Alessandra L. Poli

Effect of sonication on the particle size of montmorillonite clays

This paper reports on the effect of sonication on SAz-1 and SWy-1 montmorillonite suspensions. Changes in the size of the particles of these materials and modifications of their properties have been investigated. The variation of the particle size has been analyzed by DLS (dynamic light scattering). In all cases the clay particles show a bimodal distribution. Sonication resulted in a decrease of the larger modal diameter, as well as a reduction of its volume percentage. Simultaneously, the proportion of the smallest particles increases. After 60 min of sonication, SAz-1 presented a very broad particle size distribution with a modal diameter of 283 nm. On the other hand, the SWy-1 sonicated for 60 min presents a bimodal distribution of particles at 140 and 454 nm. Changes in the properties of the clay suspensions due to sonication were evaluated spectroscopically from dye-clay interactions, using Methylene Blue. The acidic sites present in the interlamellar region, which are responsible for dye protonation, disappeared after sonication of the clay. The changes in the size of the scattering particles and the lack of acidic sites after sonication suggest that sonication induces delamination of the clay particles.

The UV/H2O2 - photodegradation of poly(ethyleneglycol) and model compounds

O mecanismo geral para a fotodegradacao do polietilenoglicol (PEG) usando o sistema H2O2/UV foi determinado por GPC e HPLC, analisando o comportamento de moleculas modelo, como por exemplo os etilenoglicois de baixa massa molar (tetra-, tri-, di-, e etilenoglicol). Apos 30 min de irradiacao, a massa molar ponderal media (Mw) do PEG degradado, analisada por GPC, cai para metade do seu valor inicial com um aumento concomitante na polidispersidade e numero medio de quebra de cadeia (S). Essa queda acentuada caracteriza uma quebra de cadeia aleatoria, formando oligomeros e etilenoglicois de menor massa molar. A analise da fotodegradacao de etilenoglicois modelo por HPLC permitiu sugerir um mecanismo que envolve processos consecutivos, em que etilenoglicois maiores originam menores, sucessivamente. A fotodegradacao do etilenoglicol formou acidos carboxilicos de baixa massa molar, como por exemplo os acidos glicolico, oxalico e formico.

Photochemical synthesis of silver nanoparticles on chitosans/montmorillonite nanocomposite films and antibacterial activity

Silver nanoparticles (AgNPs) were synthetized on chitosans/montmorillonite nanocomposite films by photochemical method. Nanocomposites were prepared using chitosans with different molar masses and deacetylation degrees, as well as modified with diethylaminoethyl (DEAE) and dodecyl groups. AgNPs formation on the films was followed by the appearance of the plasmon band around 440nm as a function of irradiation time. TEM images revealed AgNPs with spherical morphology for all nanocomposites. For nanocomposites using modified chitosans, the AgNPs synthesis occurred quickly (1.5h) while for the others films it was above 11h. The film of modified chitosan with dodecyl and DEAE groups presented smaller and more uniform nanoparticles size along mixture of exfoliated and intercalated structures. This modified chitosan is an amphiphilic compound that can act controlling the size/shape of the AgNPs. The results of antibacterial activity suggested that all nanocomposite-AgNPs films inhibited the growth of Escherichia coli and Bacillus subtilis.

Photo-fenton degradation of poly(Ethyleneglycol)

PEG samples needed 490 min to reduce their —w by 50%, whereas under UV irradiation, only 10 min were necessary. The exponential decay of —w with a concomitant increase in polydispersity and number of average chain scission, characterized a random chain scission mechanism. The degradation products of PEG in both systems showed the presence of lower molecular weight products, including smaller ethyleneglycols and formic acid. The mechanism involves consecutive processes, were the larger ethyleneglycols give rise, successively, to smaller ones. This suggests that the mechanism involves successive scissions of the polymer chain. Irradiated samples decomposed faster than those kept in the dark This study proves that the foto-Fenton method associated with UV-light is a good reactant for PEG photodegradation.

Thermal properties of poly (methyl methacrylate)/organomodified montmorillonite nanocomposites obtained by in situ photopolymerization

The organoclay/poly(methyl methacrylate) (PMMA) nanocomposites were prepared by in situ photopolymerization method using two solvents, ethanol and acetonitrile. The influences of organoclay loading, solvent nature and length of attached surfactant (C8 or C16) on thermal and mechanical properties were studied by thermogravimetric analysis and dynamic mechanical analysis. Alkylammonium surfactants with C8 and C16 chain lengths were evaluated as clay modifiers. All the nanocomposites prepared in acetonitrile exhibited improvement in their thermal stability, mainly due to the interaction between the clay and the polymer which is maximized by the exfoliated clay structure. In the case of PMMA and nanocomposites synthesized in ethanol, the thermal stability of polymer and nanocomposites remained practically the same once the clay structure is predominantly of the intercalated type. In comparison with pure PMMA, glass transition temperature and storage modulus of polymer are notably increased by the presence of clay. It was found that the chain length of surfactant attached to the SWy-1 clay affects the Tg values. Glass transition temperatures of nanocomposites SWy-1-C16/PMMA were significantly higher than the values obtained for nanocomposites SWy-1-C8/PMMA. This can be attributed to the modifying agent C16, which has a greater hydrophobic chain length. The organic tail can provide a better dispersion of the MMA monomer in the organoclay, resulting in a nanocomposite with predominant exfoliated structure. Another significant factor to be considered was the effect of solvent used in the nanocomposite preparation. Considering nanocomposites with the same chain length (C8 or C16), Tg values obtained for nanocomposites prepared with ethanol is higher than those observed for those prepared in acetonitrile. This was attributed to the influence of the average molecular weight; once the nanocomposites prepared in ethanol exhibited higher polymeric chains.

Estudo espectroscópico do equilíbrio entre espécies hexacoordenada e pentacoordenada dos monômero d nativo e reconstituído da hemoglobina extracelular gigante de Glossoscolex paulistus em meio alcalino

UV-Vis and fluorescence spectroscopic studies of native and reconstituted d monomers from Glossoscolex paulistus were performed in alkaline pH. In addition to the presence of aquomet and hemichrome hexacoordinated species, a pentacoordinated species occurs. This latter species, which presents an imidazolate group of axial histidine as fifth ligand, is identified by the blue-shifted low-intensity Soret band and LMCT band. The pentacoordinated species must occur due to partial polypeptide unfolding.

Hemes férricos pentacoordenados e hexacoordenados dos monômeros d nativo e reconstituído da hemoglobina extracelular de Glossoscolex paulistus: estudos espectroscópicos em meio ácido

UV-Vis and fluorescence spectroscopic studies of the native and reconstituted d monomers of Glossoscolex paulistus were performed in acid medium. The coexistence of distinct species shows the complexity of the equilibria. Besides the hexacoordinate low spin hemichrome, with bands at 535 and 565 nm, a pentacoordinate high spin hemichrome is identified by the blue-shifted low intensity Soret band (371 nm) and the LMCT band (643 nm). The pentacoordinate hemichrome must be related to the partial unfolding of the polypeptide.

Photochemical Deposition of Silver Nanoparticles on Clays and Exploring Their Antibacterial Activity

Photochemical method was used to synthesize silver nanoparticles (AgNPs) in the presence of citrate or clay (SWy-1, SYn-1, and Laponite B) as stabilizers and Lucirin TPO as photoinitiator. During the photochemical synthesis, an appearance of the plasmon absorption band was seen around 400 nm, indicating the formation of AgNPs. X-ray diffraction results suggested that AgNPs prepared in SWy-1 were adsorbed into interlamellar space, and moreover, showed some clay exfoliation. In the case of SYn-1, AgNPs was not intercalated. For the AgNP/Lap B sample, the formation of an exfoliated structure occurred. Transmission electron microscopy revealed the spherical shape of AgNPs for all samples. The particle sizes obtained for AgNP/SWy-1, AgNP/SYn-1, and AgNP/Lap B were 2.6, 5.1, and 3.8 nm, respectively. AgNPs adsorbed on SYn-1 reveal nonuniform size and aggregation of some particles. However, AgNP/SWy-1 and AgNP/Lap B samples are more uniform and have diameters smaller than those prepared with SYn-1. This behavior is due to the ability to exfoliate these clays. The antibacterial activities of pure clays, AgNP/citrate, and AgNP/clays were investigated against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). AgNPs in the presence of clays (AgNPs/SYn-1 and AgNPs/SWy-1) showed a lower survival index percentage compared to those obtained for pure clays and AgNPs. The AgNP/SWy-1 sample showed good antibacterial activity against both tested species and the lowest survival index of 3.9 and 4.3 against E. coli and S. aureus, respectively. AgNPs are located in the interlayer region of the SWy-1, which has acid sites. These acidic sites may contribute to the release of Ag(+) ions from the surface of AgNPs. On the other hand, Laponite B and AgNP/Lap B samples did not demonstrate any bactericidal activity.

Behaviour of pseudoisocyanine in macromolecular and hydrotropic solutions

The photophysical behaviour of the dye pseudoisocyanine (PIC) was studied in macromolecular (sodium alginate) and hydrotropic (sodium toluenesulphonate and sodium styrenesulphonate) solutions. It was found that the placement of the dye in these media affected the spectroscopic properties and the lifetimes of its excited states. The effects are traced down to the interactions of the dye molecules with the macromolecules or their placement in the hydrotropic aggregates, inducing the formation of J-aggregates of the dye or constraining its internal movements reducing the rate of internal conversion. The lifetimes of the triplet state of the dye in alginate were of the order of 20-30 ns in freshly prepared solutions, whereas when leaving the solution to rest for some hours the decay became biexponential with the growing of a species with lifetime around 150 ns. The latter is assigned to dye molecules placed in the alginate network.

Influência de estabilizantes na degradação foto-oxidativa de filmes de compósitos de SWy-1/poli(óxido de etileno)

Poly(ethylene oxide) (PEO) with SWy-1 montmorillonite and stabilizers (2-hydroxybenzophenone and tinuvin 770) composites were prepared by intercalation in solution method. The films of the composites were exposed to UV irradiation. The photodegradation products were monitored by FTIR (Fourier Transform Infrared Spectroscopy) and SEC (Size Exclusion Chromatography) method. The PEO pure showed higher degradation rate coefficients, k d , compared to the others samples. The system that showed the lowest value for the degradation rate coefficients (k d = 1.9×10 –6 mol g –1 h –1 ) was the PEO/SWy-1 5% composite with 0.25% of tinuvin 770. In this case, the stabilization of the PEO matrix can be attributed to clay associated with tinuvin 770. The clay scatters and absorbs UV irradiation and tinuvin acts as a stabilizer of the HALS (hindered amine light stabilizers) kind.