Threadlike Micelles Research Articles

Molecular insights into the pH-induced self-assembly of CTAB/PPA system

Self-assembly systems of stimuli-responsive molecules possess the capability of altering its morphologies and properties when receiving external signals (variations in pH value, temperature, light, etc), which has generated emerging applications, and therefore, attracted great interests. However, revealing mechanisms of stimuli-induced self-assembly morphology transition is still in its infancy and experimentally challenging. In this work, we studied the pH-induced self-assembly of cetyltrimethylammonium bromide (CTAB) and potassium phthalic acid (PPA) mixtures by coarse-grained (CG) molecular dynamics (MD) simulation, aiming to give insights into the pH-responsive self-assembly behavior. We simulated the self-assembly of CTAB/PPA mixtures, and obtained spherical, rod-like, and threadlike micelles depending on pH value and PPA/CTAB ratio. Besides, we found that the length of rod-like micelles fluctuated with the increase of PPA/CTAB ratio, and that the self-assembling morphology shifted between rod-like and threadlike micelles when fixing the PPA/CTAB ratio at 4:6 and varying the pH value. With analyzing the net charge number around the CTAB micelles, the arrangement and distribution of CTAB and PPA molecules in micelles, we concluded that the electrostatic shielding and space location-obstruct of PPA play a key role in the pH-induced self-assembly. This work revealed molecular-level information and mechanisms of the pH-induced self-assembly of CTAB/PPA mixtures, which is expected to trigger further understanding and studies on pH-responsive self-assembly behavior.

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.

Comparison of the effects of methyl- and chloro-substituted salicylate counterions on drag reduction and rheological behavior and micellar formation of a cationic surfactant

We studied drag reduction, rheological behavior, and cryogenic transmission electron microscopy (cryo-TEM) imaging of aqueous solutions of a cationic surfactant, Arquad 16–50 (5 mM) (commercial cetyltrimethylammonium chloride, CTAC), with two series of isomer counterions at 5 mM: 3-, 4-, and 5-methylsalicylate, and 3-, 4-, and 5-chlorosalicylate. All these systems demonstrated viscoelastic behavior at room temperature and showed effective drag reduction in the temperature range of 20–70 °C. The additional Cl- or CH3- group on the salicylate extended the upper effective drag reduction temperature limit 5–10 °C higher than that of sodium salicylate solution. The 4-methylsalicylate, and 3- and 5-chlorosalicylate solutions are effective drag reducers down to 5 °C, the lowest temperature tested in the flow loop. All of the methyl isomers had very similar rheological behaviors, while the chloro isomers exhibited dramatic differences. The 5-chlorosalicylate system had near zero N 1, but did show viscoelastic behavior by showing recoil after swirling and overshoot at shear startup. Cryo-TEM images show that four of the isomer systems formed threadlike micellar nanostructures, but 3-methylsalicylate aggregated into vesicles and open vesicles. The 3-chloro and 5-chlorosalicylate showed vesicles with a few spherical micelles and threadlike micelles. It is postulated that the vesicles were transformed into threadlike micelles under shear and that this structure made them effective drag reducers.

Alkyltrimethylammonium bromide-induced aggregate transition of carboxylate gemini surfactant with a p-dibenzenediol spacer from networks to thread-like micelles to vesicles

The gemini surfactants, O,O’-bis(sodium 2-dodecyl-carboxylate)-p-dibenzenediol and O,O’-bis(sodium 2-decylcarboxylate)-p-dibenzenediol (denoted as C12ϕ2C12 and C10ϕ2C10, respectively), were previously found to form network-like aggregates in very dilute solution. In the present work, the effect of alkyltrimethylammonium bromides, CnNBr, on the network-like aggregates formed by C12ϕ2C12 and C10ϕ2C10 was studied using dynamic light scattering, fluorescence probing, steady-state or frequency sweep rheology, and freeze-fractured TEM measurements. These network-like aggregates were easily transformed into core-shell micelles as CnNBr was added, including spherical micelles and even thread-like micelles, upon increased concentration of CnNBr. On further increasing CnNBr concentration, vesicles were formed. The phase diagrams of aggregate transition were plotted. All of these were attributed to the improvement of added CnNBr on the effective molecule geometry under the base of original columnar-like molecules of C12ϕ2C12/C10ϕ2C10.

Rings and loops in perflurosurfactants viscoelastic solutions

Abstract The structure of micellar solutions has been a topic of intense research. Of particular interest is the relation between rheological properties and the nanostructure of the corresponding micelles. At compositions surrounding the 2nd CMC, the onset of micellar growth, spherical micelles typically transition into short rodlike micelles, and these assemblies continue to grow into long, entangled threadlike micelles, avoiding micellar ends. Another possibility to avoid unfavorable micellar ends is to form small closed structures—rings and loops. Here, we disclose with cryo-TEM the formation of these assemblies near the 2nd CMC, for a perfluorosurfactant C 8 F 17 SO 3 N(C 2 H 5 ) 4 , and we correlate the observations with electric birefringence (EB) data that show 4 separate processes near that critical composition.

Patterned Threadlike Micelles and DNA-Tethered Nanoparticles: A Structural Study of PEGylated Cationic Liposome-DNA Assemblies.

The self-assembly of oppositely charged biomacromolecules has been extensively studied due to its pertinence in the design of functional nanomaterials. Using cryo electron microscopy (cryo-EM), optical light scattering, and fluorescence microscopy, we investigated the structure and phase behavior of PEGylated (PEG: poly(ethylene glycol)) cationic liposome-DNA nanoparticles (CL-DNA NPs) as a function of DNA length, topology (linear and circular), and ρ(chg) (the molar charge ratio of cationic lipid to anionic DNA). Although all NPs studied exhibited lamellar internal nanostructure, NPs formed with short (∼2 kbps), linear, polydisperse DNA were defect-rich and contained smaller domains. Unexpectedly, we found distinctly different equilibrium structures away from the isoelectric point. At ρ(chg) > 1, in the excess cationic lipid regime, threadlike micelles rich in PEG-lipid were found to coexist with NPs, cationic liposomes, and spherical micelles. At high concentrations these PEGylated threadlike micelles formed a well-ordered, patterned morphology with highly uniform intermicellar spacing. At ρ(chg) < 1, in the excess DNA regime and with no added salt, individual NPs were tethered together via long, linear DNA (48 kbps λ-phage DNA) into a biopolymer-mediated floc. Our results provide insight into what equilibrium nanostructures can form when oppositely charged macromolecules self-assemble in aqueous media. Self-assembled, well-ordered threadlike micelles and tethered nanoparticles may have a broad range of applications in bionanotechnology, including nanoscale lithograpy and the development of lipid-based multifunctional nanoparticle networks.

Self-assembly of multi-responsive poly(N-isopropylacrylamide)-b-poly(N,N-dimethylaminopropylacrylamide) in aqueous media

Stimuli responsive double hydrophilic diblock copolymers poly(N-isopropylacrylamide)-b-poly(N,N-dimethylaminopropylacrylamide) (PNIPAM-b-PDMPA) with constant degree of polymerization (DP) of PNIPAM (=127) and varying DP of PDMPA (=31 and 66) were synthesized by RAFT polymerization, and characterized by NMR, FTIR and GPC. In dilute aqueous solutions, the copolymers remain as unimers in equilibrium with loosely bound aggregates and form aggregates with PNIPAM core in the presence of NaCl or at temperatures above the LCST of PNIPAM. However, at higher temperatures i.e. above cloud point temperature (CPT) of PNIPAM127-b-PDMPA66, core–shell micelles are formed with PNIPAM core. The CPT of both block copolymers decrease in the presence of NaCl and in acidic pH, but increases with increase in DP of PDMPA. The copolymer showed enhanced surface activity in the presence of NaCl. The temperature dependent micellization observed from NMR is supported by turbidity and fluorescence. PDMPA is highly protonated in acidic pH; cryo-TEM and SANS confirm the formation of threadlike micelles in the presence of SDS.

Aggregation behavior of sodium lauryl ether sulfate with a positively bicharged organic salt and effects of the mixture on fluorescent properties of conjugated polyelectrolytes.

The aggregation behavior of anionic single-chain surfactant sodium lauryl ether sulfate containing three ether groups (SLE3S) with positively bicharged organic salt 1,2-bis(2-benzylammoniumethoxy)ethane dichloride (BEO) has been investigated in aqueous solution, and the effects of the BEO/SLE3S aggregate transitions on the fluorescent properties of anionic conjugated polyelectrolyte MPS-PPV with a larger molecular weight and cationic conjugated oligoelectrolyte DAB have been evaluated. Without BEO, SLE3S does not affect the fluorescent properties of MPS-PPV and only affects the fluorescent properties of DAB at a higher SLE3S concentration. With the addition of BEO, SLE3S and BEO form gemini-like surfactant (SLE3S)2-BEO. When the BEO/SLE3S molar ratio is fixed at 0.25, with increasing the BEO/SLE3S concentration, the BEO/SLE3S mixture forms large, loosely arranged aggregates and then transforms to closely packed spherical aggregates and finally to long thread-like micelles. The photoluminescence (PL) intensity of MPS-PPV varies with the morphologies of the BEO/SLE3S aggregates, while the PL intensity of DAB is almost independent of the aggregate morphologies. The results demonstrate that gemini-like surfactants formed through intermolecular interactions can effectively adjust the fluorescent properties of conjugated polyelectrolytes.

Distinctive effect of maleic acid and fumaric acid on structural transitions in cationic micellar solution

Isomeric additives, maleic and fumaric acids (MA and FA) showed a distinctive influence on the structural transition induced by n-octanol in aqueous cetyltrimethylammonium bromide (CTAB) solution. With MA, significant increase in viscosity was observed on successive addition of n-octanol. Samples with high n-octanol concentration showed viscoelasticity. However, no such dramatic change was observed with FA. Rheological studies revealed the existence of entangled threadlike micelles in CTAB/MA/n-octanol system. Newtonian flow behaviour exhibited by the FA-containing samples indicated the presence of spherical micelles. The existence of threadlike micelles in MA containing samples was further confirmed by cryogenic transmission electron microscopy and small-angle neutron scattering studies. The favourable geometry and better acidic strength of MA are regarded as the factors synergistically facilitating unidimensional micellar growth in the presence of n-octanol. In contrast, the stronger hydrophobicity of FA resulting from trans geometry and feeble ionization, modifies the solvent properties and retards micellar growth.

PH-switchable structural evolution in aqueous surfactant-aromatic dibasic acid system.

Structural transitions triggered by pH in an aqueous micellar system comprising of a cationic surfactant (cetylpyridinium chloride) and an aromatic dibasic acid (phthalic acid) was investigated. Reversible switching between liquid-like and gel-like states was exhibited by the system on adjusting the solution pH. Self-assembled structures, responsible for the changes in flow properties were identified using rheology, light scattering techniques and 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 the growth of spheroidal/short cylindrical micelles into long and entangled structures. Light scattering profiles also supported the notion of pH-induced microstructural transitions in the solution. Cryo-TEM images confirmed the presence of spheroidal/short cylindrical micelles in the low-viscosity sample whereas very long and entangled thread-like micelles in the peak viscosity sample. pH-dependent changes in the micellar binding ability of phthalic acid is proposed as the key factor regulating the morphological transformations and related flow properties of the system.

Ultrasonic transformation of micelle structures: Effect of frequency and power

A comprehensive investigation on the effect of ultrasonic frequency and power on the structural transformation of CTABr/NaSal micelles has been carried out. Sonication of this micelle system at various ultrasonic frequencies and power resulted in the formation and separation of two types of micelles. High viscoelastic threadlike micelles of ∼2nm in diameter and several μm in length and tubular micelles possessing a viscosity slightly above that of water with ∼30–50nm diameter and few hundred nm length. The structural transformation of micelles was induced by the shear forces generated during acoustic cavitation. At a fixed acoustic power of 40W, the structural transformation was found to decrease from 211 to 647kHz frequency due to the decreasing shear forces generated, as evidenced by rheological measurements and cryo-TEM images. At 355kHz, an increase in the structural transformation was observed with an increase in acoustic power. These findings provide a knowledge base that could be useful for the manipulation of viscosity of micelles that may have applications in oil industry.

A drag reducing surfactant threadlike micelle system with unusual rheological responses to pH

A pH-responsive threadlike micellar system was developed by mixing alkyl bis(2-hydroxyethyl)methylammonium chloride (EO12) and trans-o-coumaric acid (tOCA). The rheological response of this system to pH is unusual in that it has viscoelasticity at both high and low pH levels, while it shows water-like behaviors at medium pH. Cryogenic transmission electron microscopy (Cryo-TEM) images confirmed the presence of TLMs at pH 3.5 and pH 9.8. This system also had DR (drag reduction) capability at low and high pH. The unusual rheological and micellar responses of this system to pH are caused by the dual pKa’s of tOCA. 1H NMR and zeta potential results support this hypothesis.

Coacervation and aggregate transitions of a cationic ammonium gemini surfactant with sodium benzoate in aqueous solution

Coacervation in an aqueous solution of cationic ammonium gemini surfactant hexamethylene-1,6-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) with sodium benzoate (NaBz) has been investigated at 25 °C by turbidity titration, light microscopy, dynamic light scattering, cryogenic temperature transmission electron microscopy (Cryo-TEM), scanning electron microscopy (SEM), isothermal titration calorimetry, ζ potential and (1)H NMR measurements. There is a critical NaBz concentration of 0.10 M, only above which coacervation can take place. However, if the NaBz concentration is too large, coacervation also becomes difficult. Coacervation takes place at a very low concentration of C12C6C12Br2 and exists in a very wide concentration region of C12C6C12Br2. The phase behavior in the NaBz concentration from 0.15 to 0.50 M includes spherical micelles, threadlike micelles, coacervation, and precipitation. With increasing NaBz concentration, the phase boundaries of coacervation shift to higher C12C6C12Br2 concentration. Moreover, the C12C6C12Br2-NaBz aggregates in the coacervate are found to be close to charge neutralized. The Cryo-TEM and SEM images of the coacervate shows a layer-layer stacking structure consisting of a three-dimensional network formed by the assembly of threadlike micelles. Long, dense and almost uncharged threadlike micelles are the precursors of coacervation in the system.

Complex Formation and Aggregate Transitions of Sodium Dodecyl Sulfate with an Oligomeric Connecting Molecule in Aqueous Solution

Anionic single-tail surfactant sodium dodecyl sulfate (SDS) and a molecule with multiple amido and amine groups (Lys-12-Lys) were used as building blocks to fabricate oligomeric surfactants through intermolecular interactions. Their interactions and the resultant complex and aggregate structures were investigated by turbidity titration, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, (1)H NMR, and 1D NOE techniques. At pH 11.0, the interaction between SDS and Lys-12-Lys is exothermic and mainly resulted from hydrogen bonding among the amido and amine groups of Lys-12-Lys and the sulfate group of SDS and hydrophobic interaction between the hydrocarbon chains of SDS and Lys-12-Lys. At pH 3.0, each Lys-12-Lys carries four positive charges and two hydrogen bonding sites. Then SDS and Lys-12-Lys form complexes Lys-12-Lys(SDS)6 and Lys-12-Lys(SDS)4 through the head groups by electrostatic attraction and hydrogen bonds assisted by hydrophobic interaction. Moreover, the complexes pack more tightly in their aggregates with the increase of the molar ratio. Especially the Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 complexes behave like oligomeric surfactants taking Lys-12-Lys as a spacer group, exhibiting a series of aggregates transitions with the increase of concentration, i.e., larger vesicles, smaller spherical micelles, and long threadlike micelles. Therefore, oligomeric surfactants Lys-12-Lys(SDS)4 and Lys-12-Lys(SDS)6 have been successfully fabricated by using a single chain surfactant and an oligomeric connecting molecule through noncovalent association.

Interconvertible Self‐Assembly and Rheological Properties of Planar Bilayers and Vesicle Gels in Anionic/Nonionic (CF/CH) Surfactant Solutions

The surface and bulk properties of aqueous mixtures of nonionic tetraethylene glycol monododecyl ether (C12EO4) and anionic perfluorolauric acid (PFLA) were studied. These mixtures exhibited pronounced synergistic effects in terms of their mixed micellization and rheological properties. Only one type of mixed micelle is formed in dilute mixed solutions. At low PFLA content, with increasing the total surfactant concentration, short thread-like micelles grow to form larger discs. On increasing the amount of PFLA, owing to the fact that the fluctuant planar bilayers of nonionic C12EO4 become charged, vesicle gels that are comprised of densely packed unilamellar and multilamellar vesicles spontaneously form, in which the fluorocarbon chains in the bilayers are in a fluid state at room temperature. The elastic properties and the yield stress of the lamellar solutions largely increase upon the addition of small amounts of PFLA and they then pass through a maximum at a saturation of effective membrane charge density. Interestingly, increasing the pH value of the gels causes a backward transition from stiff vesicles to flexible planar bilayers. On increasing the amount of NaOH, the bilayers exhibit greater flexibility, owing to the decrease in membrane charges, along with a large decrease in the solution viscosity and their elastic properties. The combination of strong stability of the vesicle gels with the interconvertible transition of bilayers and vesicles is expected to be of practical use for controlled drug delivery and release.

Fusion of Bolaamphiphile Micelles: A Method to Prepare Stable Supported Biomimetic Membranes

Supported biomimetic membranes (SBMs) on solid substrates have been commonly prepared from vesicle-forming double-tail lipids, such as zwitterionic phospholipids, using the method of vesicle fusion. Here we report on the preparation of SBMs on silica surfaces via a similar process of "micelle fusion" from a cationic single-tail bolaamphiphile GLH-20 that forms spherical and elongated thread-like micelles in solution. We demonstrate that, in contrast to zwitterionic phospholipids, GLH-20 self-assembles into a stable contiguous SBM at both low and high ionic strengths. The cationic charge of GLH-20 promotes the formation of a stable SBM through enhanced double-layer interactions with the negatively charged silica surface. It is also shown that spinach aquaporin PM-28 was successfully incorporated within bolaamphiphile SBM in a manner similar to SBMs prepared by vesicle/proteoliposome fusion; thereby the inherent curvature of the micelle surface does not inhibit protein reconstitution. The results suggest that SBMs based on charged bolaamphiphiles might be an attractive platform for applications such as water purification and biosensors, where the stability and low defect rate of SBMs in diverse conditions are crucial for achieving desired performance.

Mechanism of Solubilization of Phospholipid Liposomes, Revisited

Solubilization of membranes by detergents, commonly described by phase diagrams, is an essential tool in membrane biochemistry and biophysics.In spite of its importance in assuring that the states of aggregation in lipid-detergent mixtures are at equilibrium, the mechanism(s) of solubilization proposed thus far are based on ambiguous analyses of indirect, not necessarily relevant evidence. Our recent analysis of the available data, particularly kinetic studies yielded better understanding of the mechanism.Specifically, at sub-solubilizing detergent concentrations, partitioning of detergent into the bilayers of small vesicles induces size growth via a disproportionation mechanism, at higher detergent concentration, in the range of co-existence of detergent-saturated vesicles and lipid-saturated mixed micelles (between the onset and completion of solubilization) electron micrographs show thread-like micelles and vesicles with pores of increasing size, in agreement with the leakage of entrapped solutes of increasing sizes. In several electro micrographs, thread-like micelles have been observed attached to perforated bilayers. These probably transient microstructures indicate that the micellization of detergent super-saturated vesicles occurs via a series of three stages: (i) bending of detergent-rich monolayers into curved thread-like cover of the perimeter of the holes, (ii) formation of thread-like micelles attached to the vesicles due to the line tension of the holes and (iii) detachment of the (most stable) mixed micelles from the vesicles. All the available data (spectroscopic, microscopic and calorimetric) are consistent with this mechanistic model.

S051032 界面活性剤の添加による歳差球体内乱流の変調([S051-03]複雑流体の流動現象(3))

Precession is the motion that the spin axis of a rotating object rotates about another axis. A precessing sphere generates and sustains turbulence of a confined fluid. Using particle image velocimetry, we investigate turbulent velocity fields on the equatorial plane of the precessing sphere. With the additions of cationic surfactant(CTAC) and counterion, sodium salicylate(NaSal), turbulence suppression is observed in a central region of the sphere. When themolar concentration of CTAC(C_D) is kept constant and that of NaSal(C_S) increases, in the range of C_S/C_D &gsim; 1, turbulence is suppressed, while such a suppression is not observed for C_S/C_D = 0.5. For C_S/C_D &gsim; 1, there are fully entangled threadlike micelles in the solution. Moreover, free salicylate ions exist and they promote reconnections of threadlike micelles. This may be the reason why turbulence suppression in a precessing sphere is sustained for a long time in the cases with rich free salicylate ions.

Ultrasound-induced formation of high and low viscoelastic nanostructures of micelles

We report on the unprecedented ultrasound-driven engineering of typical wormlike micelles to well-aligned, much higher viscoelastic threadlike micelles and lower viscoelastic tubular micelles that have potential practical applications in medicine and oilfield industries. The sonication of the CTAB/salicylate micelle system at 211 kHz in aqueous solutions generated two nanostructures with specific but contradicting properties: stably aligned threadlike micelles of ∼2 nm in diameter and several μm in length with a high viscoelasticity and low viscoelastic tubular micelles of ∼30 nm in diameter and a few hundred nanometres in length. The tubular micelles are relatively unstable structures formed by the coalescence of vesicular intermediates generated during sonication. Acoustic cavitation-generated physical effects are primarily responsible for the generation of these novel nanostructures. This study also provides the first ever experimental evidence in support of a well-known theoretical model that was proposed in the 1990s on the structural transformation of micelles.

Construction of a highly viscoelastic anionic wormlike micellar solution by carboxylate gemini surfactant with a p-dibenzenediol spacer

The viscoelastic wormlike micellar solutions formed by the carboxylate gemini surfactant, O,O′-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol (referred to as C12ϕ2C12), have been investigated by steady-state and dynamic frequency-sweep rheological techniques. Another system using its homologue, O,O′-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C12ϕC12), was also examined for comparison. Both C12ϕ2C12 and C12ϕC12 formed wormlike micelles without any additives. The C* corresponding to the onset of the semi-dilute regime was 48 mmol L−1 for C12ϕ2C12 and 160 mmol L−1 for C12ϕC12, respectively, showing the considerably high ability and efficiency of C12ϕ2C12 in forming wormlike micelles. The wormlike micelles started to entangle with each other at 60 mmol L−1 for C12ϕ2C12 and 300 mmol L−1 for C12ϕC12. The average length of the C12ϕ2C12 wormlike micelles was estimated to be about 1.0–1.9 μm, significantly longer than the C12ϕC12 ones. The solution viscosity of C12ϕ2C12 at 200 mmol L−1 reached 613 Pa s, much higher than 24.2 Pa s for C12ϕC12 at 500 mmol L−1. The mechanisms of forming wormlike micelles in both cases were revealed by dynamic light scattering measurements. Differently from the normal micellar growth that C12ϕC12 displayed, C12ϕ2C12 was found to form large network-like aggregates at very low concentrations. These aggregates could more conveniently transform into threadlike (wormlike) micelles by continuously increasing the concentration of C12ϕ2C12, from which a new approach was realised for highly viscoelastic anionic wormlike micellar solutions.