Adsorption states of alkali metal ions it three kinds of 2:1 type clay minerals are systematically investigated via first-principles-based metadynamics. Our reconstructed free energy surfaces in a two-dimensional space of coordination numbers specifically employed as collective variables for describing the interlayer cations: show that an inner-sphere (IS) complex is preferentially formed for Cs+ in the 2:1 type trioctahedral clay minerals with saponite-like compositions, where lighter alkali metal ions show a tendency to form an outer-sphere one instead. The strong preference for an IS complex observed for Cs+ is found to result partially from the capability of recognizing selectively Cs+ ions at the basal 0 atoms with the Lewis basicity significantly enhanced by the isomorphic substitution in tetrahedral sheets.