Thermodynamics of ion exchange coupled with swelling reactions in hydrated clay minerals.

Crystalline hydrates of swelling clay minerals (smectites) exhibit a strong coupling between their ion exchange and hydration/dehydration reactions. The uptake or removal of water from smectite interlayers as a result of a change in the environmental conditions also leads to the partitioning of cations. Three factors, the solid ion composition, the solid basal spacing/water content, and the aqueous solution composition, are all implicated in controlling the thermodynamics of ion exchange.

Electrophoretic and potentiometric signatures of multistage CaCO nucleation.

Hypothesis: Calcium carbonate nucleation is often a complex and multistep process that is difficult to follow in situ. The time-resolved electrochemical and electrophoretic methods can provide a new insight into the nucleation pathway.

Experiments: Here, we used a combination of speciation calculations with time-resolved electrophoretic and potentiometric methods to monitor calcium carbonate precipitation from a slightly supersaturated solution.

A structural coarse-grained model for clays using simple iterative Boltzmann inversion.

Cesium-137 is a major byproduct of nuclear energy generation and is environmentally threatening due to its long half-life and affinity for naturally occurring micaceous clays. Recent experimental observations of illite and phlogopite mica indicate that Cs is capable of exchanging with K bound in the anhydrous interlayers of layered silicates, forming sharp exchange fronts, leading to interstratification of Cs- and K-illite. We present here a coarse-grained (CG) model of the anhydrous illite interlayer developed using iterative Boltzmann inversion that qualitatively and quantitatively reproduces features of a previously proposed feedback mechanism of ion exchange.

Dynamical inversion of the energy landscape promotes non-equilibrium self-assembly of binary mixtures.

When driven out of equilibrium, many diverse systems can form complex spatial and dynamical patterns, even in the absence of attractive interactions. Using kinetic Monte Carlo simulations, we investigate the phase behavior of a binary system of particles of dissimilar size confined between semiflexible planar surfaces, in which the nanoconfinement introduces a non-local coupling between particles, which we model as an activation energy barrier to diffusion that decreases with the local fraction of the larger particle. The system autonomously reaches a cyclical non-equilibrium state characterized by the formation and dissolution of metastable micelle-like clusters with the small particles in the core and the large ones in the surrounding corona.

Direct Exchange Mechanism for Interlayer Ions in Non-Swelling Clays.

The mobility of radiocesium in the environment is largely mediated by cation exchange in micaceous clays, in particular Illite-a non-swelling clay mineral that naturally contains interlayer K and has high affinity for Cs. Although exchange of interlayer K for Cs is nearly thermodynamically nonselective, recent experiments show that direct, anhydrous Cs-K exchange is kinetically viable and leads to the formation of phase-separated interlayers through a mechanism that remains unclear. Here, using classical atomistic simulations and density functional theory calculations, we identify a molecular-scale positive feedback mechanism in which exchange of the larger Cs for the smaller K significantly lowers the migration barrier of neighboring K, allowing exchange to propagate rapidly once initiated at the clay edge.