Orientations of chain axes and transition moments in Langmuir-Blodgett monolayers determined by polarized FTIR-ATR spectroscopy

FTIR-ATR dichroic ratios with and without a rotation of the ATR plates were measured for calcium or cadmium or lead stearate LB monomolecular films on germanium, hydrophilic silicon, and hydrophobic silicon plates. Various models for uniaxial or biaxial orientations of chain axes and transition moments are presented and critically evaluated. A biaxial orientation distribution was observed for calcium stearate. The average tilt and azimuthal angles of the chain axes were found to be 36° and 42°, respectively. Those angles of the transition moments of the CH₂ group were 66° and 46°, respectively. Cadmium and lead stearate monolayers had nearly uniaxial chain and moment distributions. Molecular orientations varied substantially with the ion type bound to the polar groups but little with the substrate surface. The average tilt angle of the chain axes was ca. 11° higher in X-type monolayers (methyl groups on the substrate surface) than in Z-type monolayers (polar groups on the surface) of lead stearate. The dichroic ratio for the cadmium stearate monolayer decreased from 0.98 at 25°C to 0.84 at 130°C. Recrystallization of the monolayer upon cooling was observed to be reversible and showed no time lag.