Influence of interlayer cations on structural properties of montmorillonites: A dispersion-corrected density functional theory study

Abstract

Dispersion-corrected density functional theory calculations are performed to investigate the structural distortions and counterion positions of montmorillonites (MMTs) with different alkali (Li+, Na+, K+) and alkaline-earth (Mg2+, Ca2+, Sr2+) cations. The determined interlayer thickness varies in a quadratic function manner with the ionic radius of counterions. Moreover, as the size of counterion increases (from Mg2+ to K+), the tetrahedral sheet thickness increases and the octahedral sheet thickness decreases slightly. In addition, the counterions have notable effect on the tetrahedral sheet distortions and the intercalation of larger cations leads to smaller rotation and tilting of tetrahedral sheets. The counterion is in the region bounded by two opposite ditrigonal cavities but is not located symmetrically between them. The cation with larger radius deviates from the center of the ditrigonal cavity near it. The valence state also affects the location of counterion in the interlayer and the higher charge cation tends to lie closer to the middle of the interlayer space with respect to the cations with the similar size.