Crystal lattice transition of behenic acid monolayer on pure water surface observed by polarization modulation infrared spectroscopy

The crystal lattice of $CH3(CH2)20COOH$ (behenic acid) monolayer on pure water subphase was investigated in situ by polarization modulation infrared reflection absorption spectroscopy. Two kinds of centered rectangular unit cells were found: rectangular parallel and orthorhombic packing of the trans zigzag planar–$(CH2)20$–chain. In the former cell the chain was tilted on the water surface and the C–C–C plane at the rectangle center was parallel to that at the corner. In the latter cell the chain was normal to the water surface and the C–C–C plane at the rectangle center was perpendicular to that at the corner. A rectangular parallel-to-orthorhombic transition was discovered for two kinks from 0.260 to 0.220 $nm2/molecule$ along the surface pressure-area isotherm at 283 K, evidenced by splitting of the $CH2$ scissoring mode. By holding the barrier at 0.240 $nm2/molecule$ and cyclically varying the monolayer temperature, a partially reversible transition from rectangular parallel to orthorhombic unit cells was observed.