Self-Assembly in the Mixtures of Surfactant and Dye Molecule Controlled via Temperature and β‑Cyclodextrin Recognition

Abstract

A new ternary system of tetradecyldimethylamine oxide (C₁₄DMAO)/4-phenylazo benzoic acid (AzoH)/H₂O was first investigated, and it was found that the self-assembly can be regulated via temperature and β-cyclodextrin (β-CD) recognition. In the temperature regulated self-assembly, the self-assembled phase structural transition between wormlike micelles and multilamellar vesicles (onions) were determined by cryogenic-transmission electron microscopy (cryo-TEM) images and ²H nuclear magnetic resonance (²H NMR) spectra. The phase structural transition temperatures (PSTT) controlled by changing the amount of AzoH were measured by differential scanning calorimetry analysis. The self-assembled phase structural transition mechanism was discussed. It is argued that the self-assembled phase structural transition is the synergetic balance among the hydrophilic headgroup, steric structures of the hydrophobic chain, and membrane charge. β-CD molecules were used as controlling hands to modulate the phase structural transition of self-assembly of the C₁₄DMAO/AzoH/H₂O system in solution via snatching C₁₄DMAO molecules. The phase structural transitions from the threadlike micellar phase to the lamellar phase and from the lamellar phase to the vesicular phase can each be controlled because of the β-CD molecular recognition. The phase structural transitions were confirmed by cryo-TEM observations and ²H NMR measurements. The rheological properties were also investigated to display the importance in the phase structural transition. It was found that the dye molecule, AzoH, is harder to enclose by β-CD than by C₁₄DMAO because of the lower complex stability constant (i.e., <i>K</i>_{C₁₄DMAO@β‑CD} ≫ <i>K</i>_AzoH@β‑CD. Therefore, the phase structural transition is mainly controlled by the inclusion of C₁₄DMAO into the hydrophobic cavity of β-CD molecules. The phase structural transition controlled via temperature and β-CD may find potential applications such as in actuators, shape memories, drug delivery systems, and drag-reducing fluids, etc