Evaluation of Molecular Orientation in a Polymeric Monolayer at the Air−Water Interface by Polarization-Modulated Infrared Spectroscopy

Polarization-modulated reflection−absorption spectroscopy (PM-IRRAS) is employed to record infrared spectra of a monolayer of (dodecyl)cellulose spread at the air−water interface. Methylene symmetric and antisymmetric stretches of the alkyl side chains are clearly visible in the monolayer spectra as positive peaks at 2852 and 2922 cm -1 . It is thus concluded that the side chains possess a net orientation perpendicular to the water surface. Bands characteristic of the cellulose backbone are also observed as positive PM-IRRAS peaks, in the region 1160−980 cm -1 . This result indicates that the rigid polysaccharide chain lies essentially parallel to the water surface. Spectra recorded as a function of molecular area reveal little change in molecular orientation during monolayer compression. Significant differences in orientation are however evident between dense films obtained by the compression of disperse monolayers and those prepared by direct spreading at low surface areas.