『Abstract
The role of electrolyte ions in the dissolution of orthoclase
(001) in 0.01 m NaOH (pOH〜2) at 84±1℃ is studied using a combination
of in-situ X-ray reflectivity (XR) and ex-situ X-ray reflection
interface microscopy (XRIM). The real-time XR measurements show
characteristic intensity oscillations as a function of time indicative
of the successive removal of individual layers. The dissolution
rate in 0.01 m NaOH increases approximately linearly with increasing
NaCl concentration up to 2 m NaCl. XRIM measurements of the lateral
interfacial topography/structure were made for unreacted surfaces
and those reacted in 0.01 m NaOH/1.0 m NaCl solution for 15, 30
and 58 min. The XRIM images reveal that the dissolution reaction
leads to the formation of micron-scale regions that are characterized
by intrinsically lower reflectivity than the unreacted regions,
and appears to be nucleated at steps and defect sites. The reflectivity
signal from these reacted regions in the presence of
NaCl in solution is significantly lower than that calculated from
an idealized layer-by-layer dissolution process, as observed previously
in 0.1 m NaOH in the absence of added electrolyte. This difference
suggests that dissolved NaCl results in a higher terrace reactivity
leading to a more three-dimensional process, consistent with the
real-time XR measurements. These observations demonstrate the
feasibility of XRIM to gain new insights into processes that control
interfacial reactivity, specifically the role of electrolytes
in feldspar dissolution at alkaline conditions.』
1. Introduction
2. Experimental details
2.1. Sample preparation and experimental conditions
2.2. X-ray reflectivity (XR)
2.3. X-ray reflectivity interface microscopy (XRIM)
3. Results
3.1. In-situ observations of orthoclase dissolution vs. electrolyte
concentration
3.2. Ex-situ observations of orthoclase (001) dissolution with
XRIM
3.2.1. Freshly cleaved surface
3.2.2. 15 min reaction
3.2.3. 30 and 58 min reactions
4. Discussion
4.1. Dissolution rate enhancement with added electrolyte
4.2. Effect of NaCl on the dissolution process
4.3. Interpretation of the XRIM images
5. Conclusions
Acknowledgments
Appendix A.
A.1. XRIM image processing to correct for different illumination
conditions
A.2. Reflectivity signal vs. interfacial topography and roughness
A.2.1. Single-layer roughness
A.2.2. Multiple layer roughness
References