Intercalation of ionic liquids into bentonite: Swelling and rheological behaviors

Abstract

In this study, direct intercalation of three different green ionic liquids (ILs) having different salt and cation sizes into bentonite interlayer is investigated. The ILs covered in this study are: 1-hexyl-3-methylimidazolium chloride [IL-1], 1-butyl-3-methylimidazolium octyl-sulfate [IL-2], and 1-butyl-3-methylimidazolium bromide [IL-3]. The aim of this IL modification of bentonite is to enhance both rheological and thermal stability of the modified bentonite. This objective is achieved by increasing the swelling of the bentonite interlayer, which is essential for many applications. The basal d-spacing of the ILs intercalated bentonite in comparison with dried bentonite powder as detected by XRD results showed that the cations of the three ILs are successfully intercalated into the interlayer of the bentonite platelets by a cation exchange mechanism and the swelling is likely influenced by the cation size, type and concentration of the ILs. The measurements of the equilibrium concentrations of the ILs in the solution suggested that adsorption is taking place on the external surfaces. This assumption is validated by Zeta potential measurements. The results from particle sizes and Zeta potential measurements have shown that bentonite intercalated with IL-1 and IL-2 are the most effective in decreasing the overall repulsive forces and increasing the attractive forces on the bentonite surface. Consequently, larger bentonite aggregates are produced by the chloride based IL intercalated bentonite rather than those intercalated with the bromide based IL or pure bentonite. In a similar manner, rheological measurements showed that the bentonite intercalated with IL-1 and IL-2 produced strong bentonite aggregates with higher G′, G″, τc and η* than those intercalated with IL-3 and the untreated bentonite samples. Rheological, Zeta Potential and XRD measurements have confirmed that green ionic liquids can be used in different applications to enhance bentonite swelling.