1-Methyl naphthalene reorientation at the air-liquid interface upon water saturation studied by vibrational broad bandwidth sum frequency generation spectroscopy

Vibrational broad bandwidth sum frequency generation spectroscopy was employed to investigate the surface structure of neat 1-methyl naphthalene (1-MN) and the reorientation of the 1-MN molecules upon saturation of the 1-MN liquid with water. The neat 1-MN liquid molecules have their aromatic rings aligned antiparallel to one another with their methyl groups alternating out of the surface and into the subsurface region from molecule to molecule. With the introduction of relatively few water molecules into the 1-MN liquid (1:336 water/1-MN) a rearrangement of the surface molecules is induced, leading to an increased number density of the methyl groups arranged such that more methyl groups are oriented in the same direction into the air phase at the air-liquid 1-MN interface. Surface tension measurements reveal an increase in the surface tension upon water saturation of the 1-MN liquid, indicating surface activity of the water in the 1-MN solution. It is also clear that the reorientation of the surface 1-MN molecules is reversible.