Dipeptide-based low-molecular-weight efficient organogelators and their application in water purification

The development of new low-molecular-weight gelators for organic solvents is motivated by several potential applications of gels as advanced functional materials. In the present study, we developed simple dipeptide-based organogelators with a minimum gelation concentration (MGC) of 6-0.15 %, w/v in aromatic solvents. The organogelators were synthesized using different L-amino acids with nonpolar aliphatic/aromatic residues and by varying alkyl-chain length (C-12 to C-16). The self-aggregation behavior of these thermoreversible organogels was investigated through several spectroscopic and microscopic techniques. A balanced participation of the hydrogen bonding and van der Waals interactions is crucial for efficient organogelation, which can be largely modulated by the structural modification at the hydrogen-bonding unit as well as by varying the alkyl-chain length in both sides of the hydrophilic residue. Interestingly, these organogelators could selectively gelate aromatic solvents from their mixtures with water. Furthermore, the xerogels prepared from the organogels showed a striking property of adsorbing dyes such as crystal violet, rhodamine 6G from water. This dye-adsorption ability of gelators can be utilized in water purification by removing toxic dyes from wastewater.     

脂肪酰谷氨酸与小分子有机凝胶

研究了脂肪酰谷氨酸作为凝胶因子在不同有机溶剂中的成胶性能。结果表明,小分子有机凝胶的形成及其稳定性与有机溶剂种类、凝胶因子浓度和凝胶因子中碳链长度密切相关。FT-IR表明,凝胶因子在有机溶剂中是通过氢键等非共价力相互作用而聚集、自我组装形成凝胶。利用光学显微镜观察发现,凝胶因子在不同有机溶剂中形成凝胶的微观结构不同。     

The Synthesis of Amide Dendritic Gelators and its Self-assembly Behavior in MMA

Three kinds of amide dendritic gelators, G₁-C₁₂-G₁, G₂-C₁₂-G₂ and gelator-1, were synthesized, and their self-assemble behavior in methyl methacrylate (MMA) was firstly investigated. The structures of the amide dendritic gelators were confirmed by ¹H-NMR and Mass spectra (MS). The gelation ability of the amide dendritic gelators was researched through tube inversion experiment, the results of which showed that different structures led to quite different gelation ability, including gel-sol temperature and critical concentration to form a stable gel. SEM experiments showed that three kinds of gelator formed different gel morphologies in MMA, all of which were nanoscale gel. All the three amide dendritic gelators could not only form stable gel network at certain temperature with different concentrations in MMA, but also in each case, an optically transparent gel was formed, which indicated dendrimers in the MMA had a good compatibility.     

New gelators of urea-containing triazine derivatives: effects of aggregation and optical features in different organic solvents

New gelators for urea-containing triazine derivatives were synthesised, and their gelation potential was examined using different organic solvents. These compounds were found to form the organogels with a variety of organic solvents, such as hexane and other solvents. The elongated alkyl tails of the gelators displayed an obvious decrease in the critical gelation concentrations of apolar solvents and an increase in the compatibility of gelation in polar solvents. The resulting thermo-reversible gels were characterised by using the dropping ball method and a number of other instruments. The melting temperature (Tm) of the gels in decalin and CCl₄ increased with the gelator concentrations. The intermolecular hydrogen bonding of gelation in different organic solvents was observed using an FT-IR spectrometer. Temperature-dependent UV–vis and fluorescence analysis showed that the organogels displayed diverse aggregations and various fluorescence effects in different organic solvents. Blue fluorescence and J-aggregation in decalin and the quenched effect and π–π stacking in CCl₄ were observed. Further, the morphological self-assembled feature in different organic solvents was studied with a scanning electron microscope, and the morphological features demonstrated that there were different aggregations in different solvents. In conductivity electrolyte experiments, the organogel electrolytes exhibited high conductivity (σ) compared with the corresponding tetrabutylammonium perchlorate (TBAP)/THF solution. The conductivity of the gel electrolytes increased with the concentration of the electrolyte salts and temperature. When the sol–gel temperature was achieved, a high ion conductivity was observed compared with the corresponding TBAP/THF solution. When the ratio of the added electrolyte salts exceeded 5%, gelation was inhibited. Furthermore, the effect of the electrolyte salts on the Tm of the gel was confirmed. The added electrolyte salts affected the gelation ability, but did not affect the sol–gel temperature.     

Bile acid alkylamide derivatives as low molecular weight organogelators: systematic gelation studies and qualitative structural analysis of the systems

A series of amino- and hydroxyalkyl amides of bile acids have been synthesized and characterized by Fourier transform infrared spectroscopy (FTIR), (1)H and (13)C nuclear magnetic resonance spectroscopy (NMR), as well as electrospray ionization mass spectrometry (ESI-MS) measurements. The ability of the synthesized molecules to promote gel formation was systematically investigated. Out of 396 combinations formed by 11 compounds and 36 different solvents, 22 gel-containing systems were obtained with 1% (w/v) gelator concentration. Apart from one exception, the gelator compounds were lithocholic acid derivatives. This challenges the general trend of bile acid-based physical gelators, according to which the gelation ability of lithocholic acid derivatives is poor. A correlation between the values of Kamlet-Taft parameters and solvent preferences for gelators was observed. The morphologies of the solid and gel structures studied with scanning electron microscopy (SEM) showed variability from fibers to spherical microscale aggregates, the latter of which are unique among bile acid-based organogels. The gels exhibited more complex behavior than was previously established with bile acid derivatives, judging by the microscale diversity present in gelating and non-gelating systems and the tendency for polymorphism. This study underlines the importance of both the molecular and colloidal scale aspects of the gelation phenomenon.     

Structure-property relationship of a class of efficient organogelators and their multistimuli responsiveness

A new class of efficient low-molecular-weight gelators composed of a 2-substituted anthraquinone and a hydrazide subunit were synthesized, and the structure-property relationships with respect to their gelation abilities in organic solvents were investigated. The toluene gels showed exceptional thermal stability. Interestingly, it was also found that the ultrasound radiation could promote 1b-e to form a stable organogels instead of precipitates in polar solvent, and the ultrasound could remarkably induce changes of the morphology of the assembly in pyridine. Moreover, the reversible gel to sol phase transition could be achieved by adding TFA and TEA. The gelators 1b-e further showed selective gelation of aromatic solvents or chloroalkanes from their mixtures with water.     

Poly(dimethylsiloxane)‐based polymer organogelators with L‐lysine derivatives as a organogelation‐causing segment

New poly(dimethylsiloxane)‐based polymer organogelators with L ‐lysine derivatives were synthesized on the basis of synthetically simple procedure, and their organogelation abilities were investigated. These polymer organogelators have a good organogelation ability and form organogels in many organic solvents. In the organogels, polymer gelators constructed a mesoporous structure with a pore size of about 1 μm formed by entanglement of the self‐assembled nanofibers. The L ‐lysine derivatives in the polymer gelators functioned as a gelation‐causing segment and the organogelation was induced by self‐assembly of the L ‐lysine segments through a hydrogen bonding interaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3817–3824, 2006     

Morphology dependence of 1,2-diphenylethylenediamine-derived organogelator templates in solvents and their influence in the production of nanostructured silica

The 1,2-diphenylethylenediamine-based neutral or cationic organogelator-templates, currently employed in the production of silica nanomaterials, were initially evaluated for their versatile gelation ability and found to be gelled in the majority of organic solvents tested. Scanning electron microscope and transmission electron microscope, images of neutral organogels made from different solvents revealed that they assembled into a plate-shaped, or rod-shaped morphology, respectively in ethanol or butan-1-ol and in acetonitrile or tetrahydrofurane. Similarly, a 1: 1 mixture (mass) of neutral and cationic gelators formed different morphologies in the solvents tested. Sol-gel polycondensation of tetraethoxy silane using either individual gels (neutral or cation) or a 1: 1 mixture of gels was explored. The experimental results and the scanning electron microscopy and transmission electron microscopy images revealed that silica nanotubes with an inner diameter of 82 nm and an outer diameter of 620 nm were obtained from the 1: 1 mixture of neutral and cationic gelator in ethanol, whereas silica nanoparticles were obtained using gels made in the other solvents tested.     

Gelation Behavior of N-Decane Organogels with Gelators C28 and C40

Organogel formation during paraffin crystallization is one of the causes of oilfield problems. We investigated the formation of n-alkane organogels having C₁₀ as liquid phase and n-octacosane (C₂₈) and n-tetracontane (C₄₀) as solid gelators. It was seen that C₁₀/C₂₈ and C₁₀/C₄₀ form organogels with onset gelation temperatures of 33°C and 48°C respectively, while C₁₀/C₂₈/C₄₀, gelify at 52°C. Replacement of C₂₈ by n-heneicosane (C₂₁) reduced the crystallization rate of C₄₀. This result indicates that the onset temperatures and crystallization rates of organogels with gelators in the range of C₂₈ to C₄₀ could be modulated through the introduction of shorter chain n-alkanes.     

Self-assembly of gelators confined within the nano-scale interlayer space of organo-montmorillonite

The self-assembly of low molecular weight gelators confined within the nano-scale interlayer space of organo-montmorillonite is likely to be different from that under normal conditions (bulk space). Four kinds of gelators, 1-methyl-2,4-bis(N'-n-octadecylureido) benzene (MBB18), 1-methyl-2,4-bis(N'-n-dodecylureido)benzene (MBB12), bis(4'-stearamido phenyl) methane (BSM18) and bis(4'-octanamido phenyl)methane (BOM8), were used to investigate gelation of organic solvents confined within the nano-scale interlayer space of organo-montmorillonite. The possible morphologies of these gelators aggregates confined within the nano-scale space of organo-montmorillonite will be discussed in comparison with that in bulk space by employing differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Herein, two types of organogels were prepared under the same conditions. One of them was formed within the interlayer space of organo-montmorillonite and another was formed in bulk space. The XRD patterns confirm that self-assembly of gelators takes place via an unusual pathway within the confined interlayer space of organo-montmorillonite, indicating that the alkyl chains of gelators adopt a parallel arrangement and do not insert into each other. This unusual arrangement of gelators confined within the interlayer space of organo-montmorillonite does lead to the different thermal effect observed by the DSC measurements. These features ultimately strengthen the thermodynamic stability of gelator aggregates, for example MBB18, and raise the gel-to-sol transition temperature, which jumps from 60 to 123 degrees C.     

Chiral Hexa‐ and Nonamethylene‐Bridged Bis(L‐Leu‐oxalamide) Gelators: The First Oxalamide Gels Containing Aggregates with a Chiral Morphology

Chiral amino acid‐ and amino alcohol‐oxalamides are well‐known as versatile and efficient gelators of various lipophilic and polar organic solvents and water. To further explore the capacity of the amino acid/oxalamide structural fragment as a gelation‐generating motif, the dioxalamide dimethyl esters 1₆Me and 1₉Me , and dicarboxylic acid 2₆OH / 2₉OH derivatives containing flexible methylene bridges with odd ( 9 ; n=7) and even ( 6 ; n=4) numbers of methylene groups were prepared. Their self‐assembly motifs and gelation properties were studied by using a number of methods (FTIR, ¹H NMR spectroscopy, CD, TEM, DSC, XRPD, molecular modeling, MMFF94, and DFT). In contrast to the previously studied chiral bis(amino acid or amino alcohol) oxalamide gelators, in which no chiral morphology was ever observed in the gels, the conformationally more flexible 1₆Me , 1₉Me , 2₆OH , and 2₉OH provide gelators that are capable of forming diverse aggregates of achiral and chiral morphologies, such as helical fibers, twisted tapes, nanotubules, straight fibers, and tapes, in some cases coexisting in the same gel sample. It is shown that the differential scanning calorimetry (DSC)‐determined gelation enthalpies could not be correlated with gelator and solvent clogP values. Spectroscopic results show that intermolecular hydrogen‐bonding between the oxalamide units provides the major and self‐assembly directing intermolecular interaction in the aggregates. Molecular modeling studies reveal that molecular flexibility of gelators due to the presence of the polymethylene bridges gives three conformations ( zz , p1 , and p2 ) close in energy, which could form oxalamide hydrogen‐bonded layers. The aggregates of the p1 and p2 conformations tend to twist due to steric repulsion between neighboring iBu groups at chiral centers. The X‐ray powder diffraction (XRPD) results of 1₆Me and 1₉Me xerogels prove the formation of p1 and p2 gel aggregates, respectively. The latter results explain the formation of gel aggregates with chiral morphology and also the simultaneous presence of aggregates of diverse morphology in the same gel system.     

Pyrene‐Based Fluorescent Ambidextrous Gelators: Scaffolds for Mechanically Robust SWNT–Gel Nanocomposites

With the rapid progress in the development of supramolecular soft materials, examples of low‐molecular‐weight gelators (LMWGs) with the ability to immobilise both water and organic solvents by the same structural scaffold are very limited. In this paper, we report the development of pyrene‐containing peptide‐based ambidextrous gelators (AGs) with the ability to efficiently gelate both organic and aqueous solvents. The organo‐ and hydrogelation efficiencies of these gelators are in the range 0.7–1.1 % w/v in various organic solvents and 0.5–5 % w/v in water at certain acidic pH values (pH 2.0–4.0). Moreover, for the first time, AGs have been utilised to prepare single‐walled carbon‐nanotube (SWNT)‐included soft nanocomposites in both hydro‐ and organogel matrices. The influence of different non‐covalent interactions such as hydrogen bonding, hydrophobic, π–π and van der Waals interactions in self‐assembled gelation has been studied in detail by circular dichroism, FTIR, variable‐temperature NMR, 2D NOESY and luminescence spectroscopy. Interestingly, the presence of the pyrene moiety in the structure rendered these AGs intrinsically fluorescent, which was quenched upon successful integration of the SWNTs within the gel. The prepared hydro‐ and organogels along with their SWNT‐integrated nanocomposites are thermoreversible in nature. The supramolecular morphologies of the dried gels and SWNT–gel nanocomposites have been studied by transmission electron microscopy, fluorescence microscopy and polarising optical microscopy, which confirmed the presence of three‐dimensional self‐assembled fibrillar networks (SAFINs) as well as the integrated SWNTs. Importantly, rheological studies revealed that the inclusion of SWNTs within the ambidextrous gels improved the mechanical rigidity of the resulting soft nanocomposites up to 3.8‐fold relative to the native gels.     

Novel deoxycholic acid alkylamide-phenylurea-derived organogelators

Three monomeric and three dimeric deoxycholic acid (DCA) alkylamido-phenylurea derivatives are designed based on known gelators and are synthesized and characterized by ¹H and ¹³C NMR spectroscopy, ESI-TOF mass spectrometry, and elemental analyses. Among them, a monomeric derivative forms supramolecular gels in CHCl₃ and chlorobenzene, whereas a dimeric derivative gels THF and higher 1-alkanols containing 7-10 carbons. The morphologies of their xerogels are studied by scanning electron microscopy (SEM). No signature of macroscopic chirality of the gels is visible.     

Hydrogen bonding driven self‐assembly of side‐chain amino acid and fatty acid appended poly(methacrylate)s: Gelation and application in oil spill recovery

We report an interesting class of fatty acid appended side‐chain phenylalanine (Phe) containing poly(methacrylate) homopolymers that undergo self‐assembly leading to gelation in selective organic hydrocarbons, due to association among the side‐chain functionalities. Fatty acids of different n‐alkyl chain lengths have been attached to the N‐terminal of the Phe‐based methacrylate and the corresponding homopolymers have been synthesized via reversible addition–fragmentation chain transfer polymerization. These homopolymers undergo gelation in selective organic hydrocarbons. The morphology of these organogels has been characterized by field emission scanning electron microscopy which revealed macroporous structure of the organogels. Viscoelastic properties of organogels and the thermoreversible gel‐to‐sol transition have been investigated by rheological measurements. Powder X‐ray diffraction study has been performed to understand the effect of long n‐alkyl chains on the gelation process. FTIR study reveals inter‐/intra‐chain hydrogen bonding which is the driving force of organogelation of the polymers in suitable solvents. In absence of hydrogen bonding interaction, hydrophobic association fails to direct the self‐assembly process and no gelation is observed. An interesting feature of the homopolymeric gelators is that it can selectively congeal the diesel part from an oil–water biphasic mixture, which might be useful in oil spill treatment. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 511–521     

Gelation mechanism and microstructure of organogels formed with various types of gelators

The small‐angle neutron scattering (SANS) and dynamic light scattering (DLS) investigation were carried out for organogels in toluene, formed by organogelators, to elucidate the relationship between the chemical structure and the gelation mechanism as well as the physical properties of the gels. Three different organogelators, that is cyclo(L ‐β‐3,7‐dimethyloctylasparaginyl‐L ‐phenylalanyl) (CPA), trans‐(1R,2R)‐bis(undecylcarbonylamino)cyclohexane (TCH), and N^ε‐lauroyl‐N^α‐stearylaminocarbonyl‐L ‐lysine ethyl ester (LEE), were chosen for comparison. The SANS intensity functions of toluene solutions of these gelators could be reduced with the concentration and were described with a scattering function for thin rods. This indicates that the gels consist of noncorrelated, rod‐like elements aggregated to each other. The characteristic features of the gelation properties, such as the critical gelation concentration, C_gel, the gelation temperature, T_gel, the gel structure, and the gelation mechanism, were different from each other. CPA had the lowest C_gel and became a gel gradually as the temperature decreased, while TCH and LEE had higher C_gels and underwent a sharp sol–gel transition. We conclude that the gelation mechanisms between the CPA and TCH solutions are different. The “CPA type” gelators form a gel by a linear extension of hydrogen‐bonded plane, while the “TCH type” gelators form a twisted wire, because of its strong helicity and crystallizability. In addition, in the latter type, a next generation of fibrils easily stacks on top of the previous ones to form larger fibrils. These models well explain the DLS results and the mechanical properties. That is, the fibrillar stems in CPA gels are rather mobile and fragile, while those in TCH and LEE are frozen and brittle. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3567–3574, 2005     

l-Lysine-Based Gelators for the Formation of Oleogels in Four Vegetable Oils

Supramolecular oleogel is a soft material with a three-dimensional structure, formed by the self-assembly of low-molecular-weight gelators in oils; it shows broad application prospects in the food industry, environmental protection, medicine, and other fields. Among all the gelators reported, amino-acid-based compounds have been widely used to form organogels and hydrogels because of their biocompatibility, biodegradation, and non-toxicity. In this study, four N^α, N^ε-diacyl-l-lysine gelators (i.e., N^α, N^ε-dioctanoyl-l-lysine; N^α, N^ε-didecanoyl-l-lysine; N^α, N^ε-dilauroyl-l-lysine; and N^α, N^ε-dimyristoyl-l-lysine) were synthesized and applied to prepare oleogels in four kinds of vegetable oils. Gelation ability is affected not only by the structure of the gelators but also by the composition of the oils. The minimum gel concentration (MGC) increased with the increase in the acyl carbon-chain length of the gelators. The strongest gelation ability was displayed in olive oil for the same gelator. Rheological properties showed that the mechanical strength and thermal stability of the oleogels varied with the carbon-chain length of the gelators and the type of vegetable oil. The microstructure of oleogels is closely related to the carbon-chain length of gelators, regardless of oil type. The highest oil-binding capacity (OBC) was obtained in soybean oil for all four gelators, and N^α, N^ε-dimyristoyl-l-lysine showed the best performance for entrapping oils.     

Anion influenced self-assembly of stilbazolium derivative and acid-sensitive property of its corresponding gel

Two stilbazolium derivatives with different counter-anions (Br^?  and OH^? ) have been designed and synthesised. Different counter-anions cause different aggregation properties. The reason of this difference was investigated through concentration- and temperature-dependent ¹H NMR and IR spectra. When the counter-anion is OH^? , the stilbazolium derivative exhibits pronounced aggregation properties. The scanning electron microscope images of xerogels show the characteristic gelation morphologies of 3D fibrous network structures. X-ray diffraction and fluorescence spectroscopy studies have been carried out, and provide more information to define the molecular packing model in gelation states. Finally, the acid-sensing abilities of the xerogel film were studied.     

Thermo‐ and Acid‐Responsive Photochromic Spironaphthoxazine‐Containing Organogelators

A series of photochromic spironaphthoxazine derivatives has been designed, synthesized, and characterized by using ¹H NMR spectroscopy, FAB mass spectrometry, and elemental analysis. Their photophysical and photochromic behavior have been investigated. Two of the compounds (G¹²‐en‐SA‐SO and G¹⁶‐en‐SA‐SO) have been shown to be capable of forming stable thermoreversible organogels in organic solvents, tested by the “stable‐to‐inversion of a test tube” method. Addition of p‐toluenesulfonic acid was found to induce the formation of stable organogels at concentrations below that of the critical gelation concentration (c.g.c.), with a concomitant change in color from colorless to purple. Transmission electron microscopy and scanning electron microscopy of the xerogels showed typical fibrous structures in the micrometer scale. The activation parameters for the bleaching reaction of G⁸‐en‐SA‐SO in the solution state and G¹⁶‐en‐SA‐SO in the gel state have been determined in ethanol through kinetic studies at various temperatures. The results showed that the rate of the bleaching reaction in the gel state was much slower than that in the solution state.     

Steric‐Structure‐Dependent Gel Formation,Hierarchical Structures,Rheological Behavior,and Surface Wettability

A series of bicholesteryl‐based gelators with different central linker atoms C, N, and O (abbreviated to GC , GN , and GO , respectively) have been designed and synthesized. The self‐assembly processes of these gelators were investigated by using gelation tests, field‐emission scanning electron microscopy, field‐emission transmission electron microscopy, UV/Vis absorption, IR spectroscopy, X‐ray diffraction, rheology, and contact‐angle experiments. The gelation ability, self‐assembly morphology, rheological, and surface‐wettability properties of these gelators strongly depend on the central linker atom of the gelator molecule. Specifically, GC and GN can form gels in three different solvents, whereas GO can only form a gel in N,N‐dimethylformamide (DMF). Morphologies from nanofibers and nanosheets to nanospheres and nanotubes can be obtained with different central atoms. Gels of GC , GN , and GO formed in the same solvent (DMF) have different tolerances to external forces. All xerogels gave a hydrophobic surface with contact angles that ranged from 121 to 152°. Quantum‐chemical calculations indicate that the GC , GN , and GO molecules have very different steric structures. The results demonstrate that the central linker atom can efficiently modulate the molecular steric structure and thus regulate the supramolecular self‐assembly process and properties of gelators.     

Water-induced physical gelation of organic solvents by N-(n-alkylcarbamoyl)-L-alanine amphiphiles

A series of amino acid-based gelators N-(n-alkylcarbamoyl)-L-alanine were synthesized, and their gelation abilities in a series of organic solvents were tested. No gelation was observed in pure solvents employed. All the amphiphilic molecules were found to form stable organogels in the solvents in the presence of a small amount of water, methanol, or urea. The volume of solvent gelled by a given amount of the gelator was observed to depend upon the volume of added water. The gelation behavior of the amphiphiles in a given solvent containing a known volume of water was compared. The effects of chirality and substitution on the acid group on the gelation ability were examined. Although the corresponding N-(n-tetradecylcarbamoyl)-DL-alanine was found to form only weak organogel in pure solvents, the achiral amphiphilic compound N-(n-tetradecylcarbamoyl)-β-alanine, however, did not form gel in the absence of water. The methyl ester of N-(n-tetradecylcarbamoyl)-L-alanine was also observed to form gels in the same solvents, but only in the presence of water. The organogels were characterized by several techniques, including (1)H NMR, Fourier transform IR, X-ray diffraction, and field emission scanning electron microscopy. The thermal and rheological properties of the organogels were studied. The mechanical strength of the organogel formed by N-(n-tetradecylcarbamoyl)-DL-alanine was observed to increase upon the addition of water. It was concluded that water-mediated intermolecular hydrogen-bonding interaction between amphiphiles caused formation of supramolecular self-assemblies.