Ultrafast FRET to Study Spontaneous Micelle‐to‐Vesicle Transitions in an Aqueous Mixed Surface‐Active Ionic‐Liquid System

The spontaneous micelle‐to‐vesicle transition in an aqueous mixture of two surface‐active ionic liquids (SAILs), namely, 1‐butyl‐3‐methylimidazolium n ‐octylsulfate ([C 4 mim][C 8 SO 4 ]) and 1‐dodecyl‐3‐methylimidazoium chloride ([C 12 mim]Cl) is described. In addition to detailed structural characterization obtained by using dynamic light scattering, transmission electron microscopy (TEM), and cryogenic TEM techniques, ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) as a donor (D) to rhodamine 6G (R6G) as an acceptor (A) is also used to study micelle–vesicle transitions in the present system. Structural transitions of SAIL micelles ([C 4 mim][C 8 SO 4 ] or [C 12 mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D –A distances, and therefore, increased timescale of FRET. In [C 4 mim][C 8 SO 4 ] micelles, FRET between C153 and R6G occurs on an ultrafast timescale of 3.3 ps, which corresponds to a D –A distance of about 15 Å. As [C 4 mim][C 8 SO 4 ] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of [C 12 mim]Cl, the timescale of FRET increases to 300 ps, which suggests an increase in the D –A distance to 31 Å. At a 0.5 molar fraction of [C 12 mim]Cl, unilamellar vesicles are formed in which FRET occurs on multiple timescales of about 250 and 2100 ps, which correspond to D –A distances of 33 and 47 Å. Although in micelles and mixed micelles the obtained D –A distances are well correlated with their radius, in vesicles the obtained D –A distance is within the range of the bilayer thickness.