Heat-set gels formed from easily accessible gelators of a succinamic acid derivative (SAD) and a primary alkyl amine (R-NH2)

Heat-Set Supramolecular Hydrogelation by Regulating the Hydrophilic-Lipophilic Balance for a Tunable Circularly Polarized Luminescent Switch

Heat-set supramolecular gels exhibited totally opposite phase behaviors of dissolution upon cooling and gelation on heating. They are commonly discovered by chance and their rational design remains a great challenge. Herein, a rational design strategy is proposed to realize heat-set supramolecular hydrogelation through regulation of the hydrophilic-lipophilic balance of the system. A newly synthesized amphiphile hydrogelator with pyrene embedded in its lipophilic terminal can self-assemble into a hydrogel through a heating and cooling cycle. However, the host-guest complex of the gelator and hydrophilic γ-cyclodextrin (γ-CyD) results in a sol at room temperature. Thus, heat-set hydrogelation is realized from the sol state in a controllable manner. Heat-set gelation mechanism is revealed by exploring critical heat-set supramolecular gelation and the related findings provide a general strategy for developing new functional molecular gels with tunable hydrophilic-lipophilic balance.

Designing Heat-Set Gels for Crystallizing APIs at Different Temperatures: A Crystal Engineering Approach

An organic salt crystallizes through different kinds of charge-assisted hydrogen-bonded networks depending on carboxylic functionality number and the degree of amine. These H-bonded packing patterns are often robust and predictable, so one can design a supramolecular salt with a certain purpose. In some cases, two different crystalline packing patterns can be found in Primary Ammonium Dicarboxylate (PAD) salts at different temperatures. Two kinds of supramolecular bonding, namely, charge-assisted hydrogen bonding and weak van der Waals interactions stabilize the two states. A small increase in the carbon chain length in a primary amine enhances the additional van der Waals interactions with the packing so that the 2D hydrogen-bonded network (HBN) transforms into a 1D HBN at room temperature. Such van der Waals interactions can be controlled by external heat, so a temperature-dependent 1D to 2D phase change is feasible. When certain moieties, such as azo and bipyridine, are introduced into the carboxylic acid backbone, the acids become insoluble in most organic solvents, raising their melting point, and resulting in heat-set gels. In the presence of an API, temperature and solvent-dependent polymorphic crystals can be grown in the heat-set gel medium and by simply cooling down the mixture, the API crystals can be separated easily.

Self-assembled sonogels formed from 1,4-naphthalenedicarbonyldinicotinic acid hydrazide

Ultrasound-induced gelation of a novel type of gelator, 1,4-naphthalenedicarbonyl- dinicotinic acid hydrazide, is reported. The gelator self-assembled into various architectures in different solvents.

Carboxymethyl chitosan‒promoted luminescence of lanthanide metallogel and its application in assay of multiple metal ions

In this work, the luminescence of lanthanide supramolecular metallogel formed by the self-assembly of 5,5',5″-(1,3,5-triazine-2,4,6-triyl)tris(azanediyl)triisophthalate (H₆L) and Tb³⁺ was efficiently promoted by carboxymethyl chitosan (CMCS). The total quantum yield of the resultant metallogel (denoted as H₆L/Tb³⁺/CMCS gel) was 9 times higher than the gel without CMCS. The average lifetime of H₆L/Tb³⁺/CMCS gel increased from 0.51 ms to 1.20 ms. More importantly, the aqueous dispersion of H₆L/Tb³⁺/CMCS xerogels showed a stable and pH-dependent luminescence. Based on the selective affinity of CMCS to different metal ions as well as with the aid of principal component analysis, H₆L/Tb³⁺ /CMCS can be used as a sensor array to distinguish 11 metal ions (P < 0.05). This work provides a new strategy for the design and development of bio-based functional luminescent lanthanide supramolecular metallogels.

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.

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.

Smart Materials for Environmental Remediation Based on Two-Component Gels: Room-Temperature-Phase-Selective Gelation for the Removal of Organic Pollutants Including Nitrobenzene/O-Dichlorobenzene, and Dye Molecules from the Wastewater

Novel two-component gel systems based on aliphatic acid-hydroxy/base interaction were developed as smart materials for environmental remediation. The G1-A16 gelator could be used directly as a powder form to selectively gel aromatic solvents (nitrobenzene and o-dichlorobenzene) from their mixtures with wastewater (containing 0.5 M sodium nitrate and 0.5 M sodium sulfate) via a simple shaking strategy at room temperature without employing co-solvents and a heating-cooling process. Meanwhile, the two-component gel system can efficiently remove the toxic dyes from the aqueous solution. The dominant factors that drive gelation in the case of the gelator and nitrobenzene or water have been studied using FT-IR, 1H NMR, and XRD. Overall, our research provides an efficient two-component approach for facilely tuning the properties of one-component gel for the realization of high-performance functionalities of gels. At the same time, our study demonstrates potential industrial application prospect in removing pollutants efficiently (such as aromatic solvents and toxic dye removal).

An efficient supramolecular adsorbent for co-adsorption of dyes and metal ions from wastewater and its application in self-healing materials

Herein, a novel type of two-component supramolecular adsorbent, 2-OA, was developed based on non-covalent interactions using tetrazolyl derivative with octadecylamine (OA) and fully characterized using a number of structural and spectral techniques. The self-assembled 2-OA gels displayed remarkable stimuli-responsiveness as well as shape-persistent and self-healing properties. In addition, it was found that the adsorbent 2-OA was able to remove dyes (10 kinds of cationic and anionic dyes) and metal ions (Cu²⁺ and Fe²⁺) simultaneously from wastewater owing to synergistic electrostatic attraction, hydrogen-bonding, and hydrophobic and coordination interactions. It also exhibited excellent co-adsorption capability to dye mixtures and binary mixtures of dyes and metal ions. In particular, the dye/metal-loaded adsorbents could be obtained easily from the aqueous phase, and recycling of the adsorbents could thus be achieved. These results suggest that the supramolecular gel 2-OA not only has great potential application in wastewater treatment but also provides a strategy for the development of intriguing self-healing materials.

Scaleable two-component gelator from phthalic acid derivatives and primary alkyl amines: acid-base interaction in the cooperative assembly

A series of phthalic acid derivatives (P) with a carbon-chain tail was designed and synthesized as single-component gelators. A combination of the single-component gelator P and a non-gelling additive n-alkylamine A through acid-base interaction brought about a series of novel phase-selective two-component gelators PA. The gelation capabilities of P and PA, and the structural, morphological, thermo-dynamic and rheological properties of the corresponding gels were investigated. A molecular dynamics simulation showed that the H-bonding network in PA formed between the NH of A and the carbonyl oxygen of P altered the assembly process of gelator P. Crude PA could be synthesized through a one-step process without any purification and could selectively gel the oil phase without a typical heating-cooling process. Moreover, such a crude PA and its gelation process could be amplified to the kilogram scale with high efficiency, which offers a practical economically viable solution to marine oil-spill recovery.

Aliphatic-Alcohol-Induced Opaque-to-Transparent Transformation and Application of Solubility Theory in a Bis-Dipeptide-Based Supramolecular Gel

A bis-dipeptide supramolecular gelator (DMPV) is prepared, based on l-valine moieties having a pyridinyl group and a long fatty diamine. It is found that the gelator can immobilize organic/water binary mixed solvents, and gel-to-gel transitions with unprecedented opaque-to-transparent transformations are observed upon using aliphatic alcohols such as methanol, ethanol, 1-propyl alcohol, and isopropanol as the organic components. Morphological investigations indicate that a reassembly process occurs, and microstructure evolutions from agglomerates to nanofibers are observed. Opaque and transparent assemblies can interconvert, and respond and restore under mechanical force and pH stimuli. Moreover, Hansen and Flory-Huggins parameters are used to investigate the effect of the solvent on the gelation performance of DMPV. This may facilitate the structure and solvent optimizations and aid the development of advanced gel systems.