Intercalation of N-methylformamide in kaolinite: In situ monitoring by near-infrared spectroscopy and X-ray diffraction

Abstract

The selective self-assembly of small molecules in the interlayer of kaolinite is a fundamentally important and technologically relevant process, typically studied ex situ by X-ray diffraction (XRD). Near-infrared (NIR) spectroscopy is now introduced to provide a complementary local structural description of intercalation with improved control of experimental conditions.New NIR- and XRD-based proxies were developed and applied to the real-time monitoring of N-methylformamide (NMF) intercalation in two reference kaolinites differing in stacking order.The commonly employed XRD-based formalism was found to overestimate reaction progress. The bonding of NMF in the interlayer was independent of reaction progress and kaolinite type. Both NIR and XRD recorded identical sigmoidal kinetics. Isothermal NIR monitoring (25–80 °C) yielded time-temperature superimposable sigmoidals with an apparent activation energy of ~60 kJ/mol, common to both samples. All NIR and XRD data series could be described as linear combinations of empty and fully intercalated kaolinite. The filling of the interlayer was too fast to be observed. The sigmoidal curves were instead modeled as the log-normally distributed response of an ensemble of intercalating entities, presumably crystallites. The multiplicative standard deviation of the distribution, which determines its steepness, is a sample-specific, temperature-independent property of kaolinite.