Development of New D,L-Methionine-based Gelators.

D,L-Methionine was chosen as a starting material for the preparation of a new gelator N-10-undecenoyl-D,L-methionylaminooctadecane (DL-Met-R18). Three oligo (dimethylsiloxane)-containing gelators, DL-Met-R18/Si3, DL-Met-R18/Si7-8, and DL-Met-R18/Si14-15, were also prepared from DL-Met-R18 by hydrosilylation reactions. Their gelation abilities were evaluated on the basis of the minimum gel concentration using nine solvents. Compound DL-Met-R18 was able to gelate liquid paraffin and silicone oil, but it crystallized in most solvents. However, DL-Met-R18/Si7-8 resulted to be the best gelator, gelling eight solvents at low concentrations. The results of gelation tests demonstrated that the ability to form stable gels decreases in the following order: DL-Met-R18/Si7-8 ≈ DL-Met-R18/Si14-15 > DL-Met-R18/Si3 >> DL-Met-R18. The aspects and thermal stabilities of the gels were investigated using three-component mixtures of solvents composed of hexadecyl 2-ethylhexanoate, liquid paraffin, and decamethylcyclopentasiloxane (66 combinations). DL-Met-R18/Si3, DL-Met-R18/Si7-8, and DL-Met-R18/Si14-15 could form gels with all these mixed solvent combinations; particularly, DL-Met-R18/Si7-8 gave rise to transparent or translucent gels. FT-IR spectra suggested that the formation of hydrogen bonds between the NH and C=O groups of the amides is one of driving forces involved in the gelation process. Aggregates comprising three-dimensional networks were studied by transmission electron microscopy. Moreover, the viscoelastic behavior of the gels was investigated by rheology measurements.

Metal Oxide/TiO2 Hybrid Nanotubes Fabricated through the Organogel Route.

Titanium dioxide (TiO2) nanotube and its hybrid nanotubes (with various metal oxides such as Ta2O5, Nb2O5, ZrO2, and SiO2) were fabricated by the sol-gel polymerization in the ethanol gels formed by simple l-lysine-based organogelator. The self-assembled nanofibers (gel fibers) formed by the gelator functioned as a template. The different calcination temperatures gave TiO2 nanotubes with various crystalline structures; e.g., anatase TiO2 nanotube was obtained by calcination at 600 °C, and rutile TiO2 nanotube was fabricated at a calcination temperature of 750 °C. In the metal oxide/TiO2 hybrid nanotubes, the metal oxide species were uniformly dispersed in the TiO2 nanotube, and the percent content of metal oxide species was found to correspond closely to the feed ratio of the raw materials. This result indicated that the composition ratio of hybrid nanotubes was controllable by the feed ratio of the raw materials. It was found that the metal oxide species inhibited the crystalline phase transition of TiO2 from anatase to rutile. Furthermore, the success of the hybridization of other metal oxides (except for TiO2) indicated the usefulness of the organogel route as one of the fabrication methods of metal oxide nanotubes.

Synthesis of Fluorescent Gelators and Direct Observation of Gelation with a Fluorescence Microscope.

Fluorescein-, benzothiazole-, quinoline-, stilbene-, and carbazole-containing fluorescent gelators have been synthesized by connecting gelation-driving segments, including l-isoleucine, l-valine, l-phenylalanine, l-leucine residue, cyclo(l-asparaginyl-l-phenylalanyl), and trans-(1R,2R)-diaminocyclohexane. The emission behaviors of the gelators were investigated, and their gelation abilities studied against 15 solvents. The minimum gel concentration, variable-temperature spectroscopy, transmission electron microscopy, scanning electron microscopy, fluorescence microscopy (FM), and confocal laser scanning microscopy (CLSM) were used to characterize gelation. The intermolecular hydrogen bonding between the N-H and C=O of amide, van der Waals interactions and π-π stacking play important roles in gelation. The colors of emission are related to the fluorescence structures of gelators. Fibrous aggregates characterized by the color of their emission were observed by FM. 3D images are produced by the superposition of images captured by CLSM every 0.1 µm to a settled depth. The 3D images show that the large micrometer-sized aggregates spread out three dimensionally. FM observations of mixed gelators are studied. In the case of gelation, two structurally related gelators with the same gelation-driving segment lead to the gelators build up of the same aggregates through similar hydrogen-bonding patterns. When two gelators with structurally different gelation-driving segments induce gelation, the gelators build up each aggregate through individual hydrogen-bonding patterns. A fluorescent reagent that was incorporated into the aggregates of gels through van der Waals interactions was developed. The addition of this fluorescent reagent enables the successful observation of nonfluorescent gelators' aggregates by FM.

Thixotropic Supramolecular Gel Based on l-Lysine Derivatives.

The dimer l-lysine derivatives, in which two Nα,Nε-diacyl-l-lysines were crosslinked by calcium ion, were synthesized through a simply synthetic procedure and their gelation properties were examined. These compounds functioned as an organogelator; especially, the gelators possessing both a linear and a branched alkyl chains had the better organogelation ability and formed the thermally stable and rigid organogel. In addition, some organogels had a thixotropic property, which were responsive to a mechanical stimulus and reversibly underwent the gel⁻sol transition at room temperature. The thixotropic behavior was confirmed by visual contact and rheological experiments. Furthermore, it was assumed the mechanism of the thixotropic behavior.

Thixotropic hydrogelators based on a cyclo(dipeptide) derivative.

Thixotropic hydrogelators have great potential in biomedical and biotechnological applications. In this study, we report new hydrogelators and their behavior during gel-sol-gel transitions. In particular, cyclo(L-O-hydroxyhexylaspartyl-L-phenylalanyl), which was synthesized with 1,6-hexanediol, formed a thermally/isothermally reversible physical gel with several solvents, including pure water, saline, alcohols, as well as 1.0 M aqueous NaCl, KCl, CaCl2, and MgCl2 solutions. TEM observations showed self-assembled fibers with diameters of 10-100 nm. FT-IR results revealed that the gels were mainly formed by hydrogen bonding and van der Waals forces; thixotropic behavior resulted from the disruption of the van der Waals forces between the alkylene chains under shearing. These results were repeatedly and reproducibly observed at room temperature, even when measurements were repeated many times.

Polymer organogelators that make supramolecular organogels through physical cross-linking and self-assembly.

This tutorial review highlights recent and current advances in polymer organogelators, which are rare compared with low molecular weight gelators. In this review, we classify polymer organogelators in three categories: the formation of supramolecular crosslinking points by conformational changes, the addition of crosslinking agents and the self-assembly of gelation-causing segments. Highly stereoregular polymers form a physical gel in organic solvents, involving conformational changes such as helix formation. The addition of cross-linking agents into polymer solutions provides stimuli-sensitive organogels. Furthermore, polymer organogelators, which consist of versatile polymers, such as poly(ethylene glycol)s, polycarbonates, polyesters, polycaprolactones, polyolefins and low molecular weight gelators, function as good organogelators that can form organogels in many organic solvents at low concentration. The organogelation properties of polymer organogelators are significantly affected by the chemical structures of the introduced low molecular weight gelators and polymer backbones, the molecular weight of the polymer backbones and the linking mode between the low molecular weight gelator segment and the polymer.

Low-molecular-weight gelators based on N(alpha)-acetyl-N(epsilon)-dodecyl-L-lysine and their amphiphilic gelation properties.

A simple amphiphilic low-molecular-weight gelator based on L-lysine, N(alpha)-acetyl-N(epsilon)-lauroyl-L-lysine (1), its alkali metal salts [Na (2) and K (3)], and two-component gelators [1 and 2 and 1 and 3] were synthesized. Compound 1 had a good hydrogelation ability that formed a pure water gel at 2 g L(-1) (0.2 wt.%) and a saline gel at 4 g L(-1) (0.4 wt.%). Two-component compounds were able to form hydrogels in aqueous solutions containing alkali metal and alkali earth metal ions in addition to pure water and saline. Although 1 formed organogels in a few organic solvents, two-component compounds also functioned as a good organogelator. The FT-IR study indicated that the driving forces for the formation of supramolecular gels were hydrogen-bonding and hydrophobic interactions. Furthermore, the thermal properties of the hydrogels are discussed.

L-lysine-based low-molecular-weight gelators.

Low-molecular-weight gelators form supramolecular gels in organic fluids, aqueous solutions and both organic and aqueous solutions through supramolecular interactions such as hydrogen-bonding, van der Waals, hydrophobic, pi-stacking, coordination, donor-acceptor and charge-transfer interactions. Molecules having chirality, especially, L-amino acids, are often used as a platform of low-molecular-weight gelators. This tutorial review highlights recent and current advances in low-molecular-weight gelators based on L-lysine. L-lysine based gelators are prepared through easy synthetic procedures, and some classes of gelators are synthesized by the introduction of various functional groups. In this review, the synthesis of organogelators, hydrogelators and amphiphilic gelators and their gelation properties are discussed.

Helical transfer through nonlocal interactions.

A pair of chiral bola-type enantiomers, ll-12PyBr and dd-12PyBr, was synthesized. They can cause physical gels in pure water. Sol-gel transcriptions were carried out to control mesoporous 1,4-phenylene-silica nanostructures and helicity using the organic self-assemblies of these amphiphiles as templates. Under acidic conditions, left-handed helical 1,4-phenylene-silica bundles were prepared using the self-assemblies of ll-12PyBr as templates. Additionally, right-handed helical silica and 1,4-phenylene-silica bundles were prepared using the self-assemblies of dd-12PyBr as templates. Under basic conditions, 1,4-phenylene-silica bundles were also obtained. However, it is hard to determine the handedness. For all of the 1,4-phenylene-silica bundles, it is interesting to find that the aromatic rings are packing in helix within the wall of the pore channels. Powder X-ray diffraction patterns indicated that the aromatic rings of 1,4-phenylene-silica bundles prepared under basic conditions showed a greater degree of order than those of 1,4-phenylene-silica bundles prepared under acidic conditions. Moreover, helical silica, 1,3-phenylene-silica, ethene-silica, and ethane-silica bundles were also prepared using the self-assemblies ll-12PyBr and dd-12PyBr as templates.

Two-component organogelators based on two L-amino acids: effect of combination of L-lysine with various L-amino acids on organogelation behavior.

A two-component organogelation system, consisting of Nepsilon-dodecyl-L-lysine esters (amine components) and N-dodecyl-L-amino acids (valine, phenylalanine, alanine, glycine, L-lysine) as acid components, was proposed, and its organogelation properties were investigated. The mixture of amine and acid components produced an organic salt compound through acid-base interactions, and it created a three-dimensional network by entangling self-assembled nanofibers through hydrogen bonding and van der Waals interactions, thus leading to the formation of organogels. The combination of Lys/Phe yielded rigid dodecane gel, and the gelators of Lys/ Lys yielded a thermally stable dodecane gel. The organogelation properties significantly depended on the combination of amine and acid components, and they could be controlled by the selection of suitable components.

From branched self-assemblies to branched mesoporous silica nanoribbons.

Branched left-handed twisted mesoporous silica nanoribbons were prepared via a template method.

Hybrid silica nanotubes with chiral walls.

Hybrid silica nanotubes with chiral aromatic rings in the walls were prepared using self-assemblies of a low molecular weight amphiphile as a template; transmission electron microscopy images of the nanotubes at different formation steps showed that the co-structure-directing agent plays an important role in this chiral transfer.

The formation of helical mesoporous silica nanotubes.

Three chiral cationic gelators were synthesized. They can form translucent hydrogels in pure water. These hydrogels become highly viscous liquids under strong stirring. Mesoporous silica nanotubes with coiled pore channels in the walls were prepared using the self-assemblies of these gelators as templates. The mechanism of the formation of this hierarchical nanostructure was studied using transmission electron microscopy at different reaction times. The results indicated that there are some interactions between the silica source and the gelator. The morphologies of the self-assemblies of gelators changed gradually during the sol-gel transcription process. It seems that the silica source directed the organic self-assemblies into helical nanostructures.

Segment sizes of supramolecular polymers of N,N',N"-tris(3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide in n-decane.

The average segment sizes of rather flexible supramolecular polymers formed by a racemic mixture of N,N',N"-tris(3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide (DO3B) in n-decane (C10) (DO3B/C10) and a mixture of DO3B and homochiral (S)-N,N',N"-tris(3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide ((S)DO3B) in C10 (DO3B:(S)DO3B/C10) were determined using mechanical relaxation techniques, including linear and nonlinear viscoelastic experiments in addition to dielectric relaxation measurements. To evaluate the average sizes of segments for the supramolecular polymers formed in these systems, the conventional rubber elasticity theory was employed for the linear viscoelastic behavior and a non-Gaussian-type three-chain model assuming chains constructed with freely joined chain segments with finite sizes for nonlinear viscoelastic responses. Concentration-independent similar segment sizes for the supramolecular polymer formed in DO3B/C10 were evaluated using both the mechanical techniques. However, the segment size of the formed supramolecular polymer increased upon increasing the component of (S)DO3B to the total amount of DO3B and (S)DO3B in DO3B:(S)DO3B/Ci0o. The formed supramolecular polymer in the racemic DO3B/C10 system possessed threefold intermolecular hydrogen bonding aligned along its stiff columnar structure, with equimolar right- and left-handed helicities connected by defective portions, which were DO3B molecules containing defects in hydrogen bond formation. In the case of DO3B:(S)DO3B/C10, the addition of the chiral (S)DO3B component induced the majority rule effect for the helicity of the system to increase the major part of helicity, which led to the reduction of the number of defects and an increase in segment seizes. Because large macrodipoles are naturally generated by helical three-fold hydrogen bonding of amide groups in a head-to-tail arrangement, as in type-A polymers possessing parallel electric dipoles along their backbones, profound dielectric behavior provided a concentration-independent segment size for a supramolecular polymer formed in both the DO3B/ C10 and DO3B:(S)DO3B/C10 systems, the value of which was about one-third of that determined mechanically. A free-rotation chain model that is slightly more realistic than the freely jointed chain model was employed to consider the discrepancy in the evaluated segment sizes. Then, helical columns consisting of nine DO3B molecules and supramolecular polymer chain portions formed by a three-column connection behaved as equivalent freely jointed (Kuhn) segments.

Organogelation by polymer organogelators with a L-lysine derivative: formation of a three-dimensional network consisting of supramolecular and conventional polymers.

Polymer compounds consisting of a L-lysine derivative and conventional polymers, such as poly(ethylene glycol), polycarbonate, polyesters, and poly(alkylene), have been synthesized and their organogelation properties examined in various solvents. These polymer compounds function as good organogelators that form organogels in many organic solvents and oils. The organogelation ability is almost independent of the polymer backbone. Observation by field-emission scanning electron microscopy (FE-SEM) demonstrates that the polymer organogelators form a supramolecular polymer with a diameter of several tens of nanometers and create a three-dimensional network in organogels. FT-IR spectroscopic analysis shows that the supramolecular polymer is mainly formed by the self-assembly of L-lysine segments through hydrogen-bonding and van der Waals interactions. Furthermore, the organogels formed by the polymer organogelators have a lower gel-sol temperature and higher gel strength than those of a low-molecular-weight model organogelator.

Control of mesoporous silica nanostructures and pore-architectures using a thickener and a gelator.

A chiral cationic thickener l-ValPyBr, which was able to enhance the viscosity of water and form loosely physical gel in mixtures of water and alcohols, was synthesized. Sol-gel polymerization of TEOS was carried out in mixtures of water and alcohols under basic conditions using the self-assemblies of l-ValPyBr as template. The left-handed twisted mesoporous silica nanoribbons, which were constructed by nanotubes in monolayer, were obtained, and they tended to self-assemble into bundle structure. Stirring under the preparation process played an important role in the formation of this bundle structure. The obtained silica nanoribbons were uniform in width, thickness, and helical pitch without combining amorphous particles. The helical pitch and pore size of the mesoporous silica nanoribbons sensitively depended on the volume ratio of alcohols to water in the reaction mixtures. With increasing volume ratio of alcohols to water in the reaction mixture, the morphologies of the obtained silica changed from left-handed twisted ribbon to coiled ribbon, then to tubular structure. A compound l-ValPyPF6, structurally related to thickener l-ValPyBr, was able to form physical gel in ethanol, THF, acetonitrile, and the mixtures of ethanol and water. Left-handed multiple helical mesoporous silica nanofibers were prepared by using the self-assemblies of l-ValPyPF6 as template in mixtures of water and alcohols under basic conditions. By controlling both the volume ratio of ethanol to water and the weight ratio of l-ValPyPF6 to TEOS, two- or three-dimensional pore-architecture constructed by porous chiral nanotubes was obtained.

Construction of superhydrophobic surfaces by fibrous aggregation of perfluoroalkyl chain-containing organogelators.

Superhydrophobic surfaces, characterized by water contact angles greater than 150 degrees, can be produced by means of intermediate organogels, which were formed by perfluoroalkyl chain-containing organogelators with volatile organic solvents.

Fabrication of TiO2 using L-lysine-based organogelators as organic templates: control of the nanostructures.

Using uncharged or negatively charged L-lysine-based organogelators as templates, the nanostructures of TiO2 are controllable.

Specialist gelator for ionic liquids.

Cyclo(l-beta-3,7-dimethyloctylasparaginyl-L-phenylalanyl) (1) and cyclo(L-beta-2-ethylhexylasparaginyl-L-phenylalanyl) (2), prepared from L-asparaginyl-L-phenylalanine methyl ester, have been found to be specialist gelators for ionic liquids. They can gel a wide variety of ionic liquids, including imizazolium, pyridinium, pyrazolidinium, piperidinium, morpholinium, and ammonium salts. The mean minimum gel concentrations (MGCs) necessary to make gels at 25 degrees C were determined for ionic liquids. The gel strength increased at a rate nearly proportional to the concentration of added gelator. The strength of the transparent gel of 1-butylpyridinium tetrafluoroborate ([C(4)py]BF(4)), prepared at a concentration of 60 g L(-1) (gelator 1/[C(4)py]BF(4)), was ca. 1500 g cm(-2). FT-IR spectroscopy indicated that a driving force for gelation was intermolecular hydrogen bonding between amides and that the phase transition from gel to liquid upon heating was brought about by the collapse of hydrogen bonding. The gels formed from ionic liquids were very thermally stable; no melting occurs up to 140 degrees C when the gels were prepared at a concentration of 70 g L(-1) (gelator/ionic liquid). The ionic conductivities of the gels were nearly the same as those of pure ionic liquids. The gelator had electrochemical stability and a wide electrochemical window. When the gels were prepared from ionic liquids containing propylene carbonate, the ionic conductivities of the resulting gels increased to levels rather higher than those of pure ionic liquids. The gelators also gelled ionic liquids containing supporting electrolytes.

Preparation of helical nanostructures using chiral cationic surfactants.

Left-handed helical short nanotubes and mesoporous nanofibers were prepared by sol-gel transcription using a new chiral cationic surfactant.