Self-assembly of azobenzene bilayer membranes in binary ionic liquid-water nanostructured media

Anionic azobenzene-containing amphiphile 1 (sodium 4-[4-(N-methyl-N- dodecylamino)phenylazo]benzenesulfonate) forms ordered bilayer membranes in binary ionic liquid (1-ethyl-3-methylimidazolium ethyl sulfate, [C ₂mim][C₂OSO₃])-water mixtures. The binary [C₂mim][C₂OSO₃]-water mixture is macroscopically homogeneous at any mixing ratio; however, it possesses fluctuating nanodomains of [C₂mim][C₂OSO₃] molecules as observed by dynamic light scattering (DLS). These nanodomains show reversible heat-induced mixing behavior with water. Although the amphiphile 1 is substantially insoluble in pure water, it is dispersible in the [C ₂mim][C₂OSO₃]-water mixtures. The concentration of [C₂mim][C₂OSO₃] and temperature exert significant influences on the self-assembling characteristics of 1 in the binary media, as shown by DLS, transmission electron microscopy (TEM), UV-vis spectroscopy, and zeta-potential measurements. Bilayer membranes with rod- or dotlike nanostructures were formed at a lower content of [C₂mim] [C₂OSO₃] (2-30 v/v %), in which azobenzene chromophores adopt parallel molecular orientation regardless of temperature. In contrast, when the content of [C₂mim][C₂OSO₃] is increased above 60 v/v %, azobenzene bilayers showed thermally reversible gel-to-liquid crystalline phase transition. The self-assembly of azobenzene amphiphiles is tunable depending on the volume fraction of [C ₂mim][C₂OSO₃] and temperature, which are associated with the solvation by nanoclusters in the binary [C ₂mim][C₂OSO₃]-water media. These observations clearly indicate that mixtures of water-soluble ionic liquids and water provide unique and valiant environments for ordered molecular self-assembly.