Sandra L. Burkett

Synthesis of hybrid inorganic–organic mesoporous silica by co-condensation of siloxane and organosiloxane precursors

Organically functionalized mesoporous silicas are prepared at room temperature by the co-condensation of tetraethoxysilane and organosiloxanes in the presence of surfactant templates; the resulting materials are the first examples of covalently linked, ordered, hybrid inorganic–organic networks.

Studies on mesoporous materials II. Synthesis mechanism of MCM-41

The solids obtained during the synthesis of MCM-41 are investigated by X-ray powder diffraction, thermogravimetric analysis and 29Si NMR spectroscopy. These data, in combination with those obtained from in situ14N NMR spectroscopy, are used to elucidate the synthesis mechanism of MCM-41. The results shown here reveal that the liquid crystalline phase, H1, is not present in the synthesis medium during the formation of MCM-41 and therefore cannot be the structure-directing agent for MCM-41. Rather, the data suggest that randomly ordered rod-like organic micelles interact with silicate species to yield approximately two or three monolayers of silica encapsulation around the external surfaces of the micelles. Subsequently, these composite species spontaneously assemble into the long-range ordered structure (hexagonal packing) characteristic of MCM-41. With further heating, the silicate species in the interstitial spaces of the ordered organic—inorganic composite phase continue to condense. Complete condensation of the silicate species is not possible because SiO− species are necessary for charge compensation of the occluded alkylammonium ions.

Hybrid lamellar nanocomposites based on organically functionalized magnesium phyllosilicate clays with interlayer reactivity

A series of layered magnesium phyllo(organo)silicate nanocomposites that contain covalently linked organic functionalities have been prepared by a one-step, direct synthesis procedure. Organoclays with allyl, epoxy, imidazole or ethylenediamino functionalities were synthesized and characterized by XRD, TEM, FTIR and solid state 29Si and 13C NMR spectroscopy. The epoxy-functionalized nanocomposite undergoes in situ cross-linking reactions with m-phenylenediamine to produce a polymer-inorganic lamellar hybrid, as well as ring-opening reactions with methyl thioglycolate to give a covalently linked hydroxysulfide derivative. The ethylenediamino-functionalized magnesium phyllo(organo)silicate showed good binding capacity for aqueous CoII.

Structure-directing effects in the crown ether-mediated syntheses of FAU and EMT zeolites

The structure-directing effects of 15-crown-5 and 18-crown-6 in the syntheses of cubic faujasite (FAU) and hexagonal faujasite (EMT), respectively, have been investigated. The conformation and distribution of sodium/crown ether complexes during synthesis and in the product materials have been characterized by Raman spectroscopy and by solid-state 1H-13C CP NMR. The balance between the structure-directing influences of alkali metal ions and crown ethers on product formation has been investigated. The structural relationships between the two polymorphs of faujasite and the specificity of the crown ether structure-directing agents to these products are discussed in relation to a proposed mechanism of synthesis.

Asymmetric α-helicity loss within a peptide adsorbed onto charged colloidal substrates

Abstract A combination of circular dichroism and solution 1H NMR spectroscopy provides a localized description of the distribution of α -helical structure within the capped peptide DDDDAAAAARRRR (4DAR5) in aqueous solution and adsorbed onto anionic and cationic colloidal substrates. The adsorption-induced conformational changes are different from those observed upon heating 4DAR5 in solution, in which case the alanine segment remains largely α -helical and the transition to a coil structure propagates from the termini. Adsorption is driven by electrostatic complementarity, which places the charged peptide segment adjacent to the substrate of opposite charge. A similar pattern of α -helicity loss is observed whether the peptide is adsorbed onto anionic or cationic colloidal silica, despite inverse orientations; significant α -helicity loss occurs within the central alanine segment and the terminal arginine segment, whereas α -helicity is retained in the aspartate segment. This pattern of adsorption-induced conformational change illustrates the complex and subtle balance among the intramolecular and intermolecular factors that influence the conformations of adsorbed peptides and proteins.

Synthesis of (alumino)silicate materials using organic molecules and self-assembled organic aggregates as structure-directing agents

Microporous zeolites can be synthesized using organic structure-directing molecules whose function is to organize inorganic species into particular topologies that then spontaneously self-assemble into the final crystalline materials. Extension of the zeolite assembly process to the use of organic molecular aggregates as structure-directing agents yields ordered mesoporous materials like MCM–41 and MCM–48. A unifying picture of the assembly processes ocurring in the syntheses of micro- and mesoporous materials is presented.

Synthesis and characterization of orderedorgano–silica–surfactant mesophases with functionalizedMCM-41-type architecture

Ordered organo–silica–surfactant mesophases containing covalently linked functionalized moieties are prepared with MCM-41-type architectures by template-directed co-condensation of tetraethoxysilane and organotrialkoxysilanes (RO) 3 Si–R′ [R = Me, Et; R′ = (CH 2 ) 3 SH, (CH 2 ) 3 NH 2 , (CH 2 ) 3 OCH 2 CH(O)CH 2 , (CH 2 ) 3 NCHNCH 2 CH 2 or CH 2 CHCH 2 ].