Concentration-Dependent Adsorption of CsI at the Muscovite–Electrolyte Interface

Abstract

The interfacial structure of muscovite in contact with aqueous CsI solutions was measured using surface X-ray diffraction for several CsI concentrations (2–1000 mM). At CsI concentrations up to 200 mM, Cs+ adsorption is likely hindered by H3O+, as both cations compete for the adsorption site above the muscovite hexagonal cavity. Above this concentration, more Cs+ adsorbs than is required to compensate the negatively charged muscovite surface, which means that coadsorption of an anion takes place. The I– anion does not coadsorb in an ordered manner. Moreover, the hydration ring and water layers do not change significantly as a function of the CsI concentration.

Ion-Specific and pH-Dependent Hydration of Mica–Electrolyte Interfaces

“…Qualitatively, this observation is consistent with the long residence times of ≈100 ms of Rb + ions on mica that were reported in recent AFM measurements 64 and with the slow ion-exchange rates in X-ray-based Na + –Rb + ion-exchange reactions. 19 According to the AFM studies 65 and recent X-ray measurements, 66 the mica surface is only partially covered by cations for approximately millimolar concentrations. The steepness of the adsorption isotherm in this concentration range in combination with lateral attractions promotes the formation of patches of adsorbed ions.…”

“…41−43 In the absence of added salt, hydronium ions are likely to compensate the intrinsic negative surface charge of mica, as has been suggested by other studies. 17,66 Interestingly, hydronium ions turn out to have a similarly destructive effect on the oscillatory hydration structure of the interfacial water as the bulky Rb + and Cs + ions. The amplitude of the oscillatory force component decreases with decreasing pH, and a slightly attractive monotonic contribution develops (Figure 3).…”

“…26,27,68 Similarly, the corresponding electron density profiles normal to the surface displayed a reasonable agreement with earlier X-ray reflectivity and surface diffraction. 20,66 Cations were found to exchange with the bulk at rates that decrease according to Na + > K + > Cs + > Li + , implying that the forces experienced by the AFM tip on typical measurement time scales (several milliseconds) represent thermally averaged configurations of both interfacial water and adsorbed ions.…”

Role of hydration energy and co-ions association on monovalent and divalent cations adsorption at mica-aqueous interface

“…Majority of studies focused on understanding the properties of ions at solid - aqueous interface have used muscovite mica surfaces in their work. The reasons are that (a) muscovite mica surfaces are easy to cleave, (b) produces atomically smooth surfaces, 22 (c) the framework carries a negative charge compensated by surface potassium ions (K + ), and (d) these surface (K + ) ions can be exchanged with other cations in the presence of aqueous salt solutions 23 – 26 . Experimental studies using surface force apparatus (SFA) on electrolyte solutions between two mica surfaces have observed short-range repulsive forces due to dehydration of ions beyond a critical concentration of salt solution 27 .…”

“…Measurements from the XRR technique provides structural information of interfacial water and ions perpendicular to the surface alone and does not provide in-plane behavior 23 . Recent surface X-ray diffraction (SXRD) studies have found co-ions as well in the Stern layer 23 , 26 . Recent AFM techniques on the mica-RbCl solution (0–4 M) are able to probe in the three dimensional structures of solutions on the surface but still has the limitation to distinguish ion hydration and free water molecules 31 .…”