Clay platelet orientation inside self-standing beidellite clay films: Effect of silica nanospheres and link with macroscopic mechanical resistance

Clay minerals and silica are essential components of soil. Understanding the multi-scale structure of a simple model system formed of (negatively charged) beidellite clay nanoplatelets and silica nanospheres is a first step in understanding the complex structure of soil. Due to their strong shape anisotropy, clay nanoplatelets exhibit preferential orientation at the microscopic level, both in aqueous dispersions and in dry deposits. Adding spherical colloids into the system changes the clay platelet organization. These changes are studied here for the case of self-standing clay films, using a small angle X-ray scattering (SAXS) beamline, equipped with a goniometer and a rotating device. We investigated orientation of clay nanoplatelets as a function of the nanoplatelet and nanosphere sizes, their surface charge and sphere/platelet number ratio. Two methodologies are developed to calculate the nanoplatelet order parameter 〈P2〉: (a) background contribution is subtracted from the azimuthal profile curve at a given wavevector q and (b) isotropic contribution is subtracted directly at the level of the 2D SAXS images. Smaller clay nanoplatelets (∼200 nm in lateral size) exhibit a better orientation than larger platelets (> ∼300 nm). Adding small positively and negatively charged nanospheres (diameter ∼ 30 nm) destroys the high degree of orientation of small clay platelets. Large spheres (diameter similar to the nanoplatelet size, ∼300 nm) destroy orientation for both types of clay platelets. A simple tensile strength setup allows to measure the breaking stress of the self-standing films to make the link between microscopic and macroscopic structure. Films of small clay platelets without spheres were mechanically the most resistant. Films with added positively charged spheres were stiffer than films with negatively charged spheres. The methodology presented here opens perspectives for the study of structure and mechanical response of clay deposits.