Asymptotic Underscreening in Concentrated Electrolytes Measured by Optical Second Harmonic Scattering of Water.

We show that with optical second harmonic scattering, one can measure the screening length in the bulk phase of aqueous electrolytes. The methodology is based on the analysis of the coherent contribution describing the water-water orientation correlation function present in the polarization-resolved optical second harmonic scattering intensity. Measurements were performed up to highly concentrated NaCl and LiCl electrolyte solutions, enabling the development of a theoretical framework valid over the whole salt concentration range. At low salt concentrations, the experimental findings adhere to the standard Debye-Hückel theory. However, at high salt concentrations, the observed screening length deviates from the Debye screening length and reaches an asymptotic value dependent on the ion size. This asymptotic screening length effectively demonstrates underscreening, but no anomalous increase of the screening length at high concentrations is observed.