The surfaces of ionic crystals are, in general, electrically charged,
with a compensating subsurface ionic space charge. Although a wide var
iety of phenomena are expected to result from the effects of this doub
le layer, the surface charge has been well-characterized only for the
silver halides. This paper reviews the origin of the space charge phen
omenon, its effects on near-surface properties of ionic crystals (and
especially, the photographic process), and the experimental techniques
that have been employed in silver halide studies. In particular, some
recent work, in which sub-surface radiotracer distributions are measu
red, have yielded quantitative data on the individual formation enthal
pies and entropies of the cation vacancy and the cation interstitial i
on.