Evidence for Nanostructures\nof at Least 20 nm in a\nPhosphonium Ionic Liquid at Room Temperature Using Fluorescence Correlation\nSpectroscopy

Fluorescence correlation spectroscopy (FCS) measurements\nare performed\non the ionic liquid (IL), tetradecyl(trihexyl) phosphonium chloride,\n[P66614<sup>+</sup>][Cl<sup>–</sup>], using fluorescent probes\nof varying sizes: ATTO 532, ∼2 nm; and 20- and 40 nm fluorescent\nbeads. The fluorescence correlation function, <i>G</i>(<i>t</i>), is analyzed in terms of a distribution of diffusion\ncoefficients using a maximum entropy method (MEM). For ATTO 532 and\nthe 20 nm beads, the fit to <i>G</i>(<i>t</i>)\nyields two well-defined distributions; for the 40 nm beads, however,\nonly one is obtained. These results are consistent with the existence\nof two nanodomains whose size is greater than or equal to 20 nm and\nless than 40 nm. The origin of such nanodomains is attributed to a\nliquid–liquid phase transition. Other groups have observed\nliquid–liquid phase transitions experimentally in a number\nof systems, including [P66614<sup>+</sup>][Cl<sup>–</sup>].\nWe suggest that because large regions (i.e., greater than 1–2\nnm) resulting from the liquid–liquid phase transition would\nbe expected to have different properties, such as viscosity, and because\ntheir presence would necessarily increase the number of interfaces\nin the IL, these regions may provide an explanation for the exceptional\nbehavior of ILs in various separation systems.