L'étude des matériaux argileux a de nombreuses applications en génie civil et environnemental. Ces applications, telles que l'enfouissement des déchets nucléaires, les risques naturels liés au gonflement et au retrait des sols, et l'extraction d'hydrocarbures, posent des défis technologiques qui nécessitent de comprendre et de prédire le comportement mécanique des argiles, en particulier sur le long terme. Les argiles gonflantes sont des matériaux complexes, poreux et multi-échelles dont les propriétés sont très sensibles à l'eau. Dans cette thèse, nous cherchons à comprendre l'effet de l'eau sur les propriétés mécaniques à court et long terme des argiles. Notre stratégie est de combiner des simulations numériques à l'échelle du nanomètre et des expériences à l'échelle du micromètre afin de mieux comprendre l'interaction entre ces échelles. Nous avons effectué des simulations moléculaires pour quantifier l'effet de l'eau et du cation interfoliaire sur les propriétés de gonflement, élastiques et de fluage des feuillets d'argile, qui à cette échelle sont inaccessibles à l'expérience. Nous avons également effectué une étude comparative de différents modèles de feuillets d'argile de complexité croissante afin de mieux comprendre les interactions qui régissent la cohésion entre les feuillets d'argile. Nous avons mesuré expérimentalement les propriétés de films d'argile autoporteurs bien ordonnés. Nous avons montré l'effet de l'humidité relative et de la nature du cation interfoliaire sur les déformations de ces films d'argile. En particulier, nous avons quantifié le gonflement de ces films induit par l'humidité en combinant microscopie électronique à balayage environnementale et corrélation d'images numériques. Nous avons également effectué des essais de traction et de fluage sur ces films à humidité contrôlée. Nous avons comparé les données obtenues par nos expériences aux résultats des simulations moléculaires. Cette comparaison permet de discuter les mécanismes élémentaires de déformation et les échelles pertinentes pour la compréhension du comportement hydromécanique des argiles / The study of clay-based materials has many applications in civil and environmental engineering. These applications include underground nuclear waste disposal, the natural risks associated to the swelling and shrinkage of soils, and the extraction of hydrocarbons. They pose significant technological challenges that require to understand and to predict the mechanical behavior of clays, in particular on the long term. Swelling clays are complex porous multi-scale materials and their properties are very sensitive to water. In this thesis, we aim at understanding the impact of water on the short-term and long-term mechanical properties of clays. Our strategy was to combine numerical simulations at the scale of the nanometer and experiments at the scale of the micrometer to have a better insight of the interplay between these scales. We performed molecular simulations to estimate the effect of water and of the interlayer cation on the swelling, elastic and creep properties of clay layers, which are inaccessible to experiments at this scale. We also carried out a comparative study of various numerical models of increasing complexity in order to better understand the interactions that governs the cohesion between the clay layers. We measured experimentally the properties of well-ordered self-supporting clay films. We investigated the impact of relative humidity and of the nature of the interlayer cation on the mechanical properties of these clay films. In particular, we quantified the humidity-induced swelling of these films by using a combination of environmental scanning electron microscopy and digital image correlation. We also performed tensile tests and creep tests on these films at controlled relative humidity. We compared the data obtained by our experiments to the results of the molecular simulations. This comparison makes it possible to discuss the elementary deformation mechanisms and the scales relevant to the understanding of the hydromechanical behavior of clays

With recent advancements in the synthesis and characterization of polymeric materials, scientists are able to create multi-scale novel polymers with various cases of chemical functionalities, diversified topologies, as well as cross-linking networks. Due to those remarkable achievements, there are a broad range of possible applications of smart polymers in catalysis, in environmental remediation, and especially in drug-delivery. Because of rising interest in developing therapeutic drug binding to specific treating target, polymer chemists are in particular interests in design and engineering the drug delivery materials to be not only bio-compatible, but also to be capable of self-assembly at various in-vivo physiological stimulus. Both experimental and theoretical work indicate that the thermodynamic properties relating to the hydrophobic effect play an important role in determining self-assembly process. At the same time, computational simulation and modeling are powerful instruments to contribute to microscopic thermodynamics' understanding toward self-assembly phenomenon. Along with statistical approaches, constructing empirical model based on simulation results would also help predict for further development of tailored nano-structured materials. My Research mainly focused on investigating physical and chemical characteristics of polymer materials through molecular dynamics simulation and probing the fundamental thermodynamic driving force of self-assembly behavior. We tried to surmount technological obstacles in computational chemistry and build an efficient scheme to identify the physical and chemical Feature of molecules, to reproduce underlying properties, to understand the origin of thermodynamic signatures, and to speed up current trial and error process in screening new materials. / acase@tulane.edu

This research is aimed at studying the ionic liquid/n-pentanol interface via video-microscopy and molecular dynamic simulations. Understanding the interfacial phenomena and interfacial transport between ionic liquids and other liquids is of interest to the development and application of ionic liquids in a number of areas. One such area is the biphasic hydroformylation of alkenes to obtain alcohol and aldehyde, in which case ionic liquid is the reaction medium where a catalyst resides. The dissolution of an ionic liquid into an alcohol was studied by microscopically observing and measuring the shrinking of a micropipette-produced droplet in real time. Although microscopic investigation of droplet dissolution has been studied before, no attempt had been made to measure the di↵usion coefficient D of the droplet species in the surrounding medium. A key finding of this work is that the Epstein-Plesset mathematical model, which describes the dissolution of a droplet/bubble in another fluid medium, can be used to measure D. Other experimental studies of the ionic liquid/alcohol system include electrical conductivity and UV-visible spectroscopy measurements of solutions of 1-hexyl-3-methylimidazolium tetrafluoroborate in n-pentanol. Those experiments were done in order to understand the molecular state of the particular ionic liquid in n-pentanol, as well as obtaining the dissociation constant K of such weak electrolyte solution. The experimental results provide an entry to the assessment of ionic liquid interaction with n-pentanol at molecular scale. Subsequently, molecular dynamics simulation was implemented for the investigation of such interaction. The computation started with simulation of the bulk phase of 1-butyl-3-methylimidazolium tetrafluoroborate, an affine ionic liquid on which molecular simulations had already been reported. A generalized probability based on Fuoss approximation for the closest ion to a distinguished countercharge ion was developed. In addition to 1-butyl-3- methylimidazolium tetrafluoroborate, the generalization was tested also on tetraethyl ammonium tetrafluoroborate in propylene carbonate from low to high concentrations, and on the corresponding primitive model. Such generalization helps us understand paring of ions in electrolyte solution, especially for elevated concentrations. Two cases of 1-hexyl-3-methylimidazolium tetrafluoroborate ionic liquid/npentanol system were studied, which are (i) liquid-liquid interface; and (ii) solution of the former in the latter. Computation of biphasic interface revealed interaction at the liquid-liquid junction, e.g., the transport of molecules from one phase to another, and lead to evaluation of di↵usion coefficient that has good agreement with experimental measurement. The simulation of dilute electrolyte solution, i.e., an ionic liquid pair in n-pentanol, gives free energy change as a function of ion separation distance. The dissociation constant K was evaluated and found to be closed to experimental value that was obtained from solution conductivity measurement. The investigation of ion dynamics, especially the memory function transformed from velocity autocorrelation function, lead to the finding of dielectric friction in the system. Furthermore, precise evaluation of D gives satisfied agreement with experimental measurement from micropipette technique. / acase@tulane.edu

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

Els materials porosos s'utilitzen àmpliament en moltes branques de la ciència i tecnologia modernes com la catàlisi, la separació de mescles, la purificació de fluids i la fabricació de membranes. Per a que els sòlids porosos puguin aplicar-se amb èxit cal disposar d'una caracterització precisa de la superfície i de les propietats estructurals, així com també una bona comprensió del comportament físico-químic dels fluids dins dels porus. Alguns materials, com les zeolites, tenen estructures poroses ben definides, però d'altres, com els òxids porosos, carbons i vidres de porus controlat, són bastant amorfs. Per això, un tema clau i, sovint, complicat, és la caracterització adequada d'aquests tipus de materials. Durant molts anys, l'adsorció de gasos s'ha emprat per estudiar les propietats de sòlids porosos, degut a que és un mètode ràpid, simple i que proporciona prou informació. Es van desenvolupar molts mètodes per extraure dades sobre la porositat i les propietats de la superfície de materials a partir d'isotermes d'adsorció. En les dues últimes dècades, amb l'ajuda dels ordinadors, cada cop més i més ràpids, l'ús de les tècniques de modelat molecular ha anat guanyant rellevància. En aquest context, l'objectiu general d'aquest treball de tesi és desenvolupar eines a escala molecular emprant la mecànica estadística i aplicant-la a la caracterització de materials adsorbents.Després d'una breu introducció en el tema (capítol 1), en el capítol 2 presentem una revisió de la metodologia bàsica emprada en aquest treball. En el capítol 3 hem implementat la teoria funcional de la densitat de mesures fonamentals o FMT (de l'anglès, Fundamental-Measured density functional theory), publicada per Kierlik i Rosinberg, per descriure l'adsorció de molècules Lennard-Jones en porus cilíndrics. Pel que sabem, aquest és el primer cop que la teoria s'aplica a la geometria cilíndrica. L'exactitud de la teoria en predir isotermes d'adsorció i perfils de densitat de partícules es compara amb simulacions Monte Carlo en el col·lectiu gran canònic per un rang ample de mides de porus. Aquesta comparació mostra que la concordança és molt bona en tots els casos. Addicionalment, s'ha aplicat la teoria a l'adsorció en porus plans per estudiar la influència de la geometria del porus en aquest fenomen. Els resultats indiquen que el confinament de la geometria cilíndrica introdueix diferències significatives en la forma de les isotermes d'adsorció i els perfils de densitat. Aquestes diferències són rellevants a l'hora de caracteritzar materials porosos. Els resultats indiquen que té lloc un comportament per capes en el porus cilíndric més petit que s'ha considerat, mentre que l'adsorció en un porus pla de la mateixa grandària necessita un potencial químic molt més alt per aconseguir una adsorció significant. A mida que el diàmetre del porus augmenta, la influència de la geometria es fa cada cop menys important, encara que es pot observar una certa desviació en la transició de condensació capil·lar. Addicionalment, per porus més amples, obtenim una adsorció multicapa amb condensació capil·lar a potencials químics alts, amb el mateix comportament qualitatiu observat en ambdues geometries. Quan el diàmetre assoleix el límit on els efectes de curvatura ja no són rellevants, el comportament quantitatiu del porus cilíndric es redueix al mateix que el del porus pla. La formació d'una capa fina adsorbent en mides de porus intermèdies i grans sembla correspondre a una transició de fase termodinàmica de segon ordre, per al rang de paràmetres utilitzat i les condicions termodinàmiques estudiades. No obstant, els resultats semblen indicar una interrelació entre aquest comportament i la transició pre-mullada (de la paraula anglesa prewetting) que s'observa en geometries semi-infinites, especialment al voltant del punt final crític de la línia pre-mullada. L'efecte del confinament és molt important en aquest comportament crossover (de pas). De la comparació de càlculs FMT amb resultats de la teoria funcional de la densitat no local, concloem que la FMT és una eina excel·lent per a l'estudi del comportament de fluids en geometries cilíndriques.En el capítol 4 s'explica com hem aplicat la FMT juntament amb un mètode de regularització per estimar la distribució de mides de porus o PSD (de l'anglès, Pore-Size Distribution) de vidres porosos model. Hem escollit aquest material perquè va ser desenvolupat mitjançant tècniques de modelat molecular, i es pot comparar directament amb la teoria utilitzada en aquest treball. Un avantatge addicional d'aquests materials model, enfront els experimentals, és que, en el primer cas, la mida i forma dels porus són ben conegudes, així com també la posició dels àtoms en la superfície, esdevenint així un material perfecte per comprovar l'exactitud dels mètodes de caracterització teòrica disponibles. Com que hi ha diferents solucions de l'equació integral d'adsorció compatibles amb la isoterma d'adsorció experimental, i diversos factors poden amagar els defectes del model molecular, hem realitzat la caracterització d'una forma sistemàtica: primer hem comprovat l'exactitud de la FMT i el model de porus independent per predir les isotermes d'adsorció "experimentals" utilitzant la PSD ja coneguda per als materials. Això s'ha efectuat amb porus individuals plans i cilíndrics. En segon lloc, un cop la isoterma d'adsorció va ser reconstruïda amb èxit, vam invertir la isoterma d'adsorció integral amb un procediment de regularització. L'exactitud del mètode d'inversió s'ha comprovat també abans d'estimar la PSD de materials diferents. En últim lloc, un cop demostrat que el mètode és correcte, l'hem utilitzat per estimar la PSD de quatre materials. També hem estudiat la influència d'escollir alguns valors particulars de paràmetres moleculars per les interaccions fluid-fluid i sòlid-fluid en el comportament adsorbent d'aquests sistemes. Hem obtingut que el model de porus independent és adequat per als quatre materials investigats en aquest treball. La geometria plana sembla representar millor que la geometria cilíndrica el comportament adsorbent global. Pel que fa a la PSD obtinguda amb el nostre procediment, s'observa que les distribucions obtingudes mitjançant la inversió de la integral estan en millor concordança amb les distribucions geomètriques que les calculades amb el mètode Barrett-Joyner-Halenda (BJH). El locus del pic està situat a la mateixa mida de por, i tots ells són unimodals, mentre que les distribucions BJH mostren un màxim localitzat sistemàticament a porus més petits, estimant per sota la PSD del material, i no són unimodals. En quan a la geometria dels porus individuals que formen el material podem dir que, encara que la PSD és més ampla que les geomètriques, l'adsorció que es prediu per un conjunt de porus plans individuals està en un acord quasi quantitatiu amb la isoterma d'adsorció experimental.Finalment, en el capítol 5 exposem com hem caracteritzat tres mostres diferents de g­alúmina, una d'elles sense tractament i les altres dues calcinades en un forn durant unes hores a 823 i 1023K. Per fer-ho hem mesurat isotermes d'adsorció de nitrogen a 77.35K en un equip Micromeretics ASAP 2000. A més, hem aprofitat les PSD's proporcionades pel programari de l'equip emprant el mètode BJH. Hem calculat isotermes teòriques mitjançant l'aproximació FMT. Hem invertit les equacions integrals d'adsorció amb el mètode de regularització i, finalment, hem obtingut les PSD's per les tres mostres d'alúmina, i les corresponents isotermes d'adsorció pels tres materials. D'aquesta forma hem observat la influència de la calcinació de l'alúmina en la seva PSD. A més, hem comprovat l'exactitud del mètode FMT/de regularització de manera sistemàtica. Quan comparem les PSD's obtingudes amb les corresponents distribucions BJH, hem verificat que, en els dos primers casos (alúmina no tractada i alúmina calcinada a 823K), el mètode BJH estima per sota la mida dels porus, proporcionant una PSD desviada cap a mides més petites. En el cas de l'alúmina calcinada a 1,023K, en la que el procés de sinterització produeix que els porus més petits desapareguin, afavorint els més grans, les PSD's del mètode BJH i les PSD's de la FMT/regularització són molt semblants. Amb això es corrobora el fet conegut de que el mètode BJH és força acurat en la regió macroporosa. Finalment, hem predit la isoterma d'adsorció d'un fluid diferent (età) a una altra temperatura (333K), en un dels materials caracteritzats (alúmina no tractada), amb l'ànim d'establir la robustesa de la PSD obtinguda. La concordança obtinguda mostra que és possible utilitzar aquest mètode de caracterització i extrapolar els resultats a altres condicions, mentre s'empri un nombre suficient de mides de porus per calcular la isoterma desitjada, i els paràmetres d'interacció sòlid-fluid es triïn adequadament. / Los materiales porosos se utilizan en muchas ramas de la ciencia y la tecnología, por ejemplo, se usan como catalizadores, en la separación de mezclas, en la purificación de fluidos, y en la fabricación de membranas. Su aplicación adecuada requiere de la caracterización precisa de sus propiedades superficiales y estructurales, además del conocimiento del comportamiento fisicoquímico de los fluidos cuando se encuentran dentro de los poros. Algunos materiales, como las zeolitas, tienen estructuras porosas bien definidas, pero otros en cambio (óxidos porosos, carbones, vidrios porosos con tamaño controlado) son bastante amorfos. Por lo tanto, una caracterización correcta de los materiales porosos es un área de estudio muy importante, la cual en algunos casos es una tarea sencilla pero en la mayoría no. Durante muchos años la adsorción de gases ha sido empleada para estudiar las propiedades de sólidos porosos, dado que es bastante fácil, simple y se puede obtener mucha información. Se han desarrollado muchos métodos para interpretar los datos experimentales y determinar la porosidad, las propiedades superficiales y la distribución de los tamaños de los poros de los materiales a partir de las isotermas de adsorción. En las dos últimas décadas, con la ayuda de las computadoras cada vez más rápidas, se ha extendido mucho el uso las técnicas de la mecánica estadística para realizar esta tarea. En este contexto, el objetivo general de esta tesis consiste en desarrollar herramientas a escala molecular utilizando la mecánica estadística para la caracterización de materiales adsorbentes.Después de una breve introducción en el tema (capítulo 1), el capítulo 2 está dedicado a hacer una revisión de la metodología básica empleada en este trabajo. En el capítulo 3 hemos implementado la teoría funcional de la densidad de medidas fundamentales (FMT, del inglés Fundamental-Measure density functional theory) de Kierlik y Rosinberg para describir la adsorción de moléculas Lennard-Jones dentro de poros cilíndricos. Hasta donde sabemos, ésta es la primera vez que esta teoría es aplicada a geometría cilíndrica. La exactitud de la teoría en predecir las isotermas de adsorción y los perfiles de la densidad es verificada por comparación con simulaciones Monte Carlo en el colectivo Gran Canónico para un amplio intervalo de tamaños de poros, observándose una buena concordancia en todos los casos. Adicionalmente, la teoría ha sido aplicada a la adsorción en poros planos para estudiar la influencia de los poros en esta propiedad. Los resultados indican que el confinamiento de la geometría cilíndrica introduce diferencias significativas en la forma de las isotermas de adsorción y de los perfiles de la densidad. Estas diferencias son relevantes para la caracterización de los materiales porosos. Nuestros resultados indican que un comportamiento de formación de capa tiene lugar en el poro cilíndrico, mientras que la adsorción en un poro plano del mismo tamaño necesita un potencial químico mucho más alto para alcanzar una adsorción significativa. Cuando el tamaño de poro se incrementa, la influencia de la geometría se vuelve menos importante, pero aún se observa un cierto desplazamiento del lugar en el cual se da la transición de la condensación capilar. Adicionalmente, para poros más anchos, tenemos formación de multicapas con condensación capilar a potenciales químicos altos, observándose el mismo comportamiento cualitativo en ambas geometrías. Cuando el diámetro alcanza el límite en donde los efectos de la curvatura ya no son relevantes, el comportamiento cuantitativo de los poros cilíndricos y de los planos es muy similar. La formación de una fina película adsorbida a tamaños de poro grandes e intermedios parece corresponder a una transición de fase termodinámica de segundo orden, para el intervalo de parámetros usado y a las condiciones termodinámicas estudiadas. Sin embargo, los resultados encontrados parecen indicar que existe una relación entre este comportamiento y el de una transición de pre-mojado observada en geometrías semi-infinitas, especialmente en la vecindad del punto final crítico de la línea de pre-mojado. El efecto del confinamiento es muy importante en este comportamiento de transición. A partir de la comparación de los cálculos hechos con FMT y los hechos con la teoría funcional de la densidad no-local, concluimos que la FMT es una excelente herramienta para el estudio del comportamiento de los fluidos en geometrías cilíndricas confinadas.En el capítulo 4 hemos aplicado la FMT en combinación con un método de regularización para estimar la distribución de tamaños de poros (PSD, del inglés Pore-Size Distribution) de materiales modelo que imitan a los vidrios porosos. Hemos elegido este material en particular porque fue desarrollado con técnicas de modelado molecular, y se puede hacer una comparación directa con la teoría aquí usada. Una ventaja adicional de estos materiales modelo, con respecto a los materiales reales, es que en este caso la forma y tamaño de los poros se conoce exactamente, además de que se sabe la posición de los átomos en la superficie, convirtiéndolo en un material ideal para verificar la exactitud de los métodos de caracterización teóricos disponibles. Dado que existen varias soluciones de la ecuación integral de adsorción compatibles con la isoterma de adsorción experimental, y que varios factores pueden ocultar los defectos del modelo molecular, hemos hecho la caracterización de una manera sistemática: primero hemos probado la exactitud de la FMT y del modelo de poros independientes para predecir las isotermas de adsorción "experimentales" usando la PSD geométrica ya conocida para estos materiales. Esto ha sido hecho tanto con los poros cilíndricos como con los planos. En segundo lugar, una vez que la isoterma de adsorción fue reconstruida, invertimos la isoterma integral de adsorción con un procedimiento de regularización. La exactitud del método de inversión ha sido verificado antes de estimar la PSD de los diferentes materiales. Finalmente, una vez que se ha establecido que el método es correcto, lo usamos para estimar las PSD's de estos cuatro materiales. Hemos estudiado también la influencia de elegir diferentes valores de los parámetros moleculares para la interacción fluido-fluido y para la sólido-fluido en el comportamiento de adsorción en estos sistemas. Los resultados indican que el modelo de poros independientes es adecuado para los cuatro materiales aquí investigados. La geometría plana parece representar el comportamiento de adsorción global mejor que la cilíndrica. En cuanto a lo que las PSD's obtenidas con nuestro procedimiento se refiere, las distribuciones resultantes a través de la inversión de la integral presentan una mejor concordancia con las distribuciones geométricas que las calculadas con el método Barrett-Joyner-Halenda (BJH). El máximo del pico está localizado en el mismo tamaño de poro, y las distribuciones son unimodales, mientras que las BJH's muestran un máximo sistemáticamente localizado a poros más pequeños, subestimando las PSD's del material, y éstas no son unimodales. Respecto a la geometría de los poros individuales que conforman el material, se puede decir, a pesar de que las PSD's son más dispersas que las geométricas, que la adsorción predicha por una colección de poros planos individuales tiene una concordancia casi cuantitativa con la isoterma de adsorción experimental.Finalmente, en el capítulo 5 hemos caracterizado tres muestras diferentes de g­alúmina, una de ellas sin ningún tratamiento, y las otras dos calcinadas en un horno durante varias horas a 823 y a 1,023K. Para ello hemos medido isotermas de adsorción de nitrógeno a 77.35K en un equipo Micromeritics ASAP 2000. Adicionalmente, hemos usado las PSD's calculadas con el método BJH que proporciona el software del mismo equipo experimental para comparar. Hemos calculado las isotermas teóricas utilizando la FMT. Hemos invertido las ecuaciones integrales de adsorción con el método de regularización y, finalmente, hemos obtenido las PSD's para las tres muestras de alúmina, y las correspondientes isotermas de adsorción. De esta manera hemos podido observar la influencia de la calcinación de la alúmina en su PSD. Más aún, hemos probado la exactitud del método combinado FMT/Regularización de una manera sistemática. Cuando hemos comparado las PSD's obtenidas con las correspondientes BJH's, hemos verificado que en los dos primeros casos (alúmina sin tratamiento y alúmina calcinada a 823K) el método BJH subestima el tamaño de los poros, dando PSD's desplazadas a tamaños de poros más pequeños. En el caso de la alúmina calcinada a 1,023K, en la cual el proceso de sinterización ha producido la desaparición de los poros más pequeños en beneficio de los grandes, las PSD's BJH y las PSD's FMT/Regularización son muy similares. Con esto corroboramos el hecho conocido de que el método BJH es bastante exacto en la región de los macroporos. Para terminar, hemos predicho la isoterma de adsorción de un fluido diferente (etano) a una temperatura también diferente (333K) en uno de los materiales caracterizados (alúmina sin tratar) con la idea de comprobar sí la PSD obtenida es transferible a otras condiciones o no. La concordancia observada muestra que es posible usar este método de caracterización y extrapolar los resultados a otras condiciones, procurando que se utilice un número suficiente de tamaños de poro diferentes para calcular la isoterma deseada, y se elijan bien los parámetros de interacción sólido-fluido. / Porous materials are widely used in many branches of modern science and technology, such as catalysis, separation of mixtures, purification of fluids and fabrication of membranes. A successful application of porous solids requires a precise characterization of their surface and structural properties, as well as a good understanding of the physical and chemical behavior of fluids inside the pores. Some materials, such as zeolites, have well defined porous structures, but others, such as porous oxides, carbons and controlled-porous glasses, are quite amorphous. Therefore, a proper characterization of this kind of materials is an important topic, and more often than not, a complicated one. For many years, gas adsorption has been used to study properties of porous solids, since it is fast, simple and informative. Many methods were developed to extract information about porosity and surface properties of materials from adsorption isotherm data. In the last two decades, with the aid of the increasingly faster computers, the use of molecular modeling techniques has been gaining relevance. In this context, the general objective of this thesis is to develop tools at the molecular level using statistical mechanics for the characterization of adsorbent materials.After a brief introduction on the topic (chapter 1), chapter 2 is devoted to a review of the basic methodology employed in this work. In chapter 3 we have implemented the Fundamental-Measure density functional theory (FMT) due to Kierlik and Rosinberg to describe the adsorption of Lennard-Jones molecules in cylindrical pores. To our best knowledge, this is the first time that this theory is applied to a cylindrical geometry. The accuracy of the theory in predicting adsorption isotherms and density profiles is checked by comparison with Grand Canonical Monte Carlo simulations for a wide range of pore sizes, showing very good agreement in all cases. In addition, the theory has been applied to the adsorption in slit-like pores to study the influence of the pore geometry on this property. The results indicate that the confinement of the cylindrical geometry introduces significant differences in the shape of the adsorption isotherms and density profiles. These differences are relevant for the characterization of porous materials. Our results indicate that a layering behavior takes place in the smallest cylindrical pore considered, while the adsorption in a planar pore of the same size needs a much higher chemical potential to achieve a significant adsorption. As the pore size increases, the influence of the geometry becomes less important, although a certain shift in the capillary condensation transition can still be observed. Additionally, for wider pores, we obtain multilayer adsorption with capillary condensation at high chemical potentials, with the same qualitative behavior observed for both geometries. When the diameter size reaches the limit where the curvature effects are not of further relevance, the cylindrical pores reduce to the same quantitative behavior of the slit-like pores. The formation of a thin adsorbed layer at intermediate and large pore sizes seems to correspond to a second order thermodynamic phase transition, for the range of parameters used and the thermodynamic conditions studied. However, the results found seem to indicate some relationship between this behavior and the prewetting transition observed in semi-infinite geometries, especially in the vicinity of the critical end point of the prewetting line. The effect of the confinement is very important in this crossover behavior. From the comparison of Fundamental-Measure density functional theory calculations versus non-local density functional theory results, we conclude that the FMT is an excellent tool for the study of the behavior of fluids in confined cylindrical geometries.In chapter 4 we have applied the FMT in conjunction with a regularization method to estimate the pore-size distribution (PSD) of model porous glasses. We have chosen this particular material because it was developed with molecular modeling techniques, and a direct comparison can be made with the theory used here. An additional advantage of these model materials, versus experimental ones, is that in this case the size and shape of the pores is well known, as well as the position of the atoms in the surface, making it a perfect material to check the accuracy of the theoretical characterization methods available. Since there are several solutions of the adsorption integral equation compatible with the experimental adsorption isotherm, and several factors can hide defects of the molecular model, we have done the characterization in a systematic manner: we have first checked the accuracy of the FMT and the independent pore model for predicting the "experimental" adsorption isotherms using the geometrical PSD already known for the materials. This has been done with individual cylindrical and slit-like pores. Secondly, once the adsorption isotherm was successfully reconstructed, we inverted the integral adsorption isotherm with a regularization procedure. The accuracy of the inversion method has also been checked before estimating the PSD of the different materials. Finally, once the method has been proved to be correct, we used it to estimate the PSD of four materials. We have also studied the influence of choosing different values of molecular parameters for the fluid-fluid and the solid-fluid interaction on the adsorption behavior of these systems. We have obtained that the independent pore model is adequate for the four materials investigated here. The slit-like geometry seems to represent the overall adsorption behavior better than the cylindrical geometry. As far as the PSD obtained with our procedure is concerned, the distributions obtained by inversion of the integral are in better agreement with the geometrical distributions than the ones calculated with the Barrett-Joyner-Halenda (BJH) method. The locus of the peak is at the same pore size, and all of them are unimodal, while the BJH distributions show a maximum systematically located at smaller pores, underestimating the PSD of the material, and they are not unimodal. Regarding the geometry of the individual pores that form the material, we can say that, although the PSD is broader than the geometrical ones, the adsorption predicted by a collection of individual slit-like pores is in almost quantitative agreement with the "experimental" adsorption isotherm.Finally, in chapter 5 we have characterized three different samples of g­alumina, one of them without treatment and the others two calcined in a furnace during several hours at 823 and 1,023K. For this we have measured adsorption isotherms of nitrogen at 77.35K in a Micromeritics ASAP 2000 apparatus. Additionally, we have used the PSD's provided by the software of the experimental equipment using the BJH method. We have calculated theoretical isotherms by the FMT approach. We have inverted the adsorption integral equations with the regularization method and, finally, we have obtained the PSD's for our three samples of alumina, and the corresponding adsorption isotherms. In this way we have observed the influence of the calcination of alumina on its PSD. Moreover, we have tested the accuracy of the FMT/Regularization method in a systematic way. When we compared the PSD's obtained with the corresponding BJH distributions, we verified that in the two first cases (untreated alumina and alumina calcined at 823K) the BJH method underestimated the size of the pores, giving PSD's shifted to smaller sizes. In the case of alumina calcined at 1,023K, in which the sintering process has produced the disappearance of the smallest pores, favoring the wider ones, the BJH PSD's and the FMT/regularization PSD's perform very similar. With this, we corroborated the known fact that the BJH method is quite accurate in the macroporous region. Finally, we have predicted an adsorption isotherm of a different fluid (ethane) at a different temperature (333K) in one of the characterized materials (untreated alumina) with the aim of establishing the robustness of the PSD obtained. The agreement obtained shows that it is possible to use this characterization method and extrapolate the results at other conditions, provided that a enough number of different pore sizes are used to calculate the desirable isotherm, and the solid-fluid interaction parameters are well chosen.

The purpose of the work is to employ molecular simulation to further extend the understanding of Self-Assembled Monolayers (SAMs), especially as it relates to three particular applications: organic-inorganic composite membranes, surface treatments in Micro-Electro-Mechanical Systems (MEMS) and organic-surface-modified Ordered Mesoporous Materials (OMMs). The first focus area for the work is the use of SAMS in organic-inorganic composite membranes for gas separations. These composite membranes, recently proposed in the literature, are based on the chemical derivatization of porous inorganic surfaces with organic oligomers. Our simulations achieve good qualitative agreement with experiment in several respects, including the improvement in the overall selectivity of the membrane and decrease in the permeance when increasing the chain length. The best improvement in the overall solubility selectivity is reached when the chains span throughout the pore. The second application focus is on the use of SAMs as coatings in MEMS devices. The work focuses on the modeling of adhesion issues for SAM coatings at the molecular level. It is shown that as the chain length is increased from 4 to 18 carbon atoms, the adhesion forces between two monolayers at the same separations decreases. The third application focus is on the use of SAMs for tailoring surface and structural properties of OMMs, in particular, porous silicas. A molecular study of structural and surface properties of a silica material with a 5 nm pore size, modified via chemical bonding of organosilanes with a range of sizes (C4, C8 and C18) is presented. Grand canonical MC simulations are employed to obtain nitrogen adsorption isotherms for unmodified and modified MCM-41 material models. Furthermore, the density profiles of alkyl chains and nitrogen molecules are analyzed to clarify the differences in the adsorption mechanisms in unmodified and modified materials. The position of the capillary condensation steps gradually shifted to lower pressure values with the increase in size of the bonded ligands, and this shift was accompanied by a gradual disappearance of the hysteresis loop. As the length of the bonded ligands is increased, a systematic decrease in the pore diameter is observed and the multi-layer adsorption mechanism in modified model materials diminishes.

A new group of nanoporous materials, metal organic frameworks (MOFs), have emerged as a fascinating alternative to more traditional nanoporous materials for membrane based gas separations. Although hundreds of different MOF structures have been synthesized in powder forms, very little is currently known about the potential performance of MOFs as membranes since fabrication and testing of membranes from new materials require a large amount of time and resources. The purpose of this thesis is to predict the macroscopic flux of multi-component gas mixtures through MOF-based membranes with information obtained from detailed atomistic simulations. First, atomically detailed simulations of gas adsorption and diffusion in MOFs combined with a continuum description of a membrane are introduced to predict the performance of MOF membranes. These results are compared with the only available experimental data for a MOF membrane. An efficient approximate method based on limited information from molecular simulations to accelerate the modeling of MOF membranes is then introduced. The accuracy and computational efficiency of different modeling approaches are discussed. A robust screening strategy is proposed to screen numerous MOF materials to identify the ones with the high membrane selectivity and to direct experimental efforts to the most promising of many possible MOF materials. This study provides the first predictions of any kind about the potential of MOFs as membranes and demonstrates that using molecular modeling for this purpose can be a useful means of identifying the phenomena that control the performance of MOFs as membranes.

A major challenge affecting the likelihood of a new drug reaching the market is poor oral bioavailability derived from low aqueous solubility. Nanocrystals are rapidly becoming a platform technology to address poor solubility issues, although several challenges including stabilisation and control of particle size distribution for nanosuspensions still need to be addressed. The aim of this study was to revisit the simplest approach of re-precipitation and to identify the critical parameters, including the effect of different stabilisers as well as process conditions. We utilised a combined approach of both experiments and molecular modelling and simulation, not only to determine the optimum parameters but also to gain mechanistic insight. The experimental studies utilised three rather distinct, relatively insoluble drugs, the hypoglycaemic glibenclamide, the anti-inflammatory ibuprofen, and the anti-malarial artemisinin. The choice of crystal growth inhibitors/stabilizers was found to be critical and specific for each drug. The effect of the process variables, temperature, stirring rate, and the solute solution infusion rate into the anti-solvent, was rationalized in terms of how these factors influence the local supersaturation attained at the earliest stages of precipitation. Coarse grained simulation of antisolvent crystallisation confirmed the accepted two step mechanism of nucleation at high supersaturation which involves aggregation of solute particles followed by nucleation. Recovery of nanocrystals from nanosuspensions is also a technical challenge. A novel approach involving the use of carrier particles to recovery the nanocrystals was developed and shown to be able to recover more than 90% of the drug nanocrystals. The phase stability of nanocrystals along with bulk crystals for the model compound glycine was explored using molecular dynamics simulation. The simulations were consistent with experimental data, a highlight being the β phase transforming to the δ phase at temperature >400K and 20kbar respectively, as expected. Nanocrystals of α, β and γ glycine, however did not show any phase transformation at high temperature. In summary the study demonstrates that standard crystallization technology is effective in producing nanocrystals with the primary challenge being physico-chemical (rather than mechanical), involving the identification of molecule-specific crystal growth inhibitors and/or stabilizers. The developed nanocrystal recovery method should enable the production of nanocrystals-based solid dosage forms. The molecular simulation studies reveal that crystal-crystal phase transformations can be predicted for hydrogen-bonded systems.

In this paper we will discuss the first successful molecular simulation studies exploring the statesteady crystal growth of sI and sII methane hydrates. Since the molecular modeling of the crystal growth of gas hydrates has proven in the past to be very challenging, we will provide a brief overview of the simulation framework we have utilized to achieve heterogeneous growth within timescales accessible to simulation. We will probe key issues concerning the nature of the solid/liquid interface for a variety of methane hydrate systems and will make important comparisons between various properties. For example, the interface demonstrates a strong affinity for methane molecules and we find a strong tendency for water molecules to organize into cages around methane at the growing interface. The dynamical nature of the interface and its microfaceted features will be shown to be crucial in the characterization of the interface. In addition to the small and large cages characteristic of sI and sII hydrates, water cages with a 51263 arrangement were identified during the heterogeneous growth of both sI and sII methane hydrate and their potential role in cross-nucleation of methane hydrate structures will be discussed. We will describe a previously unidentified structure of methane hydrates, designate structure sK, consisting of only 51263 and 512 cages, and will also show that a polycrystalline hydrate structure consisting of sequences of sI, sII and sK elements can be obtained. In this paper we will also detail a variety of host defects observed within the grown crystals. These defects include vacant cages, multiple methane molecules trapped in large cages, as well as one or more water molecules trapped in small and large cages. Finally, preliminary results obtains for THF and CO2 hydrates will be presented and their behaviour contrasted to that of methane hydrate.

Molecular modeling has come a long way during the past decades and in the current thesis modeling of biological membranes is the focus. The main method of choice has been classical Molecular Dynamics simulations and for this technique a model Hamiltonian, or force field (FF), has been developed for lipids to be used for biological membranes. Further, ways of more accurately simulate the interactions between solutes and membranes have been investigated. A FF coined Slipids was developed and validated against a range of experimental data (Papers I-III). Several structural properties such as area per lipid, scattering form factors and NMR order parameters obtained from the simulations are in good agreement with available experimental data. Further, the compatibility of Slipids with amino acid FFs was proven. This, together with the wide range of lipids that can be studied, makes Slipids an ideal candidate for large-scale studies of biologically relevant systems. A solute's electron distribution is changed as it is transferred from water to a bilayer, a phenomena that cannot be fully captured with fixed-charge FFs.  In Paper IV we propose a scheme of implicitly including these effects with fixed-charge FFs in order to more realistically model water-membrane partitioning. The results are in good agreement with experiments in terms of free energies and further the differences between using this scheme and the more traditional approach were highlighted. The free energy landscape (FEL) of solutes embedded in a model membrane is explored in Paper V. This was done using biased sampling methods with a reaction coordinate that included intramolecular degrees of freedom (DoF). These DoFs were identified in different bulk liquids and then used in studies with bilayers. The FELs describe the conformational changes necessary for the system to follow the lowest free energy path. Besides this, the pitfalls of using a one-dimensional reaction coordinate are highlighted.