Phase Transition of Octadecylurea at the Air−Water Interface Studied by Infrared External Reflection Spectroscopy

Abstract

External infrared reflection spectroscopy was applied to study a phase transition of the monolayer of octadecylurea (OU) at the air−water interface. Upon compression at a constant temperature (6 °C in this study) the monolayer undergoes the transition from a condensed phase (β phase) with a larger limiting area (0.250 nm2/molecule) to another condensed phase (α phase) with a smaller limiting area (0.195 nm2/molecule). The formation of the β phase and the phase transition accompany discrete IR spectral changes in the νas(CH2), νs(CH2), amide I, amide II, and δ(NH2) vibration regions. The spectral changes indicated that (i) upon compression of the monolayer to form the β phase the alkyl side chain is converted from an irregular state containing gauche conformations to an ordered one consisting of the all-trans conformation and at the same time the peptide bond in the head group is transformed from a fully hydrated state to a hydrogen-bonded one and (ii) upon further compression to the α phase and/or a colla...