Surface treatment of clay minerals — thermal stability, basal-plane spacing and surface coverage

Abstract

Oxidative thermal degradation of organically modified montmorillonite (OM) and muscovite was studied using conventional and high-resolution TGA. Analysis of the ultra thin alkylammonium film covering the mineral surface revealed a correlation between thermal stability and extent of cation exchange. In the final stage of the exchange, some alkylammonium molecules are intercalated between the ionically bonded ones in a tail-to-tail arrangement leading to a local bilayer. This local bilayer as well as the non-reacted ammonium salt molecules decompose at lower temperatures than the self-assembled monolayer (SAM) and decrease the thermal stability of the OM, i.e. the quality of SAMs can be monitored by Hi-Res TGA. Quantitative analysis of the organic monolayer was not always possible by TGA due to incomplete oxidation and superimposition of different mass-loss events. However, the mass-loss across the first degradation event gave a good estimate of the surface coverage. In the case of montmorillonite, quantitative analysis was possible by taking the mass-loss due to physisorbed water and to dehydroxylation of the mineral into consideration. Both mass-loss and basal-plane spacing increased with augmented cation exchange, reaching a saturation value at ≈100% of the CEC. The thermal stability of the organic monolayer depends on its chemical structure and purity as well as on the nature of the substrate. In general, dialkylammonium SAMs are more stable than the monoalkyl derivatives. Neither the decomposition onset temperature nor the temperature of maximum mass-loss rate is useful as an index for the stability of alkylammonium SAMs. The time needed for an isothermal mass-loss equal to 5% of that occurring across the first decomposition event was used as an index to compare the stability of different monolayers. The thermal stability of pure alkylammonium SAMs on montmorillonite allows compounding with many commercially available polymers.