Sept. 2015: charge regulation in AFM measurements - new publication in Nanoscale
Solid surfaces immersed in aqueous solutions almost invariably acquire a finite surface charge. PCF has been developing systematic methods to measure electrostatic forces in ambient electrolytes for several years. Simultaneously, extensive effort was invested into a systematic analysis of the data by a self-consistent numerical analysis of the force curves that couples the classical Poisson-Boltzmann theory of the electric double layer to adsorption and desorption equilibria. In the recent publication in Nanoscale by Cunlu Zhao et al., we describe a systematic manner to extract equilibrium constants of such adsorption/desorption reactions from the experimental data. The work shows in particular that consistent values for the equilibrium constants can be achieved for both deprotonation and cation adsorption reactions of monovalent ions on silica surfaces. For more complex gibbsite surfaces, quantitative local measurements of the surface charge with a lateral resolution of approximately 20nm are possible, yet, an unequivocal identification of the relevant surface speciation reactions is shown to require additional measurements over a broader range of fluid compositions, in particular a broader pH range. The publication is available here.
(left): Force distance curves between an AFM tip made of silica and a silica surface (red symbols) and a gibbsite surface (blue symbols). The data are analyzed using constant charge (CC) and constant potential (CP) boundary conditions (dashed lines) as well as using a self-consistent charge regulation boundary condition (solid lines) that reproduces the data down to the shortest distances. (right): merit function of fit quality for the equilibrium constants for deprotonation, pKH, and Na+ adsorption, pKC, on silica.