Abstract
We present a method to determine the local surface charge of solid-liquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces.
Highlights
In recent years, high resolution imaging and spectroscopy techniques in Atomic Force Microscopy (AFM) have generated unprecedented insights into structure and dissipation in liquids in the vicinity of solid surfaces
The novelty of our approach is twofold: (i) we demonstrate the applicability and significancy of the charge regulation concept for AFM measurements with sharp tips and correspondingly high lateral resolution. (ii) We go beyond the common constant regulation approach[30,31] and extract directly equilibrium constants of surface speciation reactions using the full non-linear Poisson
The force vs. distance curves calculated using the theoretical model depend on a number of parameters, including the radius of the AFM tip R, the Hamaker constant A, the site density Γ, the capacitance of the Stern layer Cs, and the equilibrium constants Ki of the surface speciation reactions considered
Summary
High resolution imaging and spectroscopy techniques in Atomic Force Microscopy (AFM) have generated unprecedented insights into structure and dissipation in liquids in the vicinity of solid surfaces. Certain organic liquids have attracted specific attention because of their model character and the simplicity of the dominant molecular interaction forces (e.g. van der Waals interactions), which gives rise – amongst others – to very pronounced and characteristic oscillatory solvation forces.[1,2,3,4,5] Compared to these systems, water and aqueous electrolytes are much more complex for several reasons including the strongly dipolar character of water molecules, the role of hydrogen bonding, the hydration of surfaces, and the almost unavoidable presence of ions.[6,7,8,9,10,11,12] In addition, solid surfaces, including AFM tips, typically acquire finite surface charges upon immersion into water These surface charges give rise to rather long range electrostatic forces that.
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