Solvation of Inorganic Salts in N,N-Dimethylformamide: A Computer Simulation Investigation

The effect of inorganic salts on the local structure of N,N-dimethylformamide (DMF), known to be of vital importance in applications, e.g., electrospinning, is investigated by molecular dynamics computer simulations. Four salts, i.e., LiCl, LiBr, MgCl₂, and CaCl₂, are considered in various concentrations covering the entire range of their solubilities. The performance of several interaction models was assessed. It is found that when proper charge scaling of the ions is performed, all models considered are able to reasonably reproduce the known experimental properties of these solutions, including their densities, the measurement of which is also reported here. The only exception in this respect is the solubility of the salts: as only the AMBER model is found to be able to simultaneously reproduce the good solubility of all four salts considered, this is the model of our choice in this study. We find that the structure of these salt solutions is primarily determined by the cation-DMF interaction, which is found to be an order of magnitude stronger than the anion-DMF interaction and which clearly destroys the weak, CH-donated hydrogen-bonding structure of neat DMF. Solvation shell DMF molecules turn by their O atom to the cation, but within this constraint, their N atom also approaches it as closely as possible. The average lifetime of cation-DMF contacts is found to be on the order of 1 ns for Li⁺ and Ca²⁺, while for Mg²⁺, it turns out to be many orders of magnitude longer than the entire length of our simulated equilibrium trajectories of 50 ns. Further, the interaction energy of contact Mg²⁺-DMF pairs falls in the order of covalent chemical bonds, the coordination number results in exactly 6.00 in every case, and the first solvation shell itself is exceptionally well ordered. All these findings strongly suggest (yet do not prove) the formation of the [Mg(DMF)₆]²⁺ hexacomplex by a Mg²⁺ ion and its six first-shell DMF neighbors.