Structure of Liquid Propionitrile at Interfaces. 2. Experiment

Propionitrile has been studied at the liquid/vapor, silica/vapor, and silica/liquid interfaces using vibrational sum-frequency generation (VSFG) spectroscopy and optical Kerr effect (OKE) spectroscopy. VSFG studies show that the alkyl tail of propionitrile tends to point into the vapor phase at the liquid/vapor and silica/vapor interfaces. At the silica/liquid interface, all vibrational resonances except for the methylene symmetric stretch exhibit strong cancellation in the VSFG signal. This result supports the existence of a lipid-bilayer-like organization at the silica/liquid interface, in agreement with simulation. OKE data for propionitrile confined in porous sol-gel glasses indicate that there is a surface layer, with a thickness of roughly 4 angstrom, that experiences inhibited orientational dynamics. The OKE data thus corroborate the picture of interfacial organization suggested by the VSFG results.