Calixarenes are macrocyclic compounds that form complexes with ions and neutral molecules.
Association with ammonium cations and calixarenes is directed by coulombic forces, hydrogen
bonding, N+-C-H---π interactions and van der Waals interactions. Calixarenes are stronger acids
than corresponding phenols and react with aliphatic amines by proton-transfer and the ammonium
cations can be placed as exo or endo-calix. The type of aggregate and the association constants are
related to the branching of the chain, the basic character of amine, and the size of the calix. Trimethylammonium
cations, like choline, bind sulfonate calix[4 and 6]arenes and calixarenes by coulombic
and N+-C-H---π interactions. Interaction of calixarenes and ammonium cations has been used
for biological sensors for neurotransmitters and peptide hormones.