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

The crystal lattice of CH3(CH2)(20)COOH (behenic acid) monolayer on pure wa ter subphase was investigated in situ by polarization modulation infrared r eflection absorption spectroscopy. Two kinds of centered rectangular unit c ells 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 t hat at the corner. A rectangular parallel-to-orthorhombic transition was di scovered for two kinks from 0.260 to 0.220 nm(2)/molecule along the surface pressure-area isotherm at 283 K, evidenced by splitting of the CH2 scissor ing mode. By holding the barrier at 0.240 nm(2)/molecule and cyclically var ying the monolayer temperature, a partially reversible transition from rect angular parallel to orthorhombic unit cells was observed. (C) 2001 American Institute of Physics.