Micelle shape transition

Key words Micelles, shape transitions, activation energy, 1-alcohols... 

Figure 1.11 Schematic illustration of the evolution of the mesophase structure as a function of the surfactant volume fraction. The micelle shape transitions indicated in dotted lines occur over a range of volume fraction while transitions between mesophases occur at constant volume fraction. Reproduced from reference 23 with permission of the Royal Society of Chemistry.

S. Ikeda, S. Ozeki and M. Tsunoda, Micelle molecular weight of dodecyldimethylammonium chloride in aqueous solutions, and the transition of micelle shape in concentrated NaCl solutions, J. Colloid Interface Sci. 73 (1980) 27-37. 

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. 

Fig. 6. Phase diagrams with shape transition boundaries for block copolymers as a function of (a) hydrophilic fraction f (from Ref 46, with permission from ACS Publication Division) and (b) water content in dioxane/water solutions (from Ref 39, with permission from ACS Publication Division). B bilayers/vesicles C cylindrical micelles S spherical micelles.

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. 

Water-insoluble molecules may be incorporated either in the core or in the palisade layer of the micelles. However, solubilization could induce a shape transition of the micelles and even a global phase transition in the thermodynamic sense. 

The somewhat smaller dimensions of DNA coils in highly concentrated surfactant solution, compared to those in a surfactant-free solution, can be explained with a consideration of two effects. First, by careful observation of unfolded DNAs in the coexistence region, namely at [NaBr] = 0.35 M, the individual DNA macromolecules in the intra-chain segregated state, as is exemplified in Fig. 5, are found. Due to the presence of the coiled unfolded part, these DNA molecules apparently reflect an unfolded state with a relatively slow Brownian motion, similar to the DNAs in the entirely unfolded coiled state. Thus, the smaller L value may be attributed to the existence of the intrachain-segregated DNAs in the solution. Second, the smaller DNA dimensions may be produced by the interaction between the long DNA chains and the rod-like micelles. At the threshold NaBr concentration, the shape transition of micelles from spherical (about 5-6 nm in diameter [49]) to rod-like (persistent length of about 44 nm at 35 °C in 0.5 M NaBr [49]) takes place. As the persistent length of the rod-like micelles is of the same... 

Abstract Viscosity, conductivity, and uln asound velocity measurements have been used to study shape transitions of micelles. Sodium dodecylsulfate, dode-cylammonium chloride, and dodecyltrimethyl-ammonium bromide all exhibit such transitions for 0.3 m aqueous solutions in the presence of 1-alcohols. The transition point is dependent upon the temperature and pressure, and on the alcohol chain length. The alcohols from hexanol upwards promote transitions to larger aggregates, the higher alcohols being the most efficient. At higher temperatures more alcohol must be added to induce shape transition. The same applies as the pressure is increased, but the effect of pressure is less pronounced. 

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]. 

Furthermore, the electrical double layer of the micelle is compressed by the enhanced ionic strength, and the repulsion between the anionic head groups of neighboring AOT molecules decreases in the presence of metal cations [119]. Therefore, more AOT molecules participate in micelle formation, and the shape transition originates from the driving force that tends to expose the maximum number of free water molecules to the anionic head groups of the AOT molecules Under these... 

In most cases block copolymers form spherical micelles in dilute solution. In only a few studies was the formation of non-spherical aggregates reported. For example, cylindrical or worm-like micelles were observed for polystyrene-polybutadiene-polystyrene (PSt-PB-PSt) triblock copolymers in ethylacetate [148], PSt-PI (polyisoprene) in N,N-dimethylformamide (DMF), or PEO-PPO-PEO triblock copolymers in aqueous solutions [149]. Conditions for the formation of non-spherical micelles currently seem to be clear only for ionic block copolymers. Due to enormous interfacial tension these systems are in a thermodynamic state close to the super-strong segregation limit (SSSL) [150]. Under these conditions, a sequence of shape transitions from spherical - cylindrical - lamellar is possible. Such transitions can be induced by increasing the ionic strength of the solution or by increasing the relative length of the core block. 

The shape change from sphere to spherocylinder—a cylinder capped by two end hemispheres— has been observed for many surfactant systems, and the rodlike micelles are the subject of the present discussion. The shape transition takes place by passing through the maximum spherical micelle whose aggregation number is m and whose standard chemical potential is /a (=m" )ls ). That is, the maximum micelle is the barrier that must be overcome for the shape transition from a sphere to a more stable long micelle. The standard chemical potential of cylindrical micelle n is divided into two parts on the basis of their surface geometry, one for the two caps of m monomers and the other for the cylinder of n - m monomers ... 

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. 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

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 ... 

---