  The surface charge of a water interface determines many fundamental processes
in physical chemistry and interface science, and it has been intensively
studied for over a hundred years. We summarize experimental methods to
characterize the surface charge densities developed so far: electrokinetics,
double-layer force measurements, potentiometric titration, surface-sensitive
nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we
elucidate physical ion adsorption and chemical electrification as examples of
electrification mechanisms. In the end, novel effects on surface
electrification are discussed in detail. We believe that this clear overview of
state of the art in a charged water interface will surely help the fundamental
progress of physics and chemistry at interfaces in the future.
