Fig. 1

SFG spectra. a O–H-stretch SFG signal for various NaCl solutions (ref. ²⁶). b SFG signal intensities, integrated from 2800 to 3600 cm⁻¹ and normalized to the curve maximum. The spectra corresponding to the marked data points are presented in the top panel, with corresponding colors. All spectra are measured under resting conditions, directly after flowing for 5 min. Dashed line: the predicted intensity variation by invoking modulation by charge screening and interference, at constant χ² and constant surface charge density, as detailed in ref. ²⁶