CO2-Responsive Rosin-Based Supramolecular Hydrogels: Diverse Chiral Nanostructures and Their Application in In Situ Synthesis of Chiral Gold Nanoparticles.

Natural small molecules have demonstrated tremendous potential for the construction of supramolecular chiral nanostructures owing to their unique molecular structures and chirality. In this study, novel CO2-responsive supramolecular hydrogels were constructed using a series of rosin-based surfactants (CnMPAN, n = 10, 12, and 14). The macroscopic properties, rheological properties, nanostructures, and intermolecular interactions of the hydrogels were investigated using differential scanning calorimetry, rotational rheometry, cryogenic transmission electron microscopy, and Fourier transform infrared spectroscopy. Interestingly, diverse nanostructures containing helical nanofibers, interwoven nanofibers, and twisted nanoribbons were formed in the hydrogels, which were rarely observed in reported supramolecular hydrogels, and the strength of the hydrogels was significantly enhanced by increasing the CnMPAN concentration and the alkyl chain length. The obtained hydrogels exhibited excellent CO2-responsiveness, with no obvious variation in the nanostructures and rheological properties after response to CO2/N2 for five cycles. Taking advantage of the chiral nanostructures of hydrogels, gold nanoparticles (GNPs) were further prepared. The average particle sizes of the resulting GNPs were as low as 2.5 nm, and the GNPs also had a chiral structure. It is worth noting that no additional reductants and UV-light irradiation were used during the reduction process of GNPs. This study emphasizes that the unique molecular structure and chirality of rosin are critical for the preparation of hydrogels with chiral nanostructures. In addition, this study enriches the applications of forest resources.

Fast and Reversible Photoresponsive Self-Assembly Behavior of Rosin-Based Amphiphilic Polymers.

Designing stimulus-responsive amphiphilic polymers with a fast photoresponsive self-assembly behavior remains a challenge. Two series of rosin-terminated and azobenzene-terminated amphiphilic polymers (PAMn and PMAn) with fast and reversible photoresponsive properties were prepared using rosin-based azobenzene groups and polyethylene glycol, respectively. Under 5-10 s of UV irradiation, the polymers showed trans-to-cis isomerization and reached a photosteady state. For the PAMn polymer, the absorbance of the absorption peak at 325 nm recovered to more than 95% of the initial value under visible light for 5-10 s, whereas that of the PMAn polymer recovered completely. Notably, the PAMn and PMAn polymers initially self-assembled to vesicles or spherical micelles, and various morphological changes were achieved by manipulating UV irradiation time, with the initial morphology again recovered under dark conditions or visible-light irradiation. Remarkably, vesicles of the PAM34 and PMA34 polymers presented an intermediate open-vesicle state before being completely deformed under UV irradiation because of the existence of a π-π interaction. Finally, the ability of PAM34 and PMA34 polymer vesicles to perform the controlled release and reversible loading of a fluorescent probe was evaluated.

Water-in-Oil Emulsion Gels Stabilized by a Low-Molecular Weight Organogelator Derived from Dehydroabietic Acid.

High concentrations of surfactants or gelators are usually necessary to prepare emulsions gels with unusual physicochemical properties. This situation may be improved by innovating the aggregate morphology in systems. Herein, a rosin-based molecule is designed and synthesized using dehydroabietic acid as the starting material (denoted as R-Lys-R). The molecule acts as an effective organogelator and can gelate several hydrocarbon compounds with a minimum gelation concentration of 0.2% (w/v). Analysis using atomic force microscopy (AFM) and circular dichroism (CD) reveals that in n-decane, R-Lys-R forms left-handed helical fibers with a cross-sectional diameter of approximately 15 nm. The directional hydrogen bonding of the amide group is helpful to the formation of aggregates. At concentrations of R-Lys-R above 2%, water-in-oil emulsions are transformed into emulsion gels owing to the aptitude of R-Lys-R in gelating the oil phase. The concentrations of the emulsifier can be adjusted to obtain emulsion gels with different formulations. This work reveals the potential of rosin derivatives for the formation of small molecular weight organogels and provides a novel method for the utilization of natural resources in soft materials and home care products.

Novel Temperature-Responsive Rosin-Derived Supramolecular Hydrogels Constructed by New Semicircular Aggregates.

A highly water-soluble rosin-based surfactant (C14-MPA-Na) was synthesized. Novel temperature-responsive supramolecular hydrogels were further prepared using C14-MPA-Na. The microstructure and the mechanical properties of the hydrogels were investigated. Unexpectedly, instead of the long one-dimensional structure, a new kind of twisted semicircular aggregate was formed in the hydrogels, which was rarely reported. Besides, the hydrogels possessed excellent shear-recovery properties. Upon heating to 40 °C, the hydrogels transformed into viscoelastic solutions, which were constructed by worm-like micelles. By adjusting the temperature, the hydrogels and the viscoelastic solutions could be freely transformed. Nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy were used to further explore the possible self-assembly mechanism of C14-MPA-Na. The curved alkane chain which partially overlapped with rosin's rigid skeleton became stretched when heated to 40 °C. The introduction of the rosin rigid skeleton endowed the supramolecular hydrogels with a novel microstructure and contributed to the development of strategies for the utilization of forest resources.

Photoresponsive Viscoelastic Solutions Based on Chiral Wormlike Micelles in Mixed Solutions Containing an Amphiphile Derived from Rosin.

A novel rosin-based photoresponsive anionic amphiphile, sodium N-azophenyl maleopimaric acid imide carboxylate (AzoMPCOONa), has been successfully synthesized. Its molecular structure was characterized by 1H and 13C NMR and mass spectrometry (MS). The photoisomerization of AzoMPCOONa was evaluated by ultraviolet (UV)-visible spectrometry and 1H NMR. The structure of AzoMPCOONa could be converted between the trans and cis isomers by irradiation with UV/visible light. Importantly, a fascinating photoresponsive viscoelastic solution was prepared by mixing AzoMPCOONa and cetyltrimethylammonium bromide (CTAB). The properties of the photoresponsive viscoelastic solution were further investigated by rheology, circular dichroism (CD), and cryogenic transmission electron microscopy (cryo-TEM). Initially, the AzoMPCOONa/CTAB system was a gel-like solution composed of entangled wormlike micelles possessing the right-handed chiral structure. After UV irradiation for 10 min, the gel-like solution transformed into a slightly viscous solution, its zero-shear viscosity dramatically reduced by 2 orders of magnitude, and the aggregates were converted into rod-like micelles and spherical micelles. In addition, the right-handed chiral structure of the aggregates disappeared. These dramatic changes in the viscosity and the aggregate structure can be attributed to the photoisomerization of the azobenzene group in AzoMPCOONa, which led to changes in the molecular geometry and the packing parameter of the AzoMPCOONa/CTAB system. Interestingly, the right-handed chiral structure of wormlike micelles also is photoresponsive. The results reveal the superiority of forest resources for preparing viscoelastic solutions.

pH-Responsive Wormlike Micelles Formed by an Anionic Surfactant Derived from Rosin.

A novel pH-responsive wormlike micellar viscoelastic solution was constructed by a rosin-based anionic surfactant (Na-MPA-AZO-Na) in the presence of cetyltrimethylammonium bromide (CTAB). The viscoelasticity, aggregate morphology, and pH-responsiveness of the pH-responsive wormlike micelles have been investigated through the method of rheology and cryogenic-transmission electron microscopy. Its corresponding mechanism has been studied using 1H NMR and 1H-1H 2D NOESY HNMR. The zero-shear viscosity (η0) of the wormlike micellar solution rapidly decreases by 3 orders of magnitude as the pH increases from 5.21 to 9.56. The viscoelastic fluids and water-like solutions can be converted by tuning the pH between 3.62 and 12.00, and the corresponding aggregates also transform between wormlike micelles and spherical micelles. In addition, the wormlike micellar cross-sectional diameter is approximately 10 nm, which is remarkably larger than that of the common wormlike micelles. The phenomenon can be attributed to the large steric volume of the rosin rigid skeleton. When the pH is 12.00, a "pseudo" Gemini surfactant is constructed by Na-MPA-AZO-Na and CTAB through the electrostatic interactions. Wormlike micelles also can be formed with the increasing concentrations. The η0 of the wormlike micellar system shows strong dependence on concentration with an exponent of 9.6 (η0 ∝ C9.6). This work further promotes new applications of forest resources.

Supramolecular Hydrogels with Chiral Nanofibril Structures Formed from ß-Cyclodextrin and a Rosin-Based Amino Acid Surfactant.

The rational combination of natural molecules is expected to provide new soft material building blocks. Herein, a rosin-based amino acid surfactant was synthesized using dehydroabietic acid and l-serine as the starting materials (denoted as R-6-Ser). Supramolecular hydrogels were formed when ß-cyclodextrin (ß-CD) was mixed with R-6-Ser at molar ratios of over 0.5:1 and above certain concentrations. The hydrogels were investigated using rheometry, small-angle X-ray scattering, CD spectroscopy, and cryo-transmission electron microscopy (cryo-TEM). The ß-CD associated with the isopropyl benzyl group of the dehydroabietic acid unit in R-6-Ser and formed R-6-Ser@ß-CD complexes. The complexes and R-6-Ser self-assembled to form elongated right-handed rigid fibers with a diameter of approximately 7-8 nm, which were responsible for the elasticity of the hydrogels. This work demonstrated the feasibility of preparing supramolecular hydrogels from a diterpenoid-based surfactant and ß-CD and provides a new means of utilizing the secretions of pine trees.

Hybrids of CO2-Responsive Water-Redispersible Single-Walled Carbon Nanotubes by a Surfactant Based on Natural Rosin.

A kind of polymerizable dispersant based on natural rosin was used to disperse single-walled carbon nanotubes (SWNTs) in aqueous solution followed by in situ free-radical polymerization to achieve a controllable SWNTs dispersion, that not only can be controlled by CO2/N2, but can also be recycled and redispersed in CO2-saturated water after drying without sonication.

Reversible dispersion and precipitation of single-walled carbon nanotubes using a pH-responsive rigid surfactant.

A pH-responsive rigid surfactant derived from rosin was used to disperse and precipitate single-walled carbon nanotubes (SWNTs) in aqueous solution. The precipitated SWNTs could be re-dispersed by changing the pH without sonication owing to the hydrophobic steric layers formed by the rigid group and π-π interactions from the azobenzene group.

CO2-Responsive Pickering Emulsions Stabilized by a Bio-based Rigid Surfactant with Nanosilica.

A novel CO2-responsive surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)propylamine imide (MPAGN), based on sustainable resource of rosin was synthesized and used to prepare a kind of CO2-responsive Pickering emulsions with nanosilica. MPAGN can be reversibly responsive to CO2 and N2 between active cationic (MPAGNH+) and inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of nanosilica, then stabilize or break emulsion. CO2-responsive behavior of MPAGN was verified by cycle change of pH and conductivity with bubbling CO2 and N2 alternately. The type of adsorption of MPAGNH+ at the particle-water interface was explained according to the adsorption isotherms. The mechanisms of stabilization, destabilization, and restabilization of Pickering emulsion were analyzed according to zeta potentials and droplet size. This Pickering emulsion can be reversible between stable and unstable by bubbling CO2 and N2 alternately. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has an environmentally friendly property.

Phase Behavior and Aggregation in a Catanionic System Dominated by an Anionic Surfactant Containing a Large Rigid Group.

The phase behavior and aggregates of a catanionic system have been investigated. The anionic surfactants in the mixed systems were sodium N-alkylmaleimidepimaric carboxylate (Cn -MPA-Na, n=12, 14, 16), which were prepared from rosin and contain a large rigid skeleton and a flexible alkane chain, and the cationic surfactant was cetyltrimethylammonium bromide (CTAB). The phase behavior of the C14 -MPA-Na/CTAB system transformed sequentially from a viscoelastic solution to an aqueous surfactant two-phase system (ASTP), an aqueous surfactant three-phase system (AS3P), and an anisotropic homogeneous phase as the concentration of C14 -MPA-Na was continuously increased from 10 to 35 mm. The C16 -MPA-Na/CTAB system showed similar phase behavior, whereas the C12 -MPA-Na/CTAB system did not form the AS3P system. The corresponding microstructures in the different phases were investigated by using rheology and cryogenic transmission electron microscopy (Cryo-TEM). The aggregates in the viscoelastic solutions are thread-like, annular, and worm-like micelles. The microstructures in the upper phase of the ASTP are worm-like micelles, and in the lower phase are spherical and rod-like micelles. The aggregates in the upper and lower phases of the AS3P are worm-like micelles and spherical and rod-like micelles, respectively. The aggregates in the middle phase of the AS3P and the anisotropic homogeneous phase are sponge-like micelles. The clear Cryo-TEM images of the sponge-like micelles are presented.

Annular and threadlike wormlike micelles formed by a bio-based surfactant containing an extremely large hydrophobic group.

A novel bio-based anionic surfactant containing a large rigid group and a flexible alkyl chain, namely, sodium N-dodecyl-maleimidepimaric carboxylate (C12-MPA-Na), was synthesized from rosin. The molecular structure of C12-MPA-Na was identified using 1H NMR, FT-IR spectroscopy and MS. Despite containing 36 carbon atoms, C12-MPA-Na showed good water solubility at room temperature. Large spherical aggregates with diameters of 100-200 nm were formed by C12-MPA-Na when its concentration was above 0.1 mM, which was slightly higher than the critical micelle concentration (0.078 mM). Annular wormlike micelles were discovered with increasing C12-MPA-Na concentration, and began to change into extremely long threadlike wormlike micelles when the C12-MPA-Na concentration reached approximately 58 mM. The viscoelastic properties of the wormlike micelle solutions were investigated using steady state and oscillatory shear sweep rheological measurements. The zero-shear viscosity (η0) strongly depended on the concentration of C12-MPA-Na, and the scaling exponent was 34.1. Cryo-TEM confirmed the formation of large spherical aggregates and wormlike micelles. 1H-1H 2D nuclear Overhauser effect spectroscopy (NOESY) was used to detect the molecular interactions of C12-MPA-Na. The results indicated that the alkyl chain of C12-MPA-Na was partially overlapped with its non-planar rigid structure in aqueous solution, and the possible aggregation process for C12-MPA-Na was proposed.

Mechanically-sensitive hydrogels formed from ß-cyclodextrin and an anionic surfactant containing a biphenyl group.

Hydrogels are important soft materials with intriguing properties. By taking advantage of the host-guest interactions and multiple molecular interactions, it is expected that novel hydrogel systems can be formed. This strategy has been implemented here, transparent hydrogels were formed by using a newly-synthesized anionic surfactant, sodium 2-(4-phenylphenoxy)dodecanoate (C12biphNa), and ß-cyclodextrin (ß-CD) in different proportions and their properties were investigated. Gelation of water occurs at extremely low surfactant concentrations (5 mM for a 1:3 C12biphNa: ß-CD system), and a single C12biphNa with its associated ß-CDs can trap about 11,000 water molecules on average. In addition, the systems are fragile to mechanical stimulus and thus show mechanical sensitivity. Cryo-TEM reveals that the hydrogels have a microstructure consisting of rigid nanowire-like aggregates (with cross-sectional diameters of about 7-8 nm) locally distributed in a parallel manner in solution. These microstructural features are responsible for the peculiar properties of the hydrogel systems presented. The inclusion complexes formed by C12biphNa and ß-CD were investigated using (1)H NMR and 2D nuclear overhauser effect spectroscopy and their aggregation state was proposed. This work enriches the connotation of nonamphiphilic self-assembly and provides inspiration for constructing new functional soft materials.