Toroidal Micelles Research Articles

Toroidal Micelles of Uniform Size from Diblock Copolymers

Uniform nanodonuts: Stable toroidal micelles that have a highly uniform size and shape spontaneously self-assemble from a selective THF/ethanol solvent mixture (see 3D AFM image). The donut-shaped micelles can be used as a template to grow gold nanoparticles, which form along the ring surface.

Cover Picture: Toroidal Micelles of Uniform Size from Diblock Copolymers (Angew. Chem. Int. Ed. 25/2009)

AbstractPure toroidal micelles of highly uniform shape and size are presented by T. Chang et al. in their Communication on page 4594 ff. The donut‐shaped micelles are prepared by spontaneous self‐assembly of a polyisoprene‐block‐poly(2‐vinylpyridine) copolymer, and are stable enough to act as a template for the growth of gold nanoparticles along the ring surface.magnified image

Toroidal Micelles of Uniform Size from Diblock Copolymers

AbstractEinheitliche Nanoringe: Stabile toroidale Micellen mit sehr einheitlicher Form und Größe bilden sich spontan aus einem selektiven THF/Ethanol‐Lösungsmittelgemisch (siehe 3D‐AFM‐Bild). Die Donut‐förmigen Micellen können als Template für das Wachsen von Goldnanopartikeln dienen, die sich entlang der Ringoberfläche bilden.magnified image

Origins of toroidal micelle formation through charged triblock copolymer self-assembly

The toroidal micelle morphology has been produced by the self-assembly of poly(acrylic acid)-block-poly(methyl acrylate)-block-polystyrene (PAA-b-PMA-b-PS) triblock copolymervia interaction with organic diamines in mixed THF/water solution. Formation of toroidal, or ring-like, micelles was controlled by the ratio of THF to water, the chain length of the polystyrene block, the type and amount of diamino counterion as well as the solution preparation procedure. Two distinct mechanisms for toroidal micelle formation are proposed. Under the appropriate solution condition, toroids can be constructed either through elimination of high-energy spherical micelles and/or cylindrical micelle endcaps, or through perforation of disc-like micelles. Slow chain dynamics of triblock copolymers in solution associated with their high molecular weight plays an important role in toroidal micelle construction. Toroidal nanostructures were characterized with both transmission electron microscopy (TEM) and cryogenic transmission electron microscopy (cryo-TEM).

Dissipative Particle Dynamics Simulations of Toroidal Structure Formations of Amphiphilic Triblock Copolymers

In this paper, the dynamic assembly of toroidal micelle structures of amphiphilic triblock copolymers in dilute solution has been investigated using dissipative particle dynamics simulations. The amphiphilic molecule is represented by a coarse-grained model, which contains hydrophilic and hydrophobic particles. Some microstructures of complex morphology having toroidal micelles have been observed in the simulations; the toroidal micelle formation is in accordance with the theoretical prediction of the toroidal structure in cylindrical micelle suspensions by Pochan et al. (Science 2004, 306, 94). These findings are very interesting, and these complex morphologies enrich our knowledge of the potential products obtained from the self-assembly of block copolymers.

Thermodynamics and bending energetics of toruslike micelles

The self-assembly of surfactants forming toruslike or toroidal micelles has been investigated from a theoretical point of view, in particular the structural behaviour and stability of tori in terms of the three bending elasticity constants spontaneous curvature ( H 0 ), bending rigidity ( k c ) and saddle-splay constant ( k ¯ c ). It is demonstrated that the size of toruslike micelles increases with an increasing bending rigidity, but is independent of both spontaneous curvature and saddle-splay constant. Similar to conventional micelles, toruslike micelles are found to be stable over bilayers as the spontaneous curvature times the surfactant layer thickness exceeds 1/4. Moreover, it is shown that toruslike micelles, in general, are favoured at the expense of long spherocylindrical micelles as a result of elimination of the unfavourable end-caps. However, conventional micelles that are able to grow with respect to both width and length (tablets) may be stable over tori as well as spheres in much wider regimes of different bending elasticity constants. As a result, toruslike micelles are predicted to be stable over conventional micelles, including rods, at large values of the effective bending constant k eff ≡ 2 k c + k ¯ c , i.e. in the same region where infinite cylinders are expected to be observed. This result is consistent with the fact that toruslike micelles have usually been observed to coexist with large networks of branched cylinders.

Density Functional Theory Studies on the Microphase Separation of Amphiphilic Comb Copolymers in a Selective Solvent

The density functional theory was used to study systematically the microphase separation of amphiphilic comb block copolymers with hydrophilic backbone and hydrophobic side chains in a selective solvent. The effects of the length of backbone and side chains, the grafted density, as well as solvent quality were investigated, and a phase diagram was mapped out. In addition, several novel morphologies, such as toroidal micelles, cage-like micelles, and "sphere-in-vesicle" nanostructure, were identified. The knowledge obtained may provide useful information for future experimental investigations.

Persistence Length of Wormlike Micelles Composed of Ionic Surfactants: Self-Consistent-Field Predictions

The persistence length of a wormlike micelle composed of ionic surfactants C(n)E(m)X(k) in an aqueous solvent is predicted by means of the self-consistent-field theory where C(n)E(m) is the conventional nonionic surfactant and X(k) is an additional sequence of k weakly charged (pH-dependent) segments. By considering a toroidal micelle at infinitesimal curvature, we evaluate the bending modulus of the wormlike micelle that corresponds to the total persistence length, consisting of an elastic/intrinsic and an electrostatic contribution. The total persistence length increases with pH and decreases with increasing background salt concentration. We estimate that the electrostatic persistence length l(p,e)(0) scales with respect to the Debye length kappa(-1) as l(p,e)(0) approximately kappa(-p) where p approximately 1.98 for wormlike micelles consisting of C(20)E(10)X(1) surfactants and p approximately 1.54 for wormlike micelles consisting of C(20)E(10)X(2) surfactants. The total persistence length l(p,t)(0) is a weak function of the head group length m but scales with the tail length n as l(p,t)(0) approximately n(x) where x approximately 2-2.6, depending on the corresponding head group length. Interestingly, l(p,t)(0) varies nonmonotonically with the number of charged groups k due to the opposing trends in the electrostatic and elastic bending rigidities upon variation of k.

Self-Assembly of Amphiphilic ABC Star Triblock Copolymers and Their Blends with AB Diblock Copolymers in Solution: Self-Consistent Field Theory Simulations

The self-assembled morphologies of amphiphilic ABC star triblock copolymers consisting of hydrophilic A blocks and hydrophobic B and C blocks and the blends with their counterpart linear AB diblock copolymers in solution are investigated by 2D real-space implementation of self-consistent field theory (SCFT) simulation. The star triblock copolymers self-assemble in solution to form various micellar structures from hamburger, to segmented wormlike, to toroidal segmented micelles, and finally to vesicles with simultaneously increasing hydrophobic lengths of blocks B and C. When the length of hydrophobic blocks B and C is asymmetric, specific bead-on-string worm micelles are found. Particularly, when the star ABC triblock copolymer is in a strong segregation regime and both B and C blocks are strongly hydrophobic, quite long segmented wormlike micelles are obtained, which had not been found in previously investigated diblock and linear ABC triblock copolymers solution. Additionally, raspberry micelles with beads dispersed on the core also occur in the strong segregation regime of bulk star ABC triblock copolymers. Furthermore, the aggregate morphology of ABC star triblock copolymers is strongly influenced by the addition of linear AB diblock copolymers. The most significant feature is that the long segmented worms will become shorter, to form hamburger micelles with the addition of AB diblock copolymers. These simulations are in good agreement with the experimental findings by Lodge's group.

Semiflexible grafted polymers in poor solvents: Toroidal, archway, and tower micelles

We study a system of grafted semiflexible polymers in a poor solvent which form toroidal or rodlike conformations in the bulk. However, because of the physical constraint of surface grafting, macrophase separation is inhibited and a number of different polymer aggregates (or micelles) form which can be related to the chains' stiffness and their affinity for each other. In contrast to the fully flexible Gaussian case, we observe a number of novel micelle structures, including tower micelles, archway micelles, and spider micelles. We also attempt to develop a phase diagram for the occurrence of these structures with respect to the variables of chain length, chain stiffness, and polymer grafting density.

Self-Consistent-Field Prediction for the Persistence Length of Wormlike Micelles of Nonionic Surfactants

The persistence length of wormlike micelles is predicted using a molecular realistic self-consistent-field theory by analyzing the curvature energy stored in toroidal micelles. It is assumed that the growth of the torus is unidimensional and its cross section is a perfect circle with radius of the torus Rt and curvature J = 1/Rt. For CnEm nonionic surfactants, it is found that the persistence length lp scales with respect to the tail group length n as lp ∼ nx where x is about 2.4−2.9. For sufficiently long hydrophobic tail length, the wormlike micelles become more rigid with increasing temperature and for short-tail surfactants, the reverse is found. Remarkably, curvatures in the order of the inverse persistence length are such that J4 and J6 terms are needed to know the bending energy for this case. This effect is more pronounced the larger the head and the smaller the tail group.

Microstructures in Aqueous Solutions of Mixed Dimeric Surfactants: Vesicle Transformation into Networks of Thread-Like Micelles

The microstructures in mixtures of two dimeric (gemini) surfactants, the dimethylene-1,2- and eicosamethylene-1,20-bis(dimethyldodecylammonium bromide), referred to as 12-2-12 and 12-20-12, have been investigated at 25 °C by electrical conductivity, spectrophotometry, digital light microscopy (DLM), and transmission electron microscopy at cryogenic temperature (cryo-TEM). This mixture was selected because 12-20-12 forms vesicles in a wide range of concentration whereas 12-2-12 forms micelles that are spherical at low concentration then rapidly elongate, branch, or give rise to toroidal micelles (rings), and finally form a network of threadlike micelles at 2 wt %. The measurements were performed keeping the 12-20-12 concentration at 0.09 wt % and progressively increasing the 12-2-12 concentration from 0.1 to 2.0 wt %. The electrical conductivity data clearly showed that in the early stages of 12-2-12 addition to the 12-20-12 vesicles, 12-2-12 was strongly adsorbed by the vesicles. Spectrophotometry and DLM showed that, under the conditions used, the vesicles were nearly eliminated at above 0.7 wt % 12-2-12. The cryo-TEM observations were performed on samples vitrified two months after preparing the mixtures. The progressive increase of the 12-2-12 content in the mixture resulted first in vesicle growth (0.1 wt %), followed by vesicle breakage into smaller vesicles (0.26 wt %), the formation of disklike micelles (0.4−0.75 wt %), then of ring-like micelles and short elongated threadlike micelles (1 wt %), the growth of those threads (1.5 wt %), and finally the formation of a network (2 wt %), where threads and rings were interconnected. The network contained also a few isolated rings. The same structures were observed when the mixtures for cryo-TEM observation were vitrified 5−7 days or two months after mixture preparation, for the mixtures containing 0.5 wt % or less, and 1 wt % or more 12-2-12. Aging of the systems was observed for mixtures containing 0.65−0.75 wt % 12-2-12. When examined 5−7 days after preparation the mixtures showed long rigid rodlike micelles and irregular ribbons. Those structures disappeared when the mixtures were allowed to equilibrate for two months. The results are discussed and a model is presented to explain the observed behavior.