Orthorhombic structure of cadmium behenate monolayers on the water surface of a Langmuir trough detected by polarization modulation infrared spectroscopy

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the structure of behenic acid monolayers on the surface of aqueous Cd2+ sub-phase in a Langmuir trough at the room temperature of 293 K. The PM-IRRAS spectra were recorded at representative points throughout 0.401∼0.100 nm2 and the surface-pressure area isotherms were recorded during compression to these points. It is found that compressed cadmium behenate (CdB) monolayers possess an orthorhombic unit cell. On the other hand, single monolayers of CdB deposited on CaF2 substrates at 20 mN m−1 possessed a triclinic parallel packing.