Photo-responsive wormlike micellar systems based on mixed cationic/anionic surfactants and mixed photo-sensitive additives

Wormlike micellar solutions formed by an anionic surfactant and a cationic surfactant with two head groups

Innovation in the molecular structure of surfactants is important for the preparation of soft materials with novel properties. In this study, we synthesized a cationic surfactant, N1,N1,N1,N1,N3,N3,N3-pentamethyl-N3-(3-stearamidopropyl)propane-1,3-diammonium bromide, hereafter referred to as C18-DQA. Unlike conventional cationic surfactants, C18-DQA contains two quaternary ammonium head groups and a long-saturated alkyl chain equal to a chain length of 21 carbon atoms. C18-DQA exhibits a low Krafft point of ∼0 °C and a water solubility >1000 mM at 25 °C. The critical micelle concentration (cmc) of C18-DQA was determined to be 0.59 mM using the Nile red method. C18-DQA was mixed with sodium laurate (SL) at different molar ratios to produce transparent solutions with excellent viscoelasticity over a wide concentration range. The 1 : 1.5 molar ratio C18-DQA/SL mixed solutions exhibited gel-like behavior for a total surfactant concentration of 2.88 wt% (75 mM). The solution with a total surfactant concentration of 300 mM (120 mM C18-DQA and 180 mM SL) achieved a maximum zero-shear viscosity (η0) of 4224 Pa s. Cryogenic transmission electron microscopy analysis revealed the formation of extremely long wormlike micelles, with a cross-sectional diameter of 5 nm and contour length >3 μm, in the mixed solutions. C18-DQA and SL molecules were drawn close by electrostatic attractions, leading to a suitable molecular geometry for the extensive growth of wormlike micelles. This work will act as an important reference for the future preparation of highly viscoelastic solutions by mixing cationic and anionic surfactants. The proposed system is also expected to have potential applications in cosmetic formulations, home care products, and oilfield fracturing fluids.

Adaptive 2D and Pseudo-2D Systems: Molecular, Polymeric, and Colloidal Building Blocks for Tailored Complexity

Two-dimensional and pseudo-2D systems come in various forms. Membranes separating protocells from the environment were necessary for life to occur. Later, compartmentalization allowed for the development of more complex cellular structures. Nowadays, 2D materials (e.g., graphene, molybdenum disulfide) are revolutionizing the smart materials industry. Surface engineering allows for novel functionalities, as only a limited number of bulk materials have the desired surface properties. This is realized via physical treatment (e.g., plasma treatment, rubbing), chemical modifications, thin film deposition (using both chemical and physical methods), doping and formulation of composites, or coating. However, artificial systems are usually static. Nature creates dynamic and responsive structures, which facilitates the formation of complex systems. The challenge of nanotechnology, physical chemistry, and materials science is to develop artificial adaptive systems. Dynamic 2D and pseudo-2D designs are needed for future developments of life-like materials and networked chemical systems in which the sequences of the stimuli would control the consecutive stages of the given process. This is crucial to achieving versatility, improved performance, energy efficiency, and sustainability. Here, we review the advancements in studies on adaptive, responsive, dynamic, and out-of-equilibrium 2D and pseudo-2D systems composed of molecules, polymers, and nano/microparticles.

Photoreversible formation of nanotubes in water from an amphiphilic azobenzene derivative

An anionic azobenzene-appended derivative of L-ValylGlycine self-assembles into nanotubes in water. Irradiation with 365 nm light provokes trans-cis isomerization of the azobenzene unit and subsequent tube disassembly. Thermal or photoinduced (457 nm light) recovery of the trans isomer restores the nanotubes.

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.