Marcos I. Oliva

Non-hydrothermal Synthesis of Cylindrical Mesoporous Materials: Influence of the Surfactant/Silica Molar Ratio

Ordered mesoporous materials of MCM-41 and SBA-15 type were synthesized under non-hydrothermal conditions using different molar ratios of surfactant/silica, ranging from 0.07 to 0.27 for MCM-41 and from 0.009 to 0.021 for SBA-15. Nitrogen adsorption/desorption isotherms at 77 K were used to evaluate the specific surface areas, with values up to 1450 m2/g and 1100 m2/g being obtained for MCM-41 and SBA-15, respectively. Scanning electron microscopy was carried out to study the morphology of the materials. The sizes of the primary mesopores and the pore thicknesses were assessed by a geometrical method using X-ray diffraction and nitrogen adsorption data. The results for the pore sizes were compared with those obtained by a recently reported method (VBS-method) and the Non-Local Density Functional Theory (NLDFT) model, both proposed to evaluate the pore-size distribution of these materials. It was found that, under the synthesis conditions, the surfactant/silica molar ratio had an important effect on the final characteristics of these materials.

Model of interactions in nanometric particles of barium hexaferrite

Abstract Magnetic interactions in nanostructured Ba hexaferrite systems with partially coupled particles are studied. In this kind of systems, there are regions with lower effective magnetic anisotropy K eff where nucleation of magnetization reversal occurs. The Preisach distribution functions were measured for samples with different irreversible susceptibility behavior. Several peaks in the Preisach distributions were identified and were associated with different particle clusters. The coercive fields of these clusters were associated with K eff . A model was developed linking K eff with the exchange-coupled volume ( β ) between two neighboring particles and the number of particles in clusters. The behavior that followed the best physical interpretation led to conclude that magnetization reversal starts in the exchange-coupled volume β between two neighboring particles and then propagates to the rest of the particle.

Magnetic interactions and Preisach distributions of nanostructured barium hexaferrite

Abstract Barium hexaferrite (phase M) samples with different nanostructures were studied. Sample M1 is composed of nanocrystals of BaFe12O19 produced after milling the elemental oxides (Fe2O3 and BaCO3) and heating in air atmosphere. Two more samples were produced by milling the same oxides and a 20% excess of α-Fe. The resulting powder (composed of phase M and a ∼20% hematite) was heat-treated in different conditions, resulting in samples MF1 (with a partially sintered structure) and MF8 (with almost completely sintered structure). In order to have an insight into the interactions in each sample, Preisach distributions were obtained using first-order reversion curves (FORCs) measurements. The Preisach distribution corresponding to M1 is a Gaussian-shaped one, with a maximum around 4.1 kOe . The distribution of MF1 has a narrow and high peak at 5.3 kOe , a number of overlapping small peaks down to 2.6 kOe and a distinct and low-intensity peak at 2 kOe . MF8 has a Preisach distribution with a succession of equally spaced distinct peaks from 4.2 to 1.5 kOe . The found Preisach distributions suggest that the interactions occur among nanocrystals inside conglomerates with different number of particles.

Drug release profiles of modified MCM-41 with superparamagnetic behavior correlated with the employed synthesis method

Mesoporous materials with superparamagnetic properties were successfully synthesized by two different methods: direct incorporation (DI) and wet impregnation (WI). The synthetized solids were evaluated as host of drugs for delivery systems and their physicochemical properties were characterized by XRD, ICP, N2 adsorption-desorption, spectroscopies of UV-Vis DR, FT-IR and their magnetic properties were measured. Indomethacin (IND) was incorporated into the materials and the kinetic of the release profiles was studied by applying the Pepas and Sahlin model. In this sense, materials modified by DI, particularly that with hydrothermal treatment, showed the higher adsorption capacity and slower release rate. This behavior could be associated to the synthesis method used that allowed a high percentage of silanol groups available in the solids surface, which can interact with the IND molecule. This feature coupled with the superparamagnetic behavior; make these materials very interesting for drug delivery systems.

Advances in the study of nano-structured Co/MCM-41 materials: surface and magnetic characterization

Co-modified mesoporous supports with MCM-41 structure and several metal loadings were successfully synthesized by a fast wet impregnation method. The nature and location of different Co species formed on the solids were inferred by TEM/SEM, XPS, XANES/EXAFS, adsorption of pyridine coupled to FT-IR spectroscopy and temperature dependence of magnetization. The presence of Co oxide species (clusters and Co3O4 nanoparticles) inside the channels of all the samples could be evidenced by TEM, XPS and XANES/EXAFS. In this sense, the surface Co/Si atomic ratios obtained by XPS were notably lower than the corresponding bulk Co/Si ratios obtained by ICP, indicating that the Co atoms are mostly incorporated inside the mesopores channels of the silica matrix. Nevertheless, by TEM, Co3O4 nanoparticles of small size segregated on the external surface of the silicate were also observed for the higher metal loadings. The temperature dependence of the magnetization performed for the Co/M(2.5) sample allowed to assign its superparamagnetic behavior to the presence of clusters and Co3O4 nanoparticles of very small size that grow inside the MCM-41 mesopores. Therefore, the analyses presented in this work indicate that a Co theoretical loading of 2.5 wt% leads to the formation of Co oxide nanospecies in the MCM-41 support with a particular superparamagnetic behavior. This sample with improved structural and magnetic properties result an attractive porous solid for drug hosting, to be applied in the field of the controlled release of medicaments.