Ionic liquid adsorption at the silica–oil interface revealed by neutron reflectometry

Abstract

Previous nanotribology measurements demonstrated that 2 mol % of the oil-miscible ionic liquid (IL) trihexyltetradecyl­(phosphonium) bis­(2,4,4-trimethylpentyl)­phosphinate (P6,6,6,14 ( i C8)2PO2) diluted in octane lubricated as effectively as pure IL. However, until now the structure and composition of the lubricating adsorbed layer, which is critical for lubrication, was unknown. Here, the unconfined structure of the IL adsorbed layer at the oil–silica interface has been studied using neutron reflectometry. Multiple neutron contrasts revealed an 8 Å thick adsorbed layer, even at 60 and 80 °C. The ratio of cations and anions in the layer was investigated by synthesizing the IL with deuterated cations and measuring its reflectivity at the oil–silica interface. At 60 °C the layer was composed of 48 ± 6 mol % P6,6,6,14 + cations, 24 ± 2 mol % ( i C8)2PO2 – anions, and 28 ± 8 mol % octane, while at 80 °C the composition was 50 ± 2 mol % P6,6,6,14 +, 28 ± 2 mol % ( i C8)2PO2 – anions, and 22 ± 2 mol % octane. These results reinforce the importance of the judicious selection of IL cations and anions for charged surfaces and support their use in high-temperature applications. © 2018 American Chemical Society.