Ultrafast FRET to Study Spontaneous Micelle‐to‐Vesicle Transitions in an Aqueous Mixed Surface‐Active Ionic‐Liquid System |
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| Authors: | Sarthak Mandal Jagannath Kuchlyan Debasis Banik Surajit Ghosh Chiranjib Banerjee Vijaykant Khorwal Nilmoni Sarkar |
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| Affiliation: | 1. Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, WB (India), Tel.: 91‐3222‐283332;2. Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076 (India) |
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| Abstract: | The spontaneous micelle‐to‐vesicle transition in an aqueous mixture of two surface‐active ionic liquids (SAILs), namely, 1‐butyl‐3‐methylimidazolium n‐octylsulfate (C4mim]C8SO4]) and 1‐dodecyl‐3‐methylimidazoium chloride (C12mim]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 (C4mim]C8SO4] or C12mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D –A distances, and therefore, increased timescale of FRET. In C4mim]C8SO4] 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 C4mim]C8SO4] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of C12mim]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 C12mim]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. |
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| Keywords: | FRET ionic liquids micelles self‐assembly vesicles |
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