Evidence for Nanostructures of at Least 20 nm in a Phosphonium Ionic Liquid at Room Temperature Using Fluorescence Correlation Spectroscopy

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