Determination of layer charge density in expandable phyllosilicates with alkylammonium ions: A combined experimental and theoretical assessment of the method

Layer charge deficit is a key crystal-chemical parameter for the classification and nomenclature of 2:1 phyllosilicates that also controls some their fundamental properties such as cation exchange capacity, expandability, water content, or rheology. The clay community has thus devoted significant efforts to determine this either from smectite/vermiculite chemical composition (the structural formula method) experimentally using alkylammonium method, theoretically allows determination mean layer density its distribution. In present study functional theory (DFT) molecular dynamics (MD) simulations are compared experimental X-ray diffraction data series synthetic saponites with ranging medium-charge smectite vermiculite. Consistency computed measured layer-to-layer distances confirms ability computational approaches accurately predict organization cations in interlayers. Thermalization DFT-optimized structure models through MD probing possible alternative interlayer configurations electrostatically-favored location ammonium heads above/below Al-substituted tetrahedra. results showed intermediate between those corresponding stable h1, h2, etc. described literature result interstratification different configurations. overall consistency method value consistent when revised equations proposed by Laird et al. (1989). validity appears however limited smectite-group minerals [layer ~0.5 ≤1.2 per O20(OH)4] most likely owing coexistence both “layered” paraffin-like higher values charge. addition, challenges quantitatively describe distributions expandable even complete cations.