Aggregate evolution in aqueous solutions of a Gemini surfactant derived from dehydroabietic acid

Toroidal micelles formed in viscoelastic aqueous solutions of a double-tailed surfactant with two quaternary ammonium head groups

Innovation in surfactant structures is an effective way to prepare new soft materials with novel applications. In this study, we synthesized a double-tailed surfactant containing two quaternary ammonium head groups (Di-C12-N2). The Di-C12-N2 solution behavior was investigated by surface tension, fluorescence, rheology, and cryo-TEM methods. Although Di-C12-N2 contained a large double-tailed hydrophobic group, the solubility of Di-C12-N2 was ∼90 mmol L-1 at 25 °C with a Krafft temperature of ∼1 °C. The increase in Di-C12-N2 concentration in the solutions led to the formation of various aggregates, including spherical micelles, worm-like micelles, multi-layered vesicles, and a rare type of small toroidal micelles. The two quaternary ammonium head groups in Di-C12-N2 led to strong electrostatic interactions between molecules, which was critical for the formation of toroidal micelles. Moreover, with an added NaCl concentration of 40 mmol L-1, the viscosity of the 5 mmol L-1Di-C12-N2 solution increased by ∼1000 times compared to the pure 5 mmol L-1Di-C12-N2 solution, revealing the high sensitivity of the unique head groups to ionic strength. This study enriches the research on the self-assembly principles of surfactants and brings new potential applications for new soft materials.

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.

Micro-simulation insights into the functional and mechanistic understanding of glycyrrhizin against asthma

Asthma is a common chronic respiratory disease, which causes inflammation and airway stenosis, leading to dyspnea, wheezing and chest tightness. Using transgelin-2 as a target, we virtually screened the lead compound glycyrrhizin from the self-built database of anti-asthma compounds by molecular docking technology, and found that it had anti-inflammatory, anti-oxidative and anti-asthma pharmacological effects. Then, molecular dynamics simulations were used to confirm the stability of the glycyrrhizin-transgelin-2 complex from a dynamic perspective, and the hydrophilic domains of glycyrrhizin was found to have the effect of targeting transgelin-2. Due to the self-assembly properties of glycyrrhizin, we explored the formation process and mechanism of the self-assembly system using self-assembly simulations, and found that hydrogen bonding and hydrophobic interactions were the main driving forces. Because of the synergistic effect of glycyrrhizin and salbutamol in improving asthma, we revealed the mechanism through simulation, and believed that salbutamol adhered to the surface of the glycyrrhizin nano-drug delivery system through hydrogen bonding and hydrophobic interactions, using the targeting effect of the hydrophilic domains of glycyrrhizin to reach the pathological parts and play a synergistic anti-asthmatic role. Finally, we used network pharmacology to predict the molecular mechanisms of glycyrrhizin against asthma, which indicated the direction for its clinical transformation.

Rheological behavior of thread-like fiber solutions formed from a rosin-based surfactant with two head groups

Wormlike micelles are conventional aggregates that exist in viscoelastic solutions. However, to achieve a solution with prominent viscoelasticity, rather high concentrations of surfactants are usually required due to the flexibility of aggregates in solution. If thread-like aggregates with rigidity can be formed by surfactants, the solutions are expected to show strong viscoelasticity at very low surfactant concentrations. Herein, A novel rosin-based quaternary ammonium surfactant with two head groups (abbreviated as R-11-3-DA) was synthesized. Cryogenic transmission electron microscopy (Cryo-TEM) images showed that flexible nanofibers with diameters of about 7-8 nm and lengths of over 1 μm were formed in the 1 : 1.5 R-11-3-DA : SL solutions. The rigidity of the aggregates seems to be inherited from the rigidity of the surfactant molecules. The novel aggregates endow the solutions with remarkable viscoelasticity at very low concentrations, with a critical overlap concentration of 0.01 wt% and a critical gelling concentration of 0.58 wt%. The rheological behavior of the solutions also shows excellent shear resistance and weak sensitivity to temperature below the critical gelation temperature (T_gel). This work reveals the advantages of viscoelastic solutions containing flexible nanofibers. The design principles of new molecular structures and system compositions can be applied to the preparation of smart soft materials based on the self-assembly of molecules.

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.

Synthesis and Characterization of a Pyrene-Labeled Gemini Surfactant Sensitive to the Polarity of Its Environment

The cationic gemini surfactant PyO-3-12 was designed to include two dimethyl ammonium groups, one dodecyl tail, and 1-pyrenemethyl hexyl ether tail into the structure of the surfactant. The pyrenyl label ensured that the fluorescence of pyrene could be employed to probe the behavior of PyO-3-12 at the molecular level. The introduction of the oxygen atom in the β-position to pyrene was found to be critical for restoring the sensitivity of the pyrenyl label to the polarity of its environment. The properties of PyO-3-12 were characterized in water by surface tension and a fluorescence methodology that involved the global model-free analysis (MFA) of the pyrene monomer and excimer fluorescence decays to provide quantitative information about the state (unassociated-vs-aggregated) of PyO-3-12. The MFA was combined with a fluorescence quenching study with 2,6-dinitrotoluene to determine the size of the PyO-3-12 micelles. PyO-3-12 was found to behave like a typical gemini surfactant, exhibiting a critical micelle concentration (CMC) of 0.38 (±0.05) mM and an aggregation number (N_agg) equal to 23 (±2). Besides allowing PyO-3-12 to probe the polarity of its environment, the oxygen atom in the β-position next to pyrene brought some pyrenyl labels closer to the interface between the micellar interior and the aqueous phase, in a process that increased the effective volume of the hydrophobic part of PyO-3-12. This led to an increase in the packing parameter of PyO-3-12 and, consequently, an increase in N_agg compared to the N_agg value of 14 (±0.2) obtained for Py-3-12, a gemini surfactant, whose chemical structure was similar to that of PyO-3-12 but without the oxygen in the β-position to pyrene. The methodology described in this study to prepare and characterize pyrene-labeled surfactants is general and can be applied to study any pyrene-labeled surfactant and its interactions with oppositely charged macromolecules.

Formation of asymmetric belt-like aggregates from a bio-based surfactant derived from dehydroabietic acid

The morphology and physicochemical properties of ordered molecular aggregates are closely related to surfactant molecules. Herein, a rosin-based amine oxide surfactant containing a large hydrophobic group (abbreviated R-10-AO) was synthesized from dehydroabietic acid, which is an important derivative of rosin. Cryogenic transmission electron microscopy (cryo-TEM) images and small-angle X-ray scattering (SAXS) showed that at a concentration of ∼5 mM, R-10-AO molecules formed flexible nanobelts with a thickness of only 2-3 nm. The width of these nanobelts was 50-150 nm and the length was more than 1 μm. The formation of the stable nanobelts arose from the strong van der Waals forces of the bulky hydrophobic portions of R-10-AO in solution, facilitating the stability of the asymmetrical aggregates. Rheological tests showed that the formed nanobelts were thermodynamically stable. The entanglement of these nanobelts led to significant viscoelasticity of the solutions. The zero-shear viscosity (η₀) of the R-10-AO solution reached 10 Pa s at a concentration of 5 mM, which is much greater than that of most wormlike micellar solutions. This work provides the inspirations of preparing aggregates with novel properties using natural products.

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.

Determination of the Aggregation Number of Pyrene-Labeled Gemini Surfactant Micelles by Pyrene Fluorescence Quenching Measurements

A cationic gemini surfactant referred to as Py-3-12 and composed of two alkylated diammonium bromide head groups, a propyl spacer, and dodecyl and 1-pyrenehexyl hydrophobic tails was synthesized. Its critical micellar concentration (CMC) was determined to equal 0.15 (±0.02) mM by surface tension and time-resolved fluorescence measurements. The state of the pyrene molecules, whether they were incorporated inside the Py-3-12 micelles or unassociated in the aqueous solution, was determined by applying the global model-free analysis (MFA) to the fluorescence decays acquired with Py-3-12 aqueous solutions. The unassociated Py-3-12 surfactants emitted as pyrene monomers and showed a long fluorescence lifetime. The excited pyrenyl groups located inside Py-3-12 micelles formed an excimer by a rapid encounter with a ground-state pyrene with an average rate constant equal to 0.69 (±0.06) ns^-1. After having the photophysical properties of Py-3-12 in aqueous solution characterized, the number (N_agg) of surfactants per micelle was determined by conducting quenching experiments with dinitrotoluene (DNT). Although DNT is fairly hydrophobic, it was found to partition itself between the Py-3-12 micelles and the aqueous phase. Fluorescence quenching experiments performed on the pyrene monomer and excimer generated by the Py-3-12 aqueous solutions yielded the concentration ([Q]_b) of DNT bound to the Py-3-12 micelles and the average number ⟨n⟩_d of DNT quenching an excimer by diffusive encounters. A combination of steady-state and time-resolved fluorescence quenching experiments on the excimer yielded the number (⟨n⟩_s) of DNT molecules that were bound to the micelles and quenched the excimer in a static manner. A plot of the sum ⟨n⟩_d + ⟨n⟩_s as a function of [Q]_b yielded an N_agg value of 14.0 (±0.2) Py-3-12 units per micelle. This study represents the first example in the literature where N_agg is determined for a micelle, where each surfactant molecule is labeled with pyrene.

Rheological Study of Gemini Surfactants with Amide Groups

In the present study, the micelle aggregation numbers and hydrodynamic radius distributions of the gemini surfactants (abbreviated as Cm-(EO)n-E-Cm (m = 12–16, n = 1, 2)) were investigated by fluorescence spectroscopy and dynamic light scattering (DLS). The rheological properties of these surfactants were determined using a rheometer. The Surfactants C14-(EO)-E-C14 and C16-(EO)-E-C16 formed worm-like micelles at a concentration of 200 mmol/L. It was found that longer hydrophobic chains or shorter linkage groups promoted the growth of worm-like micelles. Then, the influence of the inorganic salt NaBr and the organic salt sodium salicylate (NaSal) on the formation and properties of wormlike micelles in aqueous C16-(EO)-E-C16 solutions at a surfactant concentration of 20 mmol/L was investigated. The results showed that NaSal could penetrate deeper into the hydrophobic group of the micelle due to its good hydrophobic interaction, which promoted micelle growth and resulted in worm-like micelles. In addition, the tested systems showed a Maxwell behavior at lower frequencies, but deviated from the Maxwell mode at higher frequencies. Finally, the formation of worm-like micelles was also verified by micro polarity measurements.

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 ∝ C^9.6). This work further promotes new applications of forest resources.

Supramolecular Hydrogels with Chiral Nanofibril Structures Formed from β-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 β-cyclodextrin (β-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 β-CD associated with the isopropyl benzyl group of the dehydroabietic acid unit in R-6-Ser and formed R-6-Ser@β-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 β-CD and provides a new means of utilizing the secretions of pine trees.

Improving enzymatic hydrolysis of acid-pretreated bamboo residues using amphiphilic surfactant derived from dehydroabietic acid

In this work, amphiphilic surfactant was obtained using dehydroabietic acid from pine rosin and then pre-adsorbed with acid-pretreated bamboo residues (AP-BR) to block the residual lignin adsorption site, which is expected to improve its enzymatic digestibility. Results from cryogenic-transmission electron microscopy (Cryo-TEM) indicated amphiphilic surfactant with PEG with polymerization degree of 34 (D-34) aggregated to form worm-like micelles, which improved enzymatic hydrolysis yield of AP-BR from 24.3% to 71.9% by pre-adsorbing with 0.8 g/L. Amphiphilic surfactants pre-adsorbed on AP-BR could reduce hydrophobicity of AP-BR, adsorption affinity and adsorption capacity of lignin for cellulase from 0.51 L/g to 0.48-0.32 L/g, from 2.9 mL/mg to 1.8-1.4 mL/mg, and from 122.3 mg/g to 101.9-21.4 mg/g, respectively. These changed properties showed compelling positive contributions (R² > 0.9) for free enzymes in the supernatants and sequently for final enzymatic hydrolysis yield, which was caused by blocking non-productively hydrophobic adsorption between lignin and cellulase.

Viscoelastic lyotropic liquid crystals formed in a bio-based trimeric surfactant system

Exploring the self-assembly of oligomeric surfactants is expected to bridge the gap between conventional and polymeric surfactants. Using the natural resource rosin as the starting material, a bio-based star-shaped trimeric quaternary ammonium surfactant (abbreviated tri-R-4-Phe) was synthesized. With three bulky dehydroabietic acid units in the hydrophobic group, tri-R-4-Phe has a molecular weight of 1684.9 and shows strong affinity towards both water and nonpolar organic compounds. In the presence of tri-R-4-Phe, C12EO3 was able to form lamellar lyotropic liquid crystals over a wide concentration range in water. The tri-R-4-Phe/C12EO3/water tertiary system was investigated by polarizing optical microscopy (POM), small angle X-ray scattering (SAXS) and rheological measurements. The investigated samples with different formulations all showed strong viscoelasticity, and the viscosity increased with the surfactant content. All samples showed interesting shear banding phenomena due to the shear induced mesoscale phase transition in tri-R-4-Phe/C12EO3/water systems. The present work reveals the unique behaviour of trimeric surfactant involved LLC systems and the result may be helpful in investigating delicate molecular self-assembly using natural resources.

Rheological Behavior of Environmentally Friendly Viscoelastic Solutions Formed by a Rosin-Based Anionic Surfactant

It is of great significance to explore novel applications of renewable resources. In this study, a rosin-based anionic surfactant (abbreviated R-11-2-Na), which contains a large hydrophobic group of 30 carbon atoms, was synthesized. R-11-2-Na forms wormlike micelles in the presence of the equimolar organic salt choline chloride, endowing solutions with strong viscoelasticity. The wormlike micellar solutions were investigated using rheology, small-angle X-ray scattering, and freeze-fracture transmission electron microscopy (FF-TEM) methods at 25 °C. Due to the strong van der Waals interactions caused by the large hydrophobic group contained in R-11-2-Na, the zero-shear viscosity (η₀) of solutions showed extremely strong dependence on the concentration with an exponent of 23.4. The cross-sectional diameter of the wormlike micelles in the present system was significantly larger than that of the wormlike micelles formed by surfactants containing conventional alkyl tails. This finding may be attributed to the steric hindrance brought by the bulky and rigid dehydroabietic acid unit in the hydrophobic part. The wormlike micelles also showed high tolerance to the organic salt concentration. The present study reveals the notable qualities of rosin-based derivatives in forming complex fluids and facilitates new utilizations of forest resources.

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.

Synthesis and Characterization of Biguanide and Biguanidium Surfactants for Efficient and Recyclable Application in the Suzuki-Miyaura Reaction

We report here the synthesis and thorough characterization of a new family of alkylbiguanides and alkylbiguanidium chlorides by ¹H and ¹³C NMR and X-ray diffraction. Their critical micelle concentration was first determined by surface tension measurements. Hexylbiguanide was then studied as a surfactant in the micellar Suzuki-Miyaura cross-coupling reaction. The unexpected low reactivity of the system at high Pd/hexylbiguanide ratios was due to the change of the size and the shape of the aggregates, observed by transmission electron microscopy. The best catalytic activity was obtained for a 1:1 Pd/hexylbiguanide ratio for which the micellar conditions were conserved. Better results were obtained for several substrates, when compared to those previously obtained with metformin under the same reaction conditions. Higher yields and a better recyclability were obtained under micellar conditions with hexylbiguanide.

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

Sonication is not required to re-disperse the precipitated SWNTs in a pH-responsive SWNTs dispersion using a rigid surfactant.