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

A coarse-grained model of clay colloidal aggregation and consolidation with explicit representation of the electrical double layer

KNOWLEDGE GAP

The aggregation of clay minerals in liquid water exemplifies colloidal self-assembly in nature. These negatively charged aluminosilicate platelets interact through multiple mechanisms with different sensitivities to particle shape, surface charge, aqueous chemistry, and interparticle distance and exhibit complex aggregation structures. Experiments have difficulty resolving the associated colloidal assemblages at the scale of individual particles. Conversely, all-atom molecular dynamics (MD) simulations provide detailed insight on clay colloidal interaction mechanisms, but they are limited to systems containing a few particles.

SIMULATIONS

We develop a new coarse-grained (CG) model capable of representing assemblages of hundreds of clay particles with accuracy approaching that of MD simulations, at a fraction of the computational cost. Our CG model is parameterized based on MD simulations of a pair of smectite clay particles in liquid water. A distinctive feature of our model is that it explicitly represents the electrical double layer (EDL), i.e., the cloud of charge-compensating cations that surrounds the clay particles.

FINDINGS

Our model captures the simultaneous importance of long-range colloidal interactions (i.e., interactions consistent with simplified analytical models, already included in extant clay CG models) and short-range interactions such as ion correlation and surface and ion hydration effects. The resulting simulations correctly predict, at low solid-water ratios, the existence of ordered arrangements of parallel particles separated by water films with a thickness up to ∼10 nm and, at high solid-water ratios, the coexistence of crystalline and osmotic swelling states, in agreement with experimental observations.

Mesoscale Simulations Reveal How Salt Influences Clay Particles Agglomeration in Aqueous Dispersions

The aggregation of clay particles is an everyday phenomenon of scientific and industrial relevance. However, it is a complex multiscale process that depends delicately on the nature of the particle-particle and particle-solvent interactions. Toward understanding how to control such phenomena, a multiscale computational approach is developed, building from molecular simulations conducted at atomic resolution to calculate the potential of mean force (PMF) profiles in both pure and saline water environments. We document how it is possible to use such a model to develop a fundamental understanding concerning the mechanism of particle aggregation. For example, using molecular dynamics simulations conducted at the mesoscale in implicit solvents, it is possible to quantify the size and shape of clay aggregates as a function of system conditions. The approach is used to emphasize the role of salt concentration, which directly affects the potentials of the mean forces between kaolinite particles. While particle agglomeration in pure water yields large aggregates, the presence of sodium chloride in the aqueous brine leads instead to a large number of small aggregates. These results are consistent with macroscopic experimental observations, suggesting that the simulation protocol developed could be relevant for preventing pore blocking in heterogeneous porous matrixes.

Approaches and Perspective of Coarse-Grained Modeling and Simulation for Polymer-Nanoparticle Hybrid Systems

Molecular modeling and simulations have emerged as effective and indispensable tools to characterize polymeric systems. They provide fundamental and essential insights to design a product of the required properties and to improve the understanding of a phenomenon at the molecular level for a particular system. The polymer-nanoparticle hybrids are materials with outstanding properties and correspondingly large applications whose study has benefited from this new paradigm. However, despite the significant expansion of modern day computational powers, investigation of the long time and large length scale phenomenon in polymeric and polymer-nanoparticle systems is still a challenging task to complete through all-atom molecular dynamics (AA-MD) simulations. To circumvent this problem, a variety of coarse-grained (CG) models have been proposed, ranging from the generic CG models for qualitative properties predictions to more realistic chemically specific CG models for quantitative properties predictions. These CG models have already delivered some success stories in the study of several spatial and temporal evolutions of many processes. Some of these studies were beyond the feasibility of traditional atomistic resolution models due to either the size or the time constraints. This review captures the different types of popular CG approaches that are utilized in the investigation of the microscopic behavior of polymer-nanoparticle hybrid systems. The rationale of this article is to furnish an overview of the popular CG approaches and their applications, to review several important and most recent developments, and to delineate the perspectives on future directions in the field.

A Coarse-Grained Model for the Mechanical Behavior of Na-Montmorillonite Clay

Sodium montmorillonite (Na-MMT) is one of the most commonly found swelling clay minerals with diverse engineering and technological applications. The nanomechanical properties of this mineral have been extensively investigated computationally utilizing molecular dynamics (MD) simulations to portray the molecular-level changes at different environmental conditions. As the environmentally found Na-MMT clays are generally sized within hundreds of nanometers, all-atomistic (AA) MD simulations of clays within such size range are particularly challenging due to computational inefficiency. Informed from atomistic modeling, a coarse-grained (CG) modeling technique can be employed to overcome the spatiotemporal limitation. The current study presents a modeling strategy to develop a computationally efficient model of Na-MMT clay with a typical size over ≃100 nm by shrinking the atomistic platelet thickness and reducing the number of center-layer atoms. Using the "strain-energy conservation" approach, the force field parameters for the CG model are obtained and the developed CG model can well preserve in-plane tension, shear, and bending behaviors of atomistic counterparts. Remarkably, the CG tactoid model of Na-MMT, a hierarchical multilayer structure, can reproduce the interlayer shear and adhesion as well as d-spacing among the clay sheets as of atomistic one to a good approximation while gaining significantly improved computational speed. Our study demonstrates the efficacy of the CG modeling framework, paving the way for the bottom-up multiscale prediction of mechanical behaviors of clay and related minerals.

Ion exchange selectivity in clay is controlled by nanoscale chemical-mechanical coupling

Ion exchange in nanoporous clay-rich media plays an integral role in water, nutrient, and contaminant storage and transport. In montmorillonite (MMT), a common clay mineral in soils, sediments, and muds, the swelling and collapse of clay particles through the addition or removal of discrete molecular layers of water alters cation exchange selectivities in a poorly understood way. Here, we show that ion exchange is coupled to the dynamic delamination and restacking of clay layers, which creates a feedback between the hydration state of the exchanging cation and the composition of the clay interlayer. Particles with different hydration states are distinct phases with unique binding selectivities. Surprisingly, equilibrium achieved through thermal fluctuations in cation concentration and hydration state leads to the exchange of both ions and individual MMT layers between particles, a process we image directly with high-resolution transmission electron microscopy at cryogenic conditions (cryo-TEM). We introduce an exchange model that accounts for the binding selectivities of different phases, which is likely applicable to many charged colloidal or macromolecular systems in which the structural conformation is correlated with the activities of water and counterions within spatially confined compartments.