IN-SITU INVESTIGATIONS OF COADSORBED CATIONIC SURFACTANTS ON CHARGED SURFACES - A SERS MICROPROBE STUDY

Surface-enhanced Raman microprobe spectroscopy (micro-SERS) has been u sed to study, in situ, the adsorption process of cetylpyridinium bromi de (CPB), cetyltrimethylammonium bromide (CTAB) and their binary mixtu res adsorbed on charged silver nanoparticle surfaces. Partial vibratio nal assignment was achieved by comparison of observed band position an d intensity in the FTIR and Raman spectra with wavenumbers and intensi ties from ab initio LCAO-MO-SCF Hartree-Fock calculations at the 6-31G level of N-methylpyridinium, butylpyridinium and heptyltrimethylammo nium. Density functional theory was used for the vibrational calculati on in the Ag-10-Br cluster model system. Information on the monolayer adsorption geometry at the charged surface can be obtained on the basi s of this assignment. Considerations of surface selection rules sugges t that the CPB surfactant molecule is adsorbed with the ionic head-gro up towards the charged surface and the long hydrophobic tail is direct ed 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. Coads orbed halide ions are very important for binding in the first monolaye r. The positively charged head-groups are Surrounded by bromide ions a nd the pyridinium/bromide and trimethylammonium/bromide surface-comple xes are strongly adsorbed even on positively charged surfaces. Competi tive coadsorption measurements of the binary surfactant mixtures have shown that the adsorption process at the solid/liquid interface depend s 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 conc entration-ratio of CPB: CTAB=1: 1000. The adsorption geometry of CPB i n the mixed monolayer of CPB and CTAB (patchy hemimicells of CPB and C TAB) is strongly influenced by the surface charge. (C) 1997 Elsevier S cience B.V.