Birnessite-type manganese oxide–alkylamine mesophases obtained by intercalation and their thermal behaviour

Abstract

Layered birnessite-type manganese oxide inserts alkylamines (decylamine–octadecylamine) by ion-exchange and intercalation mechanisms and the concomitant expansion is compatible with formation of a head-to-tail layer of interdigitated amine. The structural arrangement of the MnO₂–alkylamine mesophases is sensitive to temperature, and at a temperature dependent on the chain length of the intercalated alkylamine, undergoes a phase transition to a more highly expanded phase. The basal spacing of the more expanded phases formed at higher temperature indicates that de-interdigitation occurs to give a bilayer organisation of neutral and protonated alkylamine species. The phase transition temperature is identified by differential scanning calorimetry. X-Ray absorption spectroscopy shows that while the local structure around manganese is not affected by intercalation, it provides evidence that this transition is accompanied by partial lowering of the oxidation state of manganese and migration of manganese into the interlayer region. This phase transition is reminiscent of some of those observed in phospholipid membranes, but seems to represent a new phenomenon for inorganic intercalates.