  For wetting films in dilute electrolyte solutions close to charged walls we
present analytic expressions for their effective interface potentials. The
analysis of these expressions renders the conditions under which corresponding
wetting transitions can be first- or second-order. Within mean field theory we
consider two models, one with short- and one with long-ranged solvent-solvent
and solvent-wall interactions. The analytic results reveal in a transparent way
that wetting transitions in electrolyte solutions, which occur far away from
their critical point (i.e., the bulk correlation length is less than half of
the Debye length) are always first-order if the solvent-solvent and
solvent-wall interactions are short-ranged. In contrast, wetting transitions
close to the bulk critical point of the solvent (i.e., the bulk correlation
length is larger than the Debye length) exhibit the same wetting behavior as
the pure, i.e., salt-free, solvent. If the salt-free solvent is governed by
long-ranged solvent-solvent as well as long-ranged solvent-wall interactions
and exhibits critical wetting, adding salt can cause the occurrence of an
ion-induced first-order thin-thick transition which precedes the subsequent
continuous wetting as for the salt-free solvent.
