A combined X-ray absorption spectroscopy and molecular dynamic simulation to study the local structure potassium ion in hydrated montmorillonite

Atomistic local structure of potassium ion adsorbed in hydrated montmorillonite (MMT) was investigated based on a combination of an extended X-ray absorption fine structure (EXAFS) spectroscopy and classical molecular dynamics (MD) simulation. The accuracy of the representative MMT atomistic model with PCFF-INTERFACE force field was validated. MD-simulated EXAFS spectra were calculated from trajectories of hydrated MMT atomic coordinates and the results were in satisfactory agreement with corresponding experimental EXAFS spectra. Interlayer spacing determined by X-ray diffraction was consistent with the mono-layer hydrated MMT structure. The first coordination shell of K⁺ ion in monohydrated MMT was formed by 5 water oxygen atoms at an average K–O_W distance of 2.85 Å and the second coordination shell of 6 oxygen atoms from both sides of the closest silicate tetrahedral sheet at K–O_MMT = 3.41 Å. For hydrated K⁺-MMT, MD and EXAFS results confirm that K⁺ counter ions form the inner-sphere surface complex and that the adsorbed sites were located with the vicinity edge of a basal oxygen hexagonal cavity in the silicate tetrahedral sheets of MMT. For higher-layer hydrated MMT, K⁺ ions can form surface complexes that are inner-sphere, outer-sphere, and transient diffuse-layer species depending on the number of intercalated water in the clay. Water molecules are of less ordered arrangement in the monohydrated MMT due to the confinement effect from the clay surface. K⁺ counter ions in the single layer hydrates are almost trapped within the cavities of the basal planes surface.