Molecular dynamics simulations of interlayer structure and mobility in hydrated Li-, Na- and K-montmorillonite clays

Abstract

Molecular dynamics (MD) simulations were carried out to study the hydrate structure and diffusion of water and cations in the clay interlayer region. The goal of this work was to examine the mechanisms by which the clay counterions influence the swelling and dispersion of shale. Three series of MD simulations were performed to study Wyoming montmorillonites containing either Na⁺, K⁺ or Li⁺ counterions and 100, 200 or 300 mg of adsorbed water per gram of clay. Here, the TIP4P interaction model for liquid water is used in the MD simulations. The TIP4P results are compared with simulations using the MCY interaction model, particularly for the K-clay hydrates. For the one- and two-layer K-hydrates, the counterion speciation and water structure observed are in agreement with the MCY model. However, we observe that the two-layer hydrate persists up to high water contents and that – contrary to the MCY model – the hypothetical three-layer hydrate is unstable, in agreement with experimental observations. If one notes that the one- and two-layer hydrates are only saturated at 56 and 96 water molecules, respectively, then the observed equilibrium layer spacings are in excellent agreement with experiment. Integration of the ion density profiles shows that the surface complexes represent 4.4 out of 6 ions for the K-hydrate and only 2 out of 6 ions for Na-hydrate. This means that K⁺ ions adsorb more strongly and screen the negatively charged surface more effectively than Na⁺ ions. Self-diffusion coefficients were calculated from mean-square displacement analysis of ions and water molecules. For the Na⁺ and Li⁺ cases, satisfactory agreement with experimental quasi-elastic neutron scattering diffusion coefficients was found. No experimental values are available for the K⁺ case, and therefore predictions are made in this report. Diffusion coefficients for water in K-smectite increase from 10⁻¹⁰ to 10⁻⁹ m² s⁻¹ with water contents increasing from 100 to 300 mg g⁻¹ of clay. We find that water diffusion rates in K-smectites are roughly twice as small as in Na- and Li-clays with the same water content. It appears that the microscopic diffusion coefficients calculated for interlayer diffusion of Na⁺ and K⁺ ions are two orders of magnitude smaller than the macroscopic values calculated from experimental work on hindered motion of water and ions through a clay membrane. This implies that the experimental clay film has a tortuous structure, with clay platelets aligned orthogonally to the main direction of mass transport, and a tortuosity factor of . The results show that the MD simulation technique is a valuable tool to study transport properties of water and ions in hydrated smectite clays.

Keywords:

• molecular dynamics
• clay swelling
• diffusion coefficient
• counterions and water
• tortuosity

Acknowledgements

I am grateful to Peter Coveney for initialising the research described in this paper and to Keith Refson for making the Moldy code available. I thank Neal Skipper, Christian Besson, Wim Briels and Bernadette Craster for stimulating discussions. John Cook is gratefully acknowledged for making Figure 1 available.