Ordering in ionic and molecular liquids: insights from wide angle X-ray scattering

 

Non-halogenated Ionic Liquids (ILs) show great promise as lubricants, due to their propensity to degrade into protective tribofilms, instead of promoting corrosion; their lubricity, electroresponsivity and bulk and interfacial nanostructuring has recently been experimentally confirmed.

Interfacial activity of both ionic and molecular species can be predicted if their bulk properties are known. To gain an insight into the ordering in ionic and molecular liquids we used wide angle X-ray scattering.

A Diamond Anvil Cell and synchrotron wide-angle X-ray scattering (WAXS), and a custom cylinder stress-strain apparatus, with specific focus on systems

pertinent to lubrication applications were used. With reference to a prototypical molecular oil-based lubricants like PAOs (poly-alpha-olefins), we show reversible glass transitions which occur repeatably for all ILs and the molecular fluid at 1.7 – 2.2 GPa, which is essential knowledge for informed deployment of lubricant materials. We also show the effect of temeprature on ordering. Analysis of the shift in d -spacing yields the stiffness of the liquid and vitreous states as a function of molecular architecture. The results of this combined approach bring some of the commonly held differences between ionic and molecular liquids into focus.