DIRECTLY OBSERVING INTERFACIAL GEOCHEMISTRY AT HYDRATED CLAY INTERFACES WITH ATOMIC RESOLUTION

The electric double layer (EDL) that develops at mineral-water interfaces is a dominant control on important geochemical processes including ion adsorption and swelling of smectite clays. Because of the challenges in direct determination of EDL structure, theoretical models are based on idealized configurations. Here, we directly visualize EDL structures of montmorillonite (MMT) interfaces at atomic-resolution using cryogenic transmission electron microscopy (cryo-TEM). Amplitude contrast generated by the alignment of MMT layers along the beam direction yields high resolution images of MMT particles and pores in both two- and three dimensions.^1,2 Feature deconvolution from exit-wave reconstruction of defocus series allows for sub-Angstrom resolution of atomic columns of individual MMT layers and the aqueous solution adjacent to them, which is structured in three dimensions.³ Chemical and structural differences between layers saturated with different alkali cations can be resolved, such as the proximity of the cation to the clay surface (i.e., the Stern layer) and the number of cation planes separating adjacent layers, which increases with increasing cation hydration energy. Observations of the EDL at the hydrated surface of MMT particles provides stringent constraints on theoretical models and will enable a new perspective on aqueous geochemistry and reactions at solid surfaces.

1. Whittaker, M. L., Lammers, L. N., Carrero, S., Gilbert, B. & Banfield, J. F. Phase separation during ion exchange controls collapse and induces ordering in hydrated montmorillonite. Under Review
2. Whittaker, M. L., Comolli, L., Gilbert, B. & Banfield, J. F. Layer size disparity in hydrated montmorillonite creates multiscale porosity networks. Submitted
3. Whittaker, M. L., Ophus, C., Gilbert, B. & Banfield, J. F. Electrolyte structure at mineral interfaces. In preparation