Adsorption of water vapor on MgCl2 × 6H2O salt surface

► The adsorption of water vapor on dry solid films of MgCl2 × 6H2O salt was studied. ► The uptake of water was found to be completely reversible. ► The adsorption data are well described by a Freundlich isotherm with the heterogeneity parameter close to 0.5. ► The isosteric enthalpy of adsorption was found to be -(44.7 ± 1.2) kJ mol−1. ► The amount of surface adsorbed water on mixed salts is drastically dependent on the salt sample composition.