Microstructural and Swelling Properties of Ca and Na Montmorillonite: (In Situ) Observations with Cryo-TEM and SAXS

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 cryotransmission 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 onedimensional swelling of the clay in water is observed with the characteristic spacing h s = δ/ϕ c , where h s is the separation between the platelets, δ = 1 nm is the effective platelet thickness, and ϕ c 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.

Organically Modified Saponites: SAXS Study of Swelling and Application in Caffeine Removal

“…This intensification indicated that the hydroxyl groups interacted with the organic molecules during functionalization, probably via hydrogen bonding with the organic cations. 24 The bands at 2925, 2845, and 1474 cm −1 , attributed to CH antisymmetric and symmetric vibrations of the CH 2 species present in the functionalized materials, confirmed that APTS and CTAB functionalized the clay. These bands were more intense in the Because saponite is very sensitive to acidic media, octahedral Mg 2+ may have dissolved in the reaction medium, but this phenomenon did not seem to occur herein.…”

Towards understanding the microstructural and structural changes in natural hierarchical materials for energy recovery: In-operando multi-scale X-ray scattering characterization of Na- and Ca-montmorillonite on heating to 1150 °C

“…The hierarchical morphology of the clays comprising the interlayer nanopores between adjacent clay sheets, mesopores between stacked clay sheets, 48 and larger macropores between clay grains, correspond to identifiable q -ranges in the combined USAXS/SAXS regime as presented in Figure 1. This is useful for reference in interpreting the USAXS/SAXS data in terms of the microstructural and structural changes in Na- and Ca-montmorillonite on heating, considered in detail in the following sections.…”

“…In the present case, for much of the temperature range of interest (30 °C to 1000 °C), all scattering features were assumed to be oblate pores or voids with an aspect ratio of 0.2, which is consistent with the platelet-like morphology of clays based on TEM investigations. 48 The scattering is assumed to arise from voids because the samples comprise pressed powdered clay pellets that are solid clay material except for the various hierarchical pore morphologies. The calibrated scattering intensity from the i th pore population, I i ( q ), normalized to the incident X-ray flux and sample volume, is proportional to its overall volume fraction normalized to the total sample volume, Φ Vi , and the mean volume of its individual pores, V Pi (at low and intermediate q ), to its surface area per unit sample volume, S Vi (at high q ), and to the scattering contrast factor, |Δ ρ | 2 .…”

“…Our assumption that the voids are oblate spheroids with aspect ratio, β = 0.2, accounts for the platelet like morphology of clays. 48 We use this approach to follow the morphologies of the clays as they dynamically change with temperature up to 1000 °C, as evident in Figures 2 and 3. More details of the pore size distribution analysis, including the basis of the maximum entropy method, and how the 4 pore populations are identified, are provided in the Supporting Information.…”

Rheological properties of clay material at the solid/solution interface formed under quasi-free swelling conditions

“…For montmorillonites dominated by small, monovalent cations the swelling in low ionic strength solutions can be sufficient to effectively form a colloidal sol (Laird, 2006;Segad et al, 2010Segad et al, , 2012a but for montmorillonite dominated by multivalent cations the swelling is restricted to a maximum mean interlayer space on the order of a nanometer (Norrish, 1954;Kjellander et al, 1988a,b;Segad et al, 2012b).…”