Molecular Dynamics Simulations and Density Functional Theory on Unraveling Photoresponsive Behavior of Wormlike Micelles Constructed by 12-2-12·2Br^– and trans-ortho-Methoxy Cinnamate

Abstract

Photoresponsive systems with controllable self-assembly morphologies and adjustable rheological properties have attracted widespread interest by researchers in the past few years. Among them, the photoresponsive systems consisting of orthomethoxycinnamic (OMCA) and Gemini surfactants are endowed with rich self-assemblies with different states and in different scales including spherical micelles, wormlike micelles, vesicles, aqueous two-phase system (ATPS), etc. All these self-assemblies display excellent photoresponsive behavior. However, the mechanism of these photoresponsive behaviors has not been unraveled systematically so far. In this study, molecular dynamics (MD) simulations, density functional theory (DFT) calculations, transmission electron microscopy, and rheology are employed to investigate the photoresponsive behaviors of wormlike micelles caused by photoisomerization of trans-OMCA in 12-2-12•2Br − /trans-OMCA solutions and to unravel the underlying mechanisms of these photoresponsive behaviors. The experimental results show that 12-2-12•2Br − /trans-OMCA micelles display photoresponsiveness after UV-light irradiation, with the transformation of micellar morphologies from wormlike micelle to spherical micelles. In MD simulations, certain micelle morphologies in experiments and the specific packing between 12-2-12•2Br − /OMCA were successfully captured. The larger three-dimensional structure and steric hindrance of cis-OMCA disturb the interior structure of micelles. The stronger hydrophilicity of cis-OMCA induces the escape of cis-OMCA from the interval of micelles to the solution. The energy results prove that trans-OMCA associates more strongly with 12-2-12•2Br − than cis-OMCA. These causes lead to the fission and repacking of wormlike micelles.

Effect of the Substituent Position on the Phase Behavior and Photoresponsive Dynamic Behavior of Mixed Systems of a Gemini Surfactant and trans-Methoxy Sodium Cinnamates

“…Previous studies have shown that the structure degradation of wormlike micelles is mainly caused by the changes in the interaction between cinnamates and surfactants because of the photoisomerization of light-sensitive molecules from trans isomers to cis isomers. − As discussed above, the binding energies of 12-3-12·2Br – and cis -MCA with different substitution positions are smaller than those of systems containing trans -MCAs. After UV irradiation, the cinnamate derivatives are transformed from trans isomers to cis isomers, and the cis isomers tend to escape from micelles to the solvent, which is unfavorable to the existence of wormlike micelles with long flexible structures.…”

“…TEM observation shows (Figure a,b) that the average size of spherical micelles is approximately 40 nm, while the length of wormlike micelles exceeds several microns. The negatively charged cinnamate ions interact with the positively charged Gemini surfactant micelles by electrostatic attraction and promote the growth of micelles to some extent, − which should be the primary cause for the micellar growth. With the further addition of cinnamates, the single phase of wormlike micelles would be transformed into coexisting two phases in the macroscopic level (II, ATPS), as exhibited in Figure c.…”

“…In this study, the MCAs with different substituent positions have different properties. Among them, the centralized potential distribution, greater hydrophobicity, and smaller molecular volume of trans -OMCA – endow it with the ability to insert into the micelle barrier layer easily. , This should be the main cause why the phase diagram of 12-3-12·2Br – / trans -OMCA/H 2 O is different from that of the other two systems. By comparison, the similar diagrams of 12-3-12·2Br – / trans -MMCA/H 2 O and 12-3-12·2Br – / trans -PMCA/H 2 O should be due to the similar solvation free energies of trans -MMCA and trans -PMCA and the analogous binding energies of trans -MMCA and the Gemini surfactant with trans -PMCA and the Gemini surfactant.…”

“…Among them, the centralized potential distribution, greater hydrophobicity, and smaller molecular volume of trans-OMCA − endow it with the ability to insert into the micelle barrier layer easily. 52,53 This should be the main cause why the phase diagram of 12 From Figure 3, the potential distributions of cis-MCAs with different substituent positions are all smaller than those of trans isomers; meanwhile, the isomerization of MCAs can be excited by UV illumination. These results mean that the molecular interaction between cis isomers and surfactants is different from that of trans isomers with surfactants, providing a theoretical foundation for the photoresponsive behavior of the studied systems.…”

“…This disadvantage of azobenzenes limits the development of photoresponsive self-assemblies; it is urgent to find a substitute for azobenzenes with good biocompatibility and no biotoxicity. Cinnamic acids and cinnamate esters are widely found in Arabidopsis plants and exhibit superior biocompatibility; they are regarded as suitable alternatives of azobenzenes in fabricating light-sensitive materials in recent years. − According to previous studies, the rationale of light-responsive behavior of surfactant/cinnamate mixed systems is the trans to cis isomerization reaction of cinnamates caused by light, which profoundly affects the properties of cinnamate molecules including molar volume and amphipathicity and further influences the interaction between the surfactant and photosensitizer. , Correspondingly, the rearrangement of surfactant aggregates may be induced by light illumination.…”