Elizabeth Fátima de Souza

Scanning electric potential microscopy imaging of polymers: electrical charge distribution in dielectrics

Scanning electric potential images of polymer surfaces are presented and compared to standard non-contact AFM images. Samples used were a latex film with a well-known distribution of chemical constituents and thus of ionic electrical charges, as well as finished industrial products. Topography and electric potential images show a variable degree of correlation, thus evidencing the independence of topographic and electrical features of the samples, in the micro- and nanoscopic scales. Domains with non-zero negative or positive electric potentials are observed, extending for a few tenths of a micron and creating an electric mosaic in the otherwise neutral polymers. Large electric potential gradients are observed, e.g. in a HDPE film.

Core-and-shell nature of Stöber silica particles

Two different samples of monodisperse Stober silica particles were examined in the analytical transmission electron microscope, using different imaging modes: bright-field, dark-field, energy-loss and elemental distribution maps. The particles (effective diameters = 141 and 36 nm) are formed by domains of variable O/Si ratio, which is consistent with a variable degree of hydration, and they coexist with particles with a high O/Si ratio measuring a few nanometers only, which appear dispersed in the picture background. Bright-field and energy-loss images of the larger particles show a core-and-shell morphology, and the shells have a higher amount of high-O/Si domains as well as contaminating carbon compounds. On the other hand, the smaller particles (effective diameter = 36 nm) are also formed by distinct domains, but their morphology is neither spherical or core-and-shell. The mechanisms for particle formation presented in the literature are discussed, considering the present findings.

Improved latex film-glass adhesion under wet environments by using an aluminum polyphosphate filler

—Non-crystalline aluminum polyphosphate (A1PP) was added as a filler to poly(vinyl acetate) latex and the resulting dispersion was used both as a glass coating and as a glass-to-glass adhesive. Latex-coated glass samples were subjected to a grid adhesion test following the ASTM D 3359 standard, in which the samples are immersed in water. A1PP improves the latex polymer film adhesion to glass, as evidenced by coating film and adhesive joint stabilities. This is explained in terms of the formation of Al3+ ion bridges between the polymer-polyphosphate bicontinuous composite film and the glass surface.

Drainage kinetics of nanochannels fabricated in water films a few molecules thick on mica at room temperature

Nanochannels of the order of 20 nm in diameter and forming arrangements that were a few micrometres wide were fabricated on nanometre-thick ice-like deposits on planar mica surfaces at room temperature. Because an atomic force microscopy tip can write lines on ice-like layers covering mica substrates in air that are stable under invariant conditions of humidity and temperature, the water films were modulated with nanochannels. By analysing the shape and morphology of the material removed after channel fabrication for various time intervals, the channel profile was shown to vary with a scale of a tenth of a second. In this configuration (hydrophobic tip and hydrophilic substrate, 65% RH), at the channel top region there were only aggregates of loose flakes formed after the film inscription but no liquid. Apparently, the Kelvin effect is responsible for the nanochannel profile variation with time, but the calculated and measured values of the drainage time constant are at variance by six orders of magnitude. This reduction of the mass transfer is associated with the small dimensions of the ∼ 10 nm-wide channels.

Anisotropic growth of water-puckered pentamers on a mica terrace.

Ice nucleation at mica terrace edges in air forms mounds of water molecules that grow larger as the step-edge height increases from a few Angstroms to hundreds of nanometers. The structures of the ice deposits at mica terrace edges were characterized by atomic force microscopy (AFM), and the edges were shown to act as nucleators for water pentamers, thereby forming a zigzag structure with lattice parameters of 0.72 ± 0.07 and 0.60 ± 0.06 nm. A three-dimensional arrangement of three pentamers of water molecules, which formed a parasol-like structure, was assembled to match the AFM images. Seven three-fused pentamers were clustered to form large hexamers that cover the entire surface. The nucleation at the edges reveals a substantially larger growth rate than that on the mica terraces; consequently, highly terraced mica slabs could be used as new and more efficient structures for seeding clouds and causing rain. On the basis of this finding, a new ice-condensation structure was designed with pyramidal features and steps of 100 nm in height and width.

Colloidal Stability by Adsorption of Ni2+ or Fe3+ Cations on Hairy Latex Particles

Polymer latexes have many industrial applications and are considered as models of colloids. The DLVO theory has been widely used to describe the colloidal stability. The adsorption of Ni2+ and Fe3+ cations on polyvinyl acetate latex particles in the presence of counterions, and the colloidal stability of the system were studied. The obtained results indicate that Ni2+ or Fe3+ cations were adsorbed on the latex particles’ hairy layers, which also helped to prevent latex coagulation. The stability of the PVAc latex within highly concentrated ionic media shows a deviation from the behavior described by the classical DLVO theory.

Avaliação do desempenho de surfactantes para a solubilização de fases líquidas não aquosas em meio aquoso

The presence of non-aqueous phase liquids (NAPLs) in the subsurface is a threat to public health as well as a serious environmental issue. NAPLs may remain adsorbed or form lenses floating on aquifers causing long-term contaminations. Surfactants may increase NAPLs solubility, enhancing the pump-and-treatment performance. Size, shape, hydration and ionization degree of the micelles define the affinity and the space available for the solubilization of a particular contaminating agent. The tests carried out at laboratory scale, taking into account the NAPL to be removed and the medium characteristics were useful to select surfactants and evaluate their efficiency as NAPLs solubilizers.

Determination of Chlorophenol in Environmental Samples Using a Voltammetric Biosensor Based on Hybrid Nanocomposite

In this work, a simple electrochemical biosensor for 4-chlorophenol was developed based on laccase immobilized on a hybrid nanocomposite (ZnO nanoparticles/chitosan), and incorporated in a carbon paste electrode. There are few biosensors in the literature for this specific pollutant because it tends to form polymeric films on the electrode, causing surface passivation or even enzyme inactivation. The carbon paste allowed the surface to be easily renewed by polishing, which amends this limitation. To optimize the experimental conditions, we used cyclic voltammetry and hydroquinone as a representative of phenolic compounds due to the high toxicity of chlorophenol, thus avoiding the generation of hazardous residues. After optimization, a calibration curve was constructed for 4-chlorophenol using differential pulse voltammetry, and a linear response was obtained from 1 to 50 µM, with a lower detection limit of 0.7 µM. The obtained biosensor showed high accuracy when employed in the analysis of industrial wastewater.

Application of Factorial Design to Optimize Cloud Point Extraction on the Determination of Metals in Eye Makeup

Experimental design is an important chemometric tool for systemic optimization, allowing the study of a number of variables concomitantly, thus reducing the number of tests, time and reagents that are used in addition to reduce the amount of generated residues. In the present work the main objective was to use a factorial design 2 with central and axial points for optimization of cloud point extraction in order to determine cadmium, nickel, lead and cobalt by flame atomic absorption spectrometry. Variables that were studied in this factorial design were the amount of complexing agent, ammonium pyrrolidine dithiocarbamate (APDC) and the amount of surfactant (Triton X-100). Analysis of the math models that were developed from the design experiments demonstrated that in the case of nickel and cobalt these variables did not contributed significantly to increase absorbance readings. On the other hand, in the determination of cadmium the results have shown that it is necessary to reduce the amount of complexing agent, and for lead, it was shown to be necessary to increase both the amounts of complexing agent and surfactant. After optimization, the cloud point extraction was carried out using eye makeup samples, which presented concentrations for nickel and lead within 4.96 and 14.03 x 10 mg.kg. These values are within the allowed range of the Agência Nacional de Vigilância Sanitária (ANVISA), which establishes the limits of 20 mg.kg and 100 mg.kg respectively for lead and other metals in artificial organic dyes for cosmetics. Although the samples studied here have shown to be in a concentration range that would exempt them from a pre-concentration step, considering the analysis by atomic absorption spectrometry, the use of cloud point extraction has shown to be effective and also recommended in cases in which the analyzed metals are present in trace amounts.

Atomic scale patterns formed during surface scanning by atomic force microscopy tips

In this work, tip sliding at the water/substrate interfacial region was used to investigate the pattern observed during image acquisition with atomic resolution in atomic force microscopy. The process responsible for the pattern formation is the oscillatory movement of the tip in the direction that is normal to scanning induced by a change in the water interfacial dielectric permittivity from e≈4 at the interface to e≈80 (bulk value) that results in a variation of the measured force acting on the tip of ≈30pN.