Activity: Talk or presentation › Oral presentation
We adopt atomic force microscopy (AFM) to investigate the chemistries of mineral surfaces in nanometer-confined electrolyte solutions. Firstly, AFM was used to measure interaction forces between tip and solid surfaces with nanometer separation in ambient electrolytes. Then a charge regulation (CR) boundary was formulated for the Poisson-Boltzmann equation to establish a linkage between the AFM measured force curves and the surface chemistries (e.g., pK values of surface (de)protonation and ion adsorption). Finally, we analyzed force vs. distance curves recorded between a silica tip and heterogeneous silica-gibbsite substrates in aqueous solutions of NaCl and KCl within the framework of CR boundary-complemented Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. By fitting experimental force vs. distance curves down to tip-sample separation of 2nm, we determined for both silica and gibbsite surfaces pK values of surface (de)protonation and ion adsorption. The various pK values determined from our AFM experiments are quite consistent with the macroscopic titration measurements tabulated in the literature. This indicates that AFM could be potentially used as a titration tool, but with an unprecedentedly high resolution.