Partial chain deuteration as an IRRAS probe of conformational order of different regions in hexadecanoic acid monolayers at the air/water interface

External infrared reflection-absorption spectroscopy (IRRAS) of hexadecanoic acid-d₁₇ (half deuterated at the tail end) monolayers at the air/water interface has been used to investigate conformational order in both halves of the molecule. The symmetric and asymmetric CH₂ stretching vibrations in the liquid-condensed phase were found to be 0.8 and 1.3-1.8 cm^-1 lower in frequency respectively than the same vibrations in hexadecanoic acid-d₃ (terminal methyl deuterated), suggesting substantially higher conformational order near the polar headgroup. Theoretical simulations of the bands reveal that the effect cannot arise from different IRRAS intensities for these vibrations in the two molecules. IRRAS measurements at a series of temperatures between 14 and 27 °C reveal a different average response to increased temperature in the two halves of the chain: conformational disorder is introduced at the tail end at a lower temperature. This work demonstrates that current state-of-the-art IRRAS measurements provide sufficient signal-to-noise ratios to monitor conformational order in different regions of amphiphile chains, thus providing a sound experimental base for comparison with theoretical models of chain structure.