromatic derivatives. Most of the hybrid amide-esters have molecular weights less than 500 D. These glycolipids were found to be effective gelators for several organic solvents, water, and aqueous solutions. Two efficient hydrogelators were also obtained at low concentrations. A few representative gels were characterized using optical microscopy, atomic force microscopy, and rheology to obtain information on their morphology and gel stability. Three gelators were also used to encapsulate naproxen sodium and toluidine blue. The sustained release of the drug from the gel to the aqueous phase was monitored by UV-vis spectroscopy. These gelators have structural flexibility that can be stimuli responsive. The esters can be hydrolyzed and several gels were converted to solutions under basic conditions. These rationally designed gelators could be utilized as stimuli-responsive smart materials with controlled release properties.