Transition Of Micelles Research Articles

The CO2 stimulus-responsive spherical-worm-like micelles transition based on anionic surfactant sodium oleate and N, N, N′, N′, N″-pentamethyldipropylenetriamine

ABSTRACTHere, we report a CO2 stimulus-responsive system with anionic surfactant sodium oleate (NaOA) and N, N, N′, N′, N″-Pentamethyldipropylenetriamine (PMDPTA). The microstructure transition from spherical micelles to worm-like micelles was confirmed by rheology measurements, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (Cyro-TEM). There were three types of CO2−responsive groups in PMDPTA. However, because of PMDPTA ionization degrees, just one responsive group of PMDPTA could be ionized by CO2 to work with OA−, resulting in structure transformation from spherical micelles to worm-like micelles. Besides, this system could be switchable between low-viscosity fluid and high viscoelastic by CO2/N2.

The pH and temperature dual-responsive micelle transition in the mixture of hydrotrope potassium phthalic acid and quaternary ammonium surfactants cetyltrimethylammonium bromide

ABSTRACTA facile and effective route to prepare a pH and temperature dual-responsive surfactant micelle has been introduced based on hydrotrope potassium phthalic acid (PPA) and quaternary ammonium surfactants cetyltrimethylammonium bromide (CTAB). The system can be switched between viscoelastic solution and water-like fluid by adjusting pH values. In this paper, different binding abilities between the hydrotrope and surfactants have been demonstrated during the process of adjusting pH because the hydrotrope has different ionization degrees with the pH variation. And with the aid of rheological measurements and cryo-TEM observation, we discussed the different effects of the hydrotrope on surfactants at various pH values in detail and analyzed the possible aggregate transition mechanism of CTAB–PPA system induced by pH. Besides, this CTAB–PPA system has temperature-responsive property when pH < 2.50. The temperature sensitivity and reversible control of rheological properties may greatly facilitate practical applications of such responsive viscoelastic solution.

Stability and dynamics of silicate/organic hybrid micelles

The formation of silicate/organic hybrid micelles is an important milestone in the two-step synthesis of mesoporous silica with polyethyleneoxide (PEO) nonionic structure directing agents (C.R. Chimie 8 (2005) 579). Unlike many inorganic/organic hybrid micelles, these objects have the inorganic component as a diffuse layer positioned at the periphery of the initial micelles and interacting with the hydrophilic polyoxyethylene palisade. We studied how this additional inorganic layer can modify the structure and dynamics of micelles prepared with different types of nonionic surfactants using steady-state and time-resolved fluorescence and dynamic light scattering. Our results show that these hybrid micelles still possess a versatile behavior, which allows them to adapt reversibly to temperature changes. This silicate layer tends to stabilize and consolidate the micelle structure, especially close to the cloud point of critical micelle temperature for polypropyleneoxide (PPO)-based triblock copolymers. Their internal structure is only slightly disturbed by the silicate layer, which reduces the molecular exchanges a little bit. Among other results, we managed to elucidate why mesoporous silica prepared with Pluronic P123, according to our synthesis, stands a dramatic structural change from wormhole to hexagonal structure at 40 °C. Our dynamic light scattering study shows that 40 °C is a critical temperature corresponding to a sphere-to-rodlike structural transition of hybrid micelles, which is not observed with pure micelles. La formation de micelles hybrides organiques/inorganiques est une étape importante dans la synthèse en deux étapes des silices mésoporeuses obtenues avec des agents structurants de type copolymère diblock polyéthylène glycol (PEG) (C. R. Chimie 8 (2005) 579). Contrairement à la plupart des objets décrits comme micelles hybrides organique/inorganique, ces objets micellaires présentent leur composant inorganique positionné comme une couche diffuse à la périphérie de la micelle initiale, et en interaction avec la couronne hydrophile formée des chaines PEG. Nous avons étudié par fluorescence en mode statique et résolu en temps, ainsi que par spectroscopie de photocorrélation, comment l'ajout d'une telle couche inorganique peut modifier la structure et la dynamique de micelles préparées avec différents types de tensioactifs non ioniques. Nos résultats démontrent que ces micelles hybrides possèdent un comportement très versatile, leur permettant de s'adapter réversiblement aux variations de température. Ces couches de silicate tendent à stabiliser et renforcer la structure micellaire, en particulier lorsqu'on s'approche du point de trouble (CP) et de la température micellaire critique (CMT) pour les copolymères triblock à base de polypropylène oxyde. Leur structure interne est déstabilisée légèrement par la couche de silicate, avec pour conséquence une faible réduction des échanges moléculaires. Entre autres résultats, nous sommes parvenus à trouver la raison pour laquelle les silices mésoporeuses préparées avec du Pluronic™ P123, selon notre mode de synthèse, sont affectées par un changement structural majeur à 40 °C, avec la formation d'une structure hexagonale en nid d'abeille. Notre étude par spectroscopie de photocorrélation montre que 40 °C correspond à une température critique avec une transition de micelle sphérique en micelle allongée, une telle transition n’étant pas observée pour les solutions micellaires pures.

Refractive index in relation to solvent effects on the amphiphilic association of n-alkylammonium carboxylates

Measurements of the refractive index of isotropic solutions of n-alkylammonium carboxylates [CH3(CH2)7 +NH3 −O2CR, R = H, n-tridecyl, n-heptadecyl, and 9-anthryl] display concentration dependence related to the degree of surfactant association in pure and mixed solvents of static relative permittivity in the range er = 2.28–110 at 298 K. Results for n-octylammonium formate comply with: (i) small reverse micelle type aggregates in low er ≤ 10 solvents, e.g., benzene; (ii) monomers and/or small association complexes in solvents of intermediate relative permittivity (er ∼ 15–45, e.g., methanol, and acetone:water (50:50 v:v)); and (iii) charged normal micelles in high er ≥ 78 solvents, e.g., water, and N-methylformamide. In water, the critical micelle concentration (cmc) of n-octylammonium formate (0.27–0.29 mol dm−3) is an order of magnitude higher than in benzene. A large anion group does not inhibit micellization in the case of n-octylammonium 9-anthracene carboxylate in chloroform. Refractometry, along with measurements of the electric conductance of some solutions, yield concordant information on micellar parameters and equilibrium processes compared to results from NMR and dielectric measurements. The solubility and Hamaker constants of n-octylammonium carboxylates in different solvents (er = 1.8–182) are described, and experimental data are considered in the context of a micelle transition model.

Formation and Reversible Morphological Transition of Bicontinuous Nanospheres and Toroidal Micelles by the Self‐Assembly of a Crystalline‐b‐Coil Diblock Copolymer

AbstractWe herein report the formation of two complex nanostructures, toroidal micelles and bicontinuous nanospheres, by the self‐assembly of the single structurally simple crystalline‐b‐coil diblock copolymer poly[bis(trifluoroethoxy)phosphazene]‐b‐poly(styrene), PTFEP‐b‐PS, in one solvent (THF) and without additives. The nature of these nanostructures in solution was confirmed by DLS and cryo‐TEM experiments. The two morphologies are related by means of a new type of reversible morphological evolution, bicontinuous‐to‐toroidal, triggered by changes in the polymer concentration. WAXS experiments showed that the degree of crystallinity of the PTFEP chains located at the core of the toroids was higher than that in the bicontinuous nanospheres, thus indicating that the final morphology of the aggregates is mostly determined by the ordering of the PTFEP core‐forming blocks.

Formation and Reversible Morphological Transition of Bicontinuous Nanospheres and Toroidal Micelles by the Self-Assembly of a Crystalline-b-Coil Diblock Copolymer.

We herein report the formation of two complex nanostructures, toroidal micelles and bicontinuous nanospheres, by the self-assembly of the single structurally simple crystalline-b-coil diblock copolymer poly[bis(trifluoroethoxy)phosphazene]-b-poly(styrene), PTFEP-b-PS, in one solvent (THF) and without additives. The nature of these nanostructures in solution was confirmed by DLS and cryo-TEM experiments. The two morphologies are related by means of a new type of reversible morphological evolution, bicontinuous-to-toroidal, triggered by changes in the polymer concentration. WAXS experiments showed that the degree of crystallinity of the PTFEP chains located at the core of the toroids was higher than that in the bicontinuous nanospheres, thus indicating that the final morphology of the aggregates is mostly determined by the ordering of the PTFEP core-forming blocks.

Pathway and energetics of the thermally-induced structural changes in microemulsions

Microemulsions are thermodynamically stable dispersions of nano-droplets stabilized by surfactants. In water-in-oil microemulsions, the nano-droplets exist in the form of swollen reverse micelles. Recent experiments demonstrated very high critical heat fluxes (CHF) in boiling of water-in-oil microemulsions, which makes these microemulsions ideal heat transfer fluids for high heat-flux thermal management applications. While the nanoscale structure of the microemulsions is inevitably changed during their boiling, little is known about such change and its energetics, which are critical for understanding the microemulsion boiling. Here we report a study of the pathway and energetics of thermally-induced structural change in water-in-oil microemulsions using microsecond-long molecular dynamics simulations. We show that, for microemulsions that are thermodynamically stable at room temperature, upon increasing their temperature to similar to that found in boiling experiments, their spherical reverse micelles transition to cylindrical wormlike reverse micelles through merging and splitting of individual reverse micelles. These structural transitions are accompanied by a gradual release of water molecules from the reverse micelles. These transitions are endothermic with the heat absorption dominated by the release of water molecules from the water core of the reverse micelles. The release rate of water molecules from the interior of reverse micelles, hence the heat absorption rate, slows down as the structural transition proceeds because the remaining water molecules in the interior bind more strongly to the polar moieties of the surfactants.

Water‐Induced Transitions from Ellipsoidal Micelles to Chain‐Like Nanostructures Self‐Assembled by the Coil‐Rod‐Coil Block Copolymer Based on Hydrogen‐Bonding Urea Groups

Herein, a novel coil‐rod‐coil triblock copolymer with the coil blocks composed of poly(ethylene glycol) methyl ether and the rigid midterm block alternatively connected with isophorone diisocyanate and isophorone diamine is developed. The triblock copolymer can self‐assemble into ellipsoidal micelles in 1‐methyl‐2‐pyrrolidinone. After the addition of a second coil‐selective solvent (water) to the micellar solutions, these ellipsoidal micelles can further transform into chain‐like nanostructures. The self‐assembly behavior is highly influenced by the additional order of the solvents, which is considerably due to the impacts of the hydrogen‐bonding urea groups and rigid motifs. The transition of the ellipsoidal micelles to chain‐like nanostructures is governed by the water molar fractions. The sizes of the chain‐like nanostructures increase first and then decrease with the growth of the water molar fractions in the mixed solutions. image

Splitting of Surface-Immobilized Multicompartment Micelles into Clusters upon Charge Inversion.

We investigate a morphological transition of surface-immobilized triblock terpolymer micelles: the splitting into well-defined clusters of satellite micelles upon pH changes. The multicompartment micelles are formed in aqueous solution of ABC triblock terpolymers consisting of a hydrophobic polybutadiene block, a weak polyanionic poly(methacrylic acid) block, and a weak polycationic poly(2-(dimethylamino)ethyl methacrylate) block. They are subsequently immobilized on silicon wafer surfaces by dip-coating. The splitting process is triggered by a pH change to strongly basic pH, which goes along with a charge reversal of the micelles. We find that the aggregation number of the submicelles is well-defined and that larger micelles have a tendency to split into a larger number of submicelles. Furthermore, there is a clear preference for clusters consisting of doublets and triplets of submicelles. The morphology of surface-immobilized clusters can be "quenched" by returning to the original pH. Thus, such well-defined micellar clusters can be stabilized and are available as colloidal building blocks for the formation of hierarchical surface structures. We discuss the underlying physicochemical principles of the splitting process considering changes in charge and total free energy of the micelles upon pH change.

From toroidal to rod‐like nanostructure, a mechanism study for the reversible morphological control on amphiphilic triblock copolymer micelles

ABSTRACTMechanism of the morphological changes between toroidal and rod‐like nanostructures of P4VP‐b‐PS‐b‐P4VP amphiphilic triblock copolymer micelles has been investigated in aqueous solution. This transition is proved to be highly reversible and tunable upon changing temperature. The toroidal structure, evolving from fibers at 20 °C, can transform to rod‐like morphology as the temperature either gradually or directly increases to 80 °C, and vice versa. However, the transition mechanisms are quite different in different temperature‐changing processes. The structure and thickness of the micelles are dependent on the specific temperature, whereas the transition mechanism is related to the method of the temperature change. These morphological changes are considered as a result from the interaction parameter between the solvent and the copolymer blocks, especially the hydrophobic block. Our research complements the external control over the reversible morphological transition of block copolymer micelles without changing the composition of the system or introducing additional influencing factors. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1450–1457

Reversible Vesicle–Spherical Micelle Transition in a Polyion Complex Micellar System Induced by Changing the Mixing Ratio of Copolymer Components

The mixing ratio dependence of the morphology of a polyion complex micelle formed of an anionic–neutral double-hydrophilic block copolymer (AP) and a cationic–neutral double-hydrophilic block copolymer (MP) in 0.1 M aqueous NaCl solution was investigated by using small-angle X-ray scattering (SAXS), electrophoretic light scattering (ELS), and isothermal titration calorimetry (ITC), under the condition that the anionic and cationic block chains are much longer than the neutral block chains. When the anionic and cationic monomer units in the solution are nearly equimolar, the net charge of the polyion complex micelle is close to zero, and the bilayer vesicle is formed. However, when the anionic or cationic monomer units in the solution are richer than the other, the polyion complex micelle is charged by including the excess block copolymer component to form the smaller spherical micelle. The morphology transition between the vesicle and spherical micelle can take place reversibly by simply adding AP or MP i...

Photoinduced formation of threadlike micelles from mixtures of a cationic surfactant and a stilbene amphiphile

This study examined the influence of ultraviolet (UV) irradiation on aqueous surfactant solutions containing an anionic stilbene derivative (sodium [4-[(E)-2-(4-butylphenyl)ethenyl]phenoxy]acetate; C4StilNa) as a photoresponsive skeleton. Prior to UV irradiation, an aqueous solution of cetyltrimethylammonium bromide (CTAB) and C4StilNa was a low-viscosity fluid forming spheroidal micelles. Exposure of the low-viscosity fluid to UV light resulted in the formation of threadlike micelles and an increase in the viscosity of the aqueous CTAB/C4StilNa solution. 1H nuclear magnetic resonance spectroscopy indicated that the photochemically induced isomerization and dimerization reactions of C4StilNa molecules had occurred in the aggregates due to UV irradiation. Overall, the structural transformation of the stilbene groups in the C4StilNa molecules with the photochemical reactions leads to an increase in the critical packing parameter and consequently a photoinduced transition of spheroidal micelles to threadlike micelles.

Nanostructural Evolution and Self-Healing Mechanism of Micellar Hydrogels

Understanding the nanoscale structure and dynamics of supramolecular hydrogels is essential for exploiting their self-healing mechanisms. We describe here nanostructural evolution and self-healing mechanism of hydrogels formed from in situ generated hydrophobically modified hydrophilic polymers and wormlike sodium dodecyl sulfate (SDS) micelles. We observe a conformational transition in wormlike SDS micelles upon addition of hydrophobic as well as hydrophilic monomers. Several hundred nanometer long SDS micelles completely disappear after the monomer addition, in favor of spherical micelles with a radius of 2.4 nm. After conversion of the monomers to hydrophobically modified polymer chains via micellar copolymerization, the spherical shape of the micelles remains intact but the radius increases to 2.8 nm. The interconnected spherical mixed micelles consisting of SDS and hydrophobic blocks of the polymer self-assemble to form a layered hydrogel structure. Self-healing response of the damaged hydrogel sampl...

Thermodynamics and Structural Evolution during a Reversible Vesicle-Micelle Transition of a Vitamin-Derived Bolaamphiphile Induced by Sodium Cholate.

Interaction of endogenous sodium cholate (SC) with dietary amphiphiles would induce structural evolution of the self-assembled aggregates, which inevitably affects the hydrolysis of fat in the gut. Current work mainly focused on the interaction of bile salts with classical double-layered phospholipid vesicles. In this paper, the thermodynamics and structural evolution during the interaction of SC with novel unilamellar vesicles formed from vitamin-derived zwitterionic bolaamphiphile (DDO) were characterized. It was revealed that an increased temperature and the presence of NaCl resulted in narrowed micelle-vesicle coexistence and enlarged the vesicle region. The coexistence of micelles and vesicles mainly came from the interaction of monomeric SC with DDO vesicles, whereas micellar SC contributed to the total solubilization of DDO vesicles. This research may enrich the thermodynamic mechanism behind the structure transition of the microaggregates formed by amphiphiles in the gut. It will also contribute to the design of food formulation and drug delivery system.

Spherical-to-wormlike micelle transition in a pseudogemini surfactant system with two types of effective pH-responsive groups

Abstract A novel pseudogemini surfactant system, which has two types of pH-responsive groups in the connection of the alkyl chain and the spacer, has been developed and investigated by adjusting pH. These two types of pH-responsive groups both worked for the micelle transition because they had different ionization degrees and electrostatic attraction forces with the aid of pH variation. The system could be promptly switched between low viscosity fluid and high viscoelastic solution. Besides, the pH-responsive ability was studied by the steady and dynamic rheological measurements, which is effective after several cycles of pH adjusting with the almost invariable magnitude level of viscosity. The transition between spherical micelle and wormlike micelle for the surfactant system at different pH values could be further confirmed by dynamic light scattering and cryogenic transmission electron microscopy characterization.

Light-responsive fluids based on reversible wormlike micelle to rodlike micelle transitions

A new family of rheological responsive fluids based on azobenzene surfactant and 5-methyl salicylic acid binary mixtures is developed due to the light-induced reversible transitions between wormlike micelles and rodlike micelles.

The Formation of pH-Sensitive Wormlike Micelles in Ionic Liquids Driven by the Binding Ability of Anthranilic Acid.

Wormlike micelles are typically formed by mixing cationic and anionic surfactants because of attractive interactions in oppositely charged head-groups. The structural transitions of wormlike micelles triggered by pH in ionic liquids composed of N-alkyl-N-methylpyrrolidinium bromide-based ILs (ionic liquids) and anthranilic acid were investigated. These structures were found responsible for the variations in flow properties identified by rheology and dynamic light scattering, and account for the structures observed with cryogenic transmission electron microscopy (Cryo-TEM). High-viscosity, shear-thinning behavior, and Maxwell-type dynamic rheology shown by the system at certain pH values suggested that spherical micelles grow into entangled wormlike micelles. Light scattering profiles also supported the notion of pH-sensitive microstructural transitions in the solution. Cryo-TEM images confirmed the presence of spherical micelles in the low-viscosity sample and entangled wormlike micelles in the peak viscosity sample. Nuclear magnetic resonance spectroscopy analysis revealed that the pH sensitivity of ionic liquid systems originated from the pH-dependent binding ability of anthranilic acid to the cationic headgroup of ionic liquids.

Micellar dipole potential is sensitive to sphere-to-rod transition

Structural transitions involving shape changes play an important role in cellular physiology. Charged micelles offer a convenient model system in which structural transitions can be suitably induced by increasing the ionic strength of the medium. In this paper, we have explored sphere-to-rod transition in charged micelles of SDS and CTAB by monitoring micellar dipole potential using the dual wavelength ratiometric approach utilizing the potential-sensitive membrane probe di-8-ANEPPS. Our results show that micellar dipole potential is sensitive to sphere-to-rod transition in charged micelles. Micellar dipole potential exhibited increase with increasing ionic strength (salt), irrespective of the nature of micellar charge, implying considerable dipolar reorganization underlying structural transitions. We interpret the increase in dipole potential due to sphere-to-rod transition because of an increase in the population of confined (nonrandom) dipoles induced by micellar organizational change. This is due to the fact that dipole potential arises due to the nonrandom arrangement of micellar dipoles and water molecules at the micelle interface. Our results constitute one of the first reports describing drastic dipolar reorganization due to micellar shape (and size) change. We envision that dipole potential measurements could provide novel insights into micellar processes that are associated with dipolar reorganization.

Multiple Responsive Fluids Based on Vesicle to Wormlike Micelle Transitions by Single-Tailed Pyrrolidone Surfactants

We report a new family of multiple responsive fluids based on the single-tailed pyrrolidone surfactants, N-ethyl-2-pyrrolidone N-alkyl amine (C(m)NP, where m = 10, 12, 14, 16, and 18). These surfactants are highly sensitive to solution pH as a result of the presence of the N-amino group in the molecules. Equilibrium surface tension results indicate that both the surface activity and micellization ability of C(m)NPs decrease with the increase of the protonation degree; i.e., they exhibit a higher critical micelle concentration (cmc) and higher surface tension at the cmc (γ(cmc)) at the acidic conditions than those at the basic conditions. The cmc values of C(m)NPs follow the well-known Klevens equation, which decrease linearly with the increase of the hydrocarbon chain length m at a given pH. More importantly, the self-assemblies of C(m)NPs are highly sensitive to pH, CO2, and CuCl2, as identified by turbidity and viscosity. The transitions between vesicles and wormlike micelles are further confirmed by rheology, static and dynamic light scattering (SLS and DLS), cryogenic transmission electron microscopy (cryo-TEM), and nuclear magnetic resonance (NMR) techniques systematically. Although the aggregate transitions induced by different factors are similar, however, the mechanisms are different. The pH- and CO2-induced transitions are attributed to variation in the protonation degree of the N-amino group; however, CuCl2-induced transitions are a result of the formation of C(m)NP and CuCl2 coordination complexes as revealed by two-dimensional (2D) nuclear Overhauser effect spectrometry (NOESY) NMR and ultraviolet-visible (UV-vis) spectra.

Synthesis and Design of Aggregation‐Induced Emission Surfactants: Direct Observation of Micelle Transitions and Microemulsion Droplets

AbstractThe direct visualization of micelle transitions is a long‐standing challenge owing to the intractable aggregation‐caused quenching of light emission in the micelle solution. Herein, we report the synthesis of a surfactant with a tetraphenylethene (TPE) core and aggregation‐induced emission (AIE) characteristics. The transition processes of surfactant micelles and the microemulsion droplets (MEDs) formed by the surfactant with a TPE core were clearly visualized by a high‐contrast fluorescence imaging method. The fluorescence intensity of the MEDs decreased as the size of MEDs increased as a result of weakening of the restriction of intramolecular rotation (RIR). The results of this study deepen our understanding of micelle‐transition processes and provide solid evidence in favor of the hypothesis that the AIE phenomenon has its origin in the RIR of fluorophores in the aggregate state.