Field-Dependent Orientation and Free Energy of D₂O at an Electrode Surface Observed via SFG Spectroscopy

Polarization-selected vibrational sum frequency generation (SFG) spectroscopy of D2O is used to obtain the orientation of the free OD bond at a monolayer graphene electrode. We modulate the interfacial field by varying the applied electrochemical potential, and we measure the resulting change in the orientation. A hyperpolarizability model is used for the orientational analysis, which assumes a quadratic free energy orienting potential in the absence of the field, whose minimum and curvature determine the average tilt angle and the Gaussian width of the orientational distribution. The average free OD tilt angle changes in an approximately linear fashion with the applied field, from 46° from normal at −0.9 V vs Ag/AgCl (E = −0.02 V/Å) to 32° at −3.9 V vs Ag/AgCl (E = −0.17 V/Å). Using this approach, we map the free energy profile for the molecular orientation of interfacial water by measuring the reversible response to an external perturbation, i.e., a torque applied by an electric field acting on the molecule’s permanent dipole moment. This allows us to extract the curvature of the free energy orienting potential of interfacial water, which is (4.0 ± 0.8) × 10–20 J/rad2 (or 0.25 ± 0.05 eV/rad2 )