In situ nanoscale evaluation of pressure-induced changes in structural morphology of phosphonium phosphate ionic liquid at single-asperity contacts

Zixuan Li; Oscar Morales-Collazo; Robert Chrostowski; Joan F Brennecke; Filippo Mangolini

doi:10.1039/d1ra08026a

In situ nanoscale evaluation of pressure-induced changes in structural morphology of phosphonium phosphate ionic liquid at single-asperity contacts

RSC Adv. 2021 Dec 22;12(1):413-419. doi: 10.1039/d1ra08026a. eCollection 2021 Dec 20.

Authors

Zixuan Li^{1

2}, Oscar Morales-Collazo³, Robert Chrostowski^{1

2}, Joan F Brennecke³, Filippo Mangolini^{1

4}

Affiliations

¹ Texas Materials Institute, The University of Texas at Austin Austin TX 78712 USA filippo.mangolini@austin.utexas.edu.
² Materials Science and Engineering Program, The University of Texas at Austin Austin TX 78712 USA.
³ McKetta Department of Chemical Engineering, The University of Texas at Austin Austin TX 78712 USA.
⁴ Walker Department of Mechanical Engineering, The University of Texas at Austin Austin TX 78712 USA.

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 ([P_6,6,6,14][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 [P_6,6,6,14][DEHP] molecules. This pressure-induced morphological change of [P_6,6,6,14][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 [P_6,6,6,14][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.