Cette thèse porte sur la problématique de réduction des émissions de gaz à effet de serre par captage et stockage du dioxyde de carbone (CO2) contenu dans les effluents industriels. Les procédés de captage concernés reposent sur l’absorption sélective du CO2 par des solutions aqueuses d’alcanolamines. Les mécanismes physico-chimiques d’absorption mis en jeu sont étudiés à l’aide de modèles thermodynamiques. Leur développement est complexe et la prédiction précise des données physico-chimiques, nécessaires à l’optimisation des procédés industriels de captage, reste difficile. Le développement d’outils permettant une représentation détaillée des structures microscopiques permettrait l’optimisation de ces modèles. Ces outils fourniraient également des informations pour l’établissement de relations structure-propriété nécessaires au design d’absorbants adaptés au captage en post-combustion. Les travaux de recherche ont porté sur l’évaluation du pouvoir prédictif des outils de simulation moléculaire et leur capacité à établir des relations entre la structure des absorbants, les interactions moléculaires et les propriétés physicochimiques macroscopiques. Les outils développés ont été construits afin de permettre leur transférabilité entre alcanolamines. L’étude repose sur des mesures calorimétriques et des travaux de simulation par dynamique moléculaire menés en parallèle. Elle porte sur des alcanolamines primaires, pures ou en solutions aqueuses, basées sur le squelette N-C-C-O, incluant la monoéthanolamine (MEA). La mise en évidence d’un effet d’ouverture des liaisons hydrogène intramoléculaires des alcanolamines en fonction de leur composition semble être à la base de la différenciation du comportement énergétique des systèmes binaires {alcanolamine + eau}. L’identification des différents types d’interactions engagés a permis de mettre en lumière un effet hydrophobe. L’ensemble des analyses explique certaines limites des modèles thermodynamiques classiques et constitue un guide pour leur amélioration, notamment par la prise en compte de l’effet de composition. / This thesis focuses on the problem of reducing greenhouse gas emissions by capture and storage of carbon dioxide (CO2) from industrial effluents. The capture processes concerned is based on the selective absorption of CO2 by aqueous solutions of alkanolamines. In industry and academia, the physico-chemical mechanisms of absorption are described using thermodynamic models. Their development is complex and the prediction of physicochemical data, which is necessary to optimize industrial capture processes, remains difficult. The development of molecular models for a detailed representation of microscopic structures would improve these models. These molecular models also provide information for the establishment of structure-property relationships which are necessary to design absorbants adapted to post-combustion capture. This doctoral research project has focused on assessing the predictive power of molecular simulation methods and their ability to establish relationships between the structure of absorbents, molecular interactions and macroscopic physico-chemical properties. The molecular interaction models were built to allow their transferability between alkanolamines. The study is based on calorimetric measurements and molecular dynamics simulation run in parallel. It covers primary alkanolamines, pure or in aqueous solutions, based on the N-C-C-O skeleton, including monoethanolamine (MEA). With varying composition of the {alkanolamine + water} mixtures, there is a competition between the intramolecular hydrogen bond of the alkanolamines (between the amino and hydroxyl group) and the hydrogen bonds with water molecules. This effect of opening of the intramolecular hydrogen bonds is related in this work with the value of the enthalpy of mixing. Also, this effect is of different magnitude for different alkanolamines and therefore the present model represents correctly different molecules. Evidence of the role of the hydrophobic effect is also given through an analysis of the different terms in the interactions. The main results of the present work are detailed analyses at the molecular level of the interactions present in the {alkanolamine + water} mixtures and how these determine the macroscopic thermodynamics of mixing. This knowledge at the molecular scale can provide a guide to the improvement of thermodynamic models.

L’oxydation des lipides est un phénomène complexe qui génère de nombreux produits qu’il est nécessaire de caractériser de manière précise afin de prévenir leur apparition. Parmi les méthodes pour analyser les produits d’oxydation, l’analyse thermique différentielle, basée sur la mesure différentielle du flux de chaleur dissipé au cours des réactions d’oxydation, est intéressante car elle n’utilise pas de solvant organique. De plus, elle peut être corrélée à l’apparition de produits primaires ou secondaires d’oxydation. Dans ce contexte, nous avons conçu et développé un microcalorimètre basé sur le principe de l’analyse thermique différentielle mais plus adaptable dans la mesure où il permet l’analyse de plusieurs échantillons simultanément dans des coupelles d’analyse de dimensions variables (variation du rapport surface/volume en relation avec les phénomène de diffusion de l’oxygène). Pour le développement du microcalorimètre, les thermopiles sont utilisées dans la configuration adiabatique. Le système de mesure des flux est stable et possède une sensibilité élevée. La validation de la méthode est réalisée par effet Joule et par comparaison des points de fusion de paraffines en calorimétrie différentielle classique. La modélisation des réactions d’oxydation est réalisée afin de mettre en évidence l’influence de l’oxygène sur les cinétiques. Pour une huile riche en acides gras polyinsaturés (huile de cameline), l’enthalpie de la réaction d’oxydation (en début de cinétique) obtenue en condition isotherme (100°C) est corrélée avec la mesure des diènes conjugués (produits primaires d’oxydation). Dans l’ensemble, le microcalorimètre développé est suffisamment sensible et fiable pour mesurer les enthalpies de réactions d’oxydation des lipides des huiles alimentaires. / Lipid oxidation results of many reactions generating numerous oxidation products that are worth characterizing in fats and oils. Among the analysis methods available, differential scanning calorimetry, based on the differential measurement of heat flux dissipated during the oxidation reactions, is convenient because it avoids the use of organic solvents. In this context, we have conceived and developed a microcalorimeter that allows the analysis of up to 5 samples simultaneously, in containers of variable sizes (allowing to vary the surface/volume ratio). For the conception of the microcalorimeter, the thermopiles are used in adiabatic configuration. The system is very stable and allowed flux measurement with a high sensitivity. The validation of the method is performed by Joule effect and by comparison of melting points of paraffins with classical differential scanning calorimetry. The modelization of the oxidation reaction is performed to point out the influence of oxygen on the kinetics. For a polyunsaturated oil (cameline oil), the enthalpy values obtained, at the beginning of the oxidation process, under isothermal condition (100°C) are correlated with the diene conjugated hydroperoxide amount. On the whole, the developed device provided an adaptable, sensitive, solvent-free and low cost method for the measurement of lipid oxidation, particularly suitable for the fast screening of a large set of samples.

Master of Science / Department of Biological and Agricultural Engineering / Donghai Wang / Currently, the majority of adhesives used for wood veneer, plywood, and composite applications are formaldehyde-based. Formaldehyde is derived from petroleum and natural gas, making it non-renewable and toxic. Therefore, extensive research has been conducted to develop bio-based adhesives to replace formaldehyde-based adhesives. Soy protein has shown great potential to partially replace formaldehyde adhesives, and canola protein has similar properties to soy protein. However, little research has been conducted on the feasibility of using canola protein for wood adhesive applications. The objective of this research was to study the adhesion performance of canola protein. Canola protein was modified with different chemical modifiers including sodium dodecyl sulfate (SDS), calcium carbonate (CaCO[subscript]3), zinc sulfate (ZnSO[subscript]4), calcium chloride (CaCl[subscript]2), and 2-octen-1-ylsuccinic anhydride (OSA) as well as combined chemical modifications. The wet, dry, and soak shear strengths of the adhesive formulations were determined. Viscosity testing, differential scanning calorimetry, and TEM and SEM imaging were used to characterize protein properties. Chemical modification with SDS (1%, 3%, and 5%), CaCO[subscript]3 (1%, 3%, and 5%), ZnSO[subscript]4 (1%), and OSA (2%, 3.5%, and 5%) improved the dry and soak shear strengths compared to unmodified canola protein. Canola protein modified with 3.5% OSA had improved wet, dry, and soak shear strengths. Combined chemical modification of canola protein did not show significant improvement on shear strength. Thermal modification of canola protein adhesives showed a trend of increasing shear strength with increasing press temperature. The data suggests that with further research, canola protein has potential to be used as a commercial adhesive or as an additive to formaldehyde-based adhesives to make them more environmentally-friendly.

Ce mémoire de thèse porte sur la caractérisation de l'effet électrocalorique des matériaux ferroélectriques solides et cristaux liquides. La découverte récente d'un effet électrocalorique qualifié de "géant" a relancé l'intérêt pour l'étude et la caractérisation des propriétés électrocaloriques des matériaux. Au-delà de la recherche des matériaux performants, ce domaine de recherche concerne également le développement des techniques de caractérisations appropriées et la réalisation des prototypes de réfrigération électrocalorique. Dans ce contexte, notre étude se focalise sur le développement de nouvelles techniques de caractérisation, la méthode photopyroélectrique (indirecte) et la calorimétrie (directe). La technique photopyroélectrique, développée au sein du laboratoire a été utilisée pour la détermination des propriétés thermiques du matériau pyroélectrique lui-même. L'exploitation de cette technique nous a permis également de déterminer les propriétés pyroélectriques du matériau, en particulier le rapport entre le coefficient pyroélectrique et la capacité calorifique, en fonction de la température et du champ électrique appliqué, nécessaire pour une évaluation indirecte de l'effet électrocalorique. Plusieurs matériaux ferroélectriques solides et liquides ont été étudiés à l'aide de cette méthode, en particulier, un monocristal de TriGlycine Sulfate et deux cristaux liquides. L'effet électrocalorique a été évalué autour de la température de transition de phase que présentent chacun de ces matériaux. Pour valider les résultats obtenus, nous avons procédé à des mesures indirectes de la polarisation par la méthode usuelle du courant de dépolarisation. Dans ce travail, nous avons également développé une technique de mesure indirecte de l'effet électrocalorique, par mesure calorimétrique, à l'aide d'un nouveau dispositif. Outre l'étude des transitions de phase et de la capacité calorifique, cet instrument permet une mesure directe de la température et de la quantité de chaleur absorbée ou cédée avec le milieu environnant. Une première étude de l'effet électrocalorique a été réalisée sur un matériau multicouche à base de titanate de baryum. Les résultats obtenus par cette approche ont été ensuite comparés à d'autres techniques directes et indirectes existantes dans la littérature. Ces deux nouvelles approches permettent d'élargir les possibilités d'étude de futurs matériaux électrocaloriques et de mesurer à la fois les propriétés thermiques et pyroélectriques nécessaires pour l'étude de l'effet électrocalorique. / This thesis work deals with the characterization of the electrocaloric effect in solid and liquid crystal ferroelectric materials. Following the 2006 discovery of Mischenko and al., the characterization techniques of the electrocaloric effect and the exploration of new caloric materials have attracted much attention. This discovery showed also that electrocaloric materials can be used for efficient innovative solution for refrigeration devices. This PhD dissertation focuses on the development of new techniques used to evaluate the electrocaloric effect by the photopyroelectric technique and by calorimetry. Few years ago, a new particular configuration based on the photopyroelectric technique, developed in our laboratory, was described for measuring thermal parameters of pyroelectric materials themselves. By means of this technique, we indirectly investigate the electrocaloric effect in ferroelectric materials by measuring the ratio of the pyroelectric coefficient to the volumetric heat capacity, as function of temperature and applied field, using Maxwell's relation. Measurements were carried out on ferroelectric solid materials (TriGlycine Sulfate) and liquid crystals. Electrocaloric effect has been evaluated around the phase transition temperature of each sample. To further validate the accuracy to the evaluated adiabatic temperature changes, we proceeded to indirect measurements by using the polarization reversal current technique. In the present work, we also developed a calorimeter in order to directly evaluate the electrocaloric effect. This technique is mainly used to measure with high resolution the heat capacity and enthalpy near phase transition temperature. In addition, this technique allows us to directly measure the temperature and the amount of heat absorbed or transferred from the material to the surrounding environment. A primary study of the electrocaloric effect was carried out on a multilayer material based on barium titanate. The results obtained by this approach have been then compared to conventional direct and indirect measurements. Those two new approaches give access to the measurement of both thermal and pyroelectric properties allowing the evaluation of the electrocaloric effect.

Student Number : 9900772K - MSc research project - School of Civil and Environmental Engineering - Faculty of Engineering and the Built Environment / The hydration of cement is an exothermic reaction, which begins almost immediately upon contact with water. This produces a large amount heat that subsequently raises the temperature of the concrete mixture, creating a temperature gradient across the member. The temperature rise associated with hydration induces thermo-mechanical stresses. These stresses can cause damage to the structure, affecting the durability and in extreme cases the functionality of the structure. If the maximum rate of heat evolution experienced can be minimised through the selection of the constituents of a concrete mixture, then the thermal stresses that develop in the concrete can be reduced. The main aim of this research is to develop a knowledge of how the heat evolution characteristics of concrete are affected by changing certain concrete mixture parameters and ingredients. The focus is on the addition of three different cement extenders and varying the water/cement ratio. This will be a step towards the development of a model for predicting the thermal properties of concrete. As a part of this investigation, a prediction model for the change in heat rate in concrete was developed. The model is intended to predict the contribution of the individual clinker crystallographic phases in cement and the heat liberated in concrete during hydration.

Les techniques d’analyse thermique et de calorimétrie sont des méthodes d’essai largement utilisées dans les laboratoires d’analyse physico-chimique, pour des finalités de recherche ou de contrôle qualité. Comme tout appareil de mesure, un analyseur thermique ou un calorimètre doit être étalonné en température et en énergie avec des matériaux de référence certifiés. Les matériaux de référence recommandés correspondent généralement aux points fixes de l'échelle internationale de température (EIT-90), à savoir gallium, indium, étain, zinc et aluminium. Il existe peu de matériaux de référence certifiés au-dessus de 420 °C, alors que certains analyseurs thermiques peuvent être utilisés jusqu'à 1000 °C, voire au-delà.L’élaboration et la certification de matériaux de référence doivent employer des méthodes de mesure très précises avec un raccordement métrologique des mesures au système international d’unités (SI). Le Laboratoire Commun de Métrologie (LCM) s'est engagé dans le développement d'un moyen de référence métrologique en calorimétrie permettant des mesures précises en enthalpie de fusion et en capacité thermique massique sur la plage de température [23 °C, 1000 °C]. La solution métrologique retenue a été de modifier un calorimètre de type Calvet, et de mettre au point des procédures d’étalonnage et de mesure afin d’atteindre des incertitudes de mesures suffisamment faibles pour la certification des matériaux de référence.Dans ce travail, un système d'étalonnage fonctionnant à haute température a été spécifiquement conçu et intégré dans le calorimètre pour permettre l'étalonnage par substitution électrique. Ce système permet de réaliser successivement des étalonnages par effet Joule et des mesures d’enthalpie de fusion, sans modification des conditions expérimentales.Ce travail comprend également le développement des systèmes d’acquisition et traitement des résultats des mesures. La détermination de l'enthalpie de fusion de plusieurs métaux (indium, étain et argent notamment) avec une recherche des facteurs d’influence sur cette grandeur, et une estimation des incertitudes de mesure. La mesure de l’enthalpie de fusion d’un alliage eutectique argent-cuivre, candidat comme matériau de référence en énergie à 779 °C, est également présentée. / Differential scanning calorimeters are widely used in many academic and industrial laboratories to study the thermal behavior of materials for research or quality control. Like any measuring device, a thermal analyzer or calorimeter must be calibrated in temperature and energy with certified reference materials. Recommended reference materials generally correspond to fixed points of the International Temperature Scale (ITS- 90), namely: gallium, indium, tin, zinc and aluminum. However, there are few certified reference materials above 420 °C, while the operating range of some thermal analyzers and calorimeters exceeds 1000 °C.The certification of reference materials insures the metrological traceability of measurements to the International System of Units (SI). The LCM-LNE has been working in the development of a metrological standard facility for accurate measurements of the enthalpy of fusion and heat capacity in the temperature range [23 °C, 1000 °C]. The metrological approach is based on the modification of a commercial Calvet calorimeter and of the procedures implemented for calibration and measurement, so as to get measurement uncertainties sufficiently low to fulfill the objectives of the certification of reference materials.A new in-situ high temperature calibration system (constituted by a resistance wire wound around the crucible containing the material sample) was integrated into the calorimeter to perform the calibration by electrical substitution. The system allows both calibration and measurement without modification of the apparatus, so that the experimental conditions during both steps remain unchanged.This work also includes the development of data acquisition system and processing of measurement results. The determination of the enthalpy of fusion of several metals (indium, tin and silver in particular) with an estimation of the measurement uncertainty has been made. The measurement of the enthalpy of fusion of a silver-copper eutectic alloy, as candidate reference material at 779 °C, is also presented.

A doença de Chagas, causada pelo Trypanosoma cruzi, é uma doença tropical que aflige milhões de pessoas, gerando consequências sócio-econômicas devastadoras. Ela tem sido considerada uma doença tropical super-negligenciada, já que os únicos fármacos disponíveis para o seu tratamento apresentam baixa eficácia e causam vários efeitos colaterais. Além disso, os mesmos foram introduzidos há mais de três décadas. Com esse cenário, é evidente a necessidade da descoberta, desenvolvimento e introdução de novos fármacos para o tratamento eficiente e seguro da doença de Chagas. A enzima gliceraldeído-3-fosfato desidrogenase (GAPDH) é um alvo biomacromolecular atraente para a descoberta de novos fármacos contra os tripanossomatídeos, em virtude das enzimas da via glicolítica exercerem um papel fundamental no fornecimento de energia para a sobrevivência do parasito. Essa enzima foi selecionada neste trabalho de tese para a realização de estudos em química medicinal com base em quiminformática com o objetivo de identificar potenciais inibidores enzimáticos e do T. cruzi. Na primeira etapa desta tese, o ensaio virtual baseado na estrutura do alvo (SBVS) foi usado na identificação e seleção dos compostos. Como resultado do planejamento in silico, vinte compostos foram selecionados e avaliados experimentalmente na segunda etapa do trabalho empregando a técnica de calorimetria de titulação isotérmica (ITC). Destes, onze inibiram a GAPDH de T. cruzi resultando numa elevada taxa de acerto (>= 20%). Os novos inibidores apresentam excelente eficiência do ligante (LE), bem como mostram ligeira seletividade pela enzima do parasito. O ensaio dos inibidores contra a forma tripomastigota do T. cruzi identificou dois compostos capazes de inibir essa forma infectiva e um deles também mostrou ser um potente inibidor da forma amastigota do parasito, além de apresentar baixa toxidez. As duas melhores classes de inibidores da GAPDH e do parasito foram selecionadas para o estabelecimento de relações quantitativas entre a estrutura química e a atividade biológica (QSAR). Estudos de QSAR 2D (HQSAR) forneceram modelos com elevada capacidade preditiva e proporcionaram a identificação de características estruturais importantes para a otimização dos ligantes a compostos-matrizes. / Chagas disease, caused by Trypanosoma cruzi, is a tropical disease, which afflicts millions of people, thus generating devastating socio-economic consequences. It has been pointed out that it is a super-neglected tropical disease, based on available drugs with low efficacy and that give rise to many side effects. In addition, these drugs were introduced three decades ago. With this scenario, it is clear the necessity of the discovery, development and introduction of new efficient drugs to treat Chagas disease. The enzyme glyceraldehyde-3-phosphate dehydrogenase is a promising target for the development of new drugs against trypanosomatides, since the enzymes of the glycolytic pathway display a fundamental role in the energy supply to parasite survival. In this thesis, this enzyme was selected for medicinal chemistry within the cheminformatics framework aiming at the identification of potential enzymatic and parasite inhibitors. In the first part, structure-based virtual screening (SBVS) methods were employed in the selection and identification of compounds. Based on the in silico design, twenty compounds were selected and experimentally evaluated in the second part using the isothermal titration calorimetry (ITC) technique. Out of these, eleven compounds inhibited the T. cruzi GAPDH, resulting in high hit rates (>= 20 %). The new selected inhibitors display excellent ligand efficiency (LE), as well as some selectivity for the parasite enzyme. The inhibitors assay against the trypomastigote form of T. cruzi was used to identify two compounds able to inhibit this infective form, and one showed to be a strong amastigote parasite inhibitor, also disclosing low cytotoxicity profile. The best two classes of GAPDH and parasite inhibitors were selected for the establishment of a quantitative structure-activity relationship (QSAR). 2D QSAR (HQSAR) studies resulted in linear models with high predictive power, amenable for the identification of important structural features in the process of hit-to-lead optimization.

Mais de quarenta proteínas humanas estão envolvidas em doenças amilóides, sendo a transtirretina (TTR) uma dessas. A dissociação do tetrâmero da TTR é a etapa limitante para sua via de agregação. Esta etapa pode ser dificultada pela ligação de pequenas moléculas a dois sítios na interface tetramérica da TTR e facilitada em variantes mutantes. A mutante V30M é a variante amiloidogênica mais frequente da TTR e, apesar da mutação não se encontrar no sítio de ligação de pequenas moléculas, pode limitar a interação de inibidores com esta proteína. Desta forma, a busca de inibidores da agregação da TTR tipo selvagem (TTRwt) e mutantes amiloidogênicas vem sendo realizada. Na presente tese, flavonóides, freqüentemente encontrados em alimentos, e agonistas sintéticos do receptor do hormônio tireoidiano, seletivos para a isoforma beta deste receptor, foram selecionados para testes de inibição da agregação da TTRwt e mutante V30M. A interação dos melhores inibidores com a TTR foi também caracterizada em detalhes. Para isto, um ensaio de agregação in vitro da TTR foi utilizado para triagem dos compostos; e a assinatura termodinâmica da interação dos melhores inibidores com a TTR, bem como as estruturas cristalográficas TTR:inibidores foram determinadas por calorimetria de titulação isotérmica e cristalografia de proteínas, respectivamente. Os compostos sintéticos, GC-1 e GC-24, mostraram-se inibidores eficazes, porém com potência moderada. Já as flavonas luteolina (LUT), apigenina (API) e crisina (CHR), a flavanona naringenina (NAR), o flavonol kaenferol (KAE) e a isoflavona genisteína (GEN) apresentaram boa potência e eficácia de inibição da agregação da TTR in vitro. Os inibidores NAR, CHR, GEN e KAE não mostraram mesma capacidade de inibição da mutante V30M e apresentaram cooperatividade negativa para ligação aos dois sítios da TTR. A posição dos inibidores no sítio, bem como a variabilidade química/ estrutural dos compostos testados parecem influenciar os mecanismos de interação com os dois sítios da TTR e a ligação à mutante V30M. Modificações no sítio da mutante V30M foram identificadas e são apontadas como a principal causa para a seletividade de alguns inibidores à forma selvagem da TTR. De modo geral, a investigação detalhada da interação de pequenas moléculas com a TTR permitiu propor as bases moleculares da cooperatividade entre os sítios desta proteína e das diferenças de interação dos inibidores com a forma selvagem e mutante V30M da TTR. / More than forty human proteins are involved in human amyloid diseases, being transthyretin (TTR) one of these. TTR tetramer dissociation is a limiting step for its aggregation pathway. This step can be delayed by small molecules binding to two binding sites in the TTR tetramer interface and enhanced in TTR amyloidogenic variants. The V30M mutant is the most frequent TTR amyloidogenic variant and, eventhough the mutation is not situated in the TTR binding site, this mutation can limit small molecule interaction with the V30M mutant. Therefore, the search for wild type (TTRwt) and amyloidogenic TTR mutant inhibitors is under investigation. In the present thesis, flavonoids, frequently found in food, and synthetic thyroid hormone receptor beta-selective agonists were selected for evaluation of their inhibition ability on TTRwt and V30M mutant aggregation. The interaction of the best inhibitors with TTR was also characterized in details. For this, an in vitro TTR aggregation assay was used for inhibitor screening, and the thermodynamic signature of interaction, as well the TTR:inhibitors crystal complexes were determined by isothermal titration calorimetry and protein crystallography, respectively. The synthetic compounds, GC-1 and GC-24, displayed high efficacy, however moderated inhibition potency. The flavones luteolin (LUT), apigenin (API), chrisin (CHR), the flavanone naringenin (NAR), the flavonol kaenferol (KAE) and the isoflavone genistein (GEN) showed good potency and efficacy of TTR aggregation inhibition in vitro. The inhibitors, CHR, NAR, GEN and KAE, had lower inhibition capacity to the V30M mutant, and displayed negative cooperativity of binding to the two TTR binding sites. The inhibitor position in the binding site, as well as the chemical/ structural variability of the tested compounds seems to influence the interaction mechanism with the two TTR binding sites and the binding to the V30M mutant. Modifications on the V30M mutant binding sites were identified and pointed as the main reasons for the selectivity of some inhibitors to the wild type TTR. Overall, the detailed inspection of small molecule interaction with TTR suggests the molecular basis of the cooperativity between the two TTR binding sites and also of the differences between the inhibitors interaction with the wild type and with the V30M mutant TTR.

Utilizando uma faixa de temperatura entre 5° a 45°C, observou-se duas regiões de diferentes tendências para com o aumento da temperatura durante as oscilações (eletro-oxidação galvanostática, mesma corrente normalizada aplicada) no sistema ácido fórmico sobre platina policristalina em meio ácido. Até 25°C, o comportamento cinético operou de modo convencional, do tipo Arrhenius, sendo que acima desse ponto crítico observou-se o fenômeno de (sobre)compensação de temperatura. O sistema foi caracterizado fazendo uso de técnicas eletroquímicas clássicas e espectroscopia de impedância eletroquímica tendo ficado evidente um ponto de inflexão a 25°C que marca uma quebra na tendência em todas as frequências do sistema (f, Hopf,Sosc) e taxa de envenenamento. Os resultados foram discutidos em termos do papel-chave das espécies de PtO, que acoplam quimicamente as dinâmicas rápidas/lentas. Fomos capazes de: (i) identificar a competição entre duas etapas de reação como responsáveis pelos dois domínios de temperatura; (ii) comparar as energias de ativação relativas dessas duas etapas; E ademais (iii) especulamos sobre o papel de uma determinada etapa de reação no conjunto de reações responsáveis pelo aumento do período oscilatório. Com ajuda de métodos deconvolutivos, reforçamos a hipótese de que as etapas por trás do drift possuem menor energia de ativação que as etapas LH durante tais dinâmicas. Também estão aqui discutidos experimentos de monitoramento de temperatura local durante as dinâmicas oscilatórias. Duas estratégias experimentais foram empregadas: uma utilizando eletrodos-termômetros a base de termistores e outra utilizando um micro calorímetro onde sensores piroelétricos monitoraram as oscilações na temperatura do eletrodo de trabalho, altamente em fase com as oscilações de potencial. Destacamos que ambas configurações foram capazes de acompanhar as diferenças de temperatura durante as dinâmicas oscilatórias (na faixa de 0,1~0,5mK por ciclo). Os resultados obtidos para a reconstrução dos fluxos de calor (ø) corroboram com o atual modelo mecanístico da eletro-oxidação oscilante de ácido fórmico em Pt em meio ácido. Observamos que as etapas de envenenamento do eletrodo são acompanhadas pelo aumento no ø enquanto a reativação do eletrodo é acompanhada pela diminuição no ø. / The oscillating electro-oxidation of formic acid on polycrystalline platinum in acidic media, as a model system, was employed to investigate the temperature effects on the coupling of fast and slow dynamics processes belonging to its oscillatory dynamics, i.e. the core oscillator (fast dynamics) and the slow term deactivation of surface caused by the oxygen place-exchange process. Using a temperature range from 5 to 45°C, we observed two disparate regions of tendencies upon temperature increment on the galvanostatic oxidation. The system exhibits conventional Arrhenius behavior for T up to 25°C and, on the other hand, T > 25° revealed the occurrence of temperature (over)compensation. The system was characterized by means of conventional electrochemical techniques and electrochemical impedance spectroscopy. Clearly, we observed an inflexion point at 25°C marked by a break on the tendency of oscillatory frequency ( f, hopf, Sosc) and poisoning rates. Results were discussed in terms of the key role of PtO species, which chemically couple slow and fast dynamics. In summary we were able to: (i) identify the competition between two reaction steps as responsible for the two temperature domains; (ii) compare the relative activation energies of these two steps; and (iii) suggest the role of a given reaction step on the period-increasing set of reactions involved in the oscillatory dynamics. In addition, we performed experiments to monitor the local temperature of the interface during oscillatory dynamics. Two experimental strategies were applied: low cost thermometers-electrodes and a more sophisticated experimental set up based on pyroelectric detection. It should be noted that both configurations were able to monitor temperature differences during oscillatory dynamics (in the range of 0.1 ~ 0.5mK per cycle). The results obtained for the reconstruction of the heat fluxes (ø) corroborate with the current mechanistic model of the oscillating electro-oxidation of formic acid in Pt in acidic medium. We observed that the poisoning stages of the electrode are accompanied by the increase in ø while the reactivation of the electrode is accompanied by the decrease in ø.

Neste trabalho estudamos a influência do tamanho molecular médio sobre a natureza da transição de fase esmética A (SA)-colestérica (C) líquido-cristalina para os compostos termotrópicos de miristato de colesterila (C14), nonanoato de colesterila (C9) e misturas binárias de C14 e C9 e caproato de colesterila (C6) e C9. Medidas de microscopia óptica foram realizadas para determinar as temperaturas de transição de fase SA C e C-isotrópica (I). Isto possibilitou a escolha dos intervalos de temperatura próximos às transições de fases, como referências para as outras técnicas utilizadas no trabalho. A natureza das transições de fase (descontínua, também denominada de primeira ordem, ou contínua) foi inferida através das análises de medidas de densidade e entálpicas. Os resultados evidenciaram que a transição passa de primeira ordem para contínua quando o tamanho molecular médio e a temperatura reduzida, r (definida como a razão entre as temperaturas de transição de fases SA C e C I ), decrescem no sistema. A ocorrência de uma transição de fase SA C contínua foi observada para a concentração de 65% de C9 na mistura C6 C9, para o valor de r ~ 0, 92. Propriedades mesoscópicas dos compostos estudados, como a distância entre as camadas esméticas na fase SA, dSA, o tamanho molecular médio, , no domínio C, e o comportamento do tamanho de correlação, , ao longo das transições de fase, foram determinadas da análise dos picos de difração de raios X. Os dados revelaram que dSA diminui com o decréscimo do tamanho molecular médio, como era esperado. Além disso, os valores de obtidos na fase C, próximos à transição I C, concordam com os tamanhos moleculares médios calculados supondo que as moléculas estejam em conformação estendida. Mais ainda, foi observado um aumento no tamanho de correlação na fase C quando a temperatura é reduzida da fase I para a fase SA, representando um aumento no grau de ordem orientacional. Contudo, o parâmetro troca abruptamente na transição C SA. Tal descontinuidade diminui com o decréscimo do tamanho molecular médio (e assim com r). Para o sistema particular composto por 65% C9 : 35% C6, onde identificamos um transição de fase contínua SA C, observamos um comportamento de (T TSAC)0,5 no domínio da fase C, em bom acordo com a teoria de campo médio. / The influence of the mean molecular length on the nature of the smectic A (SA)- cholesteric (C) liquid crystal phase transition has been studied for cholesteryl myristate (C14), cholesteryl nonanoate (C9) and binary mixtures of C14 and C9 and cholesteryl caproate (C6) and C9. Optical microscopy was carried out to determine the temperatures of the SAC as weel as the C - isotropic (I) phase transitions. This allowed the choice of the optimum temperature interval near the phase transitions. Information concerning the nature of the transitions (descontinuous, also referred to as first order, or continuous) was evaluated through the analysis of the density and enthalpic measurements. The results have evidenced that the transition crosses over from first order to continuous when both the mean molecular length and the reduced temperature, r (ratio between SAC and CI phase transition temperatures), decrease in the system. The occurrence of a continuous SA C phase transition is observed for a concentration very near 63.1 molar percent of C9 in the C6 C9 mixture (65 wt% C9 : 35 wt% C6), at r ~ 0.92. Mesoscopic properties of the systems, as the distance between the smectic layers in the SA phase, dSA, the mean molecular length, , in the C domain, and the correlation length, , behavior along the phase transitons, were determined from the X-ray difraction peaks. The data have revealed that dSA decreases as the mean molecular length is reduced, as it was expected. Moreover, values obtained in the C phase near the I C phase transition agree with the mean molecular lengths calculated from the corresponding extended molecule lengths. In addition, an increase in the correlation length in the C phase is observed, as the temperature is reduced from I to SA phases, representing an increase in the orientational order. Nevertheless, the parameter jumps in the C SA phase transition. Such discontinuity decreases as the mean molecular length (and hence the reduced temperature) diminishes. For the particular system composed of 65 wt% C9 : 35 wt% C6, where a continuous SA C phase transition was identified, a behavior of (T TSAC)0,5 in the C domain was observed, in good agreement with the mean field theory.