Beyond the Diffuse Layer Theory: A Molecular Analysis of the Structural, Dynamical, and Mechanical Properties of Charged Solid/Liquid Interfaces

Abstract

This work focuses on the long-range gap detected during the swelling of charged solid/liquid interfaces neutralized by monovalent counterions. A molecular model is used to investigate the influence of the long-range electrostatic coupling on the thermodynamical properties of charged solid/ liquid interfaces at separations varying between 20 and 40 Å, corresponding to the transition between discontinuous crystalline swelling, detected below 20 Å, and the continuous osmotic swelling, detected at separations larger than 40 Å. While the primitive model perfectly reproduces the stability of these charged interfaces in the osmotic swelling regime, it becomes inappropriate in the crystalline swelling regime, since it neglects the organization of the confined water molecules which is responsible for the discontinuous crystalline swelling. In that context, we study the influence of the valence of the neutralizing counterions and the location of the electric charges embedded within the charged lamellae on the structural, dynamical, and mechanical properties of these interfacial systems. This molecular approach clearly identifies two different environments accessible to the neutralizing sodium counterions, corresponding respectively to solvent separated and contact ionic pairing. The mobility of the confined counterions and water molecules is quantified by numerical simulations of molecular dynamics. Finally, the interfacial free energy provides, without introducing any additional parameter, the first theoretical interpretation of the coexistence between swollen and collapsed charged interfaces neutralized by monovalent counterions, in agreement with experimental evidence.

Influence of Tetrahedral Layer Charge on the Fixation of Cesium in Synthetic Smectite

“…63,64 The hydration of the clay layers and their neutralizing cations, neglected by the diffuse layer theory, is responsible for such a behavior. 41 As displayed in Figure 5a, the free energy for Na-Sap-0.8 showing different local minima is compatible with the coexistence of various hydrates in water-saturated conditions, including large d 001 values. This concurs with the WAXS analyses performed in the present study (Table 1) and with previous X-Ray diffraction measurements on low-charge natural smectite.…”

“…It appears to vary with both layer charge and interlayer cation nature. Such behavior was previously shown to result from the surface localization of clay electric charges that significantly modifies the variation of interfacial free energy as a function of clay layer-to-layer distance . As shown from Table , accumulation of Cs + at the clay surface drastically reduces the apparent electric charge of clay particles, thus affecting their swelling behavior.…”

“…Our results do not match that prediction because the primitive model is very simplified and does not take into account the ion/water and ion/clay layer hydration forces, in addition to the water/water interactions. As previously shown, a molecular analysis of such charged interfaces leads to completely different balance between global electrostatic repulsion (including ion/ion, ion/clay, clay/clay, ion/water, clay/water, and water/water electrostatic couplings) and Lennard-Jones attraction (between the same couples of elements). As a consequence, the van der Waals dispersion forces, at the basis of the attractive part of the Lennard-Jones potential, overcomes the interfacial repulsion induced by the global electrostatic coupling.…”

“…Such behavior was previously shown to result from the surface localization of clay electric charges that significantly modifies the variation of interfacial free energy as a function of clay layer-to-layer distance. 52 As shown from Table 2, accumulation of Cs + at the clay surface drastically reduces the apparent electric charge of clay particles, thus affecting their swelling behavior. Furthermore, increasing the degree of tetrahedral substitutions enhances significantly the fraction of ionic pairing, thus reducing the apparent surface charge density of the clay layer.…”

“…39 By contrast, at small layer-to-layer distances (i.e., <40 Å), a molecular model of the clay/water interface is required to understand its thermodynamics. 40,41 For that purpose, the classical Clay Force Field 42 with modifications suggested by Ferrage et al 16 in addition to the SPC model of flexible water 43 Molecular Dynamics simulations. After thermalisation of the GCMC simulations, one equilibrium configuration of the confined ions and water molecules was selected for each layer charge, interlayer cation and layer-to-layer distance to initialize MD simulations.…”