Abstract
In this work, we perform atomic force microscopy (AFM) experiments to evaluate in situ the dependence of the structural morphology of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P6,6,6,14][DEHP]) ionic liquid (IL) on applied pressure. The experimental results obtained upon sliding a diamond-like-carbon-coated silicon AFM tip on mechanically polished steel at an applied pressure up to 5.5 ± 0.3 GPa indicate a structural transition of confined [P6,6,6,14][DEHP] molecules. This pressure-induced morphological change of [P6,6,6,14][DEHP] IL leads to the generation of a lubricious, solid-like interfacial layer, whose growth rate increases with applied pressure and temperature. The structural variation of [P6,6,6,14][DEHP] IL is proposed to derive from the well-ordered layering of the polar groups of ions separated by the apolar tails. These results not only shed new light on the structural organization of phosphonium-based ILs under elevated pressure, but also provide novel insights into the normal pressure-dependent lubrication mechanisms of ILs in general.
Highlights
Ionic liquids (ILs) consist of organic cations and weakly coordinating anions
The atomic force microscopy (AFM) experiments performed at 111 Æ 1 C indicated a variation in nanoscale structural morphology of [P6,6,6,14] [DEHP] IL upon scanning at an applied average normal pressure of 5.5 Æ 0.3 GPa
An increase in contrast in the friction force maps was detected (Fig. 2), which indicated a reduction in friction force with pressure-induced changes in structural morphology of [P6,6,6,14][DEHP] IL occurring upon scanning at 5.5 Æ 0.3 GPa and 111 Æ 1 C
Summary
Ionic liquids (ILs) consist of organic cations and weakly coordinating anions. Paper high pressure (0.6 GPa).[41] Saouane et al evaluated the solidstate polymorphism of the same IL using single-crystal X-ray diffraction, Raman spectroscopy, and optical microscopy.[57] The experimental results indicated the existence of three polymorphs, which was proposed to originate from the conformational exibility of [C4mim] cations together with the rotational disorder of [PF6] anions. Sharma et al employed MD simulations to identify structural variations in 1-alkyl-1-methylpyrrolidinium bis(tri uoromethylsulfonyl)amide ([Pyrr1,n][NTf2], where n 1⁄4 8 or 10) and demonstrated the susceptibility of both apolar and polar groups to changes in applied pressure, while showing that [NTf2] anions could undergo conformational changes.[42]
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