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

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 nm 2 /molecule) to another condensed phase (α phase) with a smaller limiting area (0.195 nm 2 /molecule). The formation of the β phase and the phase transition accompany discrete IR spectral changes in the ν as (CH 2 ), ν s (CH 2 ), amide I, amide II, and δ(NH 2 ) 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 collapsed state the alkyl side chains are changed again to the irregular state and a part of the peptide bonds is converted to the hydrated state. These results are contrasted with the case of the monolayer of N -methyl-N-octadecylurea (MOU) at the air−water interface, which gives the amide I and δ(NH 2 ) bands characteristic of a fully hydrated state irrespective of the molecular area. Thus, the IR spectral changes provided direct proofs for the explanation that the endothermic first-order β → α phase transition of the OU monolayer takes place through a partial collapse of the two-dimensional ordered array consisting of the hydrogen-bonded network between the peptide bonds.