Gelator Molecules Research Articles

A new family of urea-based low molecular-weight organogelators for environmental remediation: the influence of structure.

Gelation processes grant access to a wealth of soft materials with tailorable properties, in applications as diverse as environmental remediation, biomedicine and electronics. Several classes of self-assembling gelators have been studied and employ non-covalent bonds to direct assembly, but recently attention has come to focus on how the overall shape of the gelator molecule impacts its gelation. Here we study a new sub-family of low molecular weight organogelators and explore how steric rearrangement influences their gelation. The gels produced are characterised with X-ray diffraction and small-angle neutron scattering (SANS) to probe their ex situ and in situ gelation mechanisms. The best examples were then tested for environmental remediation applications, gelling petrol and oils in the presence of water and salts.

Synthesis of a new series of multifunctional dialkyl 2-(1-(alkylamino)-1,3-dioxo-3-phenylpropan-2-yl)malonates as low molecular weight supramolecular organogelators using five-component reaction

Dialkyl 2-(1-(alkylamino)-1,3-dioxo-3-phenylpropan-2-yl)malonates have been synthesized using a one-pot, pseudo five-component condensation reaction of isocyanide, Meldrum's acid, phenylglyoxal and two alcohol molecules in CH2Cl2 at ambient temperature. It was found that these structures could be used as low molecular weight supramolecular organogelators. The gelation behaviors of these organic gelators have been investigated in various organic solvents. The results have shown that dioctyl 2-(1-(cyclohexylamino)-1,3-dioxo-3-phenylpropan-2-yl)malonate in n-dodecane is the best organogelator (0.2 wt%). Scanning electron microscopy and investigation of the rheological properties have indicated that gelator molecules create three dimensional networks via entangling self-assembled micro- and nano-rods. Hence the organogelation is induced by trapping solvent molecules in nano- and micro-spaces in the gel network.

The Metal Effect on Self-Assembling of Oxalamide Gelators Explored by Mass Spectrometry and DFT Calculations.

Gels formed by self-assembly of small organic molecules are of wide interest as dynamic soft materials with numerous possible applications, especially in terms of nanotechnology for functional and responsive biomaterials, biosensors, and nanowires. Four bis-oxalamides were chosen to show if electrospray ionization mass spectrometry (ESI-MS) could be used as a prediction of a good gelator and also to shed light on the gelation processes. By inspecting the gelation of several solvent, we showed that bis(amino acid)oxalamide 1 proved to be the most efficient, also being able of forming the largest observable assemblies in the gas phase. The formation of singly charged assemblies holding from one up to six monomer units is the outcome of the strong intermolecular H-bonds, particularly among terminal carboxyl groups. The variation of solvents from polar aprotic towards polar protic did not have any significant effects on the size of the assemblies. The addition of a salt such as NaOAc or Mg(OAc)2, depending on the concentration, altered the assembling. Computational analysis at the DFT level aided in the interpretation of the observed trends and revealed that individual gelator molecules spontaneously assemble to higher aggregates, but the presence of the Na+ cation disrupts any gelator organization since it becomes significantly more favorable for gelator molecules to bind Na+ cations up to the 3:1 ratio than to self-assemble, being fully in line with experimental observations reported here. Graphical Abstract ᅟ.

Amphiphilic Peptide-Based Supramolecular, Noncytotoxic, Stimuli-Responsive Hydrogels with Antibacterial Activity.

A series of peptides with a long fatty acyl chain covalently attached to the C-terminal part and a free amine (-NH2) group at the N-terminus have been designed so that these molecules can be assembled in aqueous medium by using various noncovalent interactions. Five different peptide amphiphiles with a general chemical formula [H2N-(CH2)nCONH-Phe-CONHC12 (n = 1-5, C12 = dodecylamine)] have been synthesized, characterized, and examined for self-assembly and hydrogelation. All of these molecules [P1 (n = 1), P2 (n = 2), P3 (n = 3), P4 (n = 4), P5 (n = 5)] form thermoresponsive hydrogels in water (pH 6.6) with a nanofibrillar network structure. Interestingly, the hydrogels obtained from compounds P4 and P5 exhibit potential antimicrobial activity against Gram-positive bacteria (Staphylococcus aureus, Bacillus subtilis) and Gram-negative bacteria (Escherichia coli). Dose-dependent cell-viability studies using MTT assay (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) by taking human lung carcinoma (A549) cells vividly demonstrates the noncytotoxic nature of these gelator molecules in vitro. Hemolytic studies show nonsignificant or little hemolysis of human erythrocyte cells at the minimum inhibitory concentration (MIC) of these tested bacteria. Interestingly, it has been found that these antibacterial noncytotoxic hydrogels exhibit proteolytic resistance toward the enzymes proteinase K and chymotrypsin. Moreover, the gel strength and gel recovery time have been successfully modulated by varying the alkyl chain length of the N-terminally located amino acid residues. Similarly, the thermal stability of these hydrogels has been nicely tuned by altering the alkyl chain length of the N-terminally located amino acid residues. In the era of antibiotic-resistant strains of bacteria, the discovery of this new class of peptide-based antibacterial, proteolytically stable, injectable, and noncytotoxic soft materials holds future promise for the development of new antibiotics.

Peptide-based ambidextrous bifunctional gelator: applications in oil spill recovery and removal of toxic organic dyes for waste water management.

A low molecular weight peptide-based ambidextrous gelator molecule has been discovered for efficient control of water pollution. The gelator molecules can gel various organic solvents with diverse polarity, e.g. n-hexane, n-octane, petroleum ether, petrol, diesel, aromatic solvents like chlorobenzene, toluene, benzene, o-xylene and even aqueous phosphate buffer of pH 7.5. These gels have been thoroughly characterized using various techniques including field emission scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction analysis, small angle X-ray scattering and rheological experiments. Interestingly, hydrogel obtained from the gelator molecule has been found to absorb toxic organic dyes (both cationic and anionic dyes) from dye-contaminated water. The gelator molecule can be reused for several cycles, indicating its possible future use in waste water management. Moreover, this gelator can selectively gel petrol, diesel, pump oil from an oil-water mixture in the presence of a carrier solvent, ethyl acetate, suggesting its efficient application for oil spill recovery. These results indicate that the peptide-based ambidextrous gelator produces soft materials (gels) with dual function: (i) removal of toxic organic dyes in waste water treatment and (ii) oil spill recovery.

Gelling Lamellar Phases of the Binary System Water-Didodecyldimethylammonium Bromide with an Organogelator.

Does the presence of a gel network influence the properties of a lyotropic liquid crystal? Does the replacement of oil by a lyotropic liquid crystal influence the properties of an organogel? To answer these questions we study gelled lyotropic liquid crystals (LLC). In the present study we show that it is possible to gel the lamellar phase of the binary system water-didodecyl dimethylammonium bromide (2C12DAB) with the organogelator 12-hydroxyoctadecanoic acid (12-HOA). We compare various properties of the gelled LLC phases with the "parent systems", i.e., with the binary organogel consisting of n-decane-12-HOA and with the nongelled LC phases, respectively. Optical and electron microscopy, differential scanning calorimetry (DSC), rheometry, as well as small and wide-angle X-ray scattering (SWAXS) proved the coexistence of an Lα phase and a 12-HOA gel network in the gelled Lα phase. However, a small influence of the Lα phase on the gel properties was seen, namely slightly lower sol-gel transition temperatures and viscoelastic moduli of the gelled Lα phase compared to the binary gel. On the other hand, the presence of the gel also has an influence on the Lα phase: the interlayer spacing of the surfactant bilayers in the gelled Lα phases is slightly larger compared to the nongelled Lα phases, which is due to mixing part of the 12-HOA molecules in the Lα bilayers. Despite this mutual influence the structures of both the Lα phase and the gel network are hardly disturbed in the gelled Lα phase, i.e., that the self-assembly of the surfactant and of the gelator molecules clearly occur in an orthogonal way.

Supramolecular gels: using an amide-functionalized imidazolium-based surfactant

Studies on supramolecular gel aggregates via supramolecular interactions are very fascinating and impressive. The interactions of gelator with solvent and inorganic salt can functionalize, activate, or control the properties of supramolecular gels. A lot of work on this area has been done, but all these contributions are only at an early stage. In this work, we tuned the gel formation conditions and properties by using low-molecular-weight gelator (LMWGs), amide-functionalized imidazolium-based surfactant, viz., N-cetyl-N′-acetamido imidazolium bromide ([N-C16, N′-CONH2-Im]Br), in different solvents and with inorganic salt additives. Under chemical (Cu2+/H2O2) and physical (temperature) stimuli, gel-sol phase transition occurred in the gel formed from 2 wt% [N-C16, N′-CONH2-Im]Br in the solvent formamide (FM). Accompanied with increased mechanical strength of the gels, morphology alternation from initial straight stripe-shaped structures to curly belt-like textures was observed in the addition of CuBr2. From Small-angle X-ray scattering (SAXS) measurements, bilayer units with interdigitated hydrocarbon tails of the gelator were testified to appear in the gels. Comparative experiments demonstrate that the π-π interaction, H-bonding, and hydrophobic interaction between the gelator molecules are the main driving forces for the gelation. The low-cost gelator, facile preparation, and coordination-driven assembly process make the robust gel a good candidate in various applications, such as material templates and drug deliveries.

Tribological behavior of laser textured steel impregnated with supramolecular gel lubricant

The friction and wear properties of supramolecular gel lubricant impregnated into the laser surface texturing steel fabricated by laser micromachining were investigated, with that impregnated with commercial oil PAO10 as a comparison. Scanning electron microscopy characterization shows the gel lubricant have been successfully pumped into the pores of laser surface texturing steel from liquid state and solidified afterwards via supermolecular assembly. The friction was measured on an Optimal SRV-IV oscillating reciprocating friction and wear tester. It is shown that laser surface texturing steel impregnated with gel lubricant exhibited outstanding friction reduction and antiwear performance than impregnated with PAO10. The gelator molecules act not only to solidify lubricating oil, but also contribute to boundary lubrication by strong adsorption on substrate surface. The instant self-assembly of gelator to solidify lubricating oils makes impregnation of lubricants easier and helps to keep lubricating oils retention for long-term functioning.

Functional Characteristics of Oleogel Prepared from Sunflower Oil with β‐Sitosterol and Stearic Acid

Abstractβ‐Sitosterol (Sit) and stearic acid (SA) were combined at varying ratios (w/w) and added to sunflower oil (SFO) at the concentration of 20 g/100 g oil for preparing edible fat‐like oleogel. The oleogel was characterized using an optical microscope, Fourier transform infrared spectroscopy (FTIR), X‐ray diffractometer (XRD), differential scanning calorimeter, and texture analyzer. The oil‐binding capacity, melting temperature, and firmness of the oleogel increased with the increase in the amount of SA in the gelator combination (Sit:SA, w/w). The microscopic analysis showed that the gel network formed based on the crystallization and self‐organization of gelator molecules, and both gelators showed an independent crystalline behavior in the oleogel. In addition, the FTIR spectra showed that the gel network formed via physical entanglements and was stabilized by non‐covalent interactions such as hydrogen bonding. Furthermore, XRD diffraction patterns indicated high lateral packing of molecular layers in oleogel prepared with the Sit and SA combination compared with oleogel prepared with a single gelator. On the other hand, for studying the effect of varying concentrations of gelator combinations, the Sit3:SA2 (w/w) combination was added to SFO at concentrations of 10, 15, 20, 25, and 30 g/100 g oil. Specific characteristics such as the oil‐binding capacity and firmness of the oleogel improved as the concentration of the gelator combination (Sit3:SA2) increased from 10 up to 30 g/100 g oil. Therefore, it can be concluded that the saturated fat alternative oleogel can be prepared from SFO with a specific Sit and SA combination ratio and concentration.

Probing Gelation and Rheological Behavior of a Self-Assembled Molecular Gel.

Molecular gels have been investigated over the last few decades; however, mechanical behavior of these self-assembled gels is not well understood, particularly how these materials fail at large strain. Here, we report the gelation and rheological behavior of a molecular gel formed by self-assembly of a low molecular weight gelator (LMWG), di-Fmoc-l-lysine, in 1-propanol/water mixture. Gels were prepared by solvent-triggered technique, and gelation was tracked using Fourier transform infrared (FTIR) spectroscopy and shear rheology. FTIR spectroscopy captures the formation of hydrogen bonding between the gelator molecules, and the change in IR spectra during the gelation process correlates with the gelation kinetics results captured by rheology. Self-assembly of gelator molecules leads to a fiber-like structure, and these long fibers topologically interact to form a gel-like material. Stretched-exponential function can capture the stress-relaxation data. Stress-relaxation time for these gels have been found to be long owing to long fiber dimensions, and the stretching exponent value of 1/3 indicates polydispersity in fiber dimensions. Cavitation rheology captures fracture-like behavior of these gels, and critical energy release rate has been estimated to be of the order 0.1 J/m2. Our results provide new understanding of the rheological behavior of molecular gels and their structural origin.

Organogelator–Cellulose Composite for Practical and Eco‐Friendly Marine Oil‐Spill Recovery

AbstractMarine oil spills pose serious threats to the ecosystem and economy. There is much interest in developing sorbents that can tackle such spills. We have developed a novel sorbent by impregnating cellulose pulp with a sugar‐derived oleogelator, 1,2:5,6‐di‐O‐cyclohexylidene‐mannitol. The gelator molecules mask the surface‐exposed hydroxyl groups of cellulose fibrils by engaging them in H‐bonding and expose their hydrophobic parts making the fibers temporarily hydrophobic (water contact angle 110°). This sorbent absorbs oil effectively, selectively and instantly from oil–water mixtures due to its hydrophobicity. Then the gelator molecules get released uniformly in the oil and later self‐assemble to fibers, as evident from SEM analysis, congealing the oil within the matrix. This hierarchical entrapment of the oil by non‐covalent polymeric fibers within a covalent polymer matrix makes the gel very strong (230‐fold increase in the yield stress) and rigid, making it suitable for practical use.

Organogelator-Cellulose Composite for Practical and Eco-Friendly Marine Oil-Spill Recovery.

Marine oil spills pose serious threats to the ecosystem and economy. There is much interest in developing sorbents that can tackle such spills. We have developed a novel sorbent by impregnating cellulose pulp with a sugar-derived oleogelator, 1,2:5,6-di-O-cyclohexylidene-mannitol. The gelator molecules mask the surface-exposed hydroxyl groups of cellulose fibrils by engaging them in H-bonding and expose their hydrophobic parts making the fibers temporarily hydrophobic (water contact angle 110°). This sorbent absorbs oil effectively, selectively and instantly from oil-water mixtures due to its hydrophobicity. Then the gelator molecules get released uniformly in the oil and later self-assemble to fibers, as evident from SEM analysis, congealing the oil within the matrix. This hierarchical entrapment of the oil by non-covalent polymeric fibers within a covalent polymer matrix makes the gel very strong (230-fold increase in the yield stress) and rigid, making it suitable for practical use.

In Situ Synthesis of a Supramolecular Hydrogelator at an Oil/Water Interface for Stabilization and Stimuli-Induced Fusion of Microdroplets.

Supramolecular hydrogels are expected to have applications as novel soft materials in various fields owing to their designable functional properties. Herein, we developed an in situ synthesis of supramolecular hydrogelators, which can trigger gelation of an aqueous solution without the need for temperature change. This was achieved by mixing two precursors, which induced the synthesis of a supramolecular gelator and its instantaneous self-assembly into nanofibers. We then performed the in situ synthesis of this supramolecular gelator at an oil/water interface to produce nanofibers that covered the surfaces of the oil droplets (nanofiber-stabilized oil droplets). External stimuli induced fusion of the droplets owing to disassembly of the gelator molecules. Finally, we demonstrated that this stimuli-induced droplet fusion triggered a synthetic reaction within the droplets. This means that the confined nanofiber-stabilized droplets can be utilized as stimuli-responsive microreactors.

In Situ Synthesis of a Supramolecular Hydrogelator at an Oil/Water Interface for Stabilization and Stimuli‐Induced Fusion of Microdroplets

AbstractSupramolecular hydrogels are expected to have applications as novel soft materials in various fields owing to their designable functional properties. Herein, we developed an in situ synthesis of supramolecular hydrogelators, which can trigger gelation of an aqueous solution without the need for temperature change. This was achieved by mixing two precursors, which induced the synthesis of a supramolecular gelator and its instantaneous self‐assembly into nanofibers. We then performed the in situ synthesis of this supramolecular gelator at an oil/water interface to produce nanofibers that covered the surfaces of the oil droplets (nanofiber‐stabilized oil droplets). External stimuli induced fusion of the droplets owing to disassembly of the gelator molecules. Finally, we demonstrated that this stimuli‐induced droplet fusion triggered a synthetic reaction within the droplets. This means that the confined nanofiber‐stabilized droplets can be utilized as stimuli‐responsive microreactors.

Reversible Quadruple Switching with Optical, Chiroptical, Helicity, and Macropattern in Self‐Assembled Spiropyran Gels

Enantiomeric glutamate gelators containing a spiropyran moiety are designed and found to self‐assemble into a nanohelix through gelation. Upon alternating UV and visible light irradiation, the spiropyran experiences a reversible change between a blue zwitterionic merocyanine state and a colorless closed ring state spiropyran in supramolecular gels. This photochromic switch causes a series of subsequent changes in the optical, chiroptical, morphological properties from supramolecular to macroscopic levels. While the solution of the gelator molecules does not show any circular dichroism (CD) signal in the region of 250–700 nm due to the fact that the chromophore is far from the chiral center, the gel shows chiroptical signals such as CD and circularly polarized luminescence (CPL) because of the chirality transfer by the self‐assembly. These signals are reversible upon alternating UV/vis irradiation. Therefore, a quadruple optical and chiroptical switch is developed successfully. During such process, the self‐assembled nanostructures from the enantiomeric supramolecular gels also undergo a reversible change between helices and fibers under the alternating UV and visible light trigger. Furthermore, a rewritable material fabricated from their xerogels on a glass is developed. Such rewritable material can be efficiently printed over 30 cycles without significant loss in contrast and resolution using UV and visible light.

Supramolecular Amino Acid Based Hydrogels: Probing the Contribution of Additive Molecules using NMR Spectroscopy.

Supramolecular hydrogels are composed of self‐assembled solid networks that restrict the flow of water. l‐Phenylalanine is the smallest molecule reported to date to form gel networks in water, and it is of particular interest due to its crystalline gel state. Single and multi‐component hydrogels of l‐phenylalanine are used herein as model materials to develop an NMR‐based analytical approach to gain insight into the mechanisms of supramolecular gelation. Structure and composition of the gel fibres were probed using PXRD, solid‐state NMR experiments and microscopic techniques. Solution‐state NMR studies probed the properties of free gelator molecules in an equilibrium with bound molecules. The dynamics of exchange at the gel/solution interfaces was investigated further using high‐resolution magic angle spinning (HR‐MAS) and saturation transfer difference (STD) NMR experiments. This approach allowed the identification of which additive molecules contributed in modifying the material properties.

Palmitoylated amino acids as low-molecular-weight gelators for ionic liquids

We report palmitoylated amino acids (glycine, l-valine, l-isoleucine, l-aspartic acid, l-glutamic acid, l-methionine, l-phenylalanine, l-lysine) as low-molecular-weight gelators for ionic liquids (ILs). The palmitoylated amino acids gelate ammonium- and imidazolium-based ILs, at gelation concentrations of 0.3–1.6 wt.%, despite their simple molecular structures. Eight types of gelators are synthesized, with palmitoylated methionine (Pal-Met) and palmitoylated phenylalanine (Pal-Phe) exhibiting high gelation ability toward the tested ILs. Scanning electron microscopy and confocal laser scanning microscopy observations suggest that the gelator molecules self-assemble into nanofibers within the ILs, which leads to gelation. The ionogels exhibit reversible thermal transition and viscoelastic properties, which are typical of gels. Fourier-transform infrared spectra recorded at various temperatures reveal that hydrogen bonding and van der Waals interactions between gelator molecules are important in the self-assembly. Incorporating gelators of varying concentrations do not affect the intrinsic conductivities of the ILs.

Preparation and magnetic properties of nitroxide radical liquid crystalline physical gels

ABSTRACTLiquid crystalline physical gels which consist of a chiral racemic nitroxide radical compound (±)-1 showing a nematic phase and a diamagnetic, chiral organic gelator (R,R)-2 were prepared to investigate the effects of the gelator on the “positive magneto-LC effects” observed in the nematic phase of (±)-1 and the molecular alignment of (±)-1 in the resulting nematic gels. With the increasing amount of the gelator which decreased the domain size, the molecular reorientation by a magnetic field (0.34 T) was considerably restricted in the nematic gel phase and the intermolecular magnetic interactions (positive magneto-LC effects) increased in the fine and random nematic polydomain structure formed by self-assembled fibers of organic gelator molecules.

Combined effect of shearing and cooling rate on the rheology of organogels developed by selected gelators

In this study we investigated the combined effect of shearing and cooling rate in the rheology of organogels developed in high oleic safflower oil by (R)-12-hydroxystearic acid (HSA), its primary amide derivative [(R)-12-hydroxyoctadecanamide, HOA], and the N-octadecyl derivative of HOA [(R)-N-octadecyl-12-hydroxyoctadecanamide, OHOA]. The experimental set up to develop the organogels involved: 1). The use of quiescent (0s−1) or shearing (300, 600, and 1200s−1) conditions during cooling the gelator solutions (2%) just until achieving the gelator's melting temperature (TM) in the vegetable oil, to then continuing the cooling under static conditions until achieving 15°C) The use of cooling rate protocols involving a constant cooling rate of 1°C/min (CR1) or 10°C/min (CR10) in the shearing and static stages, or variable cooling rates in each stage (i.e., VR1–10 or VR10–1). The elasticity of the organogels (G′) was measured while cooling under static conditions, once the systems achieved 15°C, and after 60min at this temperature. The rheological results obtained at 15°C showed a cooling rate and molecular weight-dependent effect of shearing on G′. We propose that the molecular relaxation time of gelator molecules, and its increase as molecular weight increases and as temperature decreases, plays an important role on the gelator's susceptibility to go through a shear induced crystallization process. Therefore, high molecular weight molecules like OHOA (551.97Da) would remain stretched by shearing longer times than HSA (300.49Da) and HOA (299.49Da). Thus, when shearing was applied while cooling at the higher cooling rate (i.e., CR10 and VR10–1), the stretched OHOA molecules would lead to the development of mesophase precursors that upon further cooling under quiescent conditions, crystallize developing a well-structured organogel. In contrast, stretched low molecular weight molecules (i.e., HSA and HOA) with shorter relaxation time would dissolve back to the isotropic state during cooling. Additionally, the rheological results of HSA and HOA organogels suggested that the shear induced crystallization process might be dependent on the gelator polarity also. These results show that the application of shear and the extent of its application as temperature decreases until achieving TM, have important implication on the self-assembly of gelator molecules, and therefore in the organization and rheology of the three-dimensional crystal network of the organogel.

Highly Fluorescent Non-Conventional Boron-Difluoride-Based π Organogel with Gelation-Assisted Piezochromism.

Triphenylamine-functionalized boron 2-(2'-pyridyl)imidazole complex bearing no alkyl chains or H-bond unit was found to be able to gelate a series of solvents, and the balanced intermolecular π-π interactions play an important role in its supramolecular self-assembly. The gelator molecule is piezochromic, and the dried gel responded to pressure more sensitively than regular crystalline powder.