Critical Gelation Concentration Research Articles

Recent Advances on Supramolecular Gels: From Stimuli-Responsive Gels to Co-Assembled and Self-Sorted Systems

Gels prepared from low-molecular-weight gelators (LMWGs) represent versatile soft materials. Self-assembly of LMWGs forms nanofibers and above critical gelation concentrations, the entanglement of which leads to self-supporting gels. Owing to the dynamic properties of the self-assembly process, stimuli-responsive LMWGs have prospered in the last decade. In addition, incorporating multiple LMWGs into one system brings the opportunity to achieve sophisticated designs and functions. This review covers recent advances in the field of supramolecular gels, from stimuli-responsive gelators to multicomponent systems that are self-sorting and/or co-assembling.

Tuning the properties of hydrogen-bonded block copolymer worm gels prepared via polymerization-induced self-assembly.

Polymerization-induced self-assembly (PISA) is exploited to design hydrogen-bonded poly(stearyl methacrylate)-poly(benzyl methacrylate) [PSMA-PBzMA] worm gels in n-dodecane. Using a carboxylic acid-based RAFT agent facilitates hydrogen bonding between neighboring worms to produce much stronger physical gels than those prepared using the analogous methyl ester-based RAFT agent. Moreover, tuning the proportion of these two types of end-groups on the PSMA chains enables the storage modulus (G′) of a 20% w/w worm gel to be tuned from ∼4.5 kPa up to ∼114 kPa. This is achieved via two complementary routes: (i) an in situ approach using binary mixtures of acid- and ester-capped PSMA stabilizer chains during PISA or (ii) a post-polymerization processing strategy using a thermally-induced worm-to-sphere transition to mix acid- and ester-functionalized spheres at 110 °C that fuse to form worms on cooling to 20 °C. SAXS and rheology studies of these hydrogen-bonded worm gels provide detailed insights into their inter-worm interactions and physical behavior, respectively. In the case of the carboxylic acid-functionalized worms, SAXS provides direct evidence for additional inter-worm interactions, while rheological studies confirm both a significant reduction in critical gelation concentration (from approximately 10% w/w to 2–3% w/w) and a substantial increase in critical gelation temperature (from 41 °C to 92 °C). It is remarkable that a rather subtle change in the chemical structure results in such improvements in gel strength, gelation efficiency and gel cohesion.

Effect of oleogelation on physical properties and oxidative stability of camellia oil-based oleogels and oleogel emulsions

In this work, the camellia oil-based oleogels and O/W oleogel emulsions were prepared with glycerol monolaurate (GML) as an oleogelator, which had a critical gelation concentration of 3 wt%. Results showed that with the increasing GML concentration, a more densely needle-like morphology was formed by the network of GML crystals within the oil, presenting the enhanced thermal stability due to the occurrence of van der Waals interactions and intermolecular hydrogen bonds. In turn, a more compact three-dimensional network was obtained in the emulsions with a larger droplet size, leading to a transition from a predominately elastic to viscous state. Moreover, a lower oxidation rate was observed by the POV and TBARS values as a function of storage time in either the oleogels or oleogel emulsions, indicating a better oxidative stability. These results contribute to a better understanding of oleogelation in camellia oil-based oleogels and oleogel emulsions for formulating food, pharmaceutical and person care products with desired physical properties and oxidative stability.

Tuning the Organogelating and Spectroscopic Properties of a C3 -Symmetric Pyrene-Based Gelator through Charge Transfer.

Two-component organogels and xerogels based on a C3 -symmetric pyrene-containing gelator have been deeply characterized through a wide range of techniques. Based on the formation of charge transfer complexes, the gelation phenomenon proved to be highly dependent on the nature of the electron poor dopant. This parameter significantly influenced the corresponding gelation domains, the critical gelation concentrations of acceptor dopants, the gel-to-sol transition temperatures, the microstructures formed in the xerogel state and their spectroscopic properties. In particular, titrations and variable-temperature UV-visible absorption spectroscopy demonstrated the key role of donor-acceptor interactions with a remarkable correlation between the phase transition temperatures and the disappearance of the characteristic charge transfer bands. The assignment of these electronic transitions was confirmed through time-dependent density functional theory (TD-DFT) calculations. Eventually, it was shown that the luminescent properties of these materials can be tuned with the temperature, either in intensity or emission wavelength.

Unexpected observation of an intermediate hexagonal phase upon fluid-to-gel transition: SDS self-assembly in glycerol

Understanding morphological transformations upon temperature-induced mesophase transitions offers mechanistic insights into the self-assembly process. We have recently reported the unexpected formation of a microfibrillar lamellar gel in SDS-glycerol mixtures above a critical gelation concentration (CGC) as low as ~2 wt%. The gel phase comprised a fibrillar structure on the microscale and a lamellar structure on the nanoscale. Here, the nanoscopic structure of the gel as a function of temperature was probed with small-angle neutron scattering (SANS). The gel underwent a gel-to-fluid transition at a critical gelation temperature, TGC = 45 °C, forming cylindrical micelles at elevated temperatures. Upon cooling, a hexagonal phase formed at ~TGC, evident from the SANS Bragg peaks. This hexagonal phase upon the fluid-to-gel transition sheds light on the gelation mechanism, in which self-assembled SDS micelles undergo a cylindrical-to-lamellar morphological transition via a hexagonal phase. This unprecedented observation also highlights the complexity of self-assembly in nonaqueous hydrogen-bonding rich media.

Effect of the Cationic Head Group on Cationic Surfactant-Based Surfactant Mediated Gelation (SMG).

The surfactant-mediated gelation (SMG) method allows us to formulate hydrogels using a water-insoluble organogelator. In this study, we formulated hydrogels using three cationic surfactants, hexadecyltrimethylammonium bromide (CTAB), hexadecyltrimethylammonium chloride (CTAC), and hexadecylpyridinium chloride (CPC)] and an organogelator (12-hydroxyoctadecanoic acid (12-HOA), and studied their structures and mechanical properties. A fiber-like structure similar to that found in the 12-HOA-based organogels was observed by optical microscopy. Small- and wide-angle X-ray scattering profiles showed Bragg peaks derived from the long- and short-spacing of the crystalline structures in the gel fibers and a correlation peak from the surfactant micelles in the small-angle region. Furthermore, the formation of micelles in the hydrogels was confirmed by UV-vis spectroscopic measurements of the gel samples in the presence of Rhodamine 6G. We concluded that the hydrogels prepared by the SMG method in the present systems are orthogonal molecular assembled systems in which two different molecular assembled structures coexist. Among the three surfactant systems, the CTAB system presented the lowest critical gelation concentration and highest sol-gel transition temperature and viscoelasticity. These differences in gel fiber formation and gel properties were discussed from the viewpoint of the degree of solubilization of the gelator molecules in micelles coexisting with gel fibers and diffusion of the gelator molecules in the gel formation process.

Self-assembly of bistriazole BDT based bolaamphiphiles into SmA phase and helical organogels

Bistriazole benzo[1,2-b:4,3-b′]dithiophene (BDT) contained bolaamphiphiles have been synthesized via Sonogashira coupling and click reactions as key steps. For such compounds, the central bent bistriazole BDT core, the flexible methylene spacer led the rodlike core together with the terminal diol groups to twist to be U-shaped polar region. Micro-segregation of U-shaped polar region from the lipophilic chains generated a SmA phase. Furthermore, these compounds can act as super organogelators to gel different kinds of solvents with the lowest critical gelation concentration (CGC) of 0.2 mg/ml. Gel morphologies with scarcely reported nanostructures including nanofibers, nanobelts, nanodisks, nanotrumpets and nanotubes were firstly observed in bolaamphiphilic self-assembly system.

Enhancing gelation properties of dioctyl phthalate using urethane and urea‐based gelators to improve mechanical properties of polyvinylchloride

AbstractThis study provides a new approach to promote the mechanical properties of polyvinylchloride/dioctyl phthalate (PVC/DOP) via gelators. It presented the effect of newly synthesized urethane and urea‐based gelators on the plasticizer DOP, and then prepared for PVC. 4,4′‐Diphenylmethane diisocyanate (MDI) was reacted with lauryl alcohol (LA), stearyl alcohol (SA), and octadecylamine (OL) to prepare LML, SMS, and SMO, respectively. These gelators were self‐assembled in the common plasticizer DOP to form gels, and then PVC was blended with these gels. SMO exhibited the lowest critical gelator concentration (CGC) in DOP. FEEM observation of xerogels exhibited micro platelet morphology with various sizes. Rheological evaluation revealed that DOPgel enhanced the gelating ability of PVC. Mechanical measurement showed that gelators improved the tensile properties of PVC in the sequence of LML > SMS > SMO. Tensile modulus and the tensile strength of PVC/DOP increased by 74.6% (from 14.6 to 25.5 MPa) and 19.8% (from 13.1 to 15.7 MPa) only by adding 3% LML. These new urethane and urea‐based gelators improved the mechanical strength of PVC without reducing its toughness.

Modifying ceramic membranes with in situ grown iron oxide nanoparticles and their use for oily water treatment

A facile technique for impregnating commercial ceramic membranes with in situ grown iron oxide nanoparticles (NPs) and their use for synthetic produced water treatment is presented. Unlike most literature, our technique led to NPs forming on the surface as well as within the pores of the membrane. The antifouling properties of the membrane were evaluated in terms of surface characteristics, hydrophilicity, fouling characteristics, organic rejection, and regeneration capacity. The contact angle and the rate of water penetration were used to determine optimum precursor concentration for which membrane hydrophilicity is improved without major pore blockage due to NP agglomeration and deposition. At such concentration, in situ generation of the NPs, mostly with sizes less than 4 nm, did not affect membrane morphology, porosity or the intrinsic hydraulic resistance. Membrane organic rejection and fouling behavior showed considerable enhancement in humic acid, a model natural organic matter also existing in produced water, retention at different transmembrane pressures. Organic rejection along with the permeate flux suggest fouling transition from concentration polarization and pore plugging to gel/cake layer formation occurs at higher critical gel concentration for the modified membranes. The steady permeate flux for modified and unmodified membranes are comparable, especially at higher humic acid concentration, and flux recovery and fouling ratios reveal enhanced antifouling properties. A new dimensionless number, membrane performance number, is introduced to couple the performance of the membrane in terms of permeate flux and contaminant rejection.

Design of cationic amphiphiles for generating self-assembled soft nanostructures, micelles and hydrogels

Design of amphiphiles to develop robust self-assembled soft nanomaterials, such as micelles and hydrogels is an interesting subject. A series of cationic amphiphilic compounds were synthesized comprising 1-ethoxy (3-pentadecyl) benzene as the hydrophobic tail. The second carbon of ethoxy was linked to quaternary head groups (trimethyl ammonium bromide (PEA), triethyl ammonium bromide (PETE), pyridinium bromide (PEPy), N-methyl morpholino bromide (PENM), N-methyl piperidine bromide (PENP)). Inclusion of benzene ring leads to a significant decrease in critical micellar concentration (CMC) as compared to other cationic surfactants, such as cetyl trimethyl ammonium bromide (CTAB). Interestingly, at higher concentration, these cationic amphiphiles were forming soft hydrogels with critical gelation concentration (CGC) from 3 to 10% (w/v). The small-angle X-ray scattering (SAXS) analysis of xerogel revealed the formation of self-assembled lamellar patterns of molecules. Further, the morphology of xerogels were also seen under a scanning electron microscope (SEM) which correlates with SAXS data. The SAXS and SEM data confirms the formation of worm-like micellar structures and entangle themselves to form a hydrogel. The cytotoxicity assay was done on HDFA, HeLa and HEK cell lines, haemolysis assay showed better haemocompatibility than CTAB. The synthesized surfactants exhibited up to 3-fold higher solubilization capability against hydrophobic molecules than CTAB.

Controlled Sol-Gel and Diversiform Nanostructure Transitions by Photoresponsive Molecular Switching of Tetraphenylethene- and Azobenzene-Functionalized Organogelators.

The implementation of stimuli-responsive materials with dynamically controllable features has long been an important objective that challenges chemists in the materials science field. We report here the synthesis and characterization of [2]rotaxanes (R1 and R1-b) with a molecular shuttle and photoresponsive properties. Axles T1 and T1-b were found to be highly efficient and versatile organogelators toward various nonpolar organic solvents, especially p-xylene, with critical gelation concentrations as low as 0.67 and 0.38 w/v %, respectively. The two molecular stations of switchable [2]rotaxanes (R1 and R1-b) can be revealed or concealed by t-butylcalix[4]arene macrocycle, thus inhibiting the gelation processes of the respective axles T1 and T1-b through the control of intermolecular hydrogen-bonding interactions. The sol-gel transition of axles T1 and T1-b could be achieved by the irradiation of UV-visible light, which interconverted between the extended and contracted forms. Interestingly, the morphologies of organogels in p-xylene, including flakes, nanobelts, fibers, and vesicles depending on the molecular structures of axles T1 and T1-b, were induced by UV-visible light irradiation. Further studies revealed that acid-base-controllable and reversible self-assembled nanostructures of these axle molecules were mainly constructed by the interplay of multi-noncovalent interactions, such as intermolecular π-π stacking, CH-π, and intermolecular hydrogen-bonding interactions. Surprisingly, our TPE molecular systems (R1, R1-b, T1, and T1-b) are nonemissive in their aggregated states, suggesting that not only fluorescence resonance energy transfer but also aggregation-caused quenching may have been functioning. Finally, the mechanical strength of these organogels in various solvents was monitored by rheological experiments.

A fast and versatile cross-linking strategy via o-phthalaldehyde condensation for mechanically strengthened and functional hydrogels.

Fast and catalyst-free cross-linking strategy is of great significance for construction of covalently cross-linked hydrogels. Here, we report the condensation reaction between o-phthalaldehyde (OPA) and N-nucleophiles (primary amine, hydrazide and aminooxy) for hydrogel formation for the first time. When four-arm poly(ethylene glycol) (4aPEG) capped with OPA was mixed with various N-nucleophile-terminated 4aPEG as building blocks, hydrogels were formed with superfast gelation rate, higher mechanical strength and markedly lower critical gelation concentrations, compared to benzaldehyde-based counterparts. Small molecule model reactions indicate the key to these cross-links is the fast formation of heterocycle phthalimidine product or isoindole (bis)hemiaminal intermediates, depending on the N-nucleophiles. The second-order rate constant for the formation of phthalimidine linkage (4.3 M−1 s−1) is over 3000 times and 200 times higher than those for acylhydrazone and oxime formation from benzaldehyde, respectively, and comparable to many cycloaddition click reactions. Based on the versatile OPA chemistry, various hydrogels can be readily prepared from naturally derived polysaccharides, proteins or synthetic polymers without complicated chemical modification. Moreover, biofunctionality is facilely imparted to the hydrogels by introducing amine-bearing peptides via the reaction between OPA and amino group.

Synthesis and self-assembly of 1,8-naphthalene-based hexacatenar mesogens

ABSTRACT A novel series of hexacatenars consisting of a central naphthalene core functionalised with 1,2,3-triazole dendritic wings at 1 and 8 positions were synthesised by Sonogashira coupling and Click reactions. POM, DSC and XRD investigations revealed that all hexacatenars could self-assemble into absolutely Colhex/p6 mm mesophase in the bulk state. Interestingly, these compounds could gel some organic solvents with low critical gelation concentration and the gel morphologies varied from 3D porous structures via nanofiber structures to 3D spherical aggregates with increasing polarity of solvents. These investigations provide a new strategy to generate supramolecular self-assembly materials.

Aromatic Stacking Facilitated Self-Assembly of Ultrashort Ionic Complementary Peptide Sequence: β-Sheet Nanofibers with Remarkable Gelation and Interfacial Properties.

Understanding peptide self-assembly mechanisms and stability of the formed assemblies is crucial for the development of functional nanomaterials. Herein, we have adopted a rational design approach to demonstrate how a minimal structural modification to a nonassembling ultrashort ionic self-complementary tetrapeptide FEFK (Phe4) remarkably enhanced the stability of self-assembly into β-sheet nanofibers and induced hydrogelation. This was achieved by replacing flexible phenylalanine residue (F) by the rigid phenylglycine (Phg), resulting in a constrained analogue PhgEPhgK (Phg4), which positioned aromatic rings in an orientation favorable for aromatic stacking. Phg4 self-assembly into stable β-sheet ladders was facilitated by π-staking of aromatic side chains alongside hydrogen bonding between backbone amides along the nanofiber axis. The contribution of these noncovalent interactions in stabilizing self-assembly was predicted by in silico modeling using molecular dynamics simulations and semiempirical quantum mechanics calculations. In aqueous medium, Phg4 β-sheet nanofibers entangled at a critical gelation concentration ≥20 mg/mL forming a network of nanofibrous hydrogels. Phg4 also demonstrated a unique surface activity in the presence of immiscible oils and was superior to commercial emulsifiers in stabilizing oil-in-water (O/W) emulsions. This was attributed to interfacial adsorption of amphiphilic nanofibrils forming nanofibrilized microspheres. To our knowledge, Phg4 is the shortest ionic self-complementary peptide rationally designed to self-assemble into stable β-sheet nanofibers capable of gelation and emulsification. Our results suggest that ultrashort ionic-complementary constrained peptides or UICPs have significant potential for the development of cost-effective, sustainable, and multifunctional soft bionanomaterials.

Temperature‐Responsive Low Molecular Weight Ionic Liquid Based Gelator: An Approach to Fabricate an Anti‐Cancer Drug‐Loaded Hybrid Ionogel

AbstractConceptualising gelators with stimuli responsiveness is advantageous to design smart materials for emerging technologies. These gelators, if are surface active and made up of active pharmaceutical ingredients, can (i) exhibit an interesting pharmaceutical profile, (ii) be useful in drug formulations and (iii) exert direct effects on cell membranes. Herein, cetylpyridinium salicylate (CetPySal) that offers higher surface activity than its precursor surfactant, cetpylpyridinium chloride (CPCl), was synthesised. CetPySal forms a temperature‐responsive ionogel at a critical gelation concentration that changes its physical appearance with temperature, i. e. opaque to transparent, owing to its structural arrangement within its supramolecular framework, that are characterized through spectrometric, calorimetric, scattering and microscopic techniques. The viscoelastic nature of the ionogels was studied through dynamic rheological measurements and the interactions that governs the phase transition were studied through FTIR spectroscopy. The hybrid pharmaceutical ionogels was successfully constructed through encapsulation of the chemotherapeutic drug imatinib mesylate within the ionogel matrix. The sustained release of the drug was investigated from this hybrid ionogel matrix. These results suggest that this new thermo‐responsive ionogel may be used to improve sustained release of drugs and open up new avenues as low‐cost gelators for biomedical applications.

The curious case of SDS self-assembly in glycerol: Formation of a lamellar gel

HypothesisHydrogen-bonding capacities of polar nonaqueous media significantly affect self-assembly behaviours of surfactants in these media. IntroductionGlycerol, a nonaqueous hydrogen-bonding solvent, is widely used in industrial formulations due to its desirable physical properties. Surfactants are ubiquitous in such applications; however, surfactant self-assembly in glycerol is not well understood. MethodsThe microscopic structure of the gel phase was studied using a series of imaging techniques: polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and environmental scanning electron microscopy (ESEM). The rheological properties of the gel were studied using viscometry and oscillation rheology measurements. Further nano-structural characterisation was carried out using small-angle neutron scattering (SANS). ResultsWe have observed the unexpected formation of a microfibrillar gel in SDS and glycerol mixtures at a critical gelation concentration (CGC) as low as ~2 wt%; such SDS gelation has not been observed in aqueous systems. The microscopic structure of the gel consisted of microfibres some mm in length and with an average diameter of D ~ 0.5 μm. The fibres in the gel phase exhibited shear-induced alignment in the viscometry measurements, and oscillation tests showed that the gel was viscoelastic, with an elastic-dominated behaviour. Fitting to SANS profiles showed lamellar nano-structures in the gel microfibres at room temperature, transforming into cylindrical-micellar solutions above a critical gelation temperature, TCG ~ 45 °C. ConclusionsThese unprecedented observations highlight the markedly different self-assembly behaviours in aqueous and nonaqueous H-bonding solvents, which is not currently well understood. Deciphering such self-assembly behaviour is key to furthering our understanding of self-assembly on a fundamental level.

Thermo-switchable de novo ionogel as metal absorbing and curcumin loaded smart bandage material

Abstract Inspired by the limitations of polymeric gels, conceptualizing low molecular weight gelator to form hydrogel that changes its properties as well applications through external stimuli of choice have attracted the attention of scientific community in recent times. Herein we had designed long alkyl chain containing ester functionalized ionic liquid, 3-methyl-1-(hexadecyloxycarbonylmethyl)imidazolium bromide (C16EMeImBr) that through self-assembling in water forms ionogel at 8.80% (w/v) critical gelation concentration and physiological pH. Ionogel, physically appeared as opaque at 25 °C, transformed into transparent at ~50 °C temperature and was characterized for the microstructural arrangement through small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) techniques. The gel formation ability of the C16EMeImBr was investigated in various solvents, in their neat form as well as solvent-water composition mixture. The fibrillar opaque ionogel have been studied for their metal (cobalt and copper) absorbing properties. Curcumin loaded ionogel was used to fabricate compact cotton patch to open the new avenues in non-woven bandage material that can work on infected wounds. The structural configuration of the curcumin-based ionogel was further investigated through molecular dynamic simulation.

Light-responsive and self-healing behavior of azobenzene-based supramolecular hydrogels

HypothesisMulti-component supramolecular hydrogels are gaining increasing interest as stimuli-responsive materials. To fully understand and possibly exploit the potential of such complex systems, the hierarchical structure of the gel network needs in-depth investigations across multiple length scales. We show that a thorough structural and rheological study represents a crucial pillar for the exploitation of this class of functional materials. ExperimentsSupramolecular hydrogels are prepared by self-assembly of hexadecyltrimethylammonium bromide (CTAB) and azobenzene-4,4′-dicarboxylic acid (AZO) in alkaline aqueous solution. The CTAB/AZO concentration was varied from ϕ = 0.25 to 4 wt% keeping the CTAB:AZO molar ratio fixed at 2:1. The systems were thoroughly studied through a combination of X-ray scattering, microscopy, rheological and spectroscopic analyses. FindingsThe CTAB/AZO solutions form a self-supporting gel with nanofibrillar structure below ~30 °C. The critical gelation concentration is ϕc = 0.45 wt%. Above this threshold, the gel elasticity and strength increase with CTAB/AZO content as ~(ϕ-ϕc)1. The hydrogels exhibit self-healing ability when left at rest after a stress-induced damage. Moreover, the light-induced isomerization of the AZO moieties provides the gel with light-responsiveness. Overall, the multi-stimuli responsiveness of the studied CTAB/AZO hydrogels makes them a solid starting point for the development of sensors for mechanical vibrations and UV/visible light exposure.

Unusual Two-Step Assembly of a Minimalistic Dipeptide-Based Functional Hypergelator.

Self-assembled peptide hydrogels represent the realization of peptide nanotechnology into biomedical products. There is a continuous quest to identify the simplest building blocks and optimize their critical gelation concentration (CGC). Herein, a minimalistic, de novo dipeptide, Fmoc-Lys(Fmoc)-Asp, as an hydrogelator with the lowest CGC ever reported, almost fourfold lower as compared to that of a large hexadecapeptide previously described, is reported. The dipeptide self-assembles through an unusual and unprecedented two-step process as elucidated by solid-state NMR and molecular dynamics simulation. The hydrogel is cytocompatible and supports 2D/3D cell growth. Conductive composite gels composed of Fmoc-Lys(Fmoc)-Asp and a conductive polymer exhibit excellent DNA binding. Fmoc-Lys(Fmoc)-Asp exhibits the lowest CGC and highest mechanical properties when compared to a library of dipeptide analogues, thus validating the uniqueness of the molecular design which confers useful properties for various potential applications.

Anthranilamide-based Short Peptides Self-Assembled Hydrogels as Antibacterial Agents

In this study, we describe the synthesis and molecular properties of anthranilamide-based short peptides which were synthesised via ring opening of isatoic anhydride in excellent yields. These short peptides were incorporated as low molecular weight gelators (LMWG), bola amphiphile, and C3-symmetric molecules to form hydrogels in low concentrations (0.07–0.30% (w/v)). The critical gel concentration (CGC), viscoelastic properties, secondary structure, and fibre morphology of these short peptides were influenced by the aromaticity of the capping group or by the presence of electronegative substituent (namely fluoro) and hydrophobic substituent (such as methyl) in the short peptides. In addition, the hydrogels showed antibacterial activity against S. aureus 38 and moderate toxicity against HEK cells in vitro.