Browsing by Author "Figueiredo, Maria Margarida Lopes"
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- Effect of extended cooking and oxygen prebleaching on the surface energy of Eucalyptus globulus kraft pulpsPublication . Carvalho, Maria da Graça Videira Sousa; Ferreira, P.J.; Santos, José M.R.C.A.; Amaral, João; Figueiredo, Maria Margarida LopesThe aim of this work was to investigate the influence of introducing an oxygen prebleaching stage and of extending the cooking process on the surface and papermaking properties of elemental chlorine-free bleached kraft pulps produced with Portuguese Eucalyptus globulus. Inverse gas chromatography was used to evaluate the surface energy as well as the Lewis acid/base character of the pulps, taking as reference one pulp cooked and bleached under standard conditions. The dispersive component of the surface tension, y~, was found to be highest for the pulp prepared under standard conditions, lowest for the pulp prepared by extended cooking and intermediate for the oxygen-prebleached pulp. Although all pulps exhibited a dominant acidic character (K)Kb > 1 ), the oxygen stage was found to enhance this character due to the higher surface concentration of oxygen-containing functional groups. Furthermore, oxygen prebleaching led to the formation of bulkier handsheets and it increased the beating energy. The opposite behaviour was found for the pulp produced by increasing the H-factor to extend the cooking.
- Modeling dissolution of sparingly soluble multisized powdersPublication . Almeida, Luís Pereira de; Simões, Sérgio; Brito, Paulo; Portugal, António; Figueiredo, Maria Margarida LopesThe dissolution of powder drugs, besides being a topic of utmost importance, especially for the sparingly soluble ones, is far from being well-explained. The purpose of the present study is, on the one hand, to obtain experimental dissolution profiles and, on the other hand, to analyze and process the data for dissolution modeling. Three different size fractions of a widely used sparingly soluble drugsibuprofenswere fully characterized with regard to its particle size distribution, specific surface area, density, solubility, and diffusion coefficient. The dissolution profiles were obtained making use of a technique that counts and sizes particlessthe Coulter counter techniqueswhich is capable of following the number and size of the particles in suspension throughout time. The knowledge of these parameters allowed a critical study of the assumptions associated with the models currently used to describe the dissolution process. It was concluded that most of the assumptions were not valid for the present experimental conditions. This motivated the proposal of a new methodology, which uses the experimentally determined characteristics of the drug and takes into account the polydisperse nature of the powder. By applying an adequate dissolution equation to each of the many size classes in which the primary particle size distribution was divided, it was possible to obtain a large agreement between the simulated and the experimental dissolution profile.
- Modelling dissolution of sparingly soluble powdersPublication . Almeida, Luís Pereira de; Brito, Paulo; Sousa, Anabela; Portugal, António; Figueiredo, Maria Margarida LopesThe present work deals with the simulation of the dissolution process of polydisperse powders. Three different size fractions of Ibuprofen were used to develop a model which assumes a diffusion layer thickness proportional to the particle diameter, up to a critical value, beyond which it is considered constant. The application of the resultant integrated equations to each size class of the primary particle size distribution leads to an almost perfect agreement between simulated and experimental dissolution profiles.
- Modelling the dissolution kinetics of ibuprofenPublication . Almeida, Luís Pereira de; Brito, Paulo; Sousa, Anabela; Portugal, António; Figueiredo, Maria Margarida Lopes
- The effects of beating, web forming and sizing on the surface energy of Eucalyptus globulus kraft fibres evaluated by inverse gas chromatographyPublication . Carvalho, Márcia; Santos, José M.R.C.A.; Martins, Alexandre A.; Figueiredo, Maria Margarida LopesIn this work attention has been focused on the effects of papermaking beating, web forming and sizing operations on the physical/chemical surface properties of bleached Eucalyptus globulus kraft fibres. Inverse gas chromatography (IGC) was used to determine the dispersive component of surface tension (cs d) as well as the acidic/basic character (according to the Lewis concept) of the solid surfaces (pulp fibres and pulp handsheets). The results have shown that the main effect of beating is to increase the fibre’s Lewis acidic character. Web forming caused a strong decrease in cs d and significant increments in the adhesion works of both acidic and basic probes, lowering the ratio between the two. Nevertheless, the surface of handsheets still exhibited a dominant acidic character. The sizing operation did not change the dispersive component of the surface tension significantly but decreased the difference between the adhesion works of the acidic and basic probes, rendering the handsheet surface less Lewis acidic and more Lewis basic. Thus, although internal sizing is expected to strongly influence liquid spreading at the paper surface and liquid penetration of the fibre’s network, it is concluded that beating and web forming lead to important changes in the surface energetic properties of the Eucalyptus globulus kraft fibres.