Formation of Microcrystals, Micropuddles, and Other Spatial Inhomogenieties in Surface Reactions under Ambient Conditions:  An Atomic Force Microscopy Study of Water and Nitric Acid Adsorption on MgO(100) and CaCO₃(101̄4)

Abstract

In this study, atomic force microscopy (AFM) is used to image freshly cleaved MgO(100) and CaCO3(104) as these surfaces undergo reaction with water and nitric acid under ambient conditions of temperature, pressure, and relative humidity. The reaction of water and nitric acid results in the formation of hydroxylated and nitrated surfaces, respectively. It is clear from the AFM images that there are spatial inhomogenieties and surface features that form on micrometer and nanometer length scales as these reactions proceed. These features, which include hillocks, patches, microcrystallites, and micropuddles, are due to surface and phase segregation as a result of facile ion mobility in the presence of adsorbed water. In addition, instabilities and oscillations in the AFM images provide an indication of liquid formation and the deliquescence (i.e., a solid to liquid-phase transition) of nitrate salts as a function of relative humidity.

Humidity-induced restructuring of the calcite surface and the effect of divalent heavy metals

“…While humidity-induced restructuring of the calcite cleavage surface has been observed in prior studies [17,20,34], the present research provides new experimental observations that further help to understand the restructuring process. Specifically, results presented in this contribution indicate that (i) the extent of the restructuring increases with the humidity level for a given exposure time, (ii) the mass that composes the humidity-induced features appears to originate at step sites, and (iii) the presence of Cd on the calcite surface inhibits the restructuring, while Pb has no discernable effect.…”

“…Far fewer studies have investigated the dynamic behavior of calcite with scanning probe microscopy under conditions of variable humidity in the absence of visible liquid water, where prior studies have shown restructuring of the calcite surface [15][16][17][18][19][20]. Stipp and co-workers initially showed using AFM that the exposure of cleavage surfaces of calcite to a humid environment resulted in nanometer scale holes and hillocks of varying sizes on terraces and cleavage steps [20].…”

An investigation of water uptake on clays minerals using ATR‐FTIR spectroscopy coupled with quartz crystal microbalance measurements

“…Mashburn et al found that as the relative humidity increased, the uptake coefficient and HNO 3 content all increased. Enhanced uptake of nitric acid has also been reported on oxide [ Goodman et al , 2001], carbonate [ Al‐Hosney and Grassian , 2005; Goodman et al , 2000, 2001; Krueger et al , 2005; Santschi and Rossi , 2006] and authentic dust samples [ Hanisch and Crowley , 2001; Laskin et al , 2005; Underwood et al , 2001] with increasing relative humidity. Reaction rates were seen to increase by an order of magnitude or more at higher relative humidity compared to dry conditions.…”

Oxygen stable isotope fractionation behavior of cerussite and hydrocerussite: New results and reconciliation of the recent literature

“…However, the volume of hydrocerussite present within the samples was not sufficient to explain the entire discrepancy considering the measured difference between hydrocerussite and cerussite fractionation. Other factors such as surface hydroxyl groups associated with calcite (e.g., Al-Abadleh et al, 2005;Krueger et al, 2005) may influence the outer surface of crystals, but not the interiors where our measurements were made. Regardless of experimental failure or kinetic fractionation effects, the contamination of the original samples and the difficulty of demonstrating that the experiments reached isotopic equilibrium suggest that the use of cerussite oxygen isotope fractionation factors presented in Melchiorre et al (2001) be discontinued in favor of the empirically-calibrated fractionation factor presented in Gilg et al (2008) and this paper.…”