Length Of Micelles Research Articles

Liquid Crystalline Phase Behavior of Well-Defined Cylindrical Block Copolymer Micelles Using Synchrotron Small-Angle X-ray Scattering

The structure and phase behavior of colloidal solutions of monodisperse rod-shaped micelles, of different lengths (ca. 300–2100 nm) and formed from poly(ferrocenylsilane)-block-polyisoprene (PFS-b-PI) diblock copolymers, have been investigated using synchrotron small-angle X-ray scattering. The dimensions of the crystalline PFS core, solvated PI corona, and the overall radial polydispersity were measured, and relationships between the characteristics of the constituent copolymers and the internal structure of the self-assembled micelles have been established. In addition, the effects of micelle length, length distribution, concentration, composition, and block length on the liquid crystalline phase behavior of the micelles have been determined. It was found that micelle dispersions exist in three distinct phases: isotropic, nematic, and hexagonally packed, depending predominantly on their concentration and aspect ratio. The results have also highlighted the importance of the coronal composition and structure in determining the high-concentration behavior of micelle dispersions.

Formation and rheological properties of wormlike micelles by N-hexadecyl-N-methylpiperidinium bromide and sodium salicylate

The formation and properties of wormlike micelles composed of a surface-active surfactant N-hexadecyl-N-methylpiperidinium bromide (C16MDB) and organic salt sodium salicylate (NaSal) at room temperature were studied. Rheological measurements and cryogenic transmission electron microscopy (cryo-TEM) were conducted to study the rheological properties and microstructures of wormlike micelles. Cryo-TEM image confirms the formation of wormlike micelles in aqueous solution. Rheological results show that wormlike micelles indicate linear viscoelasticity and follow the Maxwell model. The Cole–Cole plots agree well with the typical characteristic of the Maxwell model at low and middle frequencies. The contour length, mesh size, and entanglement length of wormlike micelles are estimated from the rheological measurements. In addition, the temperature effect on the rheological properties of wormlike micelles is also studied. Through comparison, the entanglement length and mesh size of wormlike micelles are nearly unchanged, while the contour length shows a sharp decrease tendency with the increase of temperature, indicating that the change of rheological properties with the temperature is due to the contour length change of wormlike micelles.

Peptide amphiphile micelles self-adjuvant group A streptococcal vaccination.

Delivery system design and adjuvant development are crucially important areas of research for improving vaccines. Peptide amphiphile micelles are a class of biomaterials that have the unique potential to function as both vaccine delivery vehicles and self-adjuvants. In this study, peptide amphiphiles comprised of a group A streptococcus B cell antigen (J8) and a dialkyl hydrophobic moiety (diC16) were synthesized and organized into self-assembled micelles, driven by hydrophobic interactions among the alkyl tails. J8-diC16 formed cylindrical micelles with highly α-helical peptide presented on their surfaces. Both the micelle length and secondary structure were shown to be enhanced by annealing. When injected into mice, J8-diC16 micelles induced a strong IgG1 antibody response that was comparable to soluble J8 peptide supplemented with two classical adjuvants. It was discovered that micelle adjuvanticity requires the antigen be a part of the micelle since separation of J8 and the micelle was insufficient to induce an immune response. Additionally, the diC16 tail appears to be non-immunogenic since it does not stimulate a pathogen recognition receptor whose agonist (Pam3Cys) possesses a very similar chemical structure. The research presented in this paper demonstrates the promise peptide amphiphile micelles have in improving the field of vaccine engineering.

Effects of salt concentrations of the aqueous peptide-amphiphile solutions on the sol-gel transitions, the gelation speed, and the gel characteristics.

Hydrogels made of peptide amphiphiles (PA) have attracted a lot of interest in biomedical fields. Considering the applications of PA hydrogels, the control of the gelation speed and the gel characteristics is essential to predominantly determine the usefulness and practicability of the hydrogels. In this work, the effects of the salt concentrations using sodium dihydrogenorthophosphate (NaH2PO4) on the sol-gel transition behaviors, especially the gelation speed and the gel characteristics of the designed PA (C16-W3K) hydrogels in aqueous solution were discussed. It was found that the original solution state before rheological testing was independent of the salt concentration, which was confirmed by observing the self-assembly structures and the peptide secondary structures of PA through transmission electron microscopy (TEM) and circular dichroism spectroscopy (CD). The PA solutions with different salt concentrations, however, presented a profound difference in the gelation speed and the gel characteristics: the solution exhibited higher gelation speeds and higher mechanical properties at higher salt concentrations. Concurrently, the density, the length of wormlike micelles, and the conformational ratio of β-sheets to α-helices in the equilibrium PA solutions all increased with the increase in the salt concentrations.

Ultralong cylindrical micelles precisely located with semiconductor nanorods by solvent evaporation-driven self-assembly.

We demonstrate the efficient incorporation of PS-tethered cadmium sulfide nanorods (PS-CdS NRs) into polystyrene-block-poly(ethylene oxide) (PS-b-PEO) ultralong cylindrical micelles by solvent evaporation-driven self-assembly. The hexadecyl trimethyl ammonium bromide (CTAB) was used as the surfactant in chloroform-in-water emulsions. It is a prerequisite to improve the enthalpic compatibility between the CdS NRs and the PS block of PS-b-PEO to accomplish their cooperative assembly. We investigated the effects including the concentration of CTAB in the aqueous solution ([CTAB]), the weight fraction of CdS NRs (f), the volume ratio of the organic solvent to water and the magnetic stirring rate on the cooperative assembly behaviors. The different morphologies of hybrid assemblies formed at varied [CTAB] were rationalized in view of the adsorption of the aqueous surfactant and block copolymer at the oil-water interfaces, thereby leading to the disparate mechanisms by which the organic-water interfacial instabilities proceeded. This work presents an extremely precise location of the CdS NRs in the centers of the cylindrical micelles, i.e. 95.2% of the CdS NRs were distributed in the central ca. 4 vol% regions of the cylinders and all of them were dispersed in the central ca. 38% radial spans. The length of the hybrid cylindrical micelles can reach tens of micrometers. The ultralong cylindrical micelles functionalized by the CdS NRs in the central cores can lower the toxicity of the CdS NRs and improve the biocompatibility, which have potential applications in fluorescence probes, labels and many others.

A mesoscopic simulation method for predicting the rheology of semi-dilute wormlike micellar solutions

We present a fast “pointer” simulation method that extends the model of Cates and coworkers for the rheology of entangled wormlike micelles. Our method includes not only reptation, breakage/rejoining, contour length fluctuations, and Rouse modes, which were included in Cates' model, but also constraint release, bending modes, and a cross-over to the tight entanglement regime, which had not been previously considered. Our method also contains correlations in micelle length across multiple breakage/rejoining cycles, not included in previous approaches. Our method uses “pointers” that track the ends of unrelaxed regions along each micelle, thereby allowing efficient simulations of relaxation dynamics for ensembles containing thousands of micelles, to obtain accurate results without preaveraging or neglecting correlations. A modified genetic algorithm is applied to transform the simulation data from the time to the frequency domain. The method can span several regimes of behavior depending on the relative rat...

Optimized rod length of polyplex micelles for maximizing transfection efficiency and their performance in systemic gene therapy against stroma-rich pancreatic tumors

Poly(ethylene glycol) (PEG) modification onto a gene delivery carrier for systemic application results in a trade-off between prolonged blood circulation and promoted transfection because high PEG shielding is advantageous in prolonging blood retention, while it is disadvantageous with regard to obtaining efficient transfection owing to hampered cellular uptake. To tackle this challenging issue, the present investigation focused on the structure of polyplex micelles (PMs) obtained from PEG–poly(l-lysine) (PEG–PLys) block copolymers characterized as rod-shaped structures to seek the most appreciable formulation. Comprehensive investigations conducted with particular focus on stability, PEG crowdedness, and rod length, controlled by varying PLys segment length, clarified the effect of these structural features, with particular emphasis on rod length as a critical parameter in promoting cellular uptake. PMs with rod length regulated below the critical threshold length of 200 nm fully exploited the benefits of cross-linking and the cyclic RGD ligand, consequently, exhibiting remarkable transfection efficiency comparable with that of ExGen 500 and Lipofectamine® LTX with PLUS™ even though PMs were PEG shielded. The identified PMs exhibited significant antitumor efficacy in systemic treatment of pancreatic adenocarcinoma, whereas PMs with rod length above 200 nm exhibited negligible antitumor efficacy despite a superior blood circulation property, thereby highlighting the significance of controlling the rod length of PMs to promote gene transduction.

How Long Cylindrical Micelles Formed after Extruding Block Copolymer in a Selective Solvent through a Small Pore Fragment back into Spherical Ones

The slow cylinder-to-sphere transition for cylindrical micelles from nanopore extrusion is quantitatively investigated by using transmission electron microscopy. The time-dependent length distribu- tion and weight-average length of cylindrical micelles from experiment were compared with those calculated ones from computer simulation on the basis of end-scission, random-scission and Gaussian-scission models. The results reveal that the cylinder-to-sphere transition involves a combination of the Gaussian-scission model and the end-scission model and the scission rate constant is nearly a linear function of micelle length but slightly increases with time for a given length. As expected, such a thermal agitation induced transition with the spherical phase as its thermodynamically stable state is different from those previously observed in the shear-induced fragmentation of long cylindrical micelles made of amphiphilic block copolymers in a selective solvent.

Tuning the Size of Cylindrical Micelles from Poly(l-lactide)-b-poly(acrylic acid) Diblock Copolymers Based on Crystallization-Driven Self-Assembly

A series of poly(l-lactide)-b-poly(acrylic acid) (PLLA-b-PAA) diblock copolymers with a range of hydrophobic or hydrophilic block lengths were designed in order to tune the size of the resultant cylindrical micelles using a crystallization-driven self-assembly (CDSA) approach. The precursor poly(l-lactide)-b-poly(tetrahydropyran acrylate) (PLLA-b-PTHPA) was synthesized by a combination of ring-opening polymerization (ROP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. The CDSA process was carried out in a tetrahydrofuran/water (THF/H2O) mixture during the hydrolysis of PTHPA block at 65 °C using an evaporation method. A majority of PLLA-b-PAA diblock copolymers resulted in the formation of cylindrical micelles with narrow size distributions (Lw/Ln < 1.30) as determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Furthermore, the length of PLLA block was found to control the length of the resultant cylindrical micelles while the length of PAA b...

Conductive, Monodisperse Polyaniline Nanofibers of Controlled Length Using Well‐Defined Cylindrical Block Copolymer Micelles as Templates

Stable colloidal dispersions of polyaniline (PAni) nanofibers with controlled lengths from about 200 nm-1.1 μm and narrow length distributions (Lw/Ln < 1.04; Lw = weight average micelle length, Ln = number average micelle length) were prepared through the template-directed synthesis of PAni using monodisperse, solution-self-assembled, cylindrical, block copolymer micelles as nanoscale templates. These micelles were prepared through a crystallization-driven living self-assembly method from a poly(ferrocenyldimethylsilane)-b-poly(2-vinylpyridine) block copolymer (PFS25 -b-P2VP425). This material was initially self-assembled in iPrOH to form cylindrical micelles with a crystalline PFS core and a P2VP corona and lengths of up to several micrometers. Sonication of this sample then yielded short cylinders with average lengths of 90 nm and a broad length distribution (Lw/Ln = 1.32). Cylindrical micelles of PFS25 -b-P2VP425 with controlled lengths and narrow length distributions (Lw/Ln < 1.04) were subsequently prepared using thermal treatment at specific temperatures between 83.5 and 92.0 °C using a 1D self-seeding process. These samples were then employed in the template-directed synthesis of PAni nanofibers through a two-step procedure, where the micellar template was initially stabilised by deposition of an oligoaniline coating followed by addition of a polymeric acid dopant, resulting in PAni nanofibers in the emeraldine salt (ES) state. The ES-PAni nanofibers were shown to be conductive by scanning conductance microscopy, whereas the precursor PFS25-b-P2VP425 micelle templates were found to be dielectric in character.

Access to Different Nanostructures via Self‐Assembly of Thiourea‐Containing PEGylated Amphiphiles

Readily water-soluble PEGylated amphiphiles containing bis-thiourea-based molecular recognition units at the interface of hydrophobic and hydrophilic blocks are developed. Self-assembly of these amphiphiles is found to be dependent on the exact chemical composition of the hydrophobic component. Elongated, spherical, and disk-like micelles are formed with the change in hydrophobic group from stearyl (2A), oleyl (2B), and dodecanol (2C), respectively. The length of the rod-like elongated micelles formed by 2A could be tuned by thermal treatment as well. Synthesis and detailed structural characterization of these amphiphiles by TEM, DSC, synchrotron SAXS techniques are reported. Organic solvent-free direct aqueous encapsulation of doxorubicin, an anticancer drug into these nanostructures is demonstrated.

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.

Nonionic Reverse Micelles near the Critical Point

We report shape, size, and internal cross-sectional structure of diglycerol monomyristate (C₁₄G₂) reverse micelles in n-hexadecane near the critical point using small-angle X-ray scattering (SAXS). Pair-distance distribution function, p(r), which gives structural information in real-space, was obtained by indirect Fourier transformation (IFT) method. The p(r) showed a clear picture of rodlike micelles at higher temperatures well above the critical point (micellar solution phase separates into two immiscible liquids at ~ 48°C). At a fixed surfactant concentration (5% C₁₄G₂), decrease in temperature increases the micellar size monotonously and surprisingly shape of the p(r) curve at 50°C; close to the critical point, mimics the shape of the two dimensional disk-like micelles indicating the onset of critical fluctuations (attractive interactions among rodlike micelles forming a weak network). A similar behavior has been observed with normal micelles in aqueous system near the critical point. When the system is heated to 60°C, shape of the p(r) curve regains rodlike structure. At fixed temperature of 60°C, increase in C₁₄G₂ concentration induced one dimensional micellar growth. Maximum length of micelles increases from ca. 23.5 to 46.0 nm upon increasing concentration from 1 to 12% keeping cross section diameter apparently unchanged at ca. 4.0 nm.

The lengths of thread-like micelles inferred from rheology

The average length of threadlike micelles in a semidilute solution has been inferred from the ratio of loss to storage modulus at a local minimum in loss modulus, which occurs at frequencies well above terminal. This method has yielded average micelle lengths of less than 1 μm or so for cetyltrimethylammonium surfactants at concentrations of around 0.1 to 1 M [Khatory et al., Langmuir 9, 1456–1464 (1993)]. Here, I use established correlations for polymer semidilute solutions to show that this estimate of micelle length is likely too small by around an order of magnitude. More accurate estimates can be obtained from the terminal relaxation time and the viscosity near the cross-over from dilute to semidilute, which show typical micelle average lengths that are well above 1 μm in length, for cetyltrimethylammonium solutions.

Polyferrocenylsilane Crystals in Nanoconfinement: Fragmentation, Dissolution, and Regrowth of Cylindrical Block Copolymer Micelles with a Crystalline Core

Two samples of rod-like micelles in decane were prepared by seeded growth from a sample of a poly(isoprene-b-ferrocenyldimethylsilane) diblock copolymer (PI1000–PFS50, where the subscripts indicate the degree of polymerization). These micelles were uniform in length with a mass/length of 1.9 molecules/nm. The longer micelles (L-1250) had a number-average length Ln = 1243 nm, whereas the shorter micelles (L-250) had Ln = 256 nm. We used transmission electron microscopy (TEM) to examine the behavior of these micelles when dilute solutions of L-1250 or L-250 or their mixtures were heated at temperatures ranging from 40 to 75 °C and then cooled to room temperature. At 55 °C, the L-1250 sample underwent kinetically controlled fragmentation to give a broad distribution of micelle lengths. At this temperature, fragmentation was much less prominent in the L-250 sample. At higher temperatures, micelles with narrow distributions of lengths were obtained in each case (Lw/Ln ≈ 1.01). This process operates under therm...

Dynamic light scattering study of cetyltrimethylammonium bromide aqueous solutions

Cetyltrimethylammonium bromide (CTAB) aqueous solutions are studied by dynamic light scattering method in a wide concentration range covering the first and second critical micelle concentrations (CMC1 and CMC2, respectively). Nonmonotonic and ambiguous behavior of diffusion coefficients D with an increase in concentrations above CMC1 is revealed. An increase in the D values in the first decade of CTAB concentration above CMC1 agrees with known published data for aqueous solutions of ionic surfactants. It is shown that an increase in the ionic strength of solution with the addition of KBr leads to a decrease in the positive slope of the dependence of diffusion coefficients on CTAB concentration up to zero at 0.05 M KBr. Two relaxation processes corresponding to large and small D values are simultaneously observed in micellar solutions, beginning with 0.03 M CTAB concentration. The data obtained are compared with published data, as well as with the results of viscosity measurements. The performed analysis indicates that the observed relaxation processes are explained by the coexistence of spherical and nonspherical micelles. It is established that micelles acquire a cylindrical shape at CTAB concentrations ranging from 0.2 to 0.25 M. Hydrodynamic radii and lengths of micelles are calculated.

Rheo-optics of cationic surfactant micellar solutions with mixed aromatic counterions

The effects of different ratios of mixed aromatic counterions of sodium salicylate (NaSal) and sodium 4-ethylbenzenesulfonate (NaEBS) at a total counterion concentration of 0.125 M on the rheo-optical behaviors of 0.05 M cetyltrimethylammonium chloride (CTAC) cationic surfactant solutions were studied. The introduction of NaEBS into CTAC–NaSal solution led to a significant deviation from the Cox–Merz rule. Mixing of the two aromatic counterions lowered the CTAC solutions’ zero shear viscosities, increased their plateau shear modulus, and decreased their relaxation time. In addition, the persistence lengths of the threadlike micelles and the mesh size of the micellar network were also significantly reduced. These effects are believed to be due to increased degree of branching and denser meshes of the micellar networks in the mixed counterion systems.

Cylindrical micelles from the living crystallization-driven self-assembly of poly(lactide)-containing block copolymers

The synthesis and self-assembly of poly(lactide)-b-poly(acrylic acid) and poly(lactide)-b-poly(dimethylaminoethylacrylate) block copolymers by a combination of ring-opening polymerization and reverse-addition fragmentation chain transfer (RAFT) polymerization is reported. Self-assembly of block copolymers containing enantiopure homochiral poly(lactide), PLA, by a simple direct dissolution methodology results in core-crystallization to afford micelles with a cylindrical morphology. Amorphous atactic PLA cores and conditions that did not promote crystallization resulted in spherical micelles. Cylindrical micelles were characterized by transmission electron microscopy (TEM) with cryo-TEM, small angle neutron scattering (SANS) and angular dependent dynamic light scattering (DLS) proving that the cylindrical morphology was persistent in solution. Manipulation of the assembly conditions enabled the length and dispersity of the resultant cylindrical micelles to be controlled.

Filament formation in carbon nanotube-doped lyotropic liquid crystals

By introducing carbon nanotubes (CNTs) into lyotropic nematic liquid crystals, strongly enhanced viscoelastic behaviour results, allowing the extraction of very thin and long filaments in which the CNTs are uniformly aligned. The filament formation requires the liquid crystallinity of the host phase and it does not take place for coarsely dispersed nanotubes or if their concentration is below a threshold value. The type of nanotube plays only a small role, single- as well as multiwall CNTs both trigger the filament formation, but spherical C60 fullerenes do not give rise to the phenomenon. We argue that individualized CNTs stiffen the rod-shaped micelles of the liquid crystal host and that the elongational flow then increases the nematic long-range order as well as the micelle length. If the CNTs are present at a sufficient concentration to connect in continuous linear chains of arbitrary extension, the micelle stiffening is ensured regardless of length, taking the system into a positive feedback loop between increasing orientational order and diverging micelle length. It is this percolation-like transition to aligned and quasi-infinite micelles stabilized by chains of nanotubes that makes the filament formation possible.

Reversible Photorheology in Solutions of Cetyltrimethylammonium Bromide, Salicylic Acid, and trans-2,4,4′-Trihydroxychalcone

We show photorheology in aqueous solutions of weakly entangled wormlike micelles prepared with cetyltrimethylammonium bromide (CTAB), salicylic acid (HSal), and dilute amounts of the photochromic multistate compound trans-2,4,4'-trihydroxychalcone (Ct). Different chemical species of Ct are associated with different colorations and propensities to reside within or outside CTAB micelles. A light-induced transfer between the intra- and intermicellar space is used to alter the mean length of wormlike micelles and hence the rheological properties of the fluid, studied in steady-state shear flow and in dynamic rheological measurements. Light-induced changes of fluid rheology are reversible by a thermal relaxation process, at relaxation rates which depend on pH and which are consistent with photochromic reversion rates measured by UV-vis absorption spectroscopy. Parameterizing viscoelastic rheological states by their effective relaxation time τ(c) and corresponding response modulus G(c), we find the light and dark states of the system to fall onto a characteristic state curve defined by comparable experiments conducted without photosensitive components. These reference experiments were prepared with the same concentration of CTAB, but different concentrations of HSal or sodium salicylate (NaSal), and tested at different temperatures.