<i>In situ</i> formation and dispersion of lanthanide complexes in wormlike micelles.

Abstract

Lanthanide-containing, water-based fluids normally suffer from low photoluminescent (PL) and/or colloidal stability, which greatly hinders their applications. Herein, we report the preparation of PL fluids which contain <i>in situ</i> formed europium complexes in aqueous solution. The strategy first relies on the construction of wormlike micelles by mixing a zwitterionic surfactant (tetradecyldimethylaminoxide, C₁₄DMAO) and a tridentate ligand for a lanthanide cation (2,6-dipicolinic acid, DPA) in water. The addition of the dual-functionalized DPA to an aqueous solution of C₁₄DMAO (100 mol L^-1) induced non-monotonic rheological changes, with the expected formation of a pseudogemini surfactant at a DPA-to-C₁₄DMAO molar ratio of approximately 1 : 2. When a third component of EuCl₃ is introduced to this system, complexes formed <i>in situ</i> between Eu³⁺ and DPA, resulting in bright red-emission. Besides DPA, C₁₄DMAO is also involved in the complexation, which squeezes out water molecules and greatly improves the PL stability of the fluid. The synergetic effect among Eu³⁺, DPA and C₁₄DMAO leads to the high colloidal stability of the fluid, opening the door for a wide range of potential applications. Further tests indicate that this strategy can be easily expanded to other lanthanide cations such as Tb³⁺.