Interfacial Water Structure at Surfactant Concentrations below and above the Critical Micelle Concentration as Revealed by Sum Frequency Generation Vibrational Spectroscopy

Abstract

Interfacial water in close proximity to an adsorbed surfactant layer plays an important role in many areas. Here, we systematically investigate the interfacial water structure at the adsorption layer of cetyltrimethylammonium bromide (CTAB) from low concentrations to its critical micelle concentration. Our sum frequency generation (SFG) spectroscopy results show that, with increasing CTAB concentration, the water SFG signals first increase, reaching maximum intensities at 0.1 mM, and then drop, whereas the ppp SFG signals of the terminal-methyl asymmetric stretch reach maximum intensities and saturate at 1 mM, which is the critical micelle concentration of CTAB. Our analysis reveals that the interfacial water layer adopts the most orderly arrangement when the interfacial potential of the adsorption layer reaches saturation and not at the surfactant concentration of adsorption saturation. Most importantly, our SFG data provide direct evidence for the antagonistic effects of the interfacial potential and thickness compression of the electrostatic field of the surfactant adsorption layer, leading to the strongest water SFG signals at 0.1 mM. Below 0.1 mM, the increased interfacial potential can have a pronounced effect on the increase of the overall dipole moment of the interfacial water layers. Above 0.1 mM, the decreased Debye screening length significantly reduces the water dipole moment. Finally, the newly proposed adsorption model cannot explain our SFG results.