Insight on the NMR study of supramolecular gels and its application to monitor molecular recognition on self-assembled fibers

Variable temperature NMR of organogelators: the intensities of a single sample describe the full phase diagram

Organogelators constitute a numerous class of compounds, able to form gels in organic solvents. Their phase diagrams are useful to understand their mechanisms of formation and their stability, but their mapping is often a tedious task. We show that liquid NMR can simplify and quicken the acquisition of phase diagrams. In liquid NMR spectra of organogels, the visible signals of the gelator represent only its soluble fraction. The intensities increase with temperature, until the gel melts. Suitable normalization of these intensities yields the solubility as a function of temperature, which is sufficient to map the phase diagram. We verified it experimentally with three organogelators, chosen because independent authors have previously mapped out their phase diagram by other techniques including DSC and rheology. We show that the curves obtained by NMR superimpose with these diagrams. A variable temperature NMR experiment with a single sample can yield the phase diagram with sensitivity of the order of 0.01 wt%.

Chiral Assembly Preferences and Directing Effects in Supramolecular Two-Component Organogels

The impact of chirality on the self-assembly of supramolecular gels is of considerable importance, as molecular-scale programming can be translated into nanostructuring and ultimately affect macroscopic performance. This paper explores the effect of chirality on the assembly of two-component gels comprised of a second-generation dendritic lysine peptide acid, containing three chiral centres, and an amine. This combination forms an acid⁻amine complex that assembles into nanofibres through peptide-peptide hydrogen bonds, leading to organogels. With achiral amines, a racemic mixture of l,l,l and d,d,d dendritic peptide acids surprisingly forms the best gels-more commonly, mixing enantiomers suppresses gelation. Thermodynamic studies demonstrate that depending on the amine, the greater stability of heterochiral gels can either be entropically or enthalpically driven. With amines possessing "R" chirality, the l,l,l peptide acid consistently forms more effective gels than its d,d,d analogue. Furthermore, in mixed gels, l,l,l sometimes imposes its assembly preference onto d,d,d. In summary, this paper demonstrates a rare example in which heterochiral gels are preferred, and also explores directing effects when each component in a two-component gel is chiral.

pH-Sensitive Compounds for Selective Inhibition of Acid-Producing Bacteria

Stimuli-responsive compounds that provide on-site, controlled antimicrobial activity promise an effective approach to prevent infections, reducing the need for systemic antibiotics. We present a novel pH-sensitive quaternary pyridinium salt (QPS), whose antibacterial activity is boosted by low pH and controlled by adjusting the pH between 4 and 8. Particularly, this compound selectively inhibits growth of acid-producing bacteria within a multispecies community. The successful antibacterial action of this QPS maintains the environmental pH above 5.5, a threshold pH, below which demineralization/erosion takes place. The design, synthesis, and characterization of this QPS and its short-chain analogue are discussed. In addition, their pH-sensitive physicochemical properties in aqueous and organic solutions are evaluated by UV-vis spectroscopy, dynamic light scattering, and NMR spectroscopy. Furthermore, the mechanism of action reveals a switchable assembly that is triggered by acid-base interaction and formed by tightly stacked π-conjugated systems and base moieties. Finally, a model is proposed to recognize the correlated but different mechanisms of pH sensitivity and acid-induced, pH-controlled antibacterial efficacy. We anticipate that successful application of these QPSs and their derivatives will provide protections against infection and erosion through targeted treatments to acid-producing bacteria and modulation of environmental pH.

Enantiopure Tertiary Urea and Thiourea Derivatives of Trianglamine Macrocycle: Structural Studies and Metallogeling Properties

Synthesis and detailed experimental and theoretical study on new urea and thiourea derivatives of chiral trianglamine are presented. In solution, the urea derivative of the trianglamine adopts cone conformation, whereas a respective thiourea derivative exists in solution predominantly as a partial cone conformer. In the crystalline phase, the thiourea trianglamine derivative adapts partial cone conformation. In the solid state, the two symmetry independent molecules of thiourea trianglamine create bilayers, containing molecules arranged in a zipper motif. The bilayers are separated by channels filled with disordered solvent molecules. The thiourea derivative of trianglimine appeared to be a simple, low molecular weight supergelator that formed stable chiral metallogels in N,N-dimethylformamide with Ag(I), Cu(I). and Cu(II) salts. The enantiomeric enrichment of the macrocycle is a necessary condition for effective gelling because neither racemic nor enantiomerically enriched samples (up to 50% ee) form metallogels. The metallogels formed from silver cations and thiourea trianglamine show reversible thixotropic property rarely observed in metallogels.

Applying low-molecular weight supramolecular gelators in an environmental setting - self-assembled gels as smart materials for pollutant removal

This review explores supramolecular gels as materials for environmental remediation. These soft materials are formed by self-assembling low-molecular-weight building blocks, which can be programmed with molecular-scale information by simple organic synthesis. The resulting gels often have nanoscale 'solid-like' networks which are sample-spanning within a 'liquid-like' solvent phase. There is intimate contact between the solvent and the gel nanostructure, which has a very high effective surface area as a result of its dimensions. As such, these materials have the ability to bring a solid-like phase into contact with liquids in an environmental setting. Such materials can therefore remediate unwanted pollutants from the environment including: immobilisation of oil spills, removal of dyes, extraction of heavy metals or toxic anions, and the detection or removal of chemical weapons. Controlling the interactions between the gel nanofibres and pollutants can lead to selective uptake and extraction. Furthermore, if suitably designed, such materials can be recyclable and environmentally benign, while the responsive and tunable nature of the self-assembled network offers significant advantages over other materials solutions to problems caused by pollution in an environmental setting.

Re-entrant supramolecular interactions in inverse-melting α-cyclodextrin·4-methylpyridine·water mixtures: an NMR study

Inverse freezing αCD·4MP·H₂O turns into a gel as αCD loses its solvation shell. First, it loses its interaction with 4MP, and then its solvation by water.

Multi-component hybrid hydrogels - understanding the extent of orthogonal assembly and its impact on controlled release

This paper reports self-assembled multi-component hybrid hydrogels including a range of nanoscale systems and characterizes the extent to which each component maintains its own unique functionality, demonstrating that multi-functionality can be achieved by simply mixing carefully-chosen constituents. Specifically, the individual components are: (i) pH-activated low-molecular-weight gelator (LMWG) 1,3;2,4-dibenzylidenesorbitol-4',4''-dicarboxylic acid (DBS-COOH), (ii) thermally-activated polymer gelator (PG) agarose, (iii) anionic biopolymer heparin, and (iv) cationic self-assembled multivalent (SAMul) micelles capable of binding heparin. The LMWG still self-assembles in the presence of PG agarose, is slightly modified on the nanoscale by heparin, but is totally disrupted by the micelles. However, if the SAMul micelles are bound to heparin, DBS-COOH self-assembly is largely unaffected. The LMWG endows hybrid materials with pH-responsive behavior, while the PG provides mechanical robustness. The rate of heparin release can be controlled through network density and composition, with the LMWG and PG behaving differently in this regard, while the presence of the heparin binder completely inhibits heparin release through complexation. This study demonstrates that a multi-component approach can yield exquisite control over self-assembled materials. We reason that controlling orthogonality in such systems will underpin further development of controlled release systems with biomedical applications.

Self-sorted Oligophenylvinylene and Perylene Bisimide Hydrogels

We describe two component hydrogels with networks composed of self-sorted fibres. The component gelators are based on 1,4-distyrylbenzene (OPV3) and perylene bisimide (PBI) units. Self-sorted gels can be formed by a slow decrease in pH, which leads to sequential assembly. We demonstrate self-sorting by NMR, rheology and small angle X-ray scattering (SAXS). Photoconductive xerogels can be prepared by drying these gels. The wavelength response of the xerogel is different to that of the PBI alone.

Substitution of Percutaneous Ethanol Injection with a Low Molecular Weight Peptide Gel Mimicking Chemoembolization for Cancer Therapy

In order to avoid the instability and quick separation between emulsifier and drug in the interventional chemoembolization, an injectable low molecular weight peptide gel (LMWG) was prepared to localize ethanol and chemotherapeutic for in situ synergistic therapy. The formation mechanism, rheological property and morphology of the LMWG were investigated by NMR, UV-vis, MS and SEM. The interaction between gelator and anticancer drug doxorubicin hydrochloride (DOX) was evaluated by fluorescence spectroscopy and its contribution on drug loading properties was demonstrated. The gel was non-toxic to both 3T3 fibroblasts and 4T1 breast cancer cells. DOX as well as ethanol were encapsulated in the gel and injected in breast cancer bearing mice with low drug dose (2.5 mg/kg body weight). The LMWG surrounded tumors act as a depot for ethanol release and release DOX to induce the apoptosis of cancer cells. With the combination of percutaneous ethanol injection (PEI) and chemotherapy, the DOX loaded LMWG exhibited great significance in necrosis of tumor tissue and exciting tumor inhibition efficiency.

Self-Assembly, Hydrogelation, and Nanotube Formation by Cation-Modified Phenylalanine Derivatives

Fluorenylmethoxycarbonyl-protected phenylalanine (Fmoc-Phe) derivatives are a privileged class of molecule that spontaneously self-assemble into hydrogel fibril networks. Fmoc-Phe-derived hydrogels are typically formed by dilution of the hydrogelator from an organic cosolvent into water, by dissolution of the hydrogelator under basic aqueous conditions followed by adjustment of the pH with acid, or by other external triggering forces, including sonication and heating. These conditions complicate biological applications of these hydrogels. Herein, we report C-terminal cation-modified Fmoc-Phe derivatives that are positively charged across a broad range of pH values and that can self-assemble and form hydrogel networks spontaneously without the need to adjust pH or to use an organic cosolvent. In addition, these cationic Fmoc-Phe derivatives are found to self-assemble into novel sheet-based nanotube structures at higher concentrations. These nanotube structures are unique to C-terminal cationic Fmoc-Phe derivatives; the parent Fmoc-Phe carboxylic acids form only fibril or worm-like micelle structures. Nanotube formation by the cationic Fmoc-Phe molecules is dependent on positive charge at the C-terminus, since at basic pH where the positive charge is reduced only fibrils/worm-like micelles are formed and nanotube formation is suppressed. These studies provide an important example of Fmoc-Phe derivatives that can elicit hydrogelation without organic cosolvent or pH modification and also provide insight into how subtle modification of structure can perturb the self-assembly pathways of Fmoc-Phe derivatives.

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.

Investigating the effects of peptoid substitutions in self-assembly of Fmoc-diphenylalanine derivatives

Low molecular weight agents that undergo self-assembly into fibril networks with hydrogel properties are promising biomaterials. Most low molecular weight hydrogelators are discovered empirically or serendipitously due to imperfect understanding of the mechanisms of self-assembly, the packing structure of self-assembled materials, and how the self-assembly process corresponds to emergent hydrogelation. Herein, the mechanisms of self-assembly and hydrogelation of N-fluorenylmethoxycarbonyl diphenylalanine (Fmoc-PhePhe), a well-studied low molecular weight hydrogelator, is probed by systematic comparison with derivatives in which Phe residues are replaced by corresponding N-benzyl glycine peptoid (Nphe) analogs. Peptoids are peptidomimetics that shift display of side chain functionality from the α-carbon to the terminal nitrogen. This alters the hydrogen bonding capacity, the side chain presentation geometry, amide cis/trans isomerization equilibrium, and β-sheet potential of the peptoid relative to the corresponding amino acid in the context of peptidic polymers. It was found that amino acid/peptoid hybrids Fmoc-Phe-Nphe and Fmoc-Nphe-Phe have altered fibril self-assembly propensity and reduced hydrogelation capacity relative to the parent dipeptide, and that fibril self-assembly of the dipeptoid, Fmoc-Nphe-Nphe, is completely curtailed. These findings provide insight into the potential of low molecular weight peptoids and peptide/peptoid hybrids as hydrogelation agents and illuminate the importance of hydrogen bonding and π-π interaction geometry in facilitating self-assembly of Fmoc-Phe-Phe.

Bipyridine based metallogels: an unprecedented difference in photochemical and chemical reduction in the in situ nanoparticle formation

Metal co-ordination induced supramolecular gelation of low molecular weight organic ligands is a rapidly expanding area of research due to the potential in creating hierarchically self-assembled multi-stimuli responsive materials. In this context, structurally simple O-methylpyridine derivatives of 4,4'-dihydroxy-2,2'-bipyridine ligands are reported. Upon complexation with Ag(i) ions in aqueous dimethyl sulfoxide (DMSO) solutions the ligands spontaneously form metallosupramolecular gels at concentrations as low as 0.6 w/v%. The metal ions induce the self-assembly of three dimensional (3D) fibrillar networks followed by the spontaneous in situ reduction of the Ag-centers to silver nanoparticles (AgNPs) when exposed to daylight. Significant size and morphological differences of the AgNP's was observed between the standard chemical and photochemical reduction of the metallogels. The gelation ability, the nanoparticle formation and rheological properties were found to be depend on the ligand structure, while the strength of the gels is affected by the water content of the gels.

Probing the surface chemistry of self-assembled peptide hydrogels using solution-state NMR spectroscopy

The surface chemistry of self-assembled hydrogel fibres - their charge, hydrophobicity and ion-binding dynamics - is recognised to play an important role in determining how the gels develop as well as their suitability for different applications. However, to date there are no established methodologies for the study of this surface chemistry. Here, we demonstrate how solution-state NMR spectroscopy can be employed to measure the surface chemical properties of the fibres in a range of hydrogels formed from N-functionalised dipeptides, an effective and versatile class of gelator that has attracted much attention. By studying the interactions with the gel fibres of a diverse range of probe molecules and ions, we can simultaneously study a number of surface chemical properties of the NMR invisible fibres in an essentially non-invasive manner. Our results yield fresh insights into the materials. Most notably, gel fibres assembled using different tiggering methods bear differing amounts of negative charge as a result of a partial deprotonation of the carboxylic acid groups of the gelators. We also demonstrate how chemical shift imaging (CSI) techniques can be applied to follow the formation of hydrogels along chemical gradients. We apply CSI to study the binding of Ca²⁺ and subsequent gelation of peptide assemblies at alkaline pH. Using metal ion-binding molecules as probes, we are able to detect the presence of bound Ca²⁺ ions on the surface of the gel fibres. We briefly explore how knowledge of the surface chemical properties of hydrogels could be used to inform their practical application in fields such as drug delivery and environmental remediation.

Rapid self-healing and anion selectivity in metallosupramolecular gels assisted by fluorine–fluorine interactions

Metal complexes from perfluoroalkylamide terpyridine self-assemble into anion selective gels, which manifest self-healing and thermal rearrangement in aqueous dimethyl sulfoxide.

On the syneresis of an OPV functionalised dipeptide hydrogel

We describe a new dipeptide hydrogel based on an oligophenylene vinylene core. After gelation, the initial network evolves, expelling solvent and resulting in syneresis. We describe this process and the effects in the bulk properties of the material.

Bio-inspired supramolecular materials by orthogonal self-assembly of hydrogelators and phospholipids

The orthogonal self-assembly of multiple components is a powerful strategy towards the formation of complex biomimetic architectures, but so far the rules for designing such systems are unclear. Here we show how to identify orthogonal self-assembly at the supramolecular level and describe guidelines to achieve self-sorting in self-assembled mixed systems. By investigating multicomponent self-assembled systems consisting of low molecular weight gelators and phospholipids, both at a molecular and a supramolecular level, we found that orthogonal self-assembly can only take place if the entities assemble via a strong and distinct set of interactions. The resulting supramolecular architectures consist of fibrillar networks that coexist with liposomes and thereby provide additional levels of compartmentalization and enhanced stability as compared to self-assembled systems of gelators or phospholipids alone.

Solid state NMR studies of gels derived from low molecular mass gelators

Since its invention more than six decades ago, nuclear magnetic resonance (NMR) spectroscopy has evolved as an inevitable part of chemical as well as structural analysis of small molecules, polymers, biomaterials and hybrid materials. In the solution state, due to the increased viscosity of complex viscoelastic fluids such as gels, liquid crystals and other soft materials, the rate of molecular tumbling is reduced, which in turn affects the chemical shift anisotropy, dipolar and quadrupolar interactions. As a consequence the solution state NMR spectra show broad lines, and therefore, extracting detailed structural information is a challenging task. In this context, solid state (SS) NMR has the ability to distinguish between a minute amount of polymorphic forms, conformational changes, and the number of non-equivalent molecules in an asymmetric unit of a crystal lattice, and to provide both qualitative as well as quantitative analytical data with a short-range order. Therefore, SS NMR has continued to evolve as an indispensable tool for structural analysis and gave birth to a new field called NMR crystallography. Solid state cross polarization (CP) and high resolution (HR) magic angle spinning (MAS) NMR spectroscopy has been used to study weak interactions in polymer gels. However, the application of SS NMR spectroscopy to study gels derived from low molecular weight gelators has been limited until recently. In this review, we will focus on the importance of solid state NMR spectroscopy in understanding and elucidating the structure of supramolecular gels derived from low molecular weight gelators with selected examples.

Gelation behaviour of a bent-core dihydrazide derivative: effect of incubation temperature in chloroform and toluene

In this work, a new kind of gelator, 1,3-bis[(3,4-dioctyloxy phenyl) hydrazide]phenylene (BP8-C), containing two dihydrazide units as the rigid bent-core, has been synthesized and investigated. It was demonstrated that BP8-C is an efficient gelator which can gel various organic solvents, such as ethanol, benzene, toluene, chloroform, etc. Both an opaque gel (O-gel) and a transparent gel (T-gel), which is more stable, were obtained with BP8-C in chloroform at different incubation temperatures. Kinetic data based on fluorescence spectra revealed that the T-gels showed a larger Avrami parameter (n = 1.44 at 20 °C) than that of the O-gels (n = 1.21 for gelation at temperatures below 0 °C). While BP8-C did form the opaque gel in toluene, gelation took longer at lower incubation temperatures and even precipitated out below 0 °C. The kinetic Avrami analysis on sols of BP8-C with different concentrations shows a two-phrase mechanism, i.e. the n values are between 0.88 and 1.74 followed by 1.69 and 3.01 throughout the temperature range of 5 °C and 35 °C for 5.34 mg mL(-1) BP8-C in toluene, indicating that the fibers formed first and then bundled to produce compact networks. We propose that supersaturation governs the formation of gel in chloroform and that the diffusion process denominates gelation in toluene. XRD and FT-IR measurements confirmed that the xerogels prepared at different temperatures in different solvents exhibited a Col(h) structure and that there are three molecules in one columnar slice. Our results indicate that the gelation process, morphology of the gels and thus the final properties of the gels depend strongly on the preparation conditions such as temperature, solvent, concentration, etc.

A thermo-sensitive supramolecular hydrogel derived from an onium salt with solution–gel–crystal transition properties

A self-assembled supramolecular hydrogel, which not only exhibits thermo-sensitive characteristics but has the property of solution–gel–crystal transition, is obtained by mixing the aqueous solutions of benzoic acid and melamine.

Influence of the chirality of short peptide supramolecular hydrogels in protein crystallogenesis

For the first time the influence of the chirality of the gel fibers in protein crystallogenesis has been studied. Enantiomeric hydrogels 1 and 2 were tested with model proteins lysozyme and glucose isomerase and a formamidase extracted from B. cereus. Crystallization behaviour and crystal quality of these proteins in both hydrogels are presented and compared.

Using solution state NMR spectroscopy to probe NMR invisible gelators

Supramolecular hydrogels are formed via the self-assembly of gelator molecules upon application of a suitable trigger. The exact nature of this self-assembly process has been widely investigated as a practical understanding is vital for the informed design of these materials. Solution-state NMR spectroscopy is an excellent non-invasive tool to follow the self-assembly of supramolecular hydrogels. However, in most cases the self-assembled aggregates are silent by conventional (1)H NMR spectroscopy due to the low mobility of the constituent molecules, limiting NMR spectroscopy to following only the initial assembly step(s). Here, we present a new solution-state NMR spectroscopic method which allows the entire self-assembly process of a dipeptide gelator to be followed. This gelator forms transparent hydrogels by a multi-stage assembly process when the pH of an initially alkaline solution is lowered via the hydrolysis of glucono-δ-lactone (GdL). Changes in the charge, hydrophobicity and relative arrangement of the supramolecular aggregates can be followed throughout the assembly process by measuring the residual quadrupolar couplings (RQCs) of various molecular probes (here, (14)NH4(+) and isopropanol-d8), along with the NMR relaxation rates of (23)Na(+). The initially-formed aggregates comprise negatively charged fibrils which gradually lose their charge and become increasingly hydrophobic as the pH falls, eventually resulting in a macroscopic contraction of the hydrogel. We also demonstrate that the in situ measurement of pH by NMR spectroscopy is both convenient and accurate, representing a useful tool for the characterisation of self-assembly processes by NMR.

Studies on supramolecular gel formation using DOSY NMR

Herein, we present the results obtained from our studies on supramolecular self-assembly and molecular mobility of low-molecular-weight gelators (LMWGs) in organic solvents using pulsed field gradient (PFG) diffusion ordered spectroscopy (DOSY) NMR. A series of concentration-dependent DOSY NMR experiments were performed on selected LMWGs to determine the critical gelation concentration (CGC) as well as to understand the behaviour of the gelator molecules in the gel state. In addition, variable-temperature DOSY NMR experiments were performed to determine the gel-to-sol transition. The PFG NMR experiments performed as a function of gradient strength were further analyzed using monoexponential DOSY processing, and the results were compared with the automated Bayesian DOSY transformation to obtain 2D plots. Our results provide useful information on the stepwise self-assembly of small molecules leading to gelation. We believe that the results obtained from these experiments are applicable in determining the CGC and gel melting temperatures of supramolecular gels.

Multicomponent low molecular weight gelators

We discuss the potential and challenges of multicomponent low molecular weight gels.

Speed versus stability – structure–activity effects on the assembly of two-component gels

Modifying the peripheral peptides dramatically changes the time required for gelation under ambient conditions, whilst an enthalpy–entropy balance means that as the temperature increases, the thermal stability of the gels is very similar.

Synthesis and spectroscopic characterization of chiral biphenyl-cholesterol gels

The synthesis of 4-(3-cholesteroxycarbonylpropyloxy)biphenyl (BO4-chol), 4-(7-cholesteroxycarbonylheptyloxy)biphenyl (BO8-chol), and 4,4'-bis(7-cholesteroxycarbonyl heptyloxy)biphenyl (BBO8-chol) is reported. These gelators form 1% and 2% (w/w) stable gels in n-octanol. The gels formed from single cholesterol gelators (BO4-chol and BO8-chol) exhibit lower phase transition temperatures (Tg) (62-65, 68-69 °C) than the gel obtained from the bischolesterol gelator BBO8-chol (96-98 °C). All three gelators form chiral gels in n-octanol as observed by induced circular dichroism (ICD) spectroscopy. The effect of two cholesterol moieties versus one cholesterol unit linked to a biphenyl molecule by a flexible chain, and the effect of the chain length on the gelation ability of these three novel gelators was investigated by circular dichroism (CD), absorption, and fluorescence spectroscopies. The gels obtained from BO4-chol and BO8-chol exhibit biphasic circular dichroism spectra with opposite chirality. The ICD spectra of both BO8-chol and BBO8-chol gels show a positive ICD band followed by a negative band at room temperature. However, while BO8-chol gel ICD absorptions decrease equally as temperature increases, BBO8-chol gel shows an inversion of the Cotton effect bands between 50 and 60 °C until completely disappearing above the phase transition temperature. SEM was used for the investigation of the morphology of the xerogels. On the basis of XRD data and molecular modeling, we propose packing modes for the formation of the organogelator aggregates.

Thermoreversible gel lubricants through universal supramolecular assembly of a nonionic surfactant in a variety of base lubricating liquids

The present paper investigates a new type of thermoreversible gel lubricant obtained by supramolecular assembly of low-molecular-weight organic gelator (LMWG) in different base oils. The LMWG is a nonionic surfactant with polar headgroup and hydrophobic tail that can self-assemble through collective noncovalent intermolecular interactions (H-bonding, hydrophobic interaction) to form fibrous structures and trap base oils (mineral oils, synthetic oils, and water) in the as-formed cavities. The gel lubricants are fully thermoreversible upon heating-up and cooling down and exhibit thixotropic characteristics. This makes them semisolid lubricants, but they behave like oils. The tribological test results disclosed that the LMWG could also effectively reduce friction and wear of sliding pairs compared with base oils without gelator. It is expected that when being used in oil-lubricated components, such as gear, rolling bearing, and so on, gel lubricant may effectively avoid base oil leak and evaporation loss and so is a benefit to operation and lubrication failure for a long time.

Application of solubility parameters in 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol organogel in binary organic mixtures

The gelation behavior of 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) in binary solvents has been systematically investigated. DMDBS is soluble in DMSO and insoluble in toluene (apolar) or 1-propanol (polar). When DMSO is added to a poor solvent at a certain volume fraction, DMDBS forms an organogel in the mixed solvent. With increasing DMSO content, the minimum gelation concentration increases and the gel-to-sol transition temperature decreases in both systems. However, compared with those in toluene-DMSO mixtures, the gelation ability and thermal stability are better in 1-propanol-DMSO mixtures. Scanning electron microscopy images reveal that the gelators aggregate to form three-dimensional networks. X-ray diffraction shows that the gel has a lamellar structure, which is different from the structure of the precipitate. Fourier transform infrared results reveal H-bonding is the main driving force for self-aggregation and indicate that stronger H-bonding interactions exist between gelators in 1-propanol-DMSO mixtures in contrast with toluene-DMSO mixtures. Attempts have been taken to correlate solvent parameters to gelation behavior in binary solvents. A Teas plot exhibits distinctly different solvent zones in the studied mixed solvents. The polar parameter (δp) indicates a narrow favorable domain for gel formation in the range of 1.64-7.99 MPa(1/2) for some apolar solvent-DMSO mixtures. The hydrogen-bonding parameter (δh) predicts that gelation occurs for values of 14.00-16.50 MPa(1/2) for some polar solvent-DMSO mixtures. The result may have potential applications in predicting the gelation behavior of 1,3:2,4-di-O-benzylidene-d-sorbitol derivatives in mixed solvents.

Insights into low molecular mass organic gelators: a focus on drug delivery and tissue engineering applications

In recent years low molecular mass organic gelators (LMOGs) have gained increasing interest as an alternative biomaterial to polymer derived gels, with potential applications in drug delivery and tissue engineering. LMOGs are small organic molecules which self-assemble in water or organic solvents forming a 3D network that entraps the liquid phase resulting in gel formation. In this review, we report the classification of LMOGs into hydrogelators and gelators of organic solvents and we discuss the techniques commonly used to characterise the gels of these gelators with particular reference to specific applications of LMOGs in drug delivery and tissue engineering.

In situ synthesis-gelation at room temperature vs. heating-cooling procedure. Fine tuning of molecular gels derived from succinic acid and L-valine

HYPOTHESIS

The reaction between succinic anhydride and a diamine derived from L-valine should afford efficiently a molecular gelator. Based on this reaction, it should be feasible to prepare molecular gels at room temperature, avoiding the conventional thermal treatment required for the solubilization of the gelator, by in situ, simultaneous, synthesis and gelation. The gels prepared by in situ and conventional heating-cooling protocols could present important differences relevant for potential practical applications of these materials.

EXPERIMENTAL

The gelator was synthesized by reaction of succinic anhydride and a diamine derived from L-valine, affording two new amide bonds. The molecular gels were studied by IR, NMR, electron microscopy, X-ray diffraction and DSC.

FINDINGS

The results indicate that different polymorphic fibrillar networks are formed depending on the gel preparation method, highlighting how the properties of molecular gels can be tuned in this way. Significant differences between thermal and in situ gels were found in properties such as thermal stability, thixotropic behavior or release of an entrapped dye. In situ synthesis-gelation has also been shown to provide gels in media such as oleic acid which cannot be jellified by conventional heating-cooling procedures.

In situ formation of steroidal supramolecular gels designed for drug release

In this work, a steroidal gelator containing an imine bond was synthesized, and its gelation behavior as well as a sensitivity of its gels towards acids was investigated. It was shown that the gels were acid-responsive, and that the gelator molecules could be prepared either by a conventional synthesis or directly in situ during the gel forming process. The gels prepared by both methods were studied and it was found that they had very similar macro- and microscopic properties. Furthermore, the possibility to use the gels as carriers for aromatic drugs such as 5-chloro-8-hydroxyquinoline, pyrazinecarboxamide, and antipyrine was investigated and the prepared two-component gels were studied with regard to their potential applications in drug delivery, particularly in a pH-controlled drug release.