Enthalpy of solvation of alkali metal salts in a protic ionic liquid: Effect of cation charge and size

Abstract

• Experimental and theoretical study of solution and solvation of mono- and divalent alkaline metal cations. • MD simulations provided coordination numbers, which showed an interesting similarity in IL and water environments. • Solvation of alkali and alkaline earth nitrate metal salts ((LiNO 3 , KNO 3 , RbNO 3 , CsNO 3 , Mg(NO 3 ) 2 and Ca(NO 3 ) 2 ) in EAN. • Analysis of the energetics and structure of solvation confirms the well-known water-like solvation properties of EAN. An experimental and theoretical study of solution and solvation of mono- and divalent alkali metal cations in the protic ionic liquid (IL) ethylammonium nitrate (EAN) is reported. High precision solution-reaction calorimetry was used to obtain the heat of solvation, which was used for the analysis of the thermodynamics. A close relation between the structure of the salts in the crystalline phase and its enthalpy of solvation in the IL is reported. A detailed picture of the molecular environments in the solvation shells around the metal cations is provided by means of molecular dynamics simulations. The analysis of the energetics and structure of solvation confirms the well-known water-like solvation properties of EAN, with the solvation shell around the metal cations in both media being very similar. On the other hand, the results show that it is energetically more favourable to solvate smaller cations with higher valence. Indeed, the simulations show that the long-range electrostatic interactions are the main contribution to solvation interaction, with the electric field at the surface of the alkali metal cations as the basic magnitude controlling it.