Fig. 2.

Experimental and theoretical partial spectral components as a function of negative and positive applied potential and surface charge. (A, Top) Experimental partial spectra reconstructed for PCA for discrete negative applied potentials, attributed to Na⁺ hydration. (A, Bottom) Residuals derived from the subtraction of the partial spectra from the total spectra in Fig. 1A. (B) Experimental partial spectra reconstructed for PCA for discrete positive applied potentials, attributed to Cl⁻ hydration. (Bottom) Residuals. (C) Experimental partial spectra reconstructed for PCA for discrete positive applied potentials, attributed to the interfacial water network. (D) Simulated spectrum of hydrated Na⁺ from ref. 39 multiplied by the variation in the number of hydration water around Na⁺ cations as a function of the negative surface charge as deduced in the present MD simulations. (E) Simulated spectrum of hydrated Cl⁻ ion from ref. 39 multiplied by the variation in the number of hydration waters around Cl⁻ as a function of the positive surface charge as deduced in the present MD simulations. (F) HB stretch mode of water molecules hydrating a hydrophobic (alcohol) surface, as in ref. 43, multiplied by the number of HBs of the 2D-HB network as a function of the positive surface charge as deduced in the present MD simulations. We point out that a feature at 70 cm⁻¹ is observed in all experimental spectra (A, B, and C), which is related to the spectrum of the synchrotron light, used as probing source. Increasing surface charge is associated with increasing voltage, though not necessarily via a 1:1 mapping.