Quantitative Determination of Molecular Chain Tilt Angles in Monolayer Films at the Air/Water Interface:  Infrared Reflection/Absorption Spectroscopy of Behenic Acid Methyl Ester

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Monolayers of behenic acid methyl ester at the air/H2O and air/D2O interfaces provide a convenient test for quantitative analysis of infrared reflection−absorption spectroscopy (IRRAS) intensities. Spectra were acquired for both s- and p-polarized radiation at angles of incidence of 35°, 40°, 45°, and 50°. The observed ∼10 cm-1 splitting (at a surface pressure of 14 mN/m) for both the methylene scissoring and rocking modes provides direct evidence for the occurrence of a perpendicular orthorhombic subcell structure and for the existence of all-trans acyl chains. Analysis of the IRRAS intensities of the methylene and carbonyl stretching vibrations reveals that, of the limited set of chain tilt angles possible with respect to the surface normal, a chain tilt angle of 0° provides by far the best fit to the data. For each vibration, data for both polarizations at all four angles were fit with a single set of three parameters: chain tilt angle, effective extinction coefficient (kmax), and the overall degree of polarization (determined by the efficiency of the polarizer and the error in its optical alignment). This last parameter was found to be important in accounting for observed IRRAS intensities, especially for p-polarized radiation close to the Brewster angle. Finally, the feasibility of using the observed unequal intensities in the components of the split methylene scissoring bands to determine the angle that the orthorhombic subcell makes in the x,y (water surface) plane is demonstrated.