  Mapping the electrostatic potential (ESP) distribution around ions in
electrolyte solution is crucial for the establishment of a microscopic
understanding of electrolyte solution properties. For solutions in the bulk
phase, it has not been possible to measure the ESP distribution on Angstrom
scale. Here we show that liquid electron scattering experiment using
state-of-the-art relativistic electron beam can be used to measure the Debye
screening length of aqueous LiCl, KCl, and KI solutions across a wide range of
concentrations. We observe that the Debye screening length is long-ranged at
low concentration and short-ranged at high concentration, providing key insight
into the decades-long debate over whether the impact of ions in water is
long-ranged or short-ranged. In addition, we show that the measured ESP can be
used to retrieve the non-local dielectric function of electrolyte solution,
which can serve as a promising route to investigate the electrostatic origin of
special ion effects. Our observations show that, interaction, as one of the two
fundamental perspectives for understanding electrolyte solution, can provide
much richer information than structure.
