Hydrothermal Synthesis of Low Layer Charge Trioctahedral Smectite

Smectites are widely used in (geo-) technical applications and are important components of soils. A definition gap exists between the uncharged non-swellable pyrophyllite and talc (ξ = 0) and the low charged swellable clay minerals (smectites) with 0.2 ≤ ξ ≤ 0.6. Furthermore, no reliable measurement method exists for ξ < 0.2. A recent theoretical study on the hydration of smectites (Emmerich et al. 2018) based on the density functional theory (DFT) indicates the existence of stable dioctahedral 2:1 layer silicates with ξ < 0.2 and substitutions either in the tetrahedral or octahedral sheet that are swellable. Therefore, our focus is to synthesize low charged smectites. The Na2O-MgO-Al2O3-SiO2-H2O system was reacted for triocthedral smectite synthesis at 200℃, for 72 h, and with stoichiometric composition corresponding to ideal layer charge of 0.18. The XRD results showed that the synthesis of trioctahedral smectite was successful with a small particle size. Particle size will be confirmed by AFM measurements. The CEC indicates a low charge in the envisaged range. According to the principle of AAM method (Lagaly. 1981), under ideal assumptions, when the layer charge value is less than 0.2, the long-chain alkylammonium ions will only form a monolayer structure (basal spacing <17.7 Å) after being intercalated into the smectite layer, which can also be proved by measuring the d001 value of synthetic smectite. Similar results obtained through our experiments. References: Emmerich et al. (2018) The Journal of Physical Chemistry C 122, 7484−7493. Lagaly. (1981) Clay Minerals. 16, 1-21.