SERS of surfactants in monolayer and multibilayer forms on an electrified Ag surface

The Raman spectra of several surfactants such as didodecyldimethylammonium bromide (DDAB), cetyltrimethylammonium bromide (CTAB), dimyristoylphosphatidyleholine (DMPC), cetylpyridinium chloride (CPC), dihexadecyl phosphate (DHP), and sodium laurate (NaL) have been investigated in the solid, multilayer, micellar, and multibilayer forms. The first four are cationic surfactants while DHP and NaL are anionic. Raman spectra show that they all exist in an all-trans configuration in the crystalline state while their structure in the micellar, monolayer, and multibilayer (film) forms is a mixture of gauche and trans configurations. Surface-enhanced Raman scattering (SERS) spectra of the above surfactants (with the exception of CPC) show an unexpected enhancement for C-H stretching modes at potentials more negative than -0.8 V versus the saturated calomel electrode along with the appearance of two new SERS vibration bands at 2710 and 2815 cm(-1). In the multibilayer film, the SERS spectra of some of these surfactants (DDAB, CTAB, DMPC) are dominated by two intense bands located at 1130 cm(-1) and 1530 cm(-1). The 1130 cm(-1) band corresponds to the all-trans alkyl structure, which is common in the Raman spectra of all of these surfactants, while the 1530 cm(-1) band is a totally new band. We associate it with a bending mode of the quaternary nitrogen methyl groups. Ab initio Hartree-Fock/6-311G calculations of CTAB in the trans and gauche configurations are used to support the band assignments.