Dissolution of Crystallites: Surface Energetic Control and Size Effects

Abstract

Traditional understanding of dissolution assumes that the reaction is spontaneous and continues until equilibrium is reached. This paper presents theoretical and experimental data to support a dissolution mechanism that involves the existence of critical conditions for dissolution, in which the reaction is accompanied by the formation of pits and the subsequent displacement of pit steps. The accompanying increase in surface roughness leads to changes in surface energy with losses of crystal mass that are positive rather than negative and the existence of critical dissolution conditions. Critical pits and dissolution steps are verified experimentally and a relationship between the size and rate of displacement of steps is also demonstrated, in which the rate decreases with size and approaches zero at a critical size, r*. These microscopic step dynamics are consistent with the observed size-effects in bulk dissolution, which cannot be explained using traditional dissolution theories. The observed size effects include self-inhibition, in which the dissolution rate decreases with extent of reaction, dissolution suppression, and periodic resumption. These interesting dissolution phenomena are only readily displayed when the sizes of dissolving crystallites fall in the same range as the critical size (i.e., within 50r*). It is interesting to note that natural biominerals and many nanoparticles fall into this category, so that their suspensions can be dynamically stabilized without dissolution in undersaturated supporting media. The current research implies that dissolution kinetics cannot be understood well without appealing to fundamental physical concepts about the energetic control of dissolution steps on a molecular level. A new dissolution model for crystallites is introduced systemically.

A novel determination of calcite dissolution kinetics in seawater

“…A 40‰ solution enrichment -$ 2 Á 10 À7 moles of carbonate dissolved -corresponds to about 30 monolayers of calcite dissolved, or a 15 nm change in surface height. This surface height is consistent with surface height changes measured in AFM studies of calcite dissolutions (Tang et al, 2004;Arvidson and Lü ttge, 2010) and are on the order of AFM tip sizes used in these experiments (4-50 nm; Teng (2004)). Thus, changes in surface area in our experiments are no greater than those observed in surface-based dissolution studies, and we can treat the surface area as constant throughout an experiment.…”

Bioreduction of hematite nanoparticles by the dissimilatory iron reducing bacterium Shewanella oneidensis MR-1

“…6) show that smaller hematite nanoparticles exhibit lower rates of Fe(III) reduction, and therefore lower dissolution rates, compared to the larger nanoparticles used in this study. While this observation is contrary to what may be expected in terms of dissolution, it is consistent with recent reports that show brushite (CaH-PO 4 Á2H 2 O) particles smaller than a critical size (defect spacing) actually show inhibited mineral dissolution (Tang et al, 2004). The exact effect that nanoparticle solubility and size has on nanohematite reduction by S. oneidensis MR-1 is not discernable from data presented here, but does warrant future exploration.…”

Size/Shape–Property Relationships of Non‐Carbonaceous Inorganic Nanoparticles and Their Environmental Implications

“…Below this radius, dissolution is inhibited. Such inhibition phenomena have been observed in a number of systems and particularly well studied for various biologically relevant calcium phosphates (Tang et al , 2003(Tang et al , , 2004a(Tang et al , , 2004b(Tang et al , , 2004c(Tang et al , , 2005Tang and Nancollas, 2002 ). It is therefore reasonable to expect the possibility of such inhibited dissolution occurring for nanoparticles of sparingly soluble compounds, because the nanoparticle dimensions may be below this critical radius.…”

“…Indeed, studies on various calcium phosphate minerals (Tang et al , 2001(Tang et al , , 2003(Tang et al , , 2004a(Tang et al , , 2004b(Tang et al , , 2004c(Tang et al , , 2005Tang and Nancollas, 2002 ) display a phenomenon of self -inhibited dissolution occurring primarily at the nanoscale, in which dissolution rates dwindle over time.…”