Potential-Dependent Superlubricity of Ionic Liquids on a Graphite Surface
Yunxiao Zhang, Mark W. Rutland, Jiangshui Luo, Rob Atkin, Hua Li
Abstract
The lubricities of four quaternary phosphonium ionic liquids—trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][(iC8)2PO2]), trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate ([P6,6,6,14][BEHP]), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide ([P6,6,6,14][TFSI]), and tributylmethylphosphonium bis(trifluoromenthylsulfonyl)imide ([P4,4,4,1][TFSI])—were measured as a function of potential on highly oriented pyrolytic graphite (HOPG) by using atomic force microscopy (AFM). The shear strength and surface contact radius values fitted from JKR model indicate the AFM probe slides on an IL boundary layer rather than bare HOPG. Frictions change as the compositions of the boundary layers switch from cation enriched to anion enriched when the potential changes from negative to positive. Superlubricity, which refers to near zero increase in friction with load, is achieved for the [P6,6,6,14]+ cation at −1.0 V and the [TFSI]− anion at +1.0 V. The lubricities of ILs are mainly influenced by three factors: the alkyl chain length, chemical composition, and ion sizes.