  In recent experiments, a sequence of changes in the wetting state (`wetting
transitions') has been observed upon increasing the temperature in systems
consisting of pentane on pure water and of hexane on brine. This sequence of
two transitions is brought about by an interplay of short-range and long-range
interactions between substrate and adsorbate. In this work, we argue that the
short-range interaction (contact energy) between hexane and pure water remains
unchanged due to the formation of a depletion layer (a thin `layer' of pure
water which is completely devoid of ions) at the surface of the electrolyte and
that the presence of the salt manifests itself only in a modification of the
long-range interaction between substrate and adsorbate. In a five-layer
calculation considering brine, water, the first layer of adsorbed hexane
molecules, liquid hexane, and vapor, we determine the new long-range
interaction of brine with the adsorbate {\em across} the water `layer'.
According to the recent theory of the excess surface tension of an electrolyte
by Levin and Flores-Mena, this water `layer' is of constant, i.e.\
salt-concentration independent, thickness $\delta$, with $\delta$ being the
hydrodynamic radius of the ions in water. Our results are in good agreement
with the experimental ones.
