Surfactant/water interactions at the air/water interface probed by vibrational sum frequency generation

The molecular structure and orientation of water molecules at an air/water interface has been measured in the presence of cationic and anionic surfactants. We have employed vibrational sum frequency generation to obtain the vibrational spectrum of both the surfactant and water molecules at the water surface. In the presence of the charged surfactant, the wafer molecules show a high degree of orientation with the directionality dependent upon the anionic or cationic nature of the surfactant headgroup. Relative to the charged surfactants, uncharged surfactants show minimal impact on interfacial water ordering. The results presented here provide valuable new insight into how the interfacial water molecules orient at the air/water interface in the presence of surfactants with different headgroups as well as how the relative order of the surfactants differ for these different amphiphiles.