The effect of crystal size variation on the rate of dissolution - A kinetic Monte Carlo study

Crystal size is an important parameter for many geochemical processes, and is often observed to have a significant influence on crystal dissolution rate. Although size versus solubility relationships are often cast in thermodynamic terms (the critical radius), the mechanistic basis for size versus dissolution rate is not understood in any detail. Here we examine the influence of size on dissolution rate and the mechanistic origin of this relationship isolated from other parameters. We use a kinetic Monte Carlo model approach to observe atomistic changes in the dissolution of four simple cubic crystals (edge dimension d = 25, 64, 125, 187 unit cells). These simulations maintain constant solution boundary conditions representing an under-saturated, far-from-equilibrium state. We observe that surface area-normalized dissolution rates increase non-linearly with decreasing initial crystal size. This relationship can be understood mechanistically, in part as a coupling of kink- and step atom-site formation. Under conditions where these two constraints are in rough balance, the overall dissolution rate is maximized and mass removal is most efficient. This relationship reflects the mutual dependence of the formation of kink and step atoms, whose high detachment frequencies exert the dominant control over the dissolution process.