**4. Conclusions**

The relationship between the composition of spinels and their general properties is undeniable, and has been widely demonstrated in the literature. Surface behavior and water–surface interactions are no exception. The composition dictates, in part, the defect structure of a material. This, in turn, controls the surface behavior. Ultimately, these aspects govern the water–surface interaction.

We have shown that changes in stoichiometry alter the water adsorption behavior in our system. In general, an increase in the Al2O3 concentration lowers both water uptake and the energy of water adsorption. Changes in the Al2O3 concentration influence the defect structure of the material, thereby changing the adsorption site population and its energetic diversity. Furthermore, the material composition affects the reducibility of the material, and thus, its ability to host more defects. These defects promote water uptake while lowering the adsorption enthalpy.

In addition to the effects of non-stoichiometry, the effects of intrinsic defects in spinel should be considered when dealing with water–surface interactions. A spinel having a lower inversion parameter (i), i.e., a material with fewer anti-site defects, was shown to adsorb fewer strongly bonded water molecules and to present lower enthalpies of adsorption, indicating that the Al cation is more active when it occupies a tetrahedral site in the spinel structure.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/13/14/3195/s1, Figure S1: Adsorption isotherms of the samples with clean surface, Figure S2: Adsorption isotherms of the samples with reduced surface, Table S1: Atomic percentage of Al and Mg oxides and hydroxides for adsorption processes with reduction, Table S2: Atomic percentage of Al and Mg oxides and hydroxides for adsorption processes with oxidation.

**Author Contributions:** Conceptualization, S.H.; samples synthesis and characterization, Y.M. and Y.S.; calorimetric measurements, Y.S.; data analysis, Y.M., Y.S., N.F. and S.H.; writing—original draft preparation, Y.M. and Y.S.; writing—review and editing, S.H.; visualization, Y.M. and Y.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially supported by the BSF United States-Israel Binational Science Foundation (grant 2010377) and the FP7-PEOPLE-2012-CIG (grant 321838-EEEF-GBE-CNS).

**Acknowledgments:** Yuval Mordekovitz gratefully acknowledges the Israeli Ministry of Science and Technology (MOST) for granting him the Ze'ev Jabotinsky scholarship.

**Conflicts of Interest:** The authors declare no conflict of interest.
