Humid mineral surfaces often possess mobile surface ions that can conduct electrical charge. Previous studies have shown that surface-conductivity is highly dependent on environmental parameters such as relative humidity (RH), but the quantification of ion mobility has remained a challenging problem. A bettering understanding of surface ion mobility will be essential for many applications, including geophysical mapping techniques like spectral induced polarization. Emerging techniques in atomic force microscopy (AFM) such as electrostatic force microscopy (EFM) provide new avenues for investigating the mobility of surface-ions. In this study we use electrically biased AFM probes to induce charge perturbations on mineral surfaces and monitor the charging process. Subsequent EFM imaging allows us to directly image the charge perturbations and observe their relaxation over time. We show that the response is a complex process that includes fast ion migration directly under the probe tip, and slower migration across micron-scale distances. However, we can reproduce and interpret these results with the aid of new finite-element models of the tip-sample system. We conducted studies on calcite and feldspar across a range of relative humidifies and found that ion diffusivity is highly dependent on RH. In the case of calcite, we estimate that D increase by several orders of magnitude from ~10-15 m2/s at 10% RH to ~10-11 m2/s at 90% RH. In the case of orthoclase, we also investigated the impact of ion speciation, and observed significant changes. The extreme sensitivity of surface-ion mobility to ion speciation and RH is rationalized by new molecular simulations of electrically driven ion motion on humid mineral surfaces, which suggest that the ion mobility is determined by the potential of mean force for exciting ions from an inner-sphere coordination state of low-mobility into to a hydrated outer-sphere configuration of much greater mobility.

Mobility of ions on humid mineral surfaces: New insights from electrostatic force microscopy

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