Multinuclear NMR Study on the Formation and Polyol-Induced Deformation Mechanisms of Wormlike Micelles Composed of Cetyltrimethylammonium Bromide and 3-Fluorophenylboronic Acid.

We had previously confirmed a glucose-responsive decrease in the viscosity of cetyltrimethylammonium bromide (CTAB) and phenylboronic acid (PBA) wormlike micelle (WLM) systems. However, the mechanisms of the formation of WLMs and the decrease in viscosity with glucose addition have not been determined. In this study, we elucidated the mechanisms using 3-fluorophenylboronic acid (3FPBA) based on B NMR and F NMR analyses. The system in 60 mM CTAB/60 mM 3FPBA at pH 7.4 demonstrated high viscoelasticity, and the formation of WLMs in the system was confirmed by rheological characteristics. The B NMR spectrum at pH 7.4 revealed that 3FPBA existed in a neutral form with sp-hybridized boron; however, the B signal disappeared in the presence of CTAB. In contrast, F NMR studies indicated that the quaternary ammonium ion of CTAB interacts with the phenyl group of 3FPBA in the sp form via cation-π interactions. PBA derivatives react with various polyols; thus, we investigated the change in the viscous system after the addition of sugar and sugar alcohols. The viscosity of the WLMs decreased with increased polyol concentration, especially those of fructose and mannitol, in which the decrease was apparent at 40-160 mM polyols. The F NMR spectra revealed that polyol addition induced decrease in the sp form of 3FPBA and increase in the sp form of 3FPBA. Based on the results, we propose the following mechanism of the polyol response: (1) The WLMs are stabilized by CTAB and 3FPBA in the sp form using cation-π interactions as the driving force. (2) When polyol is added to the system, the sp form of 3FPBA decreases and its sp form increases. (3) This change means that the structural component of WLMs decreases, which induces the disruption of WLMs, and the viscosity decreases. The formation and deformation mechanisms of the WLMs determined in this study are notable because 3FPBA interacts as a neutral compound, whereas CTAB often interacts with anionic aromatic compounds to form WLMs. Without F NMR measurements, these mechanisms would not have been discovered.