In-situ investigations of coadsorbed cationic surfactants on charged surfaces: a SERS microprobe study

Abstract

Surface-enhanced Raman microprobe spectroscopy (micro-SERS) has been used to study, in situ, the adsorption process of cetylpyridinium bromide (CPB), cetyltrimethylammonium bromide (CTAB) and their binary mixtures adsorbed on charged silver nanoparticle surfaces. Partial vibrational assignment was achieved by comparison of observed band position and intensity in the FTIR and Raman spectra with wavenumbers and intensities from ab initio LCAO-MO-SCF Hartree-Fock calculations at the 6–31G * level of N-methylpyridinium, butylpyridinium and heptyltrimethylammonium. Density functional theory was used for the vibrational calculation in the Ag 10 Br cluster model system. Information on the monolayer adsorption geometry at the charged surface can be obtained on the basis of this assignment. Considerations of surface selection rules suggest that the CPB surfactant molecule is adsorbed with the ionic head-group towards the charged surface and the long hydrophobic tail is directed away from the surface. In contrast, the CTAB surfactant is adsorbed with the head-group pointing to the surface and the hydrocarbon chain is adsorbed flat with the tail running parallel to the surface. Coadsorbed halide ions are very important for binding in the first monolayer. The positively charged head-groups are surrounded by bromide ions and the pyridinium/bromide and trimethylammonium/bromide surface-complexes are strongly adsorbed even on positively charged surfaces. Competitive coadsorption measurements of the binary surfactant mixtures have shown that the adsorption process at the solid/liquid interface depends greatly on the surface charge and the relative concentration of the CPB/CTAB mixture. A fully equilibrated monolayer coverage of the same amount of CPB and CTAB is observed from a bulk surfactant mixture concentration-ratio of CPB: CTAB=1: 1000. The adsorption geometry of CPB in the mixed monolayer of CPB and CTAB (patchy hemimicells of CPB and CTAB) is strongly influenced by the surface charge.