Toward Accurate Adsorption Energetics on Clay Surfaces

Abstract

Clay minerals are ubiquitous in nature, and the manner in which they interact with their surroundings has important industrial and environmental implications. Consequently, a molecular-level understanding of the adsorption of molecules on clay surfaces is crucial. In this regard computer simulations play an important role, yet the accuracy of widely used empirical force fields (FF) and density functional theory (DFT) exchange-correlation functionals is often unclear in adsorption systems dominated by weak interactions. Herein we present results from quantum Monte Carlo (QMC) for water and methanol adsorption on the prototypical clay kaolinite. To the best of our knowledge, this is the first time QMC has been used to investigate adsorption at a complex, natural surface such as a clay. As well as being valuable in their own right, the QMC benchmarks obtained provide reference data against which the performance of cheaper DFT methods can be tested. Indeed using various DFT exchange-correlation functionals yields a very broad range of adsorption energies, and it is unclear a priori which evaluation is better. QMC reveals that in the systems considered here it is essential to account for van der Waals (vdW) dispersion forces since this alters both the absolute and relative adsorption energies of water and methanol. We show, via FF simulations, that incorrect relative energies can lead to significant changes in the interfacial densities of water and methanol solutions at the kaolinite interface. Despite the clear improvements offered by the vdW-corrected and the vdW-inclusive functionals, absolute adsorption energies are often overestimated, suggesting that the treatment of vdW forces in DFT is not yet a solved problem.

Interactions between large molecules pose a puzzle for reference quantum mechanical methods

“…Second, a stochastic quantum method that computes the energy for the many-electron wavefunction directly is known as fixednode diffusion Monte Carlo (FN-DMC) 15 . This method has seen a surge of use in recent years, particularly for predicting large molecules and periodic systems with non-covalent interactions 10,16,17 , such as molecular crystals 18,19 and adsorption on 2D materials 16,[20][21][22] . The accuracy and suitability of FN-DMC in complex non-covalently bound extended materials has been established through excellent agreement with a wealth of different experiments.…”

Hydrogen Bonding and Nuclear Quantum Effects in Clays

“…H-bonding plays a crucial role in their properties, either in their interactions with water 17,[22][23][24][25] or, in the case of so-called 1:1 clays whose layers comprise an H-bond donating and an H-bond accepting sheet, in the cohesive forces holding these layers together 13,26,27 . Theoretical studies have been carried out at multiple scales, from quantum-mechanical calculations intended to find the ground-state structure of clays and characterize the types of H-bonding [27][28][29][30][31][32] to molecular dynamics simulations, either using ab initio descriptions of the electron structure to study clay wetting 14,24 or acidbase chemistry 33,34 or larger-scale studies with classical forcefields 26,35 to model diffraction 36 or vibrational spectroscopy 37 experiments.…”

“…On the other hand, quantum-mechanical treatments are generally more transferable, but require compromises to be made, either in the level of theory used, in the size of system investigated or in the timescales of events to be studied. While methods like DFT are well-established for describing H-bonded systems, the results of these calculations depend sensitively on the level of theory used 27,31,[41][42][43][44] . In particular, the general consensus is that dispersion effects must be included to fully capture the physics of clays 27,31 .…”

“…While methods like DFT are well-established for describing H-bonded systems, the results of these calculations depend sensitively on the level of theory used 27,31,[41][42][43][44] . In particular, the general consensus is that dispersion effects must be included to fully capture the physics of clays 27,31 . Despite great improvements in computational treatments of clay materials in recent years, current methods are not able to describe all experiments equally well.…”

Oil-Water Transport in Clay-Hosted Nanopores: Effects of Long-Range Electrostatic Forces

“…Our modeling relies on the use of the ClayFF and OPLS All-Atom force field to describe illite 53,54 and organic components 55 respectively, while the flexible SPC model and the shake algorithm are applied to model water. Lorentz-Berthelot mixing rules describe the interactions between different atoms.…”