Abstract
Adsorption of Rb+ on the (104) plane of single crystal calcite was investigated to estimate the charge of the ionic crystal calcite–water interface. The adsorbed Rb+ coverage was quantified as a function of Rb concentration (1–100 mM) in calcite-saturated solutions at pH 8.3 by using in situ resonant anomalous X-ray reflectivity in transmission cell geometry. The Rb+ coverages for all solution conditions were small with the maximum ion coverage (Γmax) of 0.12(4) Rb+/nm2 estimated by the best-fit Langmuir isotherm model. This result provides an estimate of the upper limit to the effective surface charge density of about −0.02 C/m2 considering that any Rb+ adsorption to the terrace plane is likely induced by electrostatic attraction. This charge density is significantly lower (by a factor of ≥10) than those estimated from macroscopic measurements, implying that any excess charge likely originates from surface defects.
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