Nanodomains and\nTheir Temperature Dependence in a\nPhosphonium-Based Ionic Liquid: A Single-Molecule Tracking Study

Ionic liquids (ILs) exhibit a unique\nnanoscale structure\n(i.e.,\nnanodomains) characterized by their organization into distinct domains.\nWe present evidence of nanodomains in trihexyl(tetradecyl)phosphonium\nchloride, [P<sub>66614</sub>][Cl], using single-molecule tracking\n(SMT) and the maximum entropy method (MEM) to analyze single-molecule\ntrajectories. The diffusion properties of ATTO 647N were assessed\nas the temperature of [P<sub>66614</sub>][Cl] increased from 20 °C\n(4020 cP), 35 °C (1239 cP), 45 °C (599 cP) to 50 °C\n(439 cP). The MEM analysis revealed a distinct two-population distribution\nof diffusion coefficients representing nanodomains in [P<sub>66614</sub>][Cl] at 20 °C (4020 cP). The slow population accounts for 16%,\nwith a diffusion coefficient of 0.104 μm<sup>2</sup>/s, while\nthe fast population constitutes 84% with a diffusion coefficient of\n0.634 μm<sup>2</sup>/s. Two diffusing populations were also\nmeasured for the chemically different probes ATTO 647N, DiD, and Nile\nBlue chloride in [P<sub>66614</sub>][Cl] at 20 °C. In contrast,\nonly a single fast population was measured in [P<sub>66614</sub>][Cl]\nat 50 °C. At a similar viscosity (640 cP) but a lower temperature\nof 20 °C, trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)-sulfonyl]imide,\n[P<sub>66614</sub>][NTf<sub>2</sub>], also showed only a single diffusing\npopulation. The elimination of the slow population and the presence\nof a single diffusing population in [P<sub>66614</sub>][Cl] as the\ntemperature increases and the viscosity decreases is consistent with\nliquid–liquid phase separation (LLPS) as a mechanism of nanodomain\nformation. In addition, the measurement of two diffusing populations\nfor three fluorophores with different chemical structures is also\nconsistent with a physical mechanism, and not a chemical mechanism,\nfor nanodomain formation.