Structural phase transitions of small molecules at air/water interfaces

Abstract

Structural phase transitions have been observed using IR–VIS sum frequency generation (SFG) at the air/acetonitrile–water and the air/propionitrile–water interfaces but not at the air/butyronitrile–water interface. The phase transitions were detected by abrupt changes in the CN vibrational frequency and in the orientation of the CN chromophore as the interface nitrile density was varied. A model based on the orientational effects of hydrogen bonding and solvation of the CN group by water molecules competing with the density and orientationally dependent dipole–dipole repulsions among the CN chromophores is used to explain the results.