Intercalation Mechanism and Interlayer Structure of Hexadecylamines in the Confined Space of Layered α-Zirconium Phosphates

Abstract

Well-defined hexadecylamine (HDA) intercalated structures, either interdigitated layers, bilayers, or hybrid layers of both, in a confined space of highly functionalized layered α-zirconium phosphates (α-ZrPs) have been prepared based on the two-step intercalation mechanism and these distinct intercalated structures can serve as a model system to investigate the interactions of two monolayers whose amphiphilic tails are adjacent to each other. At the first intercalation step, the electrostatic interaction between HDAs and α-ZrP is dominant and results in an interdigitated layer structure (d001 = 3.0 nm) and the interdigitated layer is saturated at around Φ = 50%, where Φ is the weight fraction of intercalated HDAs in the intergallery of α-ZrP. For Φ higher than 50%, the bilayer structure (d001 = 4.3 nm) emerges due to further hydrophobic interaction between HDAs initially grafted to α-ZrP and unanchored HDAs and the relative fraction of the bilayer structure over the interdigitated layer increases with the increase in the intercalated amount of HDAs. The intriguing morphology of α-ZrP tactoids intercalated with HDAs in coexisting bilayers and interdigitated layers is observed by using microtomed TEM and the two-step intercalation has also been verified with TGA and FT-IR. Also, a structural transition from the bilayers to the interdigitated layers is monitored by using in situ synchrotron WAXS showing that the hydrophobically intercalated HDAs are selectively deintercalated at a relatively low decomposition temperature around 220 °C.