Preparation and Characterization of Luminescent Alkyl β-D-galactoside–curcuminoid Dye Composites <i>via</i> Micelle-to-fiber Transition in Aqueous Systems

The precise understanding and control of the phase behavior of surfactant aqueous systems have enabled the development of diverse materials in the fields of foods, cosmetics, and pharmaceuticals. In this study, we investigated aqueous systems of the bio-based surfactants alkyl β-d-galactosides (CnGals, where n represents the chain length; n = 6, 8, 10, 11, and 12). First, we confirmed that CnGals aqueous systems formed micelle solutions upon heating and hydrogel upon cooling. In particular, decyl β-d-galactoside (C10Gal) exhibited a homogeneous fiber morphology at low concentrations (0.1 wt.%), whereas larger n values afforded an inhomogeneous solid matrix and smaller n values required higher concentrations or did not form a fiber structure. In addition, the C10Gal micelles solubilized curcuminoid dyes such as demethoxycurcumin and curcumin and allowed preparing luminescent fibers upon cooling the solution. The removal of water afforded the corresponding C10Gal-dye solid composite with a high luminescence quantum yield. Powder X-ray diffraction and differential thermal analysis suggested the formation of a stable adduct or cocrystal with a unique interaction between the curcuminoid dyes and C10Gal. Therefore, we successfully prepared an environmentally friendly, highly luminescent organic solid via micelle-to-fiber transition of an environmentally friendly surfactant induced by a mild temperature change.