Solid-liquid interfaces are ubiquitous. Despite the unquestioned relevance, many scientific research fields rely on simplified or macroscopic descriptions, discarding the molecular nature of the constituents. However, it is the specific molecular interactions and the complex chemistry in the interfacial region that are fundamental in phenomena such as heterogeneous catalysis, electrochemistry, crystal growth, membrane transport and body-antibody recognition. In particular, the models used to describe the so-called electrical double layer of ionic distributions at charged surfaces in liquid, are based on continuum assumptions. For example, the Gouy-Chapman-Stern model assumes that a dense, homogenously distributed layer with a thickness of a hydrated ion (referred to as the "Stern layer"), is adsorbed onto the charged surface, proceeded by an exponentially decaying diffuse ionic cloud, in which ions are point charges in a continuous dielectric media. In this thesis, I will first illustrate how the assumption of a homogeneous ion lateral density in the Stern Layer fails to describe the actual solid-water interface of several relevant surfaces. Then I will show how the lateral organization of the ions indeed plays a role as important as the vertical modulation of the charge distribution. This is because the molecular nature of the ions, the surface, and the water molecules themselves each contribute to the precise lateral organization of the constituents. As result, the ions can reside on discrete surface sites, defined both laterally and vertically in space. Moreover, the water-mediated interaction can impart a correlation in the reciprocal lateral distribution of the ions. To achieve this level of description, high-resolution atomic force microscopy (AFM) is used. Unlike most of other experimental techniques, AFM is able to characterize interfaces locally at the atomic/molecular level. In fact, in the AFM small-amplitude regime, detection of even single ions adsorbed at the surface of the solid is possible by measuring the perturbation they induce on the local solvation environment. This capability represents an invaluable tool for the exploration of phenomena occurring at the Stern layer of solid-liquid interfaces. Following, I will investigate the properties of several solid-aqueous interfaces in the presence of different ionic concentrations and species, ranging from hard (mica) to soft systems such as organic self-assembled monolayers, lipid bilayers, and the dynamic surface restructuring of calcite when covered with fatty acid molecules. The experimental study, combined with molecular dynamic simulations, reveals a water-induced ion-ion attractive interaction for some ionic species. These results show that water alone is the driving force to induce order within the Stern layer, creating hydration-correlation effects on mica, self-assembled monolayers and possibly lipid bilayers. The local modulation of the hydration properties of the surface is fundamental to highly dynamic systems such as calcite, where steps act as nucleating points for the processes of dissolution and growth. The specific interaction of foreign ions and fatty acids modify these processes as well as the equilibrium between the crystal and the solution. This thesis provides clear experimental evidence of new mechanisms developing at solid-liquid interfaces paving the way for a deeper and more conscious understanding of the colloidal properties of materials.
EPFL2015
