Proton Microenvironment and Interfacial Structure of Sulfonic-Acid-Functionalized Ionic Liquids

Sulfonic-acid-functionalized ionic liquids (SFILs) have shown promising performance in catalytic reactions and development of new proton conductive materials. In this work, molecular dynamic simulation based on the ion pair charge approach has been performed to investigate the structural characteristic of several typical SFILs in both bulk liquids and the vacuum-liquid interfacial region. The results showed that anions play a critical role in determining the microenvironment of the terminal sulfonic acid proton in SFILs, and less basic anions lead to a much weaker association of sulfonic acid protons with anions. A significant aggregation of sulfonic acid side chains existed in SFILs because of the strong interaction between different sulfonic acid groups. A sharp change in the ordering preference of alkyl tails and pyridine rings at vacuum-liquid interfaces was observed after the introduction of a sulfonic acid group to the side chain of the cation of ionic liquids, and the properties of the anions have a remarkable influence on the preferential location of anions in the interfacial region. These results can aid in understanding of the physiochemical properties of SFILs and thus facilitate the development of new SFILs and proton conductive materials.