(Invited) Controlling Electric Fields and Hydration at Electrochemical Junctions Using Surfactants

Tuning the electrostatics and the water structure at the nm and sub-nm scales are two important frontiers towards the broader goal of controlling reactivity at interfaces. Surfactants offer a way to influence both the electrostatics and hydration of an electrochemical interface. Here we report measurements of local electric fields generated between layers of surfactants and an electrode without application of an external potential. We estimate the value of the electric field by measuring the vibrational frequency shift of a well-defined Stark probe that is adsorbed on the electrode. The field generated by the common quaternary ammonium surfactants in aqueous media is estimated to be in the order of 1 V/nm. We show that modest quantities of surfactants (50 mM) can produce polarizing fields similar to those produced by an electrode poised at -0.8 V vs. Ag/AgCl. Furthermore, we show that under electrochemical conditions, surfactants offer a way of modulating the water content near the electrode. In a mixture of water and DMSO, there are two forces that act upon the surfactants to drive them to the interface: they hydrophobic forces and the electrostatic forces from the electrode. We measure the surfactants at the interface as a function of these variables using surface enhanced IR spectroscopy, and arrive at quantitative comparisons between these two effects. These results help in understanding the design principles of modifying interfacial reactions via tailored surfactants.