Interlayer configuration of ionic liquids in a Ca-montmorillonite as evidenced by FTIR, TG-DTG, and XRD analyses

Abstract

Ionic liquids (ILs) have attracted much attention in the scientific community during the past two decades. However, studies of interactions between Earth materials and ILs have been limited. In this study, the uptake and interlayer configuration of ILs on a high charge Ca-montmorillonite (MMT) were systematically studied using TG-DTG, FTIR, and XRD analyses. The intercalation protected the ILs from thermal decomposition, resulted in an increase in their thermal stability in comparison to ILs adsorbed on the surfaces. The results from TG-DTG, FTIR, and XRD all pointed to a change in mechanisms and interlayer configurations as the chain length, input amounts, and initial concentrations of ILs changed. A higher decomposition temperature in TG analyses and a large blue shift of selected FTIR bands agreed well with the cation exchange mechanism at lower IL loading levels. Accompanying cation exchange the interlayer water was completely removed. Hydrophobic interactions and admicelle adsorption were responsible for IL uptake beyond the cation exchange capacity of the MMT. A pseudo-trilayer configuration was suggested based on XRD analyses for the intercalation of ILs with a chain length of 12 carbons at the sorption maximum and that of 16 carbons at medium sorption levels. A paraffin-like conformation with an almost vertical orientation was suggested for the configuration of IL with a chain length of 16 carbons in the interlayer of MMT at the sorption maximum.