Competitive and displacement adsorption of cetylpyridinium chloride and p-nitrophenol on charged surfaces: A surface-enhanced Raman microprobe Scattering (SERS) study

Abstract

Micro-surface-enhanced Raman scattering (micro-SERS) has been used to study, in situ, the competitive and displacement adsorption of cetylpyridinium chloride (CPC) and p-nitrophenol (PNP) on charged silver micro particle surfaces. The investigations have shown that this surface spectroscopy is an extremely powerful technique for monitoring the interfacial behavior of CPC and PNP, for assessing adsorbate orientation, displacement kinetics, competitive adsorption and for probing the effect of various environmental factors on the adsorbate/substrate interaction. Considerations of surface selection rules suggest that the CPC surfactant molecule is adsorbed with the ionic head group towards the charged surface and the long hydrophobic tail is directed away from the surface. Coadsorbed halide ions are very important for the binding in the first monolayer. The positively charged nitrogen atom of the pyridinium group is surrounded by chloride ions and this pyridinium/chloride surface complex is strongly adsorbed even on the positively charged surface. The p-nitrophenol is transformed to p-nitrophenolate when adsorbed on the charged surface. Therefore, the p-nitrophenol is bound to the surface through the ionized Ph-O⁻ group by releasing its proton and is oriented perpendicularly to the surface with the NO₂ group extending into solution. The competitive coadsorption measurements have shown that the adsorption process depends greatly on the surface charge, the relative concentration of the CPC/PNP mixture and the counterion concentration. The kinetic displacement studies have demonstrated that the cationic surfactant is able to remove pre-adsorbed p-nitrophenol in a second-time scale.