MOLECULES AT LIQUID AND SOLID-SURFACES

The discovery of structural phase transitions in a series of alkanenit riles, acetonitrile (CH3CN), propionitrile (CH3CH2CN), nonadecanitrile [CD3(CH2)(19)CN], but not butyronitrile (CH3CH2CH2CN), at the air/wat er is described. Using sum frequency spectroscopy, the phase transitio n is manifested by an abrupt change in the orientation and an abrupt c hange in the vibrational frequency of the CN head group of the interfa ce nitrile molecules. The competition between the distance dependent n itrile-nitrile dipolar interactions and the hydrogen bonding and solva tion of the CN moieties is used to describe the phase transitions. A n ew application of second harmonic and sum frequency generation to stud y the interfaces of centrosymmetric microscopic particles is presented . Examples of this new use to the adsorption of molecules to a polysty rene microsphere/aqueous interface and the polarization of bulk water molecules by a charged microsphere of polystyrene sulfate are discusse d.