Observations of interfacial population and organization of surfactants with sum frequency generation and surface tension

The complementary interface-sensitive methods of sum frequency generation (SFG) and surface tension have been used to investigate surfactants at the air/water interface. SFG yields information on the density and orientation of surfactants at the interface, whereas surface tension yields information on the surface excess of surfactants, which, however, is not their interfacial density. The rapid rise in the SFG signal to its maximum value at 1 mM, which remained essentially constant thereafter, was attributed to the formation of large surfactant domains. Above 1 mM, the domains, all of which have the same density, dominated the SFG signal. As a consequence, the SFG signal remained essentially constant. The surface excess had an equally rapid rise to its maximum value at 1 mM, which remained constant until reaching a concentration of 3 mM, where it dropped to a zero value, indicating that a full surfactant monolayer had been formed. This coincides with the formation of bulk micelles, which is commonly referred to as the critical micelle concentration. The orientation of the surfactant carbonyl chromophore was obtained from polarization measurements of the SFG signal and showed a small change at and above a 1 mM concentration. The SFG results and the surface tension results, though inherently different, were found to be consistent with each other.