Micellar shape transition

PS-PMMA Mw = 640-150(1.24-69% PMMA-PS PMMA Mv — 110.50% Toluene -f p-tymene (selective for PS) Micellar shape transition as a function of toluene// -cymene mixture composition. SLS Kotaka el al (1978)... 

It is well known that the geometrical form of the micelles varies with the type of surfactants and the concentration of the solutions (micellar shape transition) [84]. The spontaneous curvature is the fundamental factor to decide the micellar shape. For high spontaneous curvature the micelle takes a spherical shape. For intermediate spontaneous curvature, the micelle takes the shape of a cylinder terminated by two hemispheres. The cylinder can be very long, reaching several micrometers. In some situations, such worm-like giant cylindrical micelles are formed by tuning the value of the spontaneous curvature. 

Phase Diagrams. A phase diagram with the boimdaries for the micellar shape transitions as a function of block copolsuner composition has been determined for PEO block copolymers (Fig. 6a). The vertical lines are the boimdaries separating different micellar shapes. As a rule of thumb, the hydrophilic fraction... 

For completeness, we now give the equations which describe the transition micellar shapes from spheres to cylinders, and examine some of their properties. 

At low surfactant concentration, is a monotonically decreasing function of s. Above the cnK the distribution function exhibits a maximum corresponding to the most probable micelle size J. Further information, including expressions for calculating 1 for rod- and disclike micelles and for transitions between the various micellar shapes, may be taken from Rusanov s article. 

Nh and Nab are the polymerization degrees of homopolymer and block copolymer, respectively. In fact, with increasing Nj, the macro phase separation is enhanced in homopolymer and block copolymer binary blends. Thomas et al. found in their small X-ray scattering [26] and TEM [27] investigations that a sphere-to-rod transition of micellar shape takes place at lower concentration with increasing unit number of solvent molecules. However, only a limited number of studies have been reported on the self-assemblies of A-B type diblock copolymer in polar solvents [28]. 

Extensive studies have been reported by Kunieda s group regarding the formation of worm-like micelles and micellar transient networks in water-surfactant-cosurfactant systems. However, for applications, it is also relevant to know the effect of additives on systems containing worm-hke micelles. It is reported that oils induce a rod-sphere transition in surfactant micellar solutions, leading to a reduction in viscosity [32]. Kunieda s group studied the solubilization of different oils in wormlike micellar solutions [19, 33]. The amount of solubilized oil, its location within the micelle, and its effect on micellar shape and size demonstrated to strongly depend on the nature of the oil and its interactions with the surfactants. 

The Vilgis and Halperin theory [46] does help shed light on the possible different morphological behaviors of micelles of coU-coU and crystalline-coil diblock copolymers. It can not be used to predict quantitatively the copolymer compositions at which the different micellar morphological transitions take place, because only scaling relations and not quantitative relations were derived for the free energies of three types of micelles. Aside from the semi-quantitative nature of the free energy expressions, the theory did not discuss tubular micelles at alL It examined only hairy disks, star-like micelles with a cubic core, and cylindrical micelles with a cubic-prism-shaped core. 

For optimal stealth properties, liposomes would have to be densely covered by a PEO layer. However, PEO liposomes are limited in their ability to integrate high molar ratios of PEO lipid because of shape transitions to micellar structure as a result of the increasing interfacial curvature and lower packing parameter. Poly-mersomes have the advantage that vesicles are entirely composed of PEO-based block copolymer amphiphiles and are not limited by PEO-driven micellization. 

Itri R, Amaral L (1993) Micellar-shape anisometry near isotropic-liquid-crystal phase transition. Phys Rev E 47 2551-2557... 

Shape transitions in aqueous micellar systems as a function of pressure and temperature... 

In aqueous solution micelles are generally thought to be spherical as long as the surfactant concentration remains close to the critical micelle concentration. Rod-like micelles may form at higher surfactant concentrations [1, 2]. Addition of a third component such as neutral salt or non-electrolytes may favour longer micellar structures, for instance rod-like micelles [3-6]. An increase in temperature, on the other hand, seems to favour spherical micelles [7, 8]. The effect of pressure on the shape transition point is not known, though it appears that the aggregation number of micelles decrease with pressure at least up to about 160 MPa [9-12]. 

Structural changes in aqueous micellar solutions can also be seen from conductivity and ultrasound speed measurements as shown in figure 3. The relative viscosity, the conductivity, and the speed of sound have all been plotted in the same figure. The speed of sound and the conductivity curves exhibit a break at the same hexanol molality as the viscosity starts increasing. It is thus possible to use any of these methods to investigate shape transitions. 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

The addition of salts to micelles gives large micelles that turn into cylindrical shapes. However, the addition of cosurfactant produces the liquid crystal phase. As a consequence, these micellar systems with added cosurfactant are found to undergo several macroscopic phase transitions in dilute solutions. These transitions are as follows ... 

Assuming that different polymorphisms can be found in the extractant systems, a better understanding also comes from other phase-separation mechanisms studied in classical amphiphilic systems such as soaps and lipids. The first, largely described here, is the phase separation resulting from increased attractive interactions. The second occurs when a sphere-to-rod transition is observed for the shape of the aggregates. The attraction between cylinders is higher than between spheres when attraction is dominated by van der Walls (VdW) forces between polar cores (119). For micellar solutions (reverse or not), the liquid-liquid phase transition cannot be unambiguously attributed to either shape or attractive interactions only, as it appears that these two effects coexist in nonionic surfactants solutions (91, 120-123). 

The driving force for formation of rod shaped SDS micelles is the elimination of water from die micellar core/water interface (31). The reduction in average headgroup area reflects the removal of water molecules between the SDS headgroups, and should affect the bands due to the asymmetric S-O stretching vibrations, as indicated in the discussion of the transition moment vectors above. 

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