Concentration-Dependent Conformation of Alkyl Tail in the Nanoconfined Space:  Hexadecylamine in the Silicate Galleries

Abstract

The conformation and melting of both hexadecylammonium ion, n -CH 3 (CH 2 ) 15 NH 3 +, and neutral hexadecylamine, n -CH 3 (CH 2 ) 15 NH 2, within the clay silicate galleries have been studied by Fourier transform infrared spectroscopy (FTIR). The conformational ordering of the alkyl tail in the confined system shows strong dependence on amine concentration and orientation. While the amine chains with orientation parallel to the silicate layers are in various disordered states, those with orientation radiating from the clay surface take mostly ordered conformation. Ammonium ion shows a similar trend of ordering dependence on the concentration as neutral amine, although the former has lower ordering than the latter, under the same concentration level. Temperature-dependent FTIR study shows that the melting behavior of confined amine corresponds to the chain melting process.