Papers by Author: Lilian Marques Silva

Abstract: The aim of this work was to produce, to characterize and test selective membranes based on derivatives from organic fluorinated/silicon compounds. The produced composites presented silicon and fluorinated species on the surface. The morphology of these non uniform surfaces showed big domains in the micrometer scale but a closer view reveals structures also in the nanometer range. Contact angle measurements showed a mildly hydrophobic and organophilic surface. Polymeric tapes treated with this composite showed an increase on permeation rate for the organic compounds. An acrylic device that employed this modified polymeric tape was manufactured and used for sample pretreatment during chemical analysis.

Abstract: The aim of this work was the production of a large surface area of hexamethyldisilazane (HMDS) plasma-deposited thin films and their applicability in a miniaturized structure useful for preliminary analysis of organic mixtures. The HMDS plasma films were produced with different surface areas and morphologies, and all films adsorbed polar and non-polar organic compounds. A low cost miniaturized structure was manufactured in glass using a Milling cutter and covered with HMDS plasma films. Good agreement was observed between simulation and experimental results on those microstructures. The observed different performance between pure and mixtures of organic compound samples suggests that the proposed system is a simple setup that could be useful for rough analysis of a fuel.

Abstract: Recently, it was demonstrated that copper thin films show good adsorption characteristics for organic polar and non-polar compounds. Also, these films when used in small cavities can favor preconcentration of these organic compounds. It is also known that copper oxide can provide catalysis of organic compounds. Therefore, the aim of this work is the study of copper thin film catalysis when used in small cavities. Copper thin films, 25 nm thick, were deposited on silicon and/or rough silicon. These films do not show oxide on the surface when analyzed by Rutherford backscattering. Also, Raman analysis of these films showed only silicon bands, due to the substrate, however infrared spectroscopy shows oxide bands for films exposed to organic compound aqueous solutions. Cavities with copper films deposited inside were tested with a continuous flow of n-hexane, acetone or 2-propanol admitted in the system. The effluent was analyzed by Quartz Crystal Microbalance. It was shown that n-hexane or acetone can be trapped. The system also shows good reproducibility. Tests of catalysis were carried out using Raman spectroscopy and heating the films up to 300°C during 3 minutes after exposure to n-hexane, 2- propanol and acetophenone – pure or saturated aqueous solution. After the exposure, Raman spectra present intense bands only for 2-propanol, indicating that adsorption easily occurs. However, after heating with all solutions it was not found only silicon bands. Raman microscopy after heating also showed copper oxide cluster formation and, eventually, graphite formation. Although the heating provides oxide copper formation, this reaction does not produce a high amount of residues, which means that catalysis is possible in this condition. Thus, a simple device using copper thin films can be useful as sample pretreatment on microTAS development.

Abstract: Films produced by plasma polymerization of ethyl ether and methyl or ethyl acetate show good adsorption characteristic for polar and non-polar organic compounds. These films when used in microchannels machined in a 3D-structure present some preconcentration of organic compounds. Therefore, the aim of this work is to investigate the physical-chemical preconcentration mechanisms on this structure. The test molecules used were n-hexane and 2-propanol. Quartz crystal microbalance and mass spectrometry were used to measure preconcentration. Two different procedures for reactant injection on the structure were used: a continuous flow during several minutes or a small amount injected on a single pulse and in a few seconds. The microchannels were also modified by the introduction of small ceramic particles for enhancement of the flow dispersion. It was possible to notice for all films a similar kinetic of retention. The main removal mechanism is adsorption. Although all films can provide the removal of the adsorbents molecules, the most important characteristic for the adsorption and/or retention is the surface condition. Thus, the retention of polar compound can be troubled if a non-polar compound was used previously. The most promising films for retention are ethyl ether and ethyl acetate when n-hexane and 2-propanol are used as test molecules. The results using n-hexane or 2-propanol point out the use of low-cost microchannels for preconcentration development.