Abstract:

In this work, light-responsive viscoelastic wormlike micelles based on cetyltrimethylammonium hydroxide (CTAOH) and cinnamic acid derivatives, including cinnamic acid (CA), 2-methoxycinnamic acid (2-MCA), 3-methoxycinnamic acid (3-MCA), 4-methoxycinnamic acid (4-MCA), 2, 3-dimethoxycinnamic acid (2, 3-DMCA), 2, 4-dimethoxycinnamic acid (2, 4-DMCA), 2, 3, 4-trimethoxycinnamic acid (2, 3, 4-DMCA), and 3, 4, 5-trimethoxycinnamic acid (3, 4, 5-DMCA), were prepared. The effects of the CA derivative structures, especially the position and number of methoxy moieties, on the formation of wormlike micelles were systematically determined. The CA derivatives facilitated the formation of long and entangled wormlike micelles. ¹H NMR results showed that the CA derivatives participated in the formation of wormlike micelles via insertion of the aromatic moieties into the aggregates. The number of methoxy moieties had a much stronger effect on the viscosity of the wormlike micelle solution than the position of this moiety. The larger the number of methoxy moiety, the smaller was the aggregate. Substituted methoxy moieties increased the steric hindrance between the surfactants and CA molecules, thus hindering the formation of large aggregates. However, the position of the methoxy moiety had a predominant effect on the UV-light-induced transition of the wormlike micelles. Specifically, the ortho-methoxy moiety in the CA molecules dramatically enhanced the efficiency of UV-light-induced trans-cis isomerization. For example, the 2-MCA/CTAOH, 3-MCA/CTAOH, and 4-MCA/CTAOH binary systems (90 mmol·L^-1/100 mmol·L^-1) were gel-like with similar viscosities of around 20 Pa·s, but after UV light irradiation, they were transformed into a fluid with lower viscosity because of the formation of smaller aggregates. However, the irradiation time required for the transition varied significantly, as suggested by the results of viscosity measurements and UV-Vis spectroscopy. The 2-MCA/CTAOH system underwent complete phase transition within 3 h, whereas continuous transitions were observed for the 3-MCA/CTAOH and 4-MCA/CTAOH systems upon irradiation for 24 h. ¹H NMR results suggested that the change in the configuration of MCA in the micelles before and after irradiation was the major cause of the abovementioned difference in the phase transition pattern. Initially, all the aromatic moieties of the trans-2-MCA molecules were deeply inserted into the hydrophobic cores of the micelles in a vertical manner, and the ionized carboxyl moiety was located in the palisade layer because of the electrostatic interactions between CTAOH and trans-2-MCA. In contrast, cis-2-MCA was inserted into the micelles in a horizontal manner, and some of the protons in the aromatic moiety were also transferred from the micellar core to the polar palisade layer. Accordingly, the CTAOH and cis-2-MCA molecules were packed loosely in the aggregates, thereby resulting in the formation of spherical micelles. Similar UV-light-induced transitions were observed for the 3-MCA/CTAOH and 4-MCA/CTAOH systems.

Key words: Cetyltrimethylammonium hydroxide, Cinnamic acid derivative, Wormlike micelle, Light response, trans-cis isomerization