Methods for performing atomic force microscopy imaging of clay minerals inaqueous solutions

Three methods were developed that allow for the imaging of any clay mineral in aqueous solutions with atomic force microscopy (AFM). The methods invol ve fixing the particles onto special substrates that do not complicate the imaging process, but hold the particles sufficiently so that they do not mo ve laterally or float away during imaging. Two techniques depend on electro static 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 techniq ue depends on adhesion to a thermoplastic film. The first electrostatic tec hnique involves a polished single crystal alpha-Al2O3 (sapphire) substrate. This was used successfully as a substrate for clay-sized minerals with hig h permanent layer charge localized on the basal planes (phlogopite and verm iculite) and when the AFM was operated in TappingMode to limit the lateral forces between the probe tip and the particles. However, electrostatic attr action 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 techniqu e 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 th e organic film is also a factor. The third technique, which does not depend on electrostatic attraction, fixes clay particles into the surface of a th ermoplastic adhesive called Tempfix. This has proven useful for fixing and imaging relatively large clay particles with well-defined morphology. The T empfix mount also requires imaging in TappingMode be cause the Tempfix is r elatively soft.