Langmuir films of nitro substituted N-benzylidene hexadecylamine Schiff bases at air/water interface—Phase transitions and molecular dynamics simulation

Abstract

Formation and packing of Langmuir monolayers of ortho, meta and para substituted N-benzylidene hexadecylamine have been studied using surface pressure–molecular area ( π– A) isotherms over a wide range of temperatures. A cusp with a plateau in the isotherms shows a first-order phase transition between the liquid expanded (LE) and liquid condensed (LC) phase in all the three compounds at air/water interface. Reflection spectra of the Langmuir monolayers in the LC phase and absorbance spectra of the corresponding Langmuir–Blodgett films (LB films) show fairly ordered films. In the ortho derivative the intramolecular H-bonding seems to compete with intermolecular H-bonding leading to a lesser hydration-induced repulsive interactions between the head groups thereby inducing better packing. The changes in enthalpy (Δ H) was used to understand the nature of the packing of the molecules in the LE–LC phase. Molecular dynamics simulation carried out on the three amphiphiles on water using amorphous cells showed that the compound with the –nitro group in the ortho position shows the highest interaction energy suggesting reduced hydration. This correlates well with the ordering in the different Langmuir films.