Surface force apparatus studies on the surface interaction of [Cnmim+][BF4−] and [Cnmim+J][PF6−] ionic liquids

Abstract

Ionic liquids have drawn wide attention for its potential in capacity system and lubricant, based on its wide electrochemical window and chemical stability. In the past, the research on ionic liquids has been concentrated on the molecular simulation about its surface tension and the discussions about the phenomenon at the gas-liquid interface. In this paper, the structure and distribution of ions of RTILs (Room Temperature Ionic Liquids) are studied by using SFA(Surface Force Apparatus). Compared with AFM(Atomic Force Microscope), SFA has higher accuracy of force measurements and a wider range of displacement. In our experiment, we explored the charge distribution of RTILs with anions of BF4” or PFó” at the solid-liquid surfaces and took the research on mechanical properties through force-distance curves measured by SFA. The influence, which is caused by the change of the length of carbon chain and different anions, on hardwall(the closest distance between two silica surfaces) and maximum amplitude of force are discussed. Based on the decay length which is larger than the expectation of traditional theory, it is proposed that the distribution of charge between two solid surfaces could be regarded as neutrally coordinated cation-anion network with small portion of dissociated ions and it is necessary to take hydrogen bonding, chain-chain interaction, volume effects and steric effects into consideration in the systems proposed. The local charge neutrality can be existed in these network with a part of dissociated ions, which accounts for the long range exponentially decay repulsive force curves fitted by Gouy-Chapman-Stern model.