Nanostructure of Polyoxometalate-Ionic Liquids: Effects of Anion Geometry and Cation Chain Length.

Polyoxometalate-ionic liquids (POM-ILs) show promise for catalysis and energy storage, but how molecular structure controls nanostructure remains unclear. This study uses small-angle X-ray scattering to examine how POM geometry and cation chain length affect self-assembly in four systems combining Keggin ([SiW11O39]8-) or Dawson ([P2W18O62]6-) polyoxometalates with tetraoctylammonium (Q8+) or hexadecyltributylphosphonium (Q16+) cations. Keggin-Q8 remained solid because the high POM charge density (-8) overwhelms the limited disorder imparted by the octyl chains. Dawson-Q8 formed a liquid with a 22.1 Å spacing, matching simple volume-fraction packing. Both Q16 systems produced liquids with an amphiphilic nanostructure. The repeat spacings of Dawson-Q16 (34.5 Å) and Keggin-Q16 (28.5 Å) exceed volume-fraction predictions by 75% and 45%, respectively, due to solvophobic self-assembly into polar/apolar domains. These results demonstrate that the POM charge density and cation chain length control liquid formation and the nanostructure.