This research demonstrates how cation chain length influences nanostructure and energy storage in polyoxometalate-ionic liquids, revealing key self-assembly mechanisms.
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.
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Tahroudi et al. (2026) studied this question.
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