Pressure Solution at the Molecular Scale

Abstract

The topological evolution of the cleavage surface of a gypsum single crystal during its dissolution in a flowing undersaturated aqueous solution has been observed with an atomic force microscope. The matter transfer from solid to liquid proceeds through the migration of atomic steps. The step velocity has been measured and appears to depend on the force applied by the tip on the surface. Whereas the high force velocity enhancement is likely to stem from corrosive wear, the speed behavior at low force (<10 nN) differs drastically and can be interpreted as a consequence of the pressure solution of the crystal induced by the tip force. The step velocity evolution with the force obeys the known kinetic law of pressure solution. Hence these experiments enable us to evidence a first atomic mechanism at the origin of pressure solution.

Importance of Mass Transport and Spatially Heterogeneous Flux Processes for in Situ Atomic Force Microscopy Measurements of Crystal Growth and Dissolution Kinetics

“…The AFM scanned area (ROI) is represented by an area with well-defined and uniform step density, 1 to 4 µm -1 , typically in the range observed in AFM experiments, 32,44,45 surrounded by a high-density stepped zone (outer active area). As mentioned above, an additional inactive buffer area (varied in the range 1 -10 µm) was introduced between the two regions.…”

“…The gross dissolution rates calculated over the geometric surface (Gsurf) were thus estimated from: (solid circles) oriented steps. Data are from the literature, with the following references: red, 44 blue, 43 green 45 and black. 32 The data in Figure 2 and Table 2 clearly demonstrate how the step displacement velocities measured via in-situ AFM experiments, for dissolution studies under similar experimental conditions (saturation level), typically span at least an order of magnitude.…”

“…This 3D model investigates the influence of convection and the impact of the AFM tip/holder and general cell geometry on mass transport in a square ROI for a setup in which the bottom wall of the AFM fluid is formed by the crystal surface under investigation, as in a number of studies. 12,15,18,19,31,35,44,45 The surface concentration at the ROI is simulated at different flow rates and for different bulk concentrations. Dissolution of an ionic crystal is a stoichiometric reaction to satisfy the assumption of an electro-neutral solution and to maintain a constant charge at the crystal surface; as such, for illustrative purposes we reduce the mass transport model to the simulation of a single species.…”

“…This paper considers such a case of faster (dissolution) kinetics, but also a configuration with much larger crystals which are studied most widely by in-situ AFM. 12,15,18,19,31,35,45,46 We show that both aspects -faster kinetics and larger crystals-significantly complicate local mass transport and interfacial fluxes, with significant implications for the quantitative analysis of AFM dissolution (and, by extension, crystal growth) data.…”

“…Yet, there is not yet a general consensus from local studies. For example, several works [42][43][44][45] have reported step velocity values for dissolution from steps with the same orientation on the basal (010) plane of gypsum, which vary by a factor of ≈ 10, for apparently similar experimental conditions. Furthermore, as we highlight herein, local dissolution rates do not match macroscopic dissolution rates; local rates are typically lower by one to two orders of magnitude (vide infra).…”