Ionic Liquids Based upon Metal Halide/Substituted Quaternary Ammonium Salt Mixtures

The synthesis of ionic liquids based upon functionalized quaternary ammonium salts and metal salts of zinc, tin, or iron is demonstrated. The freezing point of these ionic liquids was studied as a function of the quaternary ammonium cation. The complex anions were identified and quantified using mass spectrometry and potentiometry. It is shown that the primary zinc anion is Zn2Cl5- with Zn3Cl7- becoming more abundant in more Lewis basic solutions. Similar results were observed for ionic liquids containing SnCl2. The surface tension was also measured and was used to explain the high viscosity of the ionic liquids in terms of the large ion:hole size ratio and the small probability of finding a hole of suitable dimensions adjacent to a given ion to permit movement. The phase behavior of a variety of quaternary ammonium halides/ZnCl2 mixtures is characterized and it is shown that the depression of freezing point is related to the increase in size of the component ions.