Solvation Reaction Field at the Interface Measured by Vibrational Sum Frequency Generation Spectroscopy

Interfacial electric fields are important in several areas of chemistry, materials sciences, and device physics. However, they are poorly understood, partly because they are difficult to measure directly and model accurately. We present both a spectroscopic experimental investigation and a theoretical model for the interfacial field at the junction of a conductor and a dielectric. First, we present vibrational sum frequency generation (VSFG) results of the nitrile (CN) stretch of 4-mercaptobenzonitrile (4-MBN) covalently attached to a gold surface and in contact with a variety of liquid dielectrics. It is found that the CN stretch frequency red-shifts with increasing dielectric constant. Second, we build a model in direct analogy to the well-known Onsager reaction field theory, which has been successful in predicting vibrational frequency shifts in bulk dielectric media. Clearly, due to the asymmetric environment, with metal on one side and a dielectric on the other, the bulk Onsager model is not applicable at the interface. To address this, we apply the Onsager model to the interface accounting for the asymmetry. The model successfully explains the red-shift of the CN stretch as a function of the dielectric constant and is used to estimate the reaction field near the interface. We show the similarities and differences between the conventional bulk Onsager model and the interfacial reaction field model. In particular, the model emphasizes the importance of the metal as part of the solvation environment of the tethered molecules. We anticipate that our work will be of fundamental value to understand the crucial and often elusive electric fields at interfaces.