Study of the Stability of 1-Alkyl-3-methylimidazolium Hexafluoroantimonate(V) Based Ionic Liquids Using X-ray Photoelectron Spectroscopy

Abstract

A series of highly pure 1-alkyl-3-methylimidazolium hexafluoroantimonate(V) ionic liquids was first investigated by XPS. Reliable binding energy values for all elements within the samples were obtained by applying a C Is fitting model previously employed for other imidazolium ionic liquids, based on setting the C is aliphatic component to 285.0 eV when alkyl chains are longer than 8 carbons. We also observed a straightforward X-ray-mediated photoreduction of Sb(V) to Sb(III) species, i.e. reduction of [SbF6](-) anion to volatile SbF3. Thus, binding energies values observed at 542.5 and 533.1 eV were attributed to Sb 3d(3/2)(V) and Sb 3d(5/2)(V), respectively, whereas values at 541.0 and 531.7 eV were found for Sb 3d(3/2)(III) and Sb 3d5/2(III), respectively. The SbF3 being formed can be lost either by evaporation and/or redistribution to the bulk. Providing the sample is cooled to freezing temperatures, the SbF3 is prevented from escaping the ionic liquid, and it accumulated on the surface of cooled [C(4)C(1)Im][SbF6] and [C(8)C(1)Im][SbF6]. Particularly in the case of prolonged X-ray exposure of a cooled [C(4)C(1)Im][SbF6], an unprecedented further reduction of Sb(III) to Sb(0) was observed. This study provides fundamental knowledge on the XPS data for antimony compounds as well as providing a step toward the surface analysis of materials composed by antimony species, notably the hexafluoroantimonate(V) anion.