1 research outputs found
Field-Dependent Orientation and Free Energy of D<sub>2</sub>O 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 )