Margarita Rivera

Studies of porosity and diffusion coefficient in porous matrices by computer simulations

We present a series of molecular dynamics simulations to study the porosity on different matrix configurations. The matrices were prepared using two different processes. In the fist method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at different densities and at different matrix particle interactions. The results showed that the matrix structure and the matrix porosity were affected by the way the porous matrices were prepared. Finally, we also investigated the diffusion of a fluid inside the matrices. The diffusion coefficient was measured by mean square displacements of the particles in the fluid. It was observed that this quantity was also affected by the kind of porous matrix employed. The calculations were performed for several fluids at different densities in the different porous matrices. From these studies we observed that the highest porosity and diffusion coefficient were found in matrices prepared with attractive particle interactions and without any template.

Adsorption and Self-Assembly of Anticancer Antibiotic Doxorubicin on Single-Walled Carbon Nanotubes

We performed a combined experimental and theoretical study of the conjugates obtained from single-walled carbon nanotubes and anticancer antibiotic doxorubicin (DOX). Atomic force microscopy (AFM) imaging at lower magnification revealed, extended regions of single-walled carbon nanotubes (SWNTS) fully covered with DOX adsorbed molecules, along with some bare parts without the adsorbed drug, thus suggesting that the DOX adsorption is a cooperative process. Ambient atmosphere scanning tunneling microscopy (STM) at higher resolutions found that individual SWNTs-DOX conjugates exhibit a periodic texture, whose most important morphological feature is alternating depressions and protrusions along the nanotube. Based on the images and profiles measured, we suggest that doxorubicin molecules self-assemble on SWNTs sidewall according to a helical pattern, in which their tetracyclic fragments are turned with respect to the nanotube axis by about 50∘. To provide an additional insight into the structure of noncovalent SWNTs-DOX conjugates, we employed density functional theory (DFT) calculations with three long-range corrected functionals: M05-2X, wB97X-D and LCBLYP, of which M05-2X yielded the most realistic results in terms of geometries and energies.

Pore matrices prepared at supercritical temperature by computer simulations: matrix characterization and studies of diffusion coefficients of adsorbed fluids

We present series of molecular dynamics simulations to study the structure of different porous matrix configurations. The present simulations are an extension of recently reported data at a temperature below the critical. Here, we show how temperature modifies the structure and porosity of pore matrices during their preparation in comparison with the previous work. Moreover, in these studies at a higher temperature, we studied in more detail the structure of the porous matrix. Matrices were prepared by two different processes. The first method consisted of a single Lennard-Jones fluid simulated at a fixed density and at a supercritical temperature. Then, the matrix configuration was taken from the last configuration of the fluid. The second method was prepared from a binary mixture, where one of the components served as a template material. The final porous matrix configuration was obtained by removing template particles from the mixture. Matrices were prepared at two different densities and at different matrix particle interactions. The volume distribution, the cluster formation and the connectivity between the particles in the pore matrix were investigated. The importance of using template particles was clear since they produced larger voids and higher porosities. On the other hand, the temperature of preparation seems to modify the size of the voids and the porosity in the matrices. For instance, at this high temperature, the matrix porosity is higher when template particles are present in the system. These results point in the opposite direction compared to those found in a previous paper at a lower temperature. The diffusion of fluids immersed in the different matrices was also analysed by calculating the mean square displacements of their particles. It was observed that this quantity was higher when matrices were prepared with template particles. These results also point to different directions in contrast with pore matrices prepared at a lower temperature. Finally, the results show that temperature plays an important role in the pore matrix formation.

Cobalt electrodeposition onto highly oriented pyrolytic graphite (HOPG) electrode from ammonium sulfate solutions

-2 M of CoSO 4 + 1M (NH 4 ) 2 SO 4 at pH 4.5 (natural pH). All solutions were prepared by using analytic grade reagents with ultra pure water (Millipore-Q system) and were deoxygenated by bubbling N 2 for 15 min before each experiment. Once the solution was deoxygenated a nitrogen atmosphere was maintained over the solutions. During the experiment the bubbling was stopped to avoid the presence of additional diffusional variables caused by the nitrogen bubbles on the electrode surface. The working electrodes were freshly cleaved HOPG surfaces. A graphite bar with an exposed area greater than the working electrode was used as counter electrode. A saturated silver electrode (Ag/AgCl) was used as the reference electrode, and all measured potentials are referred to this scale. The electrochemical experiments were carried out in a BAS potentiostat connected to a personal computer running the BAS100W software to allow the control of experiments and data acquisition. In order to verify the electrochemical behavior of the electrode in the electrodeposition bath, cyclic voltammetry was performed in the 0.600 to -1.300 V potential range. The kinetic mechanism of Co deposit onto HOPG was studied under potentiostatic conditions by the analysis of the experimental potentiostatic current density transients obtained with the potential step technique. In all cases, the perturbation of the potential electrode always started at 0.600 V because this is the null current zone in where the Co deposition had not still begun. Thus, the application of this potential guarantees that the electrochemical signals recorded during the cathodic polarization are caused by the cobalt electroreduction on the electrode. 20

Electrochemical and atomic force microscopy investigations of new materials from N-trifluoromethanesulfonyl-1-azahexa-1,3,5-trienes derivatives

Thin films have been created on the surface of highly oriented pyrolytic graphite electrodes from chemically synthesized p-donor species by using the cyclic voltammetry technique and in situ electrochemistry-atomic force microscopy. Results indicate that it is possible to form new compounds called molecular materials from the coupling of electro-oxidized N-trifluoromethanesulfonyl-1-azahexa-1,3,5-triene species and a [Cr(C2O4)3]3- anion. Different techniques such as AFM, SEM and EDS were employed to characterize the films morphology and composition. Results indicate that the new materials exhibited a particular electrochemical response and a different film texture depending on the chemical composition of the starting compounds.

Diffusion of charged fluids in charged porous matrices

Computer simulations to study the diffusion of charged fluids in charged porous matrices were performed. The matrix was prepared using a template, i.e., it was simulated from a neutral binary mixture of charged particles where one of the components served as the template and the other as the matrix. The final porous matrix was obtained by removing template particles from the mixture. Then, the diffusion of a charged fluid immersed in the porous matrix was studied showing that in some cases, it increased as the fluid density increased. The results can be explained in terms of possible traps created in the matrix which capture mobile particles. Pair correlation functions of the fluid particles were studied and they showed strong structure for trapped particles indicating low motion of such mobile charges.

Conceptions and first results on the electrocrystallization behaviour of ferritin.

The role of electrochemical processes on Fe and CdSO(4) in the crystallization of horse spleen ferritin has been investigated using the cyclic voltammetry technique. It was found that although both species exhibit important redox properties in the presence of an external applied potential, CdSO(4) played a leading role not only in the nucleation process but also in the growth behaviour and morphology of ferritin crystals.

Electrical Properties of Multi-Pyrene/Porphyrin-Dendrimers

Dendrimers bearing pyrene donor groups have been obtained and act as efficient light-harvesting antennae capable of transferring light energy through space from their periphery to their core. The light-harvesting ability increases with each generation due to an increase in the number of peripheral pyrenes. In order to evaluate the photovoltaic properties of the compounds, thermal evaporated thin films were produced and the voltage response in the presence of visible light was obtained. The energy transfer efficiency was found to be almost quantitative for the first and second generations. The dendrimers have the potential to become integral components of molecular photonic devices.

N-(Chalcogen)phosphorylated (chalcogen)ureas of zinc and cadmium(II): SSPs for group 12–16 thin films

The bis-chelates of zinc (7–11) and cadmium (12–17) with general formula [M{R2P(X)NC(Y)NR′-η2-X,Y}2] were obtained from ligands 1–6 (iPr2P(X)NHC(Y)NC4H8: 1: X = S, Y = S; 2: X = S, Y = O; 3: X = O, Y = S; 4: X = S, Y = Se, 5: X = Se, Y = S; Ph2P(X)NC(Y)NC5H10: 6: X = S, Y = S). 4 and cadmium complex 15 are the first examples of this ligand system with carbon-bonded selenium as a donor atom. All the compounds were characterized by FTIR and multi-nuclear NMR spectroscopies, and microanalysis. The molecular structures of complexes 1, 7, 8, 11, and 15 were elucidated by single crystal X-ray diffraction. Thermal gravimetric analysis (TGA) and thermolysis experiments were carried out for the metal complexes. Selected pyrolysis residues were analyzed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and X-ray powder diffraction (XRPD). Remarkably, cadmium complex 13 gave a few crystals of CdS encrusted on the surface of CdO. Preliminary studies using the O,S-mixed cadmium complexes (13 and 14) as single source precursors (SSPs) were carried out for the deposition of thin films via AA-CVD. The results show that CdS films are obtained from both complexes, and in the case of 14 (the complex containing a PO fragment), persistent high carbon content was detected.

Morphological and local magnetic properties of cobalt clusters electrodeposited onto indium tin oxide substrates

Magnetic force microscopy was employed in order to investigate the local magnetic distribution of individual cobalt clusters electrodeposited onto indium tin oxide surfaces. In addition, the size and shape of the clusters were analyzed by using scanning electron microscopy and atomic force microscopy at various electrodeposition potential values or growth stages. From these results, the cluster magnetic state at different cluster dimensions was identified and the critical cluster size for the magnetic transition from the single to the multi domain state was obtained. By analyzing the local magnetic states of the aggregates in terms of a theoretical single-domain ferromagnetic model, various magnetic parameters were inferred. Finally, in order to validate the experimental results, a micromagnetic simulation study was employed which correctly confirmed the experimental magnetic transition diagram, and also, provided some insights of other intrinsic magnetic properties of the aggregates.