Molecular dynamics simulation on surface hydration of different cationic montmorillonite

Abstract

In order to reveal the mechanism of surface hydration differences for different types of montmorillonite crystals, the hydration processes of sodium, potassium, and calcium montmorillonite were simulated by molecular dynamics. These simulation results show that with the increase of the number of water molecules, the interlayer spacing of montmorillonite expands in a step-by-step manner, accompanied by volume expansion, decrease in density, and increase in self-diffusion coefficients of water molecules and cations. In addition, as the water molecular layer accumulates, the peak values of the radial distribution function between Na⁺/K⁺/Ca²⁺ ions and Ow/Hw (oxygen or hydrogen atoms in water molecules) gradually decrease. The degree of polymerization of water intensifies before decreasing, while the elastic modulus and acoustic velocity are gradually decreasing. It is worth noting that Na⁺ ion shows the highest tendency to hydrate, followed by Ca²⁺, and then K⁺. Among the cations studied, Ca²⁺ ion has the highest hydration coordination number, hydration number and hydration radius. As a result, calcium montmorillonite exhibits the widest intensity range and the largest acoustic velocity. These findings can provide references for engineering practices such as oil and gas exploration, tunnel excavation, slope stabilization, and deep geological disposal.

Keywords

montmorillonite / surface hydration / molecular dynamics simulation / radial distribution function / self-diffusion coefficient / elasticity modulus

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