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In recent experiments, a sequence of changes in the wetting state (`wettingtransitions') has been observed upon increasing the temperature in systemsconsisting of pentane on pure water and of hexane on brine. This sequence oftwo transitions is brought about by an interplay of short-range and long-rangeinteractions between substrate and adsorbate. In this work, we argue that theshort-range interaction (contact energy) between hexane and pure water remainsunchanged due to the formation of a depletion layer (a thin `layer' of purewater which is completely devoid of ions) at the surface of the electrolyte andthat the presence of the salt manifests itself only in a modification of thelong-range interaction between substrate and adsorbate. In a five-layercalculation considering brine, water, the first layer of adsorbed hexanemolecules, liquid hexane, and vapor, we determine the new long-rangeinteraction of brine with the adsorbate {\em across} the water `layer'.According to the recent theory of the excess surface tension of an electrolyteby Levin and Flores-Mena, this water `layer' is of constant, i.e.\salt-concentration independent, thickness $\delta$, with $\delta$ being thehydrodynamic radius of the ions in water. Our results are in good agreementwith the experimental ones.Comment: 27 pages, 2 tables, 4 figure

Effect of ion hydration on the first-order transition in the sequential wetting of hexane on brine