Molecular dynamics simulations in slab geometry and surface tension
measurements were performed for aqueous solutions of magnesium acetate and
magnesium nitrate at various concentrations. The simulations reveal a strong affinity of
acetate anions for the surface, while nitrate exhibits only a very weak surface propensity,
and magnesium is per se strongly repelled from the air/water interface. CH3COO- also
exhibits a much stronger tendency than NO3- for ion pairing with Mg2+ in the bulk and
particularly in the interfacial layer. The different interfacial behavior of the two anions is
reflected in opposite concentration dependence (beyond 0.5 M) of surface tension of the
corresponding magnesium salts. Measurements, supported by simulations, show that
surface tension of Mg(NO3)2(aq) increases with concentration as for other inorganic salts.
However, in the case of Mg(OAC)2(aq) the surface tension isotherm exhibits a turnover
around 0.5 M, after which it starts to decrease, indicating a positive net solute excess in the
interfacial layer at higher concentrations.