Mapping Charge-Mosaic Surfaces in Electrolyte Solutions Using Atomic Force Microscopy: New Chapter in Colloid Science

Friday January 22
Chem. Sci. & Engineering Room 101
10:00 a.m.

Presenter: Dr. Jaroslaw Drelich, Michigan Technological University, Department of Materials Science and Engineering

Abstract: Colloidal forces dominate stability of particles in aqueous environment and often dictate strategies in wet processing of minerals and other materials.  The most successful approach to the problem of net interactions between two interfaces in these systems was proposed by Derjaguin, Landau, Verway and Overbeek and is known as the DLVO theory.  This Theory treats the total interaction force between two surfaces in a liquid medium as an arithmetic sum of two components:  van der Waals and electrostatic (electrical double layer) forces.  The DLVO theory has been used as a meanfield approach, where only one surface potential and one Hamaker constant are used to describe the colloidal forces.  On a contrary, a vast majority of surfaces of particles and materials in technological systems are of a heterogeneous (mosaic) nature composed of microscopic and submicroscopic domains of different surface characteristics.  In these systems, the interactions can be dominated by heterogeneities rather than average surface character.  Attractions can be stronger, by orders of magnitude, than would be expected from the classical mean-field model when areaaveraged surface charge or potential is employed.  To detect heterogeneities in surface charge, analytical tools which provide accurate and spatially resolved information about material surface potential—particularly at microscopic and sub-microscopic resolutions—are needed. A novel AFM-based technique for mapping surface charge domains on heterogeneous surfaces was recently introduced by our research team.  It relies on recording colloidal force curves over multiple locations on the substrate surface using small probes.  The experiments are conducted in electrolyte solutions with different ionic strengths and pH values.  The force-distance curve measurements are carried out stepwise across phases of different surface characteristic.  Surface charge densities and surface potentials are then calculated by fitting the experimental data with a DLVO theoretical model.  The surface charge characteristics of the heterogeneous substrate are determined from the recorded colloidal force curves, allowing for the surface charge variation to be mapped.  In this presentation, the AFM technique will be briefly introduced and its use in determination of local surface charges for a multi-phase rock and bitumen will be reviewed.

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