How Water's Properties Are Encoded in Its Molecular Structure and Energies.

How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties

Fluids in Nanoscopic Space Revealed by Nuclear Magnetic Resonance (NMR)

The behavior of fluids in nanoscopic space is of enormous importance in various fields from biology to geophysics. At such length scales, fluids have a high surface-to-volume ratio and are subject to geometrical restrictions, leading to new phenomena that are not observed at bigger length scales. Fundamental understanding of the new sciences requires detailed characterizations of the structure, dynamics, and phase diagrams of confined fluids. Nuclear magnetic resonance (NMR) has been proven to be a versatile tool for this purpose. In this dissertation, NMR methods are developed for the study of the fluids in nanoscopic spaces. A combined methodology of NMR-detected isotherm and NMR imaging capability are implemented to study microscopic processes of water sorption on activated carbons. The NMR imaging capability with sub-nanometer scale spatial resolution is based on the natural field gradient inside the pore space of conjugated systems owing to the diamagnetic response of ring currents. Specifically, two distinct growth mechanisms are identified: gradual growth of water clusters and cooperative growth by pore-bridging. While the desorption process is predominantly associated with a single water cluster shrinking in size. The relationship between the macroscopic sorption isotherm and microscopic molecular configurations is elucidated as well. In situ NMR methods are then developed to study water effects on chemical warfare agent simulants (CWAS) capture in MOFs. Firstly, the CWAS adsorption capacity in MOFs is decreased by the presence of water. More importantly, we find that the preadsorbed water significantly decelerates the transport of CWAS, which could be a rate-limiting step in decontamination applications. Additionally, I investigate aqueous alcohols within hydrophobic nanopores using NMR. The self-assembly of water and alcohol into stable structures at hydrophobic interfaces is directly observed. This demixing phenomenon remains remarkably stable from -60 to 90 and its driving mechanisms are discussed. Moreover, microscopic segregation substantially influences macroscopic properties. Last but not least, an NMR-based isotherm technique is developed for quantifying hydrophilic and hydrophobic characteristics of reservoir rocks. Water isotherms of the pristine rocks provide information on two important rock properties: wettability and pore size distribution. Overall, the NMR approach offers unique insights into the molecular mechanisms of nanoconfined fluids.Doctor of Philosoph

Connecting theory and simulation with experiment for the study of diffusion in nanoporous solids

Nanoporous solids are ubiquitous in chemical, energy, and environmental processes, where controlled transport of molecules through the pores plays a crucial role. They are used as sorbents, chromatographic or membrane materials for separations, and as catalysts and catalyst supports. Defined as materials where confinement effects lead to substantial deviations from bulk diffusion, nanoporous materials include crystalline microporous zeotypes and metal–organic frameworks (MOFs), and a number of semi-crystalline and amorphous mesoporous solids, as well as hierarchically structured materials, containing both nanopores and wider meso- or macropores to facilitate transport over macroscopic distances. The ranges of pore sizes, shapes, and topologies spanned by these materials represent a considerable challenge for predicting molecular diffusivities, but fundamental understanding also provides an opportunity to guide the design of new nanoporous materials to increase the performance of transport limited processes. Remarkable progress in synthesis increasingly allows these designs to be put into practice. Molecular simulation techniques have been used in conjunction with experimental measurements to examine in detail the fundamental diffusion processes within nanoporous solids, to provide insight into the free energy landscape navigated by adsorbates, and to better understand nano-confinement effects. Pore network models, discrete particle models and synthesis-mimicking atomistic models allow to tackle diffusion in mesoporous and hierarchically structured porous materials, where multiscale approaches benefit from ever cheaper parallel computing and higher resolution imaging. Here, we discuss synergistic combinations of simulation and experiment to showcase theoretical progress and computational techniques that have been successful in predicting guest diffusion and providing insights. We also outline where new fundamental developments and experimental techniques are needed to enable more accurate predictions for complex systems

Polymer Structure and Dynamics in Nano-Confinements: Polymer Nanocomposiets and Cylindrical Confinement

Polymers have been used for variety of products for decades, and the usage of polymer products is still growing. Innovative methods (e.g. adding other materials) have been created to improve properties of polymer products to fulfill targeted requirements for applications and many of these strategies impose confinement on polymers. As nanotechnology and manufacturing technology advance, the confinement lengths keep shrinking and approaching the size of a single chain. While the final properties of polymer products are important for the applications, understanding how polymers behave at the microscopic scale is also critical for manufacturing and designing polymer products, especially when the manufacturing methods or the final states of polymers impose nano-confinement. To understand how polymers behave in nano-confinement, two main types of confinement are studied in this dissertation: polymer nanocomposites involving spherical and cylindrical nanoparticles and nanoconfinement directly imposed by impenetrable planar and cylindrical walls. Polymer structure can be affected when adding nanoparticles into polymer matrices, which may lead to a change in dynamics. Small angle neutron scattering is applied to study how polymer structure is affected by carbon nanotubes (CNTs). Polymer chains retain a Gaussian chain conformation, and the chain size expands (~ 30% for 10 wt% SWCNT loading) when the chain size (Rg) is larger than the radius of the filler (r) and the SWCNT mesh size is comparable to Rg. Chain expansion is not observed for MWCNT, where r ~ Rg. Moreover, when the SWCNT mesh is anisotropic the polymer conformation is anisotropic with greater expansion perpendicular to the SWCNT alignment, which is the direction with small mesh size. The temperature dependence of polymer tracer diffusion is investigated. In MWCNT/PS nanocomposites, a diffusion minimum is observed with increasing nanotube concentration at 7 temperatures from 152°C to 214°C. The diffusion minimum is shallower at higher temperature, which indicates the mechanism that slows polymer diffusion is less pronounced at higher temperatures. At fixed concentration the temperature dependence data fit the WLF equation. The temperature dependence of polymer tracer diffusion in silica/PS nanocomposites also obeys the WLF equation. However, the monotonic decrease of the tracer diffusion when silica concentration increases is more pronounced at higher temperature, which shows an opposite trend than the MWCNT/PS system. The thermal expansion coefficients of free volume (αf), obtained by fitting the temperature dependence data to the WLF equation, slightly increases when silica concentration increases. In contrast, the αf obtained from the time-temperature superposition of the rheology data decreases with silica concentration increases and shows an abrupt change at the percolation concentration of silica NPs. This finding suggests that the mechanical response of silica NPs contributes to the linear viscoelastic response. The impacts of nanoconfinement imposed by impenetrable planar or cylindrical walls were investigated by molecular dynamics simulations and experiments. The polymer conformation in thin film or cylindrical confinement is compressed parallel to the confining direction and slightly elongated perpendicular to the confining direction. The number of entanglement per polymer (Z) decreases as the pore diameter decreases. A theory, which assumes that the preferential orientation of the end-to-end vector can be directly transferred to the preferential orientation of primitive path steps, compares favorably to our simulations as a function of pore diameter. From the simulation, we also found that the local relaxation is accelerated along the cylindrical axis and is retarded perpendicular to the cylindrical axis. Combining the change in chain conformation, entanglement density, and the local relaxation, we found an increase for the center-of-mass polymer diffusion (Drep) in cylindrical confinement via the reptation model. The center-of-mass diffusion coefficients (DMSD) are also directly calculated from the mean-squared displacement in the diffusive regime, and are compared to Drep. At modest confinements, Drep agrees with DMSD, which suggests the applicability of the reptation model. At strong confinement, Drep \u3e DMSD implies the limitations of the reptation model. The center-of-mass diffusion coefficient (Dexp) is also measured experimentally using diffusing deuterated polystyrene into porous anodized aluminum oxide membranes pre-infiltrated by polystyrene. As the pore diameter decreases Dexp increases in qualitative agreement with the molecular dynamics simulations (Drep and DMSD). The local relaxations of polymers in cylindrical confinement are measured experimentally using QENS. When polystyrene is confined in cylindrical nanopores, the segmental relaxations slow down non-monotonically with pore size. This trend is also observed for EISF, which measured the fraction of non-diffusing hydrogen within the probing time scale of QENS. At last, we found that when d/Ree \u3e 2, hydrogen has the lowest MSD. When the pore size is decreasing to 2 \u3e d/Ree \u3e 1, MSD is slightly higher but still lower than that for bulk PS. When the pore size is further decrease to d/Ree \u3c 1, MSD decreases again. This non-monotonic change of MSD can be explained by combining the effect of cylindrical confinement on the local segmental relaxation and non-diffusing hydrogen. This thesis provides the first study of polymer structure in polymer nanocomposites with high-aspect ratio nanoparticles and the first systematic computer simulation study for polymer confined in cylindrical confinements. These studies contribute to the understanding of the physics of confined polymers and correlations between changes in structure and dynamics

Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å