Importance of Mass Transport and Spatially Heterogeneous Flux Processes for in Situ Atomic Force Microscopy Measurements of Crystal Growth and Dissolution Kinetics

It is well-established that important information about the dissolution and growth of crystals can be obtained by the investigation of step movement on single-crystal faces via in situ AFM. However, a potential drawback of this approach for kinetic measurements is that the small region of investigation may not be representative of the overall surface. It is shown that the investigation of local processes without accounting for the processes outside the region of interest can lead to significant misinterpretation of the data collected. Taking the case of gypsum dissolution as an example, we critically analyze literature data and develop 3 different finite element method models that treat in detail the coupled mass transport surface kinetic problem pertaining to dissolution processes in a typical AFM environment. It is shown that mass transport cannot be neglected when performing in situ AFM on macroscopic surfaces even with high-convection fluid cells. Moreover, crystal dissolution kinetics determined by AFM is mainly influenced by processes occurring in areas of the surface outside the region of interest. When this is recognized, and appropriate models are applied, step velocities due to dissolution are consistent with expectations based on macroscopic measurements, and the kinetic gap that is often apparent between nanoscale and macroscopic measurements is closed. This study provides a framework for the detailed analysis of AFM kinetic data that has wide utility and applicability.