The mediated rheological properties of emulsions stabilized by thread-like mesoporous silica nanoparticles in combination with CTAB.

The combination of hydrophilic particles and surfactants provides a simple method to stabilize Pickering emulsions. The type and concentration of the particles and surfactants play important roles in the microstructure and rheological properties of the resulting emulsions. Herein, stable n-octane-in-water Pickering emulsions with tunable rheological properties were prepared using thread-like mesoporous silica nanoparticles (TMSNPs) and cetyltrimethylammonium bromide (CTAB) as emulsifiers. The CTAB concentration (CCTAB) highly affected the properties of emulsions, which were divided into three regions according to the results of large-amplitude oscillatory shear responses. In the low CCTAB range (0.03 mmol L-1 ≤ CCTAB ≤ 0.1 mmol L-1), the emulsions gelled with a high storage modulus . With CCTAB increasing, the value of emulsions, measured by the small-amplitude oscillatory shear, decreased from approximately 1000 Pa at 0.03 mmol L-1 to 100 Pa at 0.3 mmol L-1 and then to 40 Pa at 3 mmol L-1. A three-dimensional percolation structure formed by cross-linking of TMSNPs in the emulsion continuous phase was observed via cryo-SEM in the low CCTAB range but not in the intermediate and high CCTAB ranges. The mechanisms showing the synergistic stability and rheological properties of these emulsions were investigated. It is attributed to the unique morphology of TMSNPs and the competitive adsorption of CTAB molecules at the oil-water interface and on the nanoparticle surface in different CCTAB ranges. Moreover, owing to the porosity and hydrogen-bonding interactions between the TMSNPs and the confinement effect of the flocculated oil droplets, the viscoelasticity of the emulsions could be mediated by adding a trace amount of acid/base. This study provides a new strategy to regulate the rheological properties of emulsions. It also expands the Pickering emulsion systems with tunable rheological properties.

Pickering Emulsions Stabilized by Mesoporous Nanoparticles with Different Morphologies in Combination with DTAB.

The morphology of nanoparticles plays a significant role in the properties and applications of Pickering emulsions. Oil-in-water (O/W) Pickering emulsions were prepared using spherical, rod-like, and thread-like mesoporous silica nanoparticles (MSNPs) in combination with the cationic surfactant dodecyltrimethylammonium bromide (DTAB) as a stabilizer. The effects of nanoparticle morphology on the stability and stimuli-responsive properties of Pickering emulsions were investigated. For spherical and rod-like MSNP systems, stable Pickering emulsions were obtained at DTAB concentrations above 0.2 mmol·L-1. Stable Pickering emulsions containing thread-like MSNPs were produced at lower DTAB concentrations of approximately 0.1 mmol·L-1. The droplets with thread-like MSNPs were extremely large with an average diameter around 700 µm at DTAB concentrations of 0.1-0.3 mmol·L-1, which were approximately 20 times larger than those of conventional droplets. Scanning electron microscopy (SEM) images showed that all three types of MSNPs were located at the O/W interfaces. Irrespective of the morphology of the MSNPs, all the stable Pickering emulsions retained their original appearance for more than 6 months. By adding NaOH and HCl alternatively, the Pickering emulsions containing spherical and rod-like MSNPs could be switched between unstable and stable states more than 60 times. The Pickering emulsions containing thread-like MSNPs, by contrast, could have their droplet size switched between large and small more than 10 times without any obvious phase separation. The high anisotropy of thread-like MSNPs contributed to the low interface curvature of the droplets. This study revealed the relationship between the morphology of MSNPs and the characteristics of Pickering emulsions. These results enrich our knowledge about the formulation of Pickering emulsions and expand their applications.

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.

Salt-Resistant Switchable Pickering Emulsions Stabilized by Mesoporous Nanosilica Hydrophobized In Situ by pH-Insensitive Surfactants.

Novel oil-in-water (O/W) Pickering emulsions (PEs) were prepared using mesoporous nanosilica in combination with a pH-insensitive cationic surfactant as a stabilizer and show an interesting sensitivity to acids and bases. Adding a suitable amount of NaOH (nNaOH/ncationic surfactant ≥ 1) led to prompt demulsification within 10 s. Upon further adding HCl solutions (nHCl/nNaOH = 1), stable PEs re-formed after homogenization. These emulsions remained stable for over 30 days after 60 cycles, switching from stable to unstable and back to stable states, and showed a high salt tolerance. A mechanism for the switching of the Pickering emulsion (PE) to unstable and back to stable states was derived and involved anionic and neutral forms of hydroxyl groups at the mesopores of the mesoporous silica nanoparticles (MSNPs). This work reveals a switchable PE system involving a pH-insensitive surfactant, in which the species of oils and cationic surfactants can be arbitrarily selected, a feature that greatly expands the applicability of PEs.

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.

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

Innovations in surfactant structure are a feasible way to probe molecular self-assembly principles. Herein, the solution behaviour of a newly synthesized Gemini surfactant derived from dehydroabietic acid, abbreviated R-(EO)-E-R, was investigated using surface tension, fluorescence, isothermal titration calorimetry (ITC), rheology, freeze-fracture transmission electron microscopy (FF-TEM) and cryogenic transmission electron microscopy (cryo-TEM) methods. R-(EO)-E-R has two large, rigid hydrophobic groups. At low concentrations, R-(EO)-E-R forms micelles with an aggregation number of approximately 10, which is smaller than those of Gemini surfactants containing flexible alkyl tails. In addition, the micellization process is less exothermic because of the rigidity of the hydrophobic portions. As the concentration increases, R-(EO)-E-R without any additives forms wormlike micelles, endowing the solution with an obvious viscoelasticity. Further increases in the concentration lead to the coexistence of single-walled vesicles, double-walled vesicles and rarely observed long, tubular vesicles. This behaviour is attributed to the two large, rigid hydrophobic groups of R-(EO)-E-R, which increase the density of the hydrophobic portion around the ionic head groups and facilitate the formation of aggregates with lower curvatures and asymmetric morphology. Surfactants containing rigid hydrophobic portions are expected to result in more delicate, self-assembled morphologies with broad applications.

Photoresponsive Foams Generated by a Rigid Surfactant Derived from Dehydroabietic Acid.

Innovation in the structure of surfactants is crucial to the construction of a surfactant-based system with intriguing properties. With dehydroabietic acid as a starting material, a nearly totally rigid azobenzene surfactant (R-azo-Na) was synthesized. The trans-R-azo-Na formed stable foams with half-lives of 636, 656, 976, and 872 min for 0.3, 1, 2, and 4 mmol·L-1 aqueous solutions, respectively. Under UV light irradiation, a fast collapse of the foams was observed, showing an in situ response. The excellent foam stability of trans-R-azo-Na leads to the extremely high photoresponsive efficiency. As revealed by dynamic surface tension and pulsed-field gradient NMR methods, an obvious energy barrier existed in the adsorption/desorption process of trans-R-azo-Na on the air/water interface. The foams formed by trans-R-azo-Na are thus stable against coarsening processes. The results reveal the unique photoresponsive behavior of a surfactant with a rigid hydrophobic skeleton and provide new insights into the structure causing aggregation of surfactants.