The Effect of Moisture on the Hydrolysis of Basic Salts

A great deal of information exists concerning the hydration of ions in bulk water. Much less noticeable, but equally ubiquitous is the hydration of ions holding on to several water molecules in nanoscopic pores or in natural air at low relative humidity. Such hydration of ions with a high ratio of ions to water molecules (up to 1:1) are essential in determining the energetics of many physical and chemical systems. Herein, we present a quantitative analysis of the energetics of ion hydration in nanopores based on molecular modeling of a series of basic salts associated with different numbers of water molecules. The results show that the degree of hydrolysis of basic salts in the presence of a few water molecules is significantly different from that in bulk water. The reduced availability of water molecules promotes the hydrolysis of divalent and trivalent basic ions (S₂^- , CO₃^2- , SO₃^2- , HPO₄^2- , SO₄^2- , PO₄^3- ), which produces lower valent ions (HS^- , HCO₃^- , HSO₃^- , H₂ PO₄^- , HSO₄^- , HPO₄^2- ) and OH^- ions. However, reducing the availability of water inhibits the hydrolysis of monovalent basic ions (CN^- , HS^- ). This finding sheds some light on a vast number of chemical processes in the atmosphere and on solid porous surfaces. The discovery has wide potential applications including designing efficient absorbents for acidic gases.