Thermodynamic transformations of entangled bulky organic monomers with long alkyl chains

Abstract

A bulky organic monomer with a long alkyl chain, as, e.g., hexadecyltrimethylammonium (HDTMA⁺), is the structural unit prerequisite for ionic-liquid engineering as well as porous materials design; however, the detailed structure in the entangled agglomerate, particularly caused by moderate temperatures, has not yet been well studied. Here, the local molecular structures in the entangled HDTMA⁺ monomers stirred at different temperatures were studied by open space analysis using positronium coupled with molecular dynamics simulations. The agglomerate stirred at 323 K exhibits the small and large open spaces with the size distributions centered at ~ 0.27 nm and ~ 0.55 nm, respectively. Both open spaces are found to be dominantly surrounded by protons. Upon increasing temperature to 423 K, the enlargements of the small and large open spaces occur broadening their size distributions, whereas the proton-rich chemical environment is maintained. The enhanced porousness at high temperatures is explained by the efficiently shrunk monomers triggered off by the lowering of HDTMA⁺ diffusivity upon coexisting with propanol molecules.