Silica structures templated on fibers of tetraalkylphosphonium salt gelators in organogels

Formation of supramolecular hydrogel microspheres via microfluidics

Supramolecular hydrogel microspheres are hydrogel particles formed by the self-assembly of hydrogelators in water, through non-covalent interactions. In this paper, we provide a novel strategy to prepare supramolecular hydrogel microspheres with diameters ranging from 15 to 105 microns by using microfluidics. Since the gelation temperature is ca. 64 degrees C, the aqueous solution containing the hydrogelator was initially set at 70 degrees C so the liquid mixture can be pumped into the microfluidic device. The hydrogelator solution then pinches off into uniform micron size droplets at the narrow orifice of the microfluidic device. While traveling downstream in the microchannel, the self-assembly process occurs inside the droplets and the droplets solidify into microsphere gels when the temperature drops to ca. 64 degrees C and below. Optical and scanning electron microscopy (SEM) demonstrate that compact, entangled, round, cage-like aggregates of hydrogelator were formed within the supramolecular hydrogel microsphere, in contrast to loose and less compact aggregates within bulk hydrogel. Thermal analysis (DSC) indicates that supramolecular hydrogel microspheres are more thermally stable and can immobilize more water molecules, owing to the compact entangled three-dimensional network structures. This observation is of particular importance for potential drug delivery and biomaterials applications.

Metathesis within self-assembled gels: transcribing nanostructured soft materials into a more robust form

This article reports the covalent capture of self-assembled gel-phase materials using alkene metathesis. Gels assembled from a gelator functionalized with peripheral alkene groups were reacted with Grubbs' second generation catalyst, added as a solution to the top of the gel and allowed to diffuse into the material for 24 h. Using this approach, the fibrillar self-assembled network was covalently captured, yielding a large amount of insoluble material that was robust, thermally stable, and highly swellable in solvents compatible with the gelator. Scanning electron microscopy demonstrated that the insoluble metathesized material contained nanoscale fibers, which were aligned into rigid fiber bundles on drying. When the gelator was assembled in the presence of a second non-cross-linkable gelator, self-sorting took place, giving rise to two independent gelator networks. Metathesis then generated an insoluble material in which the individual gel fibers of the cross-linkable gelator were captured, whereas the nonreactive gelator could be washed away. Intriguingly, using this approach appeared to hinder the alignment of gel fibers into rigid fiber bundles. Instead, individual, well-defined, robust gelator nanofibers were visualized in the dried materials. In addition, the material synthesized this way appeared to be even more highly porous and swellable on the addition of solvent. In summary, this article demonstrates that metathesis is an effective way to capture nanostructured gel-phase materials covalently, with the judicious choice of additives helping to control the morphology and behavior of the materials generated. This approach to nanofabrication could ultimately give rise to nanostructured polymeric materials with a wide range of applications.

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.

Confinement effects on the self-assembly of 1,3:2,4-Di-p-methylbenzylidene sorbitol based organogel

1,3:2,4-di- p-methylbenzylidene sorbitol (MDBS) is a small organic molecule that is capable of inducing self-assembly in a wide variety of organic solvents and of forming organogels. In this paper, we present a novel approach to tune the network architectures of organogels by utilizing geometric confinement while varying the gelator concentration. Self-assembly of MDBS in propylene carbonate (PC) is investigated in a series of microchannels with widths varying from 20 to 80 mum and the gelator concentration varying from 2 to 7 wt %. We demonstrate by optical microscopy and scanning electron microscopy (SEM) that a transition from fibrillar structure to sheaflike spherulite structure occurs when (a) the channel width is increased for fixed gelator concentrations and (b) gelator concentration is increased for fixed channel widths. A phase diagram is built based on these observations. Polarized microscopy and transmission electron microscopy (TEM) images are also obtained for organogel under unconfined condition to display the spherulite structures viewed under different length scales. The thermal properties of the organogel are measured by differential scanning calorimetry (DSC) to verify the structural difference obtained under confined and unconfined conditions and the structure stability. Our results provide a novel strategy to control the topological structure of self-assembled systems and to modify their thermal properties via geometric confinement.

Use of fluorescence spectroscopy to study polymeric materials with porous structure based on imprinting by self-assembled fibrillar networks

Different polymeric materials have been prepared from the organogels formed by a polymerizable methacrylic mixture (methyl methacrylate/ethylene glycol dimethacrylate, 1:1, w/w) and the macrocyclic pseudopeptide 1. The use of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide as a very efficient radical initiator allows polymeric materials in which the structure of the fibrils formed by self-assembly of the organogelator 1 is truly preserved to be obtained. Removal of the pseudopeptidic molecule provides materials with a porous structure reflecting that of the original self-assembled fibrils. The use of fluorescent probes such as rhodamine B and pyrene greatly facilitate the study of the porous structures formed and, accordingly, that of the morphology of the original fibrils. Those studies reveal the presence of a permanent porosity and the organization of the substructures as a porous network. This confirms the existence of a nucleation and growth mechanism for the generation of the fibrils, giving rise to the formation of spherulitic structures. Those spherulites are additionally linked by connections of variable size. A series of diffusion experiments allowed establishment of a direct dependence of the inner porosity of the materials on the amount of self-organizing template used for their preparation.

Fabrication of size-tunable TiO2 tubes using rod-shaped calcite templates

Titania tubes with tunable wall thickness were produced by the sol-gel reaction of titanium(IV) n-butoxide in the presence of rod-shaped calcite particles that act as templates. A shell of amorphous titania was deposited around the calcite particles by sol-gel synthesis. The titania was crystallized to the anatase or rutile phase by sintering at different temperatures, and then acid etching was used to remove the calcite core, leaving hollow titania tubes. The influences of several parameters on the final particle formation were investigated, including calcite templates, surfactant, the method of adding reagents, and catalyst. The average width of the prepared titania tubes ranges from nearly 100 nm to 1 microm, with wall thickness ranging from approximately 70 to 300 nm. A possible growth mechanism of the titania tubes is presented. The ability to control titania tube size and crystal structure is important for photocatalysis, photovoltaics, and other applications. The fabrication approach presented is applicable to form tubes of other oxide materials by sol-gel synthesis.

Rodlike silica and titania objects templated on extremely dilute aqueous dispersions of self-assembled sodium lithocholate nanotubes

Simple methods for preparation of mesoporous rodlike silica and titania nanoobjects, some with exceedingly high aspect ratios, are described. They involve hydrolytic sol-gel processes using nanotubes from aqueous assemblies of very dilute (0.1 wt%) sodium lithocholate as templates. Shearing of the lithocholate nanotubes results in aligned, templated silica rods. The relative rates of lithocholate self-assembly and of polymerization of the titania precursors, especially, appear to be important factors in templating efficiency.