Abstract
Aqueous dispersions of pure sodium and calcium smectite clays with platelet sizes on the order of a few hundred nanometers were characterized using a combination of cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). With monovalent sodium counterions the clay is dispersed as individual platelets, as seen by cryo-TEM, that order into a nematic phase. From SAXS a one-dimensional swelling of the clay in water is observed with the characteristic spacing hs = δ/ϕc, where hs is the separation between the platelets, δ = 1 nm is the effective platelet thickness, and ϕc is the clay volume fraction in the sample. In calcium montmorillonite, on the other hand, cryo-TEM images clearly show the presence of tactoids, where the platelets have aggregated into stacks with a periodic spacing of 2 nm. From imaging a large number of tactoids the distribution function f(N) for the number of platelets per tactoid was estimated, and the average number ⟨N⟩ ≈ 10. The characteristic 2 nm spacing as well as the small number of platelets per tactoid was also confirmed by SAXS. The present study demonstrates that cryo-TEM, with carefully prepared specimen, is a very useful technique to characterize clay dispersions, particularly in aggregated systems.
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