Quantum theory of interfacial tension quantitatively predicts spontaneous charging of nonpolar aqueous interfaces

•Spontaneous charging of aqueous nonpolar interfaces challenges Debye dielectrics.•A quantum non-Debye theory of interfacial tension is developed.•The minimization of the interfacial tension promotes hydroxide ion adsorption.

The spontaneous negative charging of aqueous nonpolar interfaces has eluded quantitative first-principle prediction, possibly because it steadfastly challenges the classical Debye dielectric picture. In this work we show that quantitative prediction requires a substantive revision of Debye's linear dielectric ansatz to incorporate an anomalous polarization component yielding electrostatic energy stored as interfacial tension and detailed enough to account for the differences in electronic structure between water and its ionized states. The minimization of this interfacial tension is due to a quantum effect resulting in the reduction in hydrogen-bond frustration that takes place upon hydroxide ion adsorption. The quantitative predictions are validated vis-à-vis measurements of the free energy change associated with hydroxide adsorption obtained using sum-frequency vibrational spectroscopy.