Nanomorphology and surface potential of layer silicates by scanning probe microscopy

The knowledge of the surface properties of layer silicates is of great importance to understand both basic and applicative technological issues, such as, for example liquid/surface interactions, microfluidity, friction or tribology and biomolecules self-assembly and adhesion. Recent developments of Scanning Probe Microscopy (SPM) have widened the spectrum of possible investigations that can be performed at a nanometric level on the surface of minerals. They range from physical properties such as surface potential and electric field topological determination to chemical and spectroscopical analysis both in air, in liquid or in a gaseous environment. After an introduction illustrating the theory and instrumentation which stand behind SPM, we present its recent achievements in the field of the characterization of nanomorphology and surface potential properties of minerals, in particular the behaviour of some layer silicates. Applications to mineral hydrophilicity/hydrophobicity determination, organic surface interactions of layer silicates, friction and nanotribology of minerals and adhesion issues on applicative basis is addressed. SPM was used to assess the force-curve, force-volume and surface potential characteristics of layer silicates by working in Electric Force Microscopy (static and dynamic EFM) and in Kelvin probe modes of operation. For instance, electric force microscopy allowed us to measure the thickness of silicate layers, to experimentally verify the layer polarity and, from frequency, amplitude, phase modulation and Kelvin analysis, to derive the electrostatic force experienced by the probe. We can then directly relate these measurements with the electrostatic force gradient at the mineral surface.