Impact of Additive Hydrophilicity on Mixed Dye-Nonionic Surfactant Micelles: Micelle Morphology and Dye Localization

The nonionic surfactant pentaethylene glycol-monododecylether C₁₂E₅ forms micelles in aqueous solutions with a lower critical solution temperature. This characteristic solution behavior of C₁₂E₅ is independent of the pH. Such micelles are used to solubilize a large variety of active guest molecules like for instance dyestuffs. An example is an acidic azo dye termed Blue used as a hair colorant. Depending on the pH, Blue gradually changes its hydrophilicity from the protonated BlueH at pH = 2 to the bivalent anion Blue^2- at pH = 13 while keeping the shape and size of Blue essentially unchanged. These features of C₁₂E₅ and Blue offer the unique chance to investigate the sole impact of a tunable hydrophilicity of a guest molecule on the solution behavior of mixed micelles of the guest and C₁₂E₅. Accordingly, the present work establishes a phase diagram of Blue-C₁₂E₅ micelles and analyzes their morphology including the spatial distribution of Blue in the micelles as a function of the hydrophilicity of Blue. Small angle neutron scattering reveals the size and shape of the micelles, and detailed contrast matching of the C₁₂E₅ supported by ¹H NMR with NOESY provided insight into the localization of Blue within the micelles as its hydrophilicity changes.