Assisted Desolvation as a Key Kinetic Step for Crystal Growth

Abstract

The crystallization of materials from a supersaturated solution is a fundamental chemical process. Although several very successful models that provide a qualitative understanding of the crystal growth process exist, in most cases the atomistic detail of crystal growth is not fully understood. In this work, molecular dynamics simulations of the morphologically most important surfaces of barite in contact with a supersaturated solution have been performed. The simulations show that an ordered and tightly bound layer of water molecules is present on the crystal surface. The approach of an ion to the surface requires desolvation of both the surface and the ion itself leading to an activated process that is rate limiting for two-dimensional nucleation to occur. However, desolvation on specific surfaces can be assisted by anions adsorbed on the crystal surface. This hypothesis, corroborated by crystallization and scanning electron microscopy studies, allows the rationalization of the morphology of barite crystals grown at different supersaturations.

Molecular dynamics simulations of the electrical double layer on smectite surfaces contacting concentrated mixed electrolyte (NaCl–CaCl2) solutions

“…simulations have provided insight into the behavior of water and ions near flat charged surfaces [2,18,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54]. In particular, these simulations have confirmed that ions confined between planar charged surfaces do form ISSC, OSSC, and diffuse swarm species [35] in agreement with most EDL models.…”

“…5a), in the diffuse swarm ( Fig. 5b), and in OSSCs ( We note in passing that metal and mineral desolvation both have been invoked as possible rate-limiting steps of metal adsorption and precipitation reactions [46]. In the case of Na + or Ca 2+ , Fig.…”

Ion desolvation as a mechanism for kinetic isotope fractionation in aqueous systems

“…Our finding of the isotopic mass dependence of k wex implies that any ligand-exchange reaction whose rate constant correlates with k wex [mineral dissolution (28,29), metal binding to organic ligands (26,27), and possibly metal attachment to mineral surfaces (13,18,19,21)] should exhibit significant kinetic isotope fractionation. For such reactions, the overall kinetic isotope fractionation (α kinetic ) has a maximum value in conditions where the forward reaction rate is much larger than the backward rate.…”

“…The last two steps entail a progressive desolvation of the ions; therefore, these steps may be rate-limited by solute dehydration (18,19). In support of this inference, molecular dynamics (MD) simulations of barium sulfate growth suggest that the kinetics of Ba 2+ attachmentwhich govern barite nucleation and growth (20)-may be ratelimited by the partial desolvation of the metal to form an innersphere surface complex (21). Likewise, experimental (22,23), theoretical (13,24), and molecular simulation studies (25) suggest that the kinetics of metal attachment at calcite surfaces may be related to the dehydration frequency of the metal near the surface.…”

Kinetics of crystal nucleation in ionic solutions: Electrostatics and hydration forces

“…The kinetics of new kink formation (surface nucleation) was hypothesized to depend on the frequency of water exchange between a building unit and a bulk fluid (Kowacz and Putnis, 2008). It has been shown that changing the properties of the aqueous solvation environment by organic additives as well as simple inorganic salts can result in modification of the reaction rates as well as modes of crystal dissolution and growth, surface features, bulk crystal morphology and impurity incorporation even if the thermodynamic driving force is kept constant (De Yoreo and Dove, 2004;Dove et al, 2005;Piana et al, 2006Piana et al, , 2007Kowacz et al, 2007). The influence of the ionic strength on precipitation kinetics was found to be in agreement with the Bronsted-Bjerrum theory that relates reaction rates with polar properties of the solvent (salt medium effect) and the consequential hydration of ions (Zuddas and Mucci, 1998).…”