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A full understanding of chemical reaction dynamics at the gas/organic-surface interface requires knowledge of energy-transfer processes that happen during the initial gas/surface collision. We have examined the influence of mass and rovibrational motion on the energy-transfer dynamics of gas-phase species scattering from model organic surfaces using theory and experiment. Molecular-beam scattering techniques were used to investigate the rare gases, Ne, Ar, Kr, and Xe, and the diatomics, N₂ and CO, in collisions with CH₃- and CF₃-terminated self-assembled monolayer (SAM) surfaces. Complementary molecular-dynamics simulations were employed to gain an atomistic view of the collisions and elucidate mechanistic details not observable with our current experimental apparatus. We developed a systematic approach for obtaining highly accurate analytic intermolecular potential-energy surfaces, derived from high-quality ab initio data, for use in our classical-trajectory simulations. Results of rare gas scattering experiments and simulations indicate mass to be the determining factor in the energy-transfer dynamics, while other aspects of the potential-energy surface play only a minor role. Additionally, electronic-structure calculations were used to correlate features of the potential-energy surface with the energy-transfer behavior of atoms and small molecules scattering from polar and non-polar SAM surfaces. Collisions of diatomic molecules with SAMs are seen to be vibrationally adiabatic, however translational energy transfer to and from rotational modes of the gas species, while relatively weak, is readily apparent. Examination of the alignment and orientation of the final rotational angular momentum of the gas species reveals that the collisions induce a stereodynamic preference for the expected "cartwheel" motion, as well as a surprising propensity for "corkscrew" or "propeller" motion. The calculated stereodynamic trends suggest that the CH₃-SAM is effectively more corrugated than the CF₃-SAM. Finally, the feasibility for collisional-energy promoted, direct gas/organic-surface reactions was interrogated using the 1,3-dipolar azide-alkyne cycloaddition reaction. We found that geometrical constraints prevented the reaction from proceeding at the probed conditions. / Ph. D.

Comprehension of reactive chemical dynamics in the gas phase and at the gas/organic-surface interface and non-reactive dynamics at the interface between hydrophobic surfaces and water requires an understanding of the fundamental atomic and molecular interactions that undergird these important phenomena. In an effort to study these regimes of chemical interaction, we have performed computational simulations that probe the dynamics of chemical systems that exemplify each of these domains. To study gas-phase chemical dynamics, we reparametrized semiempirical Hamiltonians so that they can accurately describe the potential energy surfaces for two distinct atom+alkane reactions. In addition to their demonstrated accuracy, these methods possess the attractive quality of being computationally inexpensive enough to afford extensive direct-dynamics trajectory studies. Our results on the dynamics of atom+alkane hydrogen-abstraction reactions have shown good agreement with experimental metrics that are as diverse as product velocity distributions, excitation functions, angular distributions and rovibrational state distributions for diatomic products of the abstraction. We have demonstrated that our reparametrized Hamiltonians are suitable for investigating gas-phase reactions with up to 15 (5 heavy) atoms and that they are appropriate for studying reactions beyond the gas phase, especially gas/surface reactions. By employing our semiempirical methods within a quantum-mechanics/molecular-mechanics hybrid scheme we are able to examine hydrogen-abstraction reactions of fluorine atoms with alkanethiolate self-assembled monolayers. Our simulations reproduce the general trends of experimental results for the cousin F+squalane reaction. Our simulations also probe the role that secondary collisions play in determining the final internal and translational energy of the product HF molecules. For instance, we determined that very few interactions with the SAM surface were required to cool rotational and translational modes of the HF product, while its vibrational energy remains unchanged on the time scale that HF molecules trap on the SAM surface. Moving beyond the gas/organic surface interface, we have also performed molecular-dynamics simulations of thin water films confined between hydrophobic SAM surfaces. These simulations illuminated the structural and dynamics behavior induced in the water films by confinement in hydrophobic environments. While most effects of the surface do not penetrate deep into the water layers we have noted that enhanced lateral diffusion of water molecules can persist in these films with > 1 nm length scales. We have elucidated a possible mechanistic precursor for the attractive forces seen in experimental measurement of the hydrophobic effect. / Ph. D.

Fabrication of surfaces with versatile functional groups is an important research area. Hence, it is essential to control and tune the surface properties in a reliable manner. Vinyl-terminated self-assembled monolayers (SAMs) offer significant flexibility for further chemical modification and can serve as a versatile starting point for tailoring of surface properties. Here a synthetic route for the preparation of vinyl-terminated trichlorosilane self-assembling molecules: 9-decenyltrichlorosilane (CH₂=CH-(CH₂)₈-SiCl₃), 10-undecenyltrichlorosilane (CH₂=CH-(CH₂)₉-SiCl₃), and 14-pentadecenyltrichlorosilane (CH₂=CH-(CH₂)₁₃-SiCl₃) is presented. These molecules were used for the preparation of SAMs in either liquid or vapour phase processes. Commercially available methyl-terminated self-assembling molecules: decyltrichlorosilane (CH₃-(CH₂)₉-SiCl₃) and octadecanetrichlorosilane (CH₃-(CH₂)₁₇-SiCl₃) were used as controls. The resultant films were characterised by X-ray photoelectron spectroscopy (XPS), contact angle analysis, ellipsometry, and atomic force microscopy (AFM). Well defined, vinyl-terminated SAMs were further chemically modified with carbenes (:CCl₂, :CBr₂, :CF₂) and hexafluoroacetone azine (HFAA). The reactions were performed in the liquid or the vapour phase. The resulting SAMs were characterised using the same methods as for the vinyl-terminated monolayers. Successful modification was confirmed by the appearance of new signals in the XPS spectrum, with simultaneous changes in water contact angle values and unchanged thickness values. Methyl-terminated SAMs were also exposed to carbenes and HFAA as a control system. These are the first examples of C-C bond formation on SAMs in the vapour phase.

Ce travail concerne l'étude de monocouches auto-assemblées (SAMs) sur l'or par simulation moléculaire. Des SAMs électroactives formées de chaines ferrocenylalcanethiols et alcanethiols et des SAMs constituées de β-cyclodextrines immobilisées sur des surfaces pouvant donner lieu à la formation de complexes d'inclusion à l'interface ont été étudiées. L'objectif était d'obtenir des grandeurs macroscopiques qui soient directement comparables aux grandeurs expérimentales. Pour cela, des simulations de dynamique moléculaire ont été couplées à des calculs de perturbation thermodynamique afin d'obtenir des grandeurs rédox et des propriétés thermodynamiques d'association. La reproduction de grandeurs expérimentales a dans un premier temps permis de valider les méthodologies de simulation et les champs de forces utilisés. Ceci a ensuite conduit à envisager la simulation moléculaire comme une technique prédictive pour l'étude de nouveaux systèmes. Les grandeurs macroscopiques obtenues ont pu être interprétées grâce à une caractérisation structurale et énergétique des processus mis en jeu. / This work concerns the study of self-assembled monolayers (SAMs) on gold surfaces by molecular simulation. Electroactive SAMs formed by both ferrocenylalkanethiol and alkanethiol chains and SAMs of immobilized β-cyclodextrins that can form inclusion complexes at the interface were investigated. The objective of this study was to use molecular simulation to reproduce macroscopic properties that can be directly compared with experimental results. Molecular dynamics simulations are coupled to perturbation methods in order to calculate redox properties and thermodynamic properties of association. The comparison with experimental data allows us to validate simulation methodologies and forcefields and to consider simulation as a predictive tool for the study of new systems. Molecular dynamic also provides a rationalization of the macroscopic properties at the atomic level by a structural and energetic analysis of the processes involved in the reactions.

Le travail suivant est consacré d'une part à l'emploi du calix[4]arène comme une plate-forme organisatrice de principes actifs pour la conception de nouvelles prodrogues. Ce concept a été développé avec des substances antibactériennes ou antivirales, choisies comme modèles. Les conjugués calixarène - anti-infectieux ainsi synthétisés sont amphiphiles et insolubles dans l'eau. Leur comportement interfacial a été étudié via l'interface eau-air, mime d'une interface hydrophile-hydrophobe physiologique, à l'aide de la technique des films monomoléculaires de Langmuir. Nos résultats indiquent que ces prodrogues étalées à l'interface eau-air peuvent libérer leurs principes actifs dans la sous-phase. La méthodologie développée pour ces études de réactivité interfaciale pourrait à l'avenir être appliquée à d'autres prodrogues à base de calix[4]arène. Un second projet a concerné le trifluoroacétate de tétra-p-(guanidinoéthyl)-calix[4]arène (CX1). Ce composé présente des propriétés antibactériennes à large spectre, couplées à une faible toxicité cellulaire. Nos travaux ont visé à mieux comprendre son mode d'action, lié à une perturbation des parois bactériennes, par une approche physico-chimique. La technique de Langmuir a donc été employée afin d'étudier les interactions entre le CX1 et des films monomoléculaires de phospholipides étalés à l'interface eau-air, utilisés comme modèles de membrane bactérienne. Nos résultats nous ont permis de proposer un mode d'organisation des membranes bactériennes sous l'influence du CX1. Nous avons ainsi apporté des précisions sur son mécanisme d'action qui pourraient être utiles dans le développement de nouveaux calixarènes antibactériens / This work begins with utilization of the calix[4]arene macrocycle as organizing platform of anti-infectious molecules shaped as prodrug. The concept has been explored using antibacterial (nalidixic acid) and antiviral (aciclovir, ganciclovir) molecules, chosen as models. The calixarene - anti-infectious conjugates synthesized have amphiphilic structure and are insoluble in aqueous media. Their interfacial behavior was studied via the air-water interface, considered as mimic of biological hydrophilic-hydrophobic interfaces, using Langmuir monolayers technique. Our results indicate that calixarene-based prodrugs spread at the air-water interface are able to release anti-infectious molecules into the subphase. The original methodology employed for interfacial reactivity studies could be applied to further calixarene-based prodrugs. A second project concerns the trifluoroacetate salt of tetra-p-(guanidinoethyl)-calix[4]arene (CX1). CX1 is antibacterial, active against various Gram-positive and Gram-negative bacteria, with low eukaryotic cell toxicity. The aim of our work was to get more insight in the mechanism of action of CX1, involving bacterial wall disruption, by a physico-chemical approach. The Langmuir monolayers technique was employed in order to study interactions between CX1 and phospholipid monolayers spread at the air-water interface, used as models of bacterial membranes. Our results led us to propose a particular reorganization mode of bacterial membranes upon interactions with CX1. This proposal gives more understanding in the mechanism of biological activity of CX1, and could be helpful in developing new antibacterial calixarene derivatives

Nanotecnologia é o conjunto de conhecimentos acumulados pelo homem que permite controlar a produção de estruturas com uma ou mais dimensões nanométricas. Desde seus primórdios na década de 70, a nanotecnologia tem estado em constante ascensão, encontrando uma diversidade enorme de aplicações, como por exemplo em medicina e na indústria optoeletrônica. Por consequência, a demanda por equipamentos tanto de preparo como de caracterização/controle tem crescido exponencialmente. O uso da radiação X no estudo de dispositivos nanoestruturados tem sido, em grande parte, possível gra- ças as fontes síncrotrons com feixes intensos. Mas a disponibilidade desses laboratórios de alta tecnologia está aquém da crescente demanda das pesquisas em nanotecnologia, as quais precisam de técnicas de análise estrutural rápidas e de fácil acesso para otimização e controle da produção de dispositivos nanoestruturados. Com foco nessa falta por técnicas de análise estrutural, esta dissertação tem como objetivo avaliar quais parâmetros básicos de nanodispositivos, com substratos monocristalinos, podem ser investigados por meio de técnicas de difração de raios X utilizando fontes compactas de radiação, bem como avaliar as limitações instrumentais. / X-ray radiation has provided a powerful tool for analyzing the structure of materials at atomic scale. While many are fascinated with the perspectives oered by advanced X-ray sources, the practical aspects of these perspectives in the actual and future scenery of nanotechnology needs to be discussed. Nanotechnology, i.e. the capacity of controlling matter at atomic-molecular scales and manufacturing structures with dimensions of a few tens of nanometers, has provided a constant challenge for structural analysis via X-ray techniques. The great diversity of materials and methods derived from nanotechnology is generating a huge demand for time of analysis, much beyond of that can be supplied by synchrotron facilities worldwide. In optimizing nanostructured materials and devices processing methods, fast and easy-access techniques to control and characterization are required. Microscopy and spectroscopy techniques are very important in this scenery, but they have intrinsic limitations that have justied the search for high-resolution techniques of structural analysis, such as those obtained by diraction of X-rays. The use of Xradiation in the study of nanostructured device has been possible by synchrotron sources due to the high intense beams. But the availability of these high tech laboratories falls short of the growing demand for nanotechnology research. In this context, this dissertation intend evaluate which basic parameters of nanodevices with single crystal substrates can be investigated by techniques of X-ray diraction using compact radiation sources as well how to evaluate the instrumental limitations .

Este trabalho visa ao entendimento da interação de capsaicinóides com membranas celulares utilizando sistemas modelo. Dentre os alcalóides derivados de plantas do gênero Capsicum, a capsaicina e a dihidrocapsaicina respondem por 90% dos capsaicinóides, que são usados como analgésicos e antiinflamatórios, devido a sua interação específica com receptores. O mecanismo neurofarmacológico já foi bastante estudado, mas o modo de ação não neural ainda não foi elucidado. Usamos extratos brutos (EBs) de pimenta malagueta e de dedo-de-moça, que têm atividade superficial, e afetaram monocamadas de Langmuir de fosfatidil colina de dipalmitoíla (DPPC) e fosfatidil glicerol de dipalmitoíla (DPPG). Tais efeitos não tiveram dependência expressiva com a carga, pois o EB de dedo-de-moça interagiu mais fortemente com o DPPC do que com o DPPG, ao passo que o contrário se verificou para o EB de malagueta. Também não houve diferença significativa entre os EBs das duas pimentas. Nas monocamadas de Langmuir representativas para a bactéria S. aureus, ambos os EBs tiveram efeito, tanto nas isotermas de pressão quanto nos resultados de espectroscopia de absorção e reflexão no infravermelho com modulação de polarização (PM-IRRAS), sem distinção significativa entre malagueta e dedo-de-moça. No entanto, as medidas de vazamento com lipossomos mostraram maior interação com o EB de dedo-de-moça, o que é consistente com a atividade bactericida para S. aureus. De fato, a concentração inibitória mínima (MIC) foi 0,13 mg mL-1 para o EB da pimenta dedo-de moça e 4,0 mg mL-1 para o EB de malagueta. Para a E. coli, os EBs interagiram com as monocamadas de Langmuir sem diferenças dignas de nota para as duas pimentas, e nas medidas de vazamento o efeito maior foi para a dedo-de-moça. Não houve efeito bactericida para nenhum dos extratos. Isso se explica porque bactérias gram-negativas, como a E. coli, têm uma camada externa protetora de lipossacarídeos (LPS). Das medidas de monocamadas de Langmuir representativas da camada de LPS, observou-se pouca incorporação dos EBs. Conclui-se, assim, que os EBs não conseguem causar rompimento da camada de LPS. Do conjunto dos resultados, infere-se que o mecanismo de ação para a S. aureus envolve solubilização parcial da membrana, e não há relação entre pungência e atividade bactericida, pois a pimenta dedo-de-moça, que é menos pungente, teve maior efeito do que a malagueta. Depreende-se, também, que a ação de extratos de pimenta deve depender da interação com receptores na membrana, o que explica porque o uso de tais extratos tem sido principalmente em aplicações tópicas. / This study is aimed at understanding the interaction of capsaicinoids with cell membranes using model systems. Among the alkaloids derived from plants of the genus Capsicum, capsaicin and dihydrocapsaicin account for 90% of capsaicinoids, which are used as analgesic and anti-inflammatory due to their interaction with specific receptors. The neuropharmacological mechanism has been well studied, but the non-neural mode of action has not been elucidated. Here, we use crude extracts (EBs) of malagueta and dedo-de-moça chilli peppers, which are surface active, and affected Langmuir monolayers of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidyl glycerol (DPPG). Such effects did not depend on the charge, since EB from dedo-de-moça interacted more strongly with DPPC than with DPPG, while the opposite applied for malagueta. In addition, there was no significant difference between the two EBs. For Langmuir monolayers representing the bacteria S. aureus, both EBs affected the surface pressure isotherms and the polarizationmodulated infrared reflection-absorption spectroscopy (PM-IRRAS) data, without significant distinction between dedo-de-moça and malagueta. However, in the leakage measurements with liposomes the EB from dedo-de-moça was more efficient in rupturing the liposome, which is consistent with the bactericidal activity for S. aureus. In fact, the minimum inhibitory concentration (MIC) was 0.13 mg mL-1 for dedo-de-moça and 4.0 mg mL-1 for malagueta. For the Langmuir monolayers mimicking the E. coli membrane, the EBs interacted much in the same way, while the EB from dedo-de-moça caused larger leakage in liposomes. There was no bactericidal effect of the EBs. This is explained by the fact that gram-negative bacteria, such as E. coli, have a protective outer layer of liposaccharides (LPS). In monolayers representing LPS, there was little incorporation of EBs, from which one infers that the EBs cannot cause disruption of the LPS layer. Taking all these results together, it appears that the mechanism of action for S. aureus involves partial solubilization of the membrane. Furthermore, there is no relationship between pungency and bactericidal activity because dedo-de-moça, which is less pungent, had greater effect than malagueta. It seems also that the action of pepper extracts must depend on the interaction with membrane receptors, which explains why the use of such extracts has been essentially in topical applications.

O estudo da interação de moléculas orgânicas com a água é de grande importância para diversas áreas das ciências, como por exemplo, na compreensão de sistemas biológicos. Uma elevada diversidade de estudos nesta área já foram realizados, no entanto, alguns aspectos do estudo de filmes finos de aminas anfifílicas não foram satisfatoriamente esclarecidos. Este trabalho aborda filmes de Langmuir de octadecilamina em função do pH e da força iônica de subfases aquosas, investigando a estrutura da monocamada orgânica e sua interação com a água, em diversos graus de ionização. Foi utilizada a Espectroscopia Vibracional por Geração de Soma de Frequências (Sum Frequency Generation - SFG), uma técnica que utiliza o princípio da óptica não-linear para estudar superfícies e interfaces, com o objetivo de investigar a conformação molecular dos filmes de Langmuir de aminas anfifílicas de cadeias longas, seu grau de ionização e a estrutura da água interagindo com a monocamada orgânica. Pôde-se acompanhar a ionização da monocamada em função do pH da subfase pela intensidade do modo de estiramento do grupo NH3+, e verificou-se que ela depende do pH local da interface, que por sua vez é alterado pela ionização da monocamada e é sensível à força iônica da subfase. A água interage com a monocamada via ligações de H com os grupos NH2/NH3+ e forma uma camada com alto grau de orientação perpendicular à superfície, mas relativamente desordenada do ponto de vista da rede de ligações de H (como na água líquida). A conformação das cadeias alquila depende da ionização do filme de Langmuir, sendo bastante compacta e ordenada a altos pHs (filme neutro) e mais expandida e desordenada a baixos pHs. Além disso, foram fabricados filmes de Langmuir-Blodgett (LB) de octadecilamina para comparar a estrutura dos filmes na superfície da água e em substratos sólidos. / The study of the interaction of organic molecules with water is of great importance for many areas of science, such as the understanding of biological systems. A large diversity of studies in this area have been performed, however some aspects of the study of thin films of amphiphilic amines have not been satisfactorily clarified. This dissertation focuses on Langmuir films of Octadecylamine as a function of pH and ionic strength of the aqueous subphase, investigating the structure of the organic monolayer and its interaction with water at various degrees of ionization. Sum-Frequency Vibrational Spectroscopy (SFG spectroscopy), a non-linear optical technique to study surfaces and interfaces, was used in order to investigate the molecular conformation of the Langmuir films, its degree of ionization and the structure of water interacting with the organic monolayer. The monolayer ionization could be monitored by the intensity of the NH3+ group stretch mode, and it was found that it depends on the local pH at the interface, which in turn is changed by the monolayer ionization and is sensitive to the subphase ionic strength. Water interacts with the monolayer by H bonds with NH2/NH3+ groups, forming a layer with a high degree of orientational order along the surface normal, but relatively disordered regarding the H-bonding network (like in the bulk liquid). The conformation of the alkyl chains depends on the monolayer ionization, being compact and all-trans at high pHs, but more expanded and disordered at low pHs. In addition, Langmuir-Blodgett (LB) films of Octadecylamine were fabricated to compare their structure on the solid substrate to those on the water surface

O ibuprofeno é um antiinflamatório não esteróide, com baixa solubilidade em água, que apresenta diversos efeitos colaterais, incluindo lesão gástrica e intestinal. Esses efeitos podem depender da interação com a membrana celular, o que nos motivou a investigar, na primeira parte deste trabalho, a incorporação do ibuprofeno em monocamadas de Langmuir como modelos de membrana celular. Monocamadas de dipalmitoil fosfatidil glicerol (DPPG) e dipalmitoil fosfatidil colina (DPPC) co-espalhadas com o ibuprofeno ou depositadas sobre subfases contendo o fármaco foram estudadas por meio das isotermas de pressão e potencial de superfície. Foram observados efeitos significativos para monocamadas de DPPC, particularmente na transição de fase líquido-expandida para líquido-condensada, com modificações relevantes na elasticidade da monocamada. Esses efeitos aumentaram com a concentração do ibuprofeno. Para os dois tipos de fosfolipídios, o ibuprofeno pôde penetrar na região hidrofóbica, o que foi confirmado por espectroscopia de reflexão e absorção no infravermelho com modulação da polarização (PM-IRRAS), indicando assim a presença de interações hidrofóbicas. A análise por microscopia no ângulo de Brewster (BAM) mostrou que o ibuprofeno impede a formação de grandes domínios de DPPC, enquanto que não foram observadas alterações significativas para o DPPG. A interação entre o ibuprofeno e o DPPG também foi confirmada após a imobilização da monocamada mista em filmes LB com alterações na absorção no UV-Vis da molécula de ibuprofeno. No que diz respeito às implicações biológicas, a ação farmacológica que depende diretamente da interação com a membrana deve ocorrer primeiramente em regiões neutras via penetração do ibuprofeno na região hidrofóbica da membrana celular. A segunda parte deste trabalho foi dedicada à interação de capsaicinóides extraídos da pimenta malagueta com monocamadas de Langmuir constituídas de DPPG e DPPC. A capsaicina é um potente analgésico de uso tópico, que pode causar dessensibilização no local de aplicação dependendo da dose e, portanto há interesse na sua incorporação em sistemas de liberação controlada, como os lipossomos. A técnica de Langmuir foi empregada para verificar essa possibilidade. Os capsaicinóides expandiram as monocamadas de DPPG e aumentaram sua elasticidade. As isotermas de potencial de superfície indicaram que os capsaicinóides provocam aumento de 10% no momento de dipolo numa concentração de 30% em mol. Para os filmes mistos de DPPC e capsaicinóides, a área mínima diminuiu e a elasticidade da monocamada aumentou. De acordo com as isotermas de potencial, os momentos de dipolo diminuíram para as monocamadas de DPPC independentemente da concentração de capsaicinóides. Esses resultados sugerem que as moléculas de DPPC são solubilizadas para a subfase na presença do fármaco. A partir destes resultados, conclui-se que os capsaicinóides podem ser incorporados em estruturas lipídicas, constituídas principalmente de DPPG, o que é relevante para uso em sistemas de liberação de fármacos. / Ibuprofen is a nonsteroidal anti-inflammatory drug, with low solubility in water, which exhibits side effects including gastric and intestinal injury, often irreversible. Some of these effects may depend on the interaction with the cell membrane, which motivated us to investigate the incorporation of ibuprofen in Langmuir monolayers as cell membrane models, in the first part of this thesis. Dipalmitoyl phosphatidyl choline (DPPC) or dipalmitoyl phosphatidyl glycerol (DPPG) monolayers co-spread with ibuprofen or deposited on ibuprofen-containing aqueous subphases were studied using surface pressure and surface potential isotherms. Significant effects were observed for DPPC monolayers, particularly at the liquid-expanded to liquid-condensed phase transition, with relevant changes in the elasticity of the monolayer. These effects increased with the ibuprofen concentration. For both types of phospholipids, ibuprofen could penetrate into the hydrophobic part of the monolayer, which was confirmed with polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), thus indicating the presence of hydrophobic interactions. BAM images showed that ibuprofen prevents the formation of large domains of DPPC, while no significant changes were observed for DPPG. The interaction between DPPG-ibuprofen was also confirmed for deposited layers in the form of LB films, with changes in the ibuprofen UV-Vis absorption. As for the biological implications, the pharmacological action depending directly on the membrane interaction should occur primarily with zwitterionic regions of the membrane via penetration of ibuprofen in the hydrophobic part of the monolayer. The second part of this thesis is dedicated to the interaction of capsaicinoids, extracted from malagueta pepper, with Langmuir monolayers of DPPC and DPPG. Capsaicin is a powerful analgesic of topical use, which can cause desensitization in the application site depending on the dose, and therefore there is interest in its incorporation in drug delivery systems, such as liposomes. The Langmuir technique was employed to verify this possibility. The capsaicinoids expanded the DPPG monolayer and increased its elasticity. Surface potential isotherms indicated that the capsaicinoids increased the average dipole moment by 10 % for 30 mol % of capsaicinoids. For the mixed films of DPPC and capsaicinoids, the minimum area decreased and the elasticity increased. According to the surface potential isotherms, the dipole moments decreased for DPPC monolayers regardless of the capsaicinoid concentrations. These results suggest that the DPPC molecules are solubilized into the subphase in the presence of the drug. From these results, it is concluded that the capsaicinoids can be incorporated into structures as the liposomes constituted mainly of DPPG, which is relevant for use in drug delivery systems.