基于甲氧肉桂酸衍生物光控分子构建光响应蠕虫状胶束的结构性质关系

Structure-Property Relationship of Light-Responsive Wormlike Micelles Using Methoxycinnamate Derivatives as Light-Switchable Molecules

本文利用十六烷基三甲基氢氧化铵和不同结构的甲氧基取代的肉桂酸衍生物构建了光响应蠕虫状胶束体系,研究了甲氧取代基的位置和数量对蠕虫状胶束结构与光响应性的影响。研究发现,肉桂酸衍生物通过将其芳香环插进胶束内核促进了蠕虫状胶束的形成。相较于甲氧基的位置,甲氧基的数量对所形成蠕虫胶束结构的影响更为显著,因为引入的甲氧基会增大表面活性剂与肉桂酸衍生物之间的空间位阻,不利于更大聚集体的形成。在光诱导蠕虫状胶束结构转变的过程中,甲氧基的取代位置则更为重要。总体而言,含有邻位甲氧基的肉桂酸衍生物体系的转变过程较快,而间位和对位甲氧基取代的肉桂酸衍生物体系的转变则较慢。

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. 1 H 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 transcis 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. 1 H 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.