Fig. 1: Friction coefficient of Li-, Na-, K-, Rb-, and Cs-montmorillonites (Mnt) under dry and wet conditions.

Friction coefficients are measured by triaxial shear experiments at effective normal stress of 40 MPa, both dry and wet, by adding distilled water (DW) or brine (1 M salts). Horizontal axis indicates the ionic radii of interlayer cations determined from six-coordinated anions⁴⁶. A vertical dashed line indicates the radius of ditrigonal Mnt cavities (1.2 Å) calculated from the radius of the cavity (2.6 Å)⁴⁷ minus the ionic radius of six-coordinated oxygen ions (1.4 Å)⁴⁶. In the DFT calculations, the size of the cavity was relaxed to be stable depending on the cationic species, but there is no clear difference between Na- and K-Mnts in the radius of the average cavity (2.6 Å).