Methods for Performing Atomic Force Microscopy Imaging of Clay Minerals in Aqueous Solutions

Barry R. Bickmore1, Michael F. Hochella Jr.1, Dirk Bosbach2 and Laurent Charlet3
1 Department of Geological Sciences, 4044 Derring Hall, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 USA
2 Institut für Mineralogie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany
3 Environmental Geochemistry Group, L.G.I.T., B.P. 53, F-38041 Grenoble Cedex 9, France

Abstract: Three methods were developed that allow for the imaging of any clay mineral in aqueous solutions with atomic force microscopy (AFM). The methods involve fixing the particles onto special substrates that do not complicate the imaging process, but hold the particles sufficiently so that they do not move laterally or float away during imaging. Two techniques depend on electrostatic attraction under circumneutral pH conditions, between the negatively charged clay particles and the high point of zero charge substrate (either aluminum oxide or polyethyleneimine-coated mica) whereas the third technique depends on adhesion to a thermoplastic film. The first electrostatic technique involves a polished single crystal α-Al2O3 (sapphire) substrate. This was used successfully as a substrate for clay-sized minerals with high permanent layer charge localized on the basal planes (phlogopite and vermiculite) and when the AFM was operated in TappingMode to limit the lateral forces between the probe tip and the particles. However, electrostatic attraction between the sapphire surface and clay minerals such as smectite and kaolinite (low or no permanent layer charge) is not sufficiently strong to adequately fix the particles for imaging. The second electrostatic technique involves a polyethyleneimine-coated mica surface designed to immobilize a larger variety of clay minerals (phlogopite, vermiculite, montmorillonite, and kaolinite), and in this technique weak bonding between the clay and the organic film is also a factor. The third technique, which does not depend on electrostatic attraction, fixes clay particles into the surface of a thermoplastic adhesive called Tempfix. This has proven useful for fixing and imaging relatively large clay particles with well-defined morphology. The Tempfix mount also requires imaging in TappingMode because the Tempfix is relatively soft.

Key Words: Atomic Force Microscopy • CMS Source Clay KGa-1 • CMS Source Clay SWy-1 • Fluid Cell • Particles • Polyethyleneimine • Sapphire • TappingMode • Tempfix

Clays and Clay Minerals; October 1999 v. 47; no. 5; p. 573-581; DOI: 10.1346/CCMN.1999.0470504
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)