UV-Responsive Behavior of Multistate and Multiscale Self-Assemblies Constructed by Gemini Surfactant 12-3-12·2Br- and trans- o-Methoxy-cinnamate

Influence of twin-headed gemini micellar system on the study of methionine amino acid with ninhydrin in buffer solution

The influence of double-headed gemini was examined in the present study by studying the amino acid methionine mixed with ninhydrin in CH₃COOH-CH₃COONa buffer solvent. The absorbance was monitored at fixed time intervals with UV-vis spectroscopy. An impact typical of surfactants was observed on the ninhydrin-methionine reaction and explained by a pseudo-phase model of micelles. The effect of different temperatures (343 to 363 K) was also determined. Based on data showing the impact of temperature on k_ψ , several relevant thermodynamic quantities, ΔH ^#, ΔS ^#, and E _a, were calculated using linear least-squares regression. In addition, the influence of the other reaction ingredients on the reaction, that is, pH and the concentration of ninhydrin and methionine, was studied. The CMC (critical micelle concentration) of pure geminis and the surfactant system with methionine and ninhydrin was evaluated at two temperatures, i.e. at 303 K and 353 K by conductivity measurements. The CMC values of pure gemini surfactants evaluated in the existing case at 303 K are concordant with the results stated before. Moreover, other parameters, including rates and binding constants, were calculated.

pH and light dual stimuli-responses of mixed system of 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecyl ammonium bromide) and trans-ortho-hydroxyl cinnamic acid

Stimuli-responsive smart supramolecular self-assembly with controllable morphology and adjustable rheological property has attracted widespread concern of scientists in recent years due to the great potential application in microfluidics, controlled release, biosensors and so on. In this study, a pH and UV light dual stimuli-responsive system was constructed by combining Gemini surfactant 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecyl ammonium bromide) (12-3(OH)-12·2Br^-) with trans-ortho-hydroxyl cinnamic acid (trans-OHCA) in aqueous solution. The phase behavior and stimuli-responsive behavior of the system including the microstructural transition, rheological property, intermolecular interaction, and isomerization reaction were explored by various experiment techniques such as rheometer, UV-vis spectrum, polarized optical microscopy (POM), transmission electron microscopy (TEM), dynamic light scattering (DLS) as well as theoretical calculation. The system displays abundant phase behaviors that supramolecular self-assemblies of different morphologies in different states such as spherical micelle, wormlike micelle, lamellar liquid crystal, and aqueous two phase system (ATPS) were observed even at lower concentration, which provide the research basis on the abundant stimuli-responsiveness of the system. The results prove that the multiple ionization and the photo-isomerization of trans-OHCA endow the system with plentiful responses to pH and UV light stimuli. It is expected that this study on the dual stimuli-responsive system with abundant self-assembly behaviors and adjustable rheological behaviors would be of theoretical and practical importance, which would provide essential guidance in designing and constructing smart materials with multiple stimuli-responses.

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

Mixed systems of the Gemini cationic surfactant trimethylene-1,3-bis (dodecyldimethylammonium bromide) (12-3-12·2Br^-) and the photosensitive additives trans-methoxy sodium cinnamates with different substituent positions (trans-ortho-methoxy cinnamate, trans-OMCA; trans-meta-methoxy cinnamate, trans-MMCA; and trans-para-methoxy cinnamate, trans-PMCA) were selected for investigating the effects of the substituting position of methoxy on the system phase diagram and UV light-responsive behavior of the wormlike micelles. The differences in phase behaviors of the selected systems were analyzed by calculating the potential distribution, molecular volume, and free energy of solvation of cinnamates and the binding energies between photosensitive additives and the surfactant. The photoresponsive behaviors of wormlike micelle solutions formed in the selected systems were studied by the rheological method and UV-vis and H nuclear magnetic resonance (¹H NMR) spectroscopy; the kinetics of photoisomerization of trans-OMCA, trans-MMCA, and trans-PMCA were studied by first-order derivative spectrophotometry. The results reveal that the methoxy substituent position has a great influence on the phase behavior and photosensitivity of the studied systems. In addition, the photoisomerization of the studied cinnamates follows the first-order opposite reaction laws; the different reaction rates play the decisive role in the photosensitivity of the wormlike micelles. This paper would afford a deeper understanding of the UV light-responsive mechanism at the molecular level and provide essential guidance in preparing smart materials with adjustable light sensitivity.

Light-Triggered Rheological Changes in a System of Cationic Wormlike Micelles Formulated with a Photoacid Generator

"Smart" fluids displaying large changes in their rheological properties in response to external stimuli have been of great interest in recent years. For example, "smart" wormlike micelles (WLMs) that respond to pH can be readily formulated by combining a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) with an aromatic compound such as 1,2-dihydroxybenzene (DHB). Here, we show that a pH-responsive aqueous formulation as mentioned above can be simultaneously made responsive to ultraviolet (UV) light by incorporating a photoacid generator (PAG) into the system. A commercially available PAG, diphenyliodonium-2-carboxylate, is used here. Upon exposure to UV light, this PAG irreversibly photolyzes into iodobenzene (IB) and benzoic acid (BA), with the formation of BA, leading to a drop in pH. WLMs formed by mixtures of CTAB, DHB, and the PAG are systematically characterized before and after UV irradiation. As the PAG photolyzes, an increase in the viscosity of WLMs occurs by a factor of 1000. We show that the ratio of the zero-shear viscosity η₀ (after UV/before UV) depends on the initial pH of the sample. The UV-induced increase in η₀ can be attributed to the growth of WLMs in solution, which in turn is influenced by both the ionization state of DHB and the presence of IB and BA.

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

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 ortho-methoxycinnamic (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.