Tom P. Caremans

Investigation of the mechanism of colloidal silicalite-1 crystallization by using DLS, SAXS, and 29Si NMR spectroscopy.

Colloidal silicalite-1 zeolite was crystallized from a concentrated clear sol prepared from tetraethylorthosilicate (TEOS) and aqueous tetrapropylammonium hydroxide (TPAOH) solution at 95 degrees C. The silicate speciation was monitored by using dynamic light scattering (DLS), synchrotron small-angle X-ray scattering (SAXS), and quantitative liquid-state (29)Si NMR spectroscopy. The silicon atoms were present in dissolved oligomers, two discrete nanoparticle populations approximately 2 and 6 nm in size, and crystals. On the basis of new insight into the evolution of the different nanoparticle populations and of the silicate connectivity in the nanoparticles, a refined crystallization mechanism was derived. Upon combining the reagents, different types of nanoparticles (ca. 2 nm) are formed. A fraction of these nanoparticles with the least condensed silicate structure does not participate in the crystallization process. After completion of the crystallization, they represent the residual silicon atoms. Nanoparticles with a more condensed silicate network grow until approximately 6 nm and evolve into building blocks for nucleation and growth of the silicalite-1 crystals. The silicate network connectivity of nanoparticles suitable for nucleation and growth increasingly resembles that of the final zeolite. This new insight into the two classes of nanoparticles will be useful to tune the syntheses of silicalite-1 for maximum yield.

Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O molar ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature.

Enantioselective Adsorption in Achiral Zeolites

Minority excluded: If mixtures of chiral compounds are adsorbed in Al-substituted MFI zeolite containing nonframework cations, a process occurs that ultimately might be used for enantioseparation. If one enantiomer is present in excess it will organize the nearby cations, which in turn influence all other cations through long-range interaction, such that all adsorption sites become effectively chiral. This suppresses the minority enantiomer.

Growth of Itraconazole Nanofibers in Supersaturated Simulated Intestinal Fluid

Many drug compounds have limited solubility in water. To enhance the oral bioavailability of such compounds, pharmaceutical formulations target the creation of a supersaturated solution. Release of the compound from ordered mesoporous silica carrier is such a means for reaching supersaturation. Little is known about the evolution of supersaturated intestinal media. The present study reveals the phase transitions of the poorly water-soluble drug itraconazole in simulated intestinal fluid under conditions corresponding to supersaturation. Electron spin resonance of n-doxylstearic acid spin probes evidenced that during supersaturation itraconazole is solubilized inside the hydrophobic core of mixed micelles composed of lecithin and bile salt. Cryogenic transmission electron microscopy revealed that the supersaturated state of itraconazole provokes the formation of nanofibers with a uniform diameter of 12 nm. The nanofiber length determined via dynamic light scattering increases from 220 to 1480 nm after 30 and 90 min, respectively. Nanofibers drastically reduced transepithelial transport of itraconazole across a Caco-2 cell monolayer mimicking the gastrointestinal absorption. Based on our study, we suggest the existence of an optimum intraluminal itraconazole supersaturation at which itraconazole absorption is enhanced but formation of itraconazole nanofibers prevented.

Kinetics of intermediate-mediated self-assembly in nanosized materials: A generic model

We propose in this paper a generic model of a nonstandard aggregation mechanism for self-assembly processes of a class of materials involving the mediation of intermediates consisting of a polydisperse population of nanosized particles. The model accounts for a long induction period in the process. The proposed mechanism also gives insight on future experiments aiming at a more comprehensive picture of the role of self-organization in self-assembly processes.