Interaction Forces Measured by Atomic Force Microscopy for Muscovite and Carboxyl-modified Microspheres as a Function of Ionic Strength

This research used atomic force microscopy (AFM) to gain a better understanding of the interactions between muscovite mica and a model colloid used commonly throughout the existing colloid transport literature. The interaction forces between a single crystal muscovite mica disc and a 2.0-μm carboxyl-modified polystyrene latex microsphere attached to a tipless silicon nitride (SiN) cantilever were measured in KCl solutions of varying ionic strength (10, 30, 100, and 300 mM) using AFM. The three main objectives of this work were to 1) directly measure the interaction forces between muscovite mica and carboxyl-modified polystyrene latex microspheres, 2) evaluate the magnitude and significance of surface charge heterogeneities on the muscovite surface by making AFM measurements at multiple spatial locations, and 3) evaluate the feasibility of measuring the secondary energy minimum using conventional AFM instrumentation. AFM measurements were shown to be qualitatively reproducible only when strict surface preparation techniques were employed and a well-developed experimental protocol was adhered to. For measurements taken at various ionic strengths, repulsive forces were observed to decrease at higher ionic strengths. The tip-sample separation distance at which repulsive forces were first observed increased at lower ionic strengths. Both of these observations are in accord with theoretical predictions of Derjaguin-LandauVerwey-Overbeek (DLVO) theory. Measurements taken at several spatial locations on the muscovite surface suggested that there is heterogeneity in the muscovite surface properties. This heterogeneity was within the range of forces predicted by DLVO calculations using bounding values of ζ-potential and Hamaker constant. Variability in the tip-sample separation distance at the primary barrier was greater at a lower ionic strength than at a higher ionic strength for both AFM measurements and DLVO predictions. With the instrumentation used in this work, excessive noise and insufficient cantilever sensitivity made measurement of the secondary energy minimum infeasible. The results suggest that AFM is a useful tool for direct measurement of interaction forces and confirmation of theoretical predictions. The major results outlined above are augmented by the content of several appendices. Background AFM information is given, including a useful description on interpretation of theoretically-calculated and measured interaction forces. The process by which numerical artifacts arose and were dealt with is detailed. Finally, force measurements conducted between a silica surface and a 2.0-μm carboxyl-modified polystyrene latex microsphere, not included in the major results, are described and commented on, including the reason for their exclusion. A data CD with all raw and manipulated data used for the figures and discussions in this work, in addition to an extensive reference list, are provided to facilitate the use of this thesis project as a reference tool for future