The effect of nanoscale structure on interfacial energy

[摘要] Interfaces are ubiquitous in nature. From solidification fronts to the surfaces of biological cells, interfacial properties determine the interactions between a solid and a liquid. Interfaces, specifically liquid-solid interfaces, play important roles in many fields of science. In the field of biology, interfaces are fundamental in determining cell-cell interactions, protein folding behavior and assembly, and ligand binding. In chemistry, heterogeneous catalysts greatly increase reaction rates of reactions occurring at the interface. In materials science, crystallization and the resulting crystal habit are determined by interfacial properties, and interfaces affect diffusion through polycrystalline materials. In nanotechnology, much work on self-assembly, molecular recognition, catalysis, electrochemistry and numerous other applications depends on the properties of interfaces. The structure and properties of interfaces have been studied experimentally using a variety of techniques including various forms of microscopy, wetting measurements, and scattering techniques. Conventionally, the typical interface considered was highly homogeneous and exhibited a uniform composition and roughness. In contrast, many of the interfaces encountered in biological or nanotechnological systems have surfaces with a greater degree of complexity. While the surface may be compositionally homogeneous over a large area, these surfaces are structured and have a complex surface topology. On a mixed interface, several different chemical groups may be present on the surface, and the chemical composition can vary on a sub-nanometer length scale. Structured systems are inherently difficult to experimentally measure. Most techniques available to characterize interfaces average properties over the entire surface and are not sensitive to nanoscale variations. Furthermore, many of these techniques are incapable of distinguishing global, surface-dependent properties from artifactual influences. Many surface characterization techniques require a large, flat, smooth surface. Preparation of mixed interfaces is an experimental challenge as well as many mixed interfaces with nanoscale structure are present on objects that are themselves nanoscale, such as proteins. Several technological hurdles exist that limit the ability to produce nanoscale mixed interfaces large enough for conventional measurements. In this thesis, the effect of surface structure on wetting behavior was investigated. Interfaces can be characterized by the energy required to form them, a quantity called interfacial energy. Models have been developed to describe the interfacial energy of mixed interfaces for a wide range of surfaces. These models only account for the composition of the surface. The wetting behavior of mixed surfaces has also been related to artifact-dependent wetting effects (namely the effect of a boundary or asperity). No attempt has been made to incorporate surface structure into a global expression of interfacial energy. This thesis will study how the structure of an interface determines the resulting interfacial energy. Surfaces prepared with chemical domains of different length scales demonstrate and interfacial energy trend with significant deviation from the current best model. Specifically, the observed trend is non-linear, unlike the conventional model, and furthermore in some cases, is non-monotonic. These deviations are shown to stem from the surfaces;; intrinsic structure and are not an artifact of the measurement process or surface defects. The deviations from the predicted trend are explained by the molecular scale structure of the solvent. The two proposed mechanisms, cavitation and confinement, arise when surface features are smaller than a solvent-dependent length. With cavitation, nonwetting surface features below a size threshold are more wetting than would be expected. With confinement, wetting patches become less wetting as their dimensions are decreased. Molecular dynamics simulations support the proposed mechanisms. Additional experimental results provide further experimental evidence of the proposed molecular-scale wetting phenomena.

[发布机构] Massachusetts Institute of Technology