Raman spectroscopy of Langmuir monolayers at the air-water interface

A new approach for the acquisition of Raman spectra of Langmuir layers at the air-water interface is presented. Colloidal Ag is grown electrochemically under layers of the anionic surfactants stearic acid and dipalmitoylphosphatidic acid sodium salt (DPPA) on an aqueous AgNO3 subphase. The Ag colloid layer supports surface-enhanced Raman scattering (SERS) of the Langmuir layers with minimal interference from the subphase. Spectra of good signal-to-noise ratio have been acquired. These spectra are interpreted in terms of the headgroup chemistry and the degree of alkane chain order as a function of surface pressure.