Light-responsive multillamellar vesicles in coumaric acid/alkyldimethylamine oxide binary systems: Effects of surfactant and hydrotrope structures.

Herein, we report a series of novel light-responsive multilamellar vesicles based on the surfactant/hydrotrope binary systems. The phase behaviors of alkyldimethylamine oxide (CmDMAO, m=10, 12, 14) and trans-coumaric acid (trans-CA) isomerides, including trans-ortho-coumaric acid (trans-OCA), trans-meta-coumaric acid (trans-MCA) and trans-para-coumaric acid (trans-PCA), show that the multilamellar vesicle (MLV) formation region is commonly presented in the trans-CA/CmDMAO systems except trans-PCA/C12DMAO. Moreover, the molecular structures of CmDMAO and trans-CA affect the multilamellar vesicle formation region significantly. Generally speaking, the bigger the m, the larger the MLV region. Various techniques such as rheology, polarized optical microscopy (POM), (1)H NMR, (2)H NMR, cryogen transmission electron microscopy (cryo-TEM) and freeze-fracture transmission electron microscopy (FF-TEM) are used to characterize the aggregate structures. The multilamellar vesicles can transform into a homogeneous and transparent micelle phase or a two-phase system in the trans-OCA/CmDMAO binary systems under UV light irradiation, which depends on the chain length of CmDMAO and the molar ratio of [trans-OCA]/[CmDMAO]. Specifically, the light-stimuli response of multilamellar vesicles in the trans-OCA/C12DMAO system is representatively studied in detail. UV-vis spectra and (1)H NMR measurements illustrate that the light-induced trans-OCA to cis-OCA isomerization is essential during the transitions and the light-induced two-phase formation is attributed to the enrichment of surfactants, because the trans-cis isomerization can not only strengthen the hydrophilicity of cis-OCA but also increase the steric hindrance between cis-OCA and C12DMAO, and thereby altering the morphology of aggregate and the rheological response of bulk phase significantly.