All interfaces between dissimilar media are characterized by asymmetry, which is manifests itself in anisoptropic molecular orientation. This means that the free enrgy potential for the interfacial molecular orientation, instead of being flat as in bulk liquid, has a minimum corresponding to the most likely orientation and a width giving rise to the orientational distribution. Here we present a method for experimentally quantifying the free energy potential for the molecular orientation of water molecules at a single-layer graphene electrode. The electric field produced by the charged plane of the graphene results in a torque on the water dipole. We measure the induced reorientation using polarization-selected Vibrational Sum Frequency Generation spectroscopy. In effect, this provides a calibration of the orientational free energy potential in the absence of the applied field against the applied -μ●E term.

Measuring the free energy orienting potential of water molecules at a graphene electrode

ACS Fall 2021

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