![]() The accuracy of these determinations has been questioned by a recent real-time study of NMF intercalation by XRD and near-infrared spectroscopy (NIR), cross-verified by thermal analysis (Andreou et al., 2021). Recently, Fashina and Deng ( 2021) suggested that, for the same size fraction, increasing stacking disorder decreases reactivity toward K-acetate intercalation and NaOH dissolution.Īll the aforementioned literature studies of intercalation capacity were based on XRD determinations of the relative intensity of the 001 reflections from pristine and intercalated interlayers. The low reactivity of kaolinite is often attributed to the fact that smaller particles display slow kinetics of intercalation (Deng et al., 2002 Lagaly et al., 2006 Uwins et al., 1993 Weiss et al., 1970). According to the literature, particle size seems to be the primary determinant of intercalation capacity. Bulk kaolinites exhibited yields between ~70 and 95%, whereas their finer fractions demonstrated systematically smaller yields, as low as ~30%. For example, the intercalation yield of nine kaolinites subjected to the same intercalation agent (N-methylformamide, NMF) and protocol were measured by Uwins et al. Smaller kaolinite particles intercalate less than their larger counterparts, if at all (Lagaly et al., 2006 and references therein). There is evidence that high-defect kaolinites have low intercalation yields, but low-defect samples with little tendency for intercalation are also known (Frost et al., 2002 Uwins et al., 1993). Distinguishing between the two is not straightforward because crystallite size is often related to crystal imperfection (Gomes, 1982 Rausell-Colom & Serratosa, 1987). Experimentally, the limited intercalation capacity of a given kaolinite should not be confused with the rate of its intercalation, despite the fact that both properties may exhibit a similar dependence on kaolinite type (Rausell-Colom & Serratosa, 1987).īoth the crystallite size and the stacking order of kaolinite have been considered as determining the intercalation capacity. Dedzo & Detellier, 2016 Seifi et al., 2016). This property determines the value of kaolinite in many technological applications (e.g. The amount of intercalation-recalcitrant material has been considered as an intrinsic, but poorly understood, property of the kaolin material. The intercalation capacity (otherwise referred to below as ‘yield’) of kaolinite for particular guest molecules should be considered at virtually infinite reaction time. A common finding in kaolinite intercalation studies is that, regardless of the type of guest molecules, there is a population of kaolinite that is resistant to intercalation (Lagaly et al., 2006 Rausell-Colom & Serratosa, 1987). As stacking order and thermal history had no detectable effect on the NMF-hosting environment, the unusual temperature dependence was attributed tentatively to the adverse effect of temperature on the adsorption of NMF on the edges of the crystallites, which is a prerequisite for intercalation.Ĭertain molecules such as hydrazine, DMSO, small amides, K-acetate, etc., are known to intercalate spontaneously the interlayer space of kaolinite (a 1:1 dioctahedral clay mineral) shifting its X-ray diffraction (XRD) 001 reflection from ~7.2 to 10–14 Å (Kloprogge, 2019 Lagaly et al., 2006). Thus, intercalation capacity was not a unique feature of the material because it depended on thermal history. Subjecting the samples to stepwise temperature changes showed that, once intercalated, the NMF could not deintercalate and was removed from equilibrium with the surrounding fluid. All kaolinites exhibited the same behavior, but the amount of inert material was in the order of their stacking-fault concentration at all temperatures: KGa-2 > Hywite > KGa-1b. Complementary thermogravimetric analysis (TGA) confirmed this unexpected trend. Increasing intercalation temperature accelerated the reaction, but the NMF uptake decreased and eventually vanished. Near-infrared spectroscopy (NIR) was employed to record the full kinetics of intercalation in closed systems with excess NMF. The purpose of the current study was to investigate systematically the N-methylformamide (NMF) intercalation capacity of three kaolinites differing in layer stacking order (KGa-1b, KGa-2, and Imerys Hywite Alum) in the 5–150☌ temperature range. Some reports have linked this lack of reactivity to particular structural features or to slow kinetics others recommended increasing intercalation temperature as a remedy. Many kaolinites are known to exhibit limited intercalation capacity which affects their usage.
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