The Density Functional Theory Study on the Adsorption and Diffusion Behavior of H2O Molecules on the Kaolinite (001) Surface in Soft Rock

Zhao, Jian. (China University of Mining and Technology) | Wang, Jia-Min (China University of Mining and Technology) | Gao, Wei (China University of Mining and Technology) | He, Man-Chao (China University of Mining and Technology)



Kaolinite is often a cause of deformation in soft-rock tunnel engineering, leading to safety problems. The mechanism of the deformation is closely related to the interaction between kaolinite and water molecules. In order to gain a better predictive understanding of the governing principles associated with this phenomenon, we investigated the adsorption of H2O on the kaolinite (001) surface using the density functional theory within the local-density approximation and a supercell approach at first. The coverage dependence of the adsorption structures and energetics was studied systematically for a wide range of coverage, [from 1/16 to 1 monolayers (ML)], and adsorption sites. The results showed that the preferred adsorption sites on the kaolinite (001) surface for H2O are the threefold hollow site with the adsorption energies ∼1.10 eV. The adsorption energy decreased with coverage, thus indicating the greater stability of surface adsorption and a tendency to form H2O islands (clusters) with increasing coverage. The results further revealed that the H2O does not adsorb on sixfold hollow site of the aluminum (001) face of the third layer of kaolinite, implying that water molecule is difficult to penetrate through the ideal kaolinite (001) surface. In addition, we calculated the energetic barriers for diffusion of H2O between the most stable and next most stable adsorption sites, which range from 0.073 to 0.129 eV. The results also showed that H2O are very easy to diffuse on kaolinite (001) surface. The other properties of the H2O /kaolinite (001) system, including the different charge distribution, the lattice relaxation, and the electronic density of states, were also studied and are discussed in detail.

1. Introduction

Soft rocks rich in clay minerals can cause harm to tunnel engineering because, when adsorbing water, the mechanical strength of the clay minerals is reduced, leading to deformation of the rocks. In order to gain a better predictive understanding of the governing principles associated with this phenomenon, the interaction between clay minerals and water molecules requires further investigation (Roland et al., 2011; Croteau et al., 2009). Kaolinite is one of the most abundant components in clay minerals. Understanding the interaction between kaolinite and water molecules is important to researchers in the fields of geophysics and geomechanics (Yoshihiko et al., 1999; He et al., 2009; Gupta and Miller, 2010; Yin and Miller, 2012). Due to the limitations of experimental methods, theoretical analysis of the mechanism from a microscopic point of view will help to solve the engineering problems. Computer simulation based on the density-functional theory (DFT) has been proven to be a powerful and reliable tool to study water-solid interfaces at the molecular level (Yang et al., 2004; Park and Sposito, 2004). The behavior of water at the kaolinite (001) surface using DFT has been investigated (Hu and Angelos, 2008). The adsorption and diffusion behavior of H2O molecules on kaolinite surface have not yet been established. So a deeper insight into the water on kaolinite surface, through detailed first-principles calculations, is needed.