Molecular dynamics simulation of the early stages of the synthesis of periodic mesoporous silica

We present results of detailed atomistic modeling of the early stages of the synthesis of periodic mesoporous silica using molecular dynamics. Our simulations lead to the proposal of a mechanism that validates several previous experimental and modeling studies and answers many controversial issues regarding the synthesis of mesoporous silicas. In particular, we show that anionic silicates interact very strongly with cationic surfactants and, significantly adsorb on the surface of micelles, displacing a fraction of previously bound bromide counterions. This induces an increase in micelle size and also enhances silica condensation at the micelle surface. The presence of larger silica aggregates in solution further promotes the growth of micelles and, by binding to surfactant molecules in different micelles, their aggregation. This work demonstrates the crucial role played by silica in influencing, by way of a cooperative templating mechanism, the structure of the eventual liquid-crystal phase, which in turn determines the structure of the porous material

Micellization of CTAB in the presence of silicate anions and the exchange between bromide and silicate at the micelle surface: A step to understand the formation of mesoporous molecular sieves at extremely low surfactant and silicate concentrations

The effect of silicate anions, from dilute aqueous tetramethylammonium silicate (TMASi) solutions (0-3.0 mmol L-1 in silicon), on the formation of hexadecyltrimethylammonium bromide (CTAB) micelles was investigated by means of a series of simple conductivity experiments. These two compounds are used in the preparation of mesoporous silicate molecular sieves. An increase in the monovalent silicate anion concentration decreases the critical micelle concentration (cmc) of CTAB, as might be expected from the decreased repulsive forces between the polar heads of the surfactant molecules. However, the decrease in cmc values is less pronounced than that observed in the presence of bromide ions, suggesting that Br- binds more strongly than Si(OH)(3)O- at the micelle surface. Through the ion-exchange formalism, a selectivity coefficient for Si(OH)(3)O-/Br- exchange of 0.30 +/- 0.03 was estimated from the conductivity data. This value compares well with that of 0.4 +/- 0.1 also determined in this work by the pyrene fluorescence quenching method. The experimental results were used to rationalize the formation of a surfactant supramolecular-templated mesoporous molecular sieve at extremely low surfactant (0.63 mmol L-1) and silicate (4.00 mmol L-1) concentrations. (c) 2006 Elsevier Inc. All rights reserved.299287488

Physical chemistry of nanostructured molecular sieves by the study of phase diagrams: the case of the cetyltrimethylammonium bromide-tetramethylammonium silicate-water system

A phase diagram for the system cetyltrimethylammonium bromide (CTAB)/tetramethylammonium silicate (TMASi)/water has been constructed in order to better understand the interactions between these precursors of the MCM-41 mesoporous molecular sieves. Three different CTAB concentration regions were analyzed: the dilute and semidilute regions, where simple surfactant species, such as monomers and spherical and nonspherical CTAB micelles, are found, and the concentrated region, involving liquid-crystalline phases. In the dilute and semidilute regions, the formation of a white nanostructured solid, having a hexagonal array similar to that found in MCM-41 materials, was observed. Precipitation of this solid requires some degree of surfactant monomer aggregation, which is favored by the presence of silicate anions. If micelles have already been formed, the material can be obtained at any CTAB concentration above a threshold concentration of silicate anions. These facts suggest that silicate anions have an important role in changing the aggregation and/or the shape of the surfactant aggregates. In the concentrated region, precipitation of the solid was not observed, but the presence of the silicate anions alter the characteristics of the liquid-crystalline phase formed by the surfactant. The system shows very complex and rich behavior and its investigation may be very useful in understanding the processes of nanostructured solid formation. (c) 2004 Elsevier Inc. All rights reserved.284268769

Ferromagnetic Resonance In Amine/organic Acid Caped Au-nanoparticles Assisted By Yttrium-oxide

In this study we report on the magnetic properties of amine/organic acid caped Au-nanoparticles (Au-NPs) assisted by yttrium-oxide (Y 2O 3) with diameters ranging between 20 and 60 nm, prepared by somewhat modified chemical route already described in the literature. The magnetization results show a ferromagnetic (FM) loop with negligible coercive field and a magnetic saturation of ∼0.1 emu/gAu at 2 K. Furthermore, an intense magnetic resonance was observed from room-T down to 4.2 K with the following features: i) the field for resonance, H R, is sample and weakly T-dependent; ii) the line width, ΔH PP, broadens at low-T; and iii) the resonance intensity remains almost constant in the entire T-region. The observed resonance is the signature of a ferromagnetic resonance (FMR). To the best of our knowledge this is the first clear report on FMR in magnetic caped Au-NPs. These features characterize our Au-NPs as FM ones. Our results are discussed in terms of the interactions established between the capping ligands and the surface of the Au-NPs which stimulate an effective hybridization of the 5d-6s conduction electrons of the Au atoms, allowing the Au 5d shell to become magnetic due to uncompensated 5d spins.9194ACCT Canada,Anaheim Center for New Energy Technologies (AC-NET),Angel Capital Association,Antenna Systems Magazine,Applied MaterialsSun, S., Murray, C.B., Weller, D., Folks, L., Moser, A., (1989) Science, 287, p. 2000Johnson, B.F.G., (2003) Top. Catal, 24, p. 147Luo, X., Morrin, A., Killard, A.J., Smyth, M.R., (2006) Electroanalysis, 18, p. 319Volokitin, Y., Sinzig, L.J., De Jongh, J.G., Schmidt, G., Varaftik, M.N., Moiseev, I.I., (1996) Nature, 384, p. 621Service, R.F., (1996) Science, 271, p. 920Schmid, G., Corain, B., (2003) Eur. J. Inorg. Chem, 17, p. 3081Nobusada, K., (2004) J. Phys. Chem. B, 108, p. 11904Liu, H., Mun, B.S., Thomton, G., Isaacs, S.R., Shon, Y.S., Ogletree, D.F., Salmeron, M., (2005) Phys. Rev. B, 72, p. 155430Zhang, P., Sham, T.K., (2003) Phys. Rev. Lett., 90, p. 245502López-Cartes, C., Rojas, T.C., Litrán, R., Martinez- Martínez, D., De La Fuente, J.M., Penades, S., Fernandez, A., (2005) J. Phys. Chem. B, 109, p. 8761Roldán, A., Illas, F., Tarakeshwar, P., Mujica, V., (2011) J. Phys. Chem. Lett., 12, p. 2996Vargas, J.M., Iwamoto, W., Holanda Jr., L.M., Oseroff, S.B., Pagliuso, P.G., Rettori, C., (2011) J. Nanosci. Nanotechnol., 11, p. 2126Tang, Y., Ouyang, M., (2007) Nature Materials, 6, p. 754Garitaonandia, J.S., Insausti, M., Goikolea, E., Suzuki, M., Cashion, J.D., Kawamura, N., Ohsawa, H., Rojo, T., (2008) Nano Letters, 8, p. 661De La Venta, J., Pucci, A., Pinel, E.F., García, M.A., Fernandez, C.J., Crespo, P., Mazzoldi, P., Hernando, A., (2007) Adv. Mater., 19, p. 875Dutta, P., Pal, S., Seehraa, M.S., Anand, M., Roberts, C.B., (2007) Appl. Phys. Lett., 90, p. 213102Lopes, G., Vargas, J.M., Sharma, S.K., Beron, F., Pirota, K.R., Knobel, M., Rettori, C., Zysler, R.D., (2010) J. Phys. Chem. C, 114, p. 10148Harbola, K.M., Sahni, V., (1988) Phys. Rev. B, 37, p. 745Hori, H., Yamamoto, Y., Iwamoto, T., Miura, T., Teranishi, T., Miyake, M., (2004) Phys. Rev. B, 69, p. 174411Munoz-Márquez, M.A., Guerrero, E., Fernández, A., Crespo, P., Hernando, A., Lucena, R., Conesa, J.C., (2010) J.Nanopart. Res., 12, p. 1307Crespo, P., Litran, R., Rojas, T.C., Multigner, M., De La Fuente, J.M., Sanchez-Lopez, J.C., Garcia, M.A., Fernandez, A., (2004) Phys. Rev. Lett., 93, p. 087204Coulthard, I., Degen, I.S., Zhu, Y., Sham, T.K., (1998) Can. J. Chem., 76, p. 1707Zhanchetet, D., Tolentino, H., Martin Alves, M.C., Alves, O.L., Ugarte, D., (2000) Chem. Phys. Lett., 323, p. 167Zhang, P., Sham, T.K., (2002) Appl. Phys. Lett., 81, p. 736Garzón, I.L., Rovira, C., Michaelian, K., Beltrán, M.R., Ordejon, P., Junquera, J., Sánchez-Portal, D., Soler, J.M., (2000) Phys. Rev. Lett., 85, p. 525

Micro Raman Spectroscopy Investigation of Patinas Formed by Exposure of Copper to Vapor of Several Aqueous Electrolyte Solutions

Universidade Federal de São Paulo, Dept Ciencias Exatas & Terra, BR-09972270 Diadema, BrazilUniversidade Federal de São Paulo, Dept Ciencias Exatas & Terra, BR-09972270 Diadema, BrazilWeb of Scienc