Dynamic nature of micellization

Resolution at tire atomic level of surfactant packing in micelles is difficult to obtain experimentally. This difficulty is based on tire fundamentally amoriDhous packing tliat is obtained as a result of tire surfactants being driven into a spheroidal assembly in order to minimize surface or interfacial free energy. It is also based upon tire dynamical nature of micelles and tire fact tliat tliey have relatively short lifetimes, often of tire order of microseconds to milliseconds, and tliat individual surfactant monomers are coming and going at relatively rapid rates. 

The above discussion emphasizes the dynamic nature of micelles, and it is important to realize that these molecules are in continuous motion and that there is a constant interchange between micelles and solution. This dynamic nature also applies to the counterions which exchange rapidly with Hfetimes in the range 10 to 10 s. Furthermore, the counterions appear to be laterally mobile and not to be associated with (single) specific groups on the micelle surfaces. 

It must be noted that the dynamic nature of micelles must especially be borne in mind in dealing with electron transfers which invariably are fast processes. The seminal work of Bruhn and Holzwarth [88], an examination of the kinetics of diffusion-controlled electron transfer reactions in micellar sodium dodecyl sulfate solutions, disclosed that sufficient heed must be paid to the continuous disintegration and reconstitution of the micelles in this time range. 

Surfactants form micelles above the critical micelle concentration (cmc) of different sizes and shapes, depending on the nature of the molecule, temperature, electrolyte concentration, etc. The dynamic nature of micellization can be described by two relaxation processes, (the life time of a monomer in the micelle) and T2 (the life time... 

The Introductory lecture was given by Professor H. Freundlich, now of University College, London. Micelles composed of amphiphilic molecules form a bewildering array of structures as a function of soap concentration, pH and salt concentration. The highly dynamic nature of micelles was stressed. 

A number of studies have focused on D-A systems in which D and A are either embedded in a rigid matrix [103-110] or separated by a rigid spacer with covalent bonds [111-118], Miller etal. [114, 115] gave the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions [114,115], Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micelles [4] and vesicles [5], wherein some particular D and A species are expected to be separated by a phase boundary. However, owing to the dynamic nature of such interfacial systems, D and A are not always statically fixed at specific locations. 

Here, V is the volume of the hydrocarbon chain(s) of the surfactant, the mean cross-sectional (effective) headgroup surface area, and 4 is the length of the hydrocarbon tail in the all-trans configuration. Surfactants with Pcone-shaped and form spherical micelles. For l/3truncated-cone-shaped, resulting in wormlike micelles (the term wormlike is preferred over rodlike to highlight the highly dynamic nature of these micelles). 

Assume that the micellar aggregate is spherical (radius rm) with the ionic groups of the amphiphile at the surface. Due to the dynamic nature of the micelle the... 

Typical polymeric pseudostationary phases include micelle polymers, polymeric surfactants, water-soluble anionic siloxanes and dendrimers [223-231]. Micelle polymers [e.g. poly(sodium 10-undecylenate), poly (sodium 10-undecenylsulfate), poly(sodium undeconylvalinate), etc.] are synthesized from polymerizable surfactant monomers at a concentration above their critical micelle concentration. These polymers have similar structures to micelles without the dynamic nature of the micelle structure. Polymeric surfactants are polymers with surfactant properties [e.g. acrylate copolymers, such as 2-acrylamide-2-methyl-l-propanesulfonic acid and alkyl methacrylamide, alkyl methacrylate or alkyl acrylate, poly (ally lamine)-supported phases, poly(ethyleneimine), etc]. Water-soluble anionic siloxane polymers are copolymers of alkylmethylsiloxane... 

Reversed micelles have very highly dynamic structures and are in rapid equilibrium with surfactant monomers. Therefore, it is usually difficult to observe their real features by microscopy. A freeze-fracture transmission electron microscope (TEM) would probably show the real picture of a reversed micellar solution because a freeze-fracture film of the reversed micelles is made by rapid cooling to — 150°C to stop instantly the dynamic nature of the structure. Figure 2(a) shows an electron micrograph of the AOT reversed micellar solution (5% w/v AOT-iso-octane solution, IV = 1) [44]. The visual observation by a... 

Second, since micelles have molecular dimensions, the number of reactants they comprise is usually small. Therefore, one has to deal with a discrete statistical distribution of reactants among the micelles instead of conventional concentrations. The overall kinetics in the ensemble of micelles is obtained by averaging the microscopic intramicellar kinetics with a given number of reactants over the occupancy distribution [367]. In the low occupancy limit this distribution is Poissonian. The dynamic nature of amphiphilic aggregates means that the number of reactants in a given micelle fluctuate with time. These fluctuations are slow (microseconds), so that the reaction inside the micelle can normally be treated as kinetically independent. 

Micellization of poly(isobutylene)-( tocfc-poly(methacrylic acid) copolymers with short hydrophobic and long PE blocks has been studied [124-127] by DLS, SLS, SANS, and pyrene titration experiments supported by cryo-TEM imaging. It was unambiguously demonstrated that at high pH, when the poly(methacrylic acid) blocks are fully ionized, the aggregation number increases whereas the hydrodynamic radius decreases as a function of salt concentration. Both dependencies can be approximated by power laws with exponents close to those predicted by theory (87), (89), which again points to the dynamic nature of these micelles. 

As microemulsion-mediated synthesis of particles is designed to take place within a finite nanosized domain (the word finite needs qualifiers because of the dynamic nature of a micelle, possible elastic behavior of a surfactant layer etc.), the average size and size distribution of the particles thus synthesized have always been of interest. This interest is both academic and practical. A survey of the literature shows that due to both these reasons, attempts have been made to understand the factors that influence the ultimate size of the synthesized particles. The factors can be broadly divided into two groups one is various case-specific factors taken together, and the other is the effect of water in the system, including the parameter w ([water]/[surfactant]). Available information on these factors is discussed below. 

Figure 5 Images of core, interfacial, and aqueous regions of ionic micelles, (a) It shows a classical micelle cartoon that represents the dynamic nature of the course and interface, but may over emphasize the minimization of water-hydrocarbon contact, (b and c) The results of united-atom (thin lines) and all-atom (thick lines) molecular dynamic simulations of decyltrimethylanunonium bromide, DeTABr containing 29 or 30 surfactants. COM = center of mass. (Structure 5(a) Reproduced from Ref. 71. American Chemical Society, 1991. Structures 5(b) and 5(c) Reproduced from Ref. 72. American Chemical Society, 2008.)...

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