Coexistence of Multilayered Phases of Confined Water: The Importance of Flexible Confining Surfaces

Flexible nanoscale confinement is critical to understanding the role that bending fluctuations play on biological processes where soft interfaces are ubiquitous or to exploit confinement effects in engineered systems where inherently flexible 2D materials are pervasively employed. Here, using molecular dynamics simulations, we compare the phase behavior of water confined between flexible and rigid graphene sheets as a function of the in-plane density, ρ_2D. We find that both cases show commensurate mono-, bi-, and trilayered states; however, the water phase in those states and the transitions between them are qualitatively different for the rigid and flexible cases. The rigid systems exhibit discontinuous transitions between an (<i>n</i>)-layer and an (<i>n</i>+1)-layer state at particular values of ρ_2D, whereas under flexible confinement, the graphene sheets bend to accommodate an (<i>n</i>)-layer and an (<i>n</i>+1)-layer state coexisting in equilibrium at the same density. We show that the flexible walls introduce a very different sequence of ice phases and their phase coexistence with vapor and liquid phases than that observed with rigid walls. We discuss the applicability of these results to real experimental systems to shed light on the role of flexible confinement and its interplay with commensurability effects

Kinetic Monte Carlo Study on the Role of Heterogeneity in the Dissolution Kinetics of Glasses

The dissolution of silicate and other oxide glasses regulates many natural processes and plays an important role in many technological applications. Despite the fact that these glasses are inherently heterogeneous, current theories of glass dissolution exclusively rely on average descriptors of the structure or chemistry of the glass. The effect that spatial fluctuations in the local structure and chemical composition of a glass has on its dissolution kinetics is not well understood. Here, we use kinetic Monte Carlo (KMC) simulations to elucidate the role that heterogeneity plays in the dissolution kinetics of a glass model system. In single-phase particles, we find that heterogeneity, far from having a monotonic effect, can slow down or speed up the dissolution depending on the average extent of disorder of the glass. In two-phase particles, we find that the dissolution kinetics of phase-separated systems is governed by the less-soluble phase, while for well-mixed systems, the dissolution can be faster than that of the equivalent single-phase system because as the more soluble phase dissolves, it leaves behind a sparse structure of the less soluble phase which can then dissolve faster. We explain our findings based on both the mechanisms of dissolution observed in the KMC simulations and theoretical arguments

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. Barrier lowering upon slight increase in layer spacing (∼0.7 Å) during Cs⁺ exchange is an example of “chemical-mechanical coupling” that likely explains the observed sharp exchange fronts leading to interstratification. Interestingly, we find that these features are thermodynamically favored even in the absence of a heterogeneous layer charge distribution