"Up" versus "down" alignment and hydration structures of solutes at the air/water interface revealed by heterodyne-detected electronic sum frequency generation with classical molecular dynamics simulation.

We unambiguously demonstrate the "up" versus "down" alignment of a pair of prototypical solute molecules adsorbed at the air/water interface for the first time using heterodyne-detected electronic sum frequency generation spectroscopy. This molecular alignment is also reproduced by classical molecular dynamics (MD) simulation theoretically. Furthermore, the MD simulation indicates distinctly different interface-specific hydration structures around the two solute molecules, which dictate the molecular alignment at the interface. It is concluded that the hydrophilicity difference between the terminal functional groups of the solute governs the molecular orientation and surrounding hydration structures at the interface.