Structure of liquid–vapor interfaces: Perspectives from liquid state theory, large-scale simulations, and potential grazing-incidence x-ray diffraction

Grazing-incidence x-ray diffraction (GIXRD) is a scattering technique that allows one to characterize the structure of fluid interfaces down to the molecular scale, including the measurement of surface tension and interface roughness. However, the corresponding standard data analysis at nonzero wave numbers has been criticized as to be inconclusive because the scattering intensity is polluted by the unavoidable scattering from the bulk. Here, we overcome this ambiguity by proposing a physically consistent model of the bulk contribution based on a minimal set of assumptions of experimental relevance. To this end, we derive an explicit integral expression for the background scattering, which can be determined numerically from the static structure factors of the coexisting bulk phases as independent input. Concerning the interpretation of GIXRD data inferred from computer simulations, we extend the model to account also for the finite sizes of the bulk phases, which are unavoidable in simulations. The corresponding leading-order correction beyond the dominant contribution to the scattered intensity is revealed by asymptotic analysis, which is characterized by the competition between the linear system size and the x-ray penetration depth in the case of simulations. Specifically, we have calculated the expected GIXRD intensity for scattering at the planar liquid–vapor interface of Lennard-Jones fluids with truncated pair interactions via extensive, high-precision computer simulations. The reported data cover interfacial and bulk properties of fluid states along the whole liquid–vapor coexistence line. A sensitivity analysis shows that our findings are robust with respect to the detailed definition of the mean interface position. We conclude that previous claims of an enhanced surface tension at mesoscopic scales are amenable to unambiguous tests via scattering experiments

Structure of liquid--vapor interfaces: perspectives from liquid state theory, large-scale simulations, and potential grazing-incidence X-ray diffraction

Grazing-incidence X-ray diffraction (GIXRD) is a scattering technique which allows one to characterize the structure of fluid interfaces down to the molecular scale, including the measurement of the surface tension and of the interface roughness. However, the corresponding standard data analysis at non-zero wave numbers has been criticized as to be inconclusive because the scattering intensity is polluted by the unavoidable scattering from the bulk. Here we overcome this ambiguity by proposing a physically consistent model of the bulk contribution which is based on a minimal set of assumptions of experimental relevance. To this end, we derive an explicit integral expression for the background scattering, which can be determined numerically from the static structure factors of the coexisting bulk phases as independent input. Concerning the interpretation of GIXRD data inferred from computer simulations, we account also for the finite sizes of the bulk phases, which are unavoidable in simulations. The corresponding leading-order correction beyond the dominant contribution to the scattered intensity is revealed by asymptotic analysis, which is characterized by the competition between the linear system size and the X-ray penetration depth in the case of simulations. Specifically, we have calculated the expected GIXRD intensity for scattering at the planar liquid--vapor interface of Lennard-Jones fluids with truncated pair interactions via extensive, high-precision simulations. The reported data cover interfacial and bulk properties of fluid states along the whole liquid--vapor coexistence line. A sensitivity analysis demonstrates the robustness of our findings concerning the detailed definition of the mean interface position. We conclude that previous claims of an enhanced surface tension at mesoscopic scales are amenable to unambiguous tests via scattering experiments

The role of local structure in dynamical arrest

Amorphous solids, or glasses, are distinguished from crystalline solids by their lack of long-range structural order. At the level of two-body structural correlations, glassformers show no qualitative change upon vitrifying from a supercooled liquid. Nonetheless the dynamical properties of a glass are so much slower that it appears to take on the properties of a solid. While many theories of the glass transition focus on dynamical quantities, a solid's resistance to flow is often viewed as a consequence of its structure. Here we address the viewpoint that this remains the case for a glass. Recent developments using higher-order measures show a clear emergence of structure upon dynamical arrest in a variety of glass formers and offer the tantalising hope of a structural mechanism for arrest. However a rigorous fundamental identification of such a causal link between structure and arrest remains elusive. We undertake a critical survey of this work in experiments, computer simulation and theory and discuss what might strengthen the link between structure and dynamical arrest. We move on to highlight the relationship between crystallisation and glass-forming ability made possible by this deeper understanding of the structure of the liquid state, and emphasize the potential to design materials with optimal glassforming and crystallisation ability, for applications such as phase-change memory. We then consider aspects of the phenomenology of glassy systems where structural measures have yet to make a large impact, such as polyamorphism (the existence of multiple liquid states), aging (the time-evolution of non-equilibrium materials below their glass transition) and the response of glassy materials to external fields such as shear.Comment: 70 page

Spontaneous uptake dynamics of gaseous and liquid n-alcohols in nanoporous Vycor Glass

This work focusses on the experimental study of transport dynamics for different n-alcohols in the spatial confinement of a three-dimensional, isotropic network of meandering, cylindrical nanopores (radius r ≈ 5 nm) in monolithic Vycor glass. Due to the high surface-to-volume ratio, interfacial effects are particularly important in nanoporous materials. Two such phenomena are the adsorption of molecules from a surrounding vapour phase as well as a spontaneous capillary rise into the pore space when in contact to a liquid. Using dielectric spectroscopy, the dynamic uptake of molecules has been studied for both individual processes and a combination thereof. As a result of the strong interactions of n-alcohols with the silica surface of the Vycor matrix, a fixed boundary layer can form on the pore walls exhibiting properties different from the remaining pore filling. The equilibrium states and kinetics of adsorption can be used to determine the extent and mobility of this layer as a function of pore loading. Moreover, since the radius of the nanopores is comparable to molecular dimensions, a significant change in the hydrodynamic boundary condition ensues for liquid transport. This has been investigated for stationary, pressure-driven flows and spontaneous capillary flows as a function of the molecule length.Im Fokus dieser Arbeit steht die experimentelle Untersuchung der Transportdynamik verschiedener n-Alkohole in der beschränkten Geometrie eines dreidimensionalen, isotropen Netzwerks von mäandrierenden, zylinderförmigen Nanoporen (Radius r ≈ 5 nm) in monolithischem Vycor-Glas. Aufgrund des hohen Oberfläche-zu-Volumen-Verhältnisses sind Grenzflächeneffekte in nanoporösen Materialien von besonderer Bedeutung. Zwei solche Phänomene sind die Adsorption von Molekülen aus einer umgebenden Dampfphase sowie ein spontanes Kapillarsteigen in den Porenraum im Kontakt mit einer Flüssigkeit. Mittels dielektrischer Spektroskopie wurde die dynamische Aufnahme von Molekülen für die einzelnen Mechanismen und eine Überlagerung beider Prozesse untersucht. Durch starke Wechselwirkungen der n-Alkohole mit der Siliziumoxidoberfläche der Vycor-Matrix kann sich an den Porenwänden eine gebundene Grenzschicht ausbilden, die sich in ihren Eigenschaften von der restlichen Porenfüllung unterscheidet. Gleichgewichtszustände und Kinetik der Gasadsorption können zur Bestimmung der Ausdehnung und Mobilität der Wandlage in Abhängigkeit der Porenfüllung genutzt werden. Da der Radius der Nanoporen zudem vergleichbar mit molekularen Größenordnungen ist, ergibt sich für den Transport von Flüssigkeiten eine signifikante Änderung der hydrodynamischen Randbedingung. Diese wurde für stationäre, druckgetriebene Flüsse und dynamische Kapillarflüsse in Abhängigkeit der Moleküllänge ermittelt

Atomic-scale understanding of oxidation mechanisms of materials by computational approaches: A review

The urgent requirement of minimising the worldwide cost of corrosion, accompanied by the increasingly pivotal role of advanced oxide materials, highlights the importance of understanding the mechanisms of material oxidation at the atomic level, which could help us to precisely control the oxidation processes. Nowadays, we are able to model and predict how the surface structures of materials evolve during oxidation based on the cross-fertilisation of various computational techniques. This review first overviews the state-of-the-art first-principles and force-field-based approaches for modelling surface reactions. Then, classical theories and recent advances in understanding the atomic-scale oxidation of bulk materials are introduced, from the initial solid-gas interactions to the subsequent oxide film growth. Defect-promoted oxidation mechanisms will be discussed in detail. Finally, distinct oxidation mechanisms of nanomaterials are discussed, including nanoparticles, nanowires, and two-dimensional materials, which are significantly different from their bulk counterparts and could result in novel oxide nanostructures with unique properties. This review provides a systematic overview of the central role of computational techniques in probing the atomic-scale oxidation mechanisms, which could further guide the synthesis of oxide-based cutting-edge materials such as ultra-thin oxide films and hollow oxide nanostructures