Reflection-absorption FT-IR spectroscopy of pentadecanoic acid at the air/water interface

Reflection–absorption Fourier transform infrared (RA FT-IR) spectra of spread monolayers of pentadecanoic acid have been obtained for a range of surface pressures at 292 and 297 K. Unpolarised and s- and p-polarised radiation at 30° angle of incidence have been used. Results in the C–H stretching region reveal disorder in the arrangement of the acyl chains with numerous gauche bonds when the monolayer is in the liquid expanded (LE) phase and highly ordered chains with mostly trans bonds in the condensed phases (LC and S). The transition from one form to the other occurs at nearly constant surface pressure in the LE↔LC coexistence region