Micelle dynamics

Recently, Muller et al. studied block and graft copolymers poly(n-butyl acrylate)-Wocfc/gra/f-poly(acrylic acid), PnBA-h/g-PAA [136]. The non-polar block/backbone has a low glass transition temperature, thus dynamic micelles were expected the ionic block/side-chains are weak anionic polyelectrolytes, thus a strong dependence of micellization on pH could be expected. The graft copolymers were synthesized by ATRP copolymerization of poly(-ferf-butyl acrylate) macromonomers with n-butyl acrylate, followed by hydrolysis of the terf-butyl acrylate side-chains to PAA [137]. The length of the PAA side chains was varied from 20 to 85 monomer units and their number from 1.5 to 10, whereas the length of the backbone was kept at ca. 130 units. 

One of the possible solutions for design of truly responsive ( dynamic ) micelles is to use copolymers with soft hydrophobic blocks. For example, at the stage of copolymer synthesis, one can incorporate a small fraction of pH-sensitive comonomer units in the hydrophobic moiety. This opens up the possibility of turning on some repulsive contributions in a net attractive domain, leading to a softer core. Alternatively, one can opt for copolymers that are made of a PE block that is linked to a thermosensitive block [11]. This gives the possibility of triggering the formation and dissociation of micelles by variations in the temperature [12]. 

Most experimental studies searching for dynamic micelles have focused on the proper choice of soft hydrophobic block to assure equilibrium,i.e., reversible association of the ionic/hydrophobic block copolymers. Copolymers with such soft... 

The micelle core is nonpolar. It contains some water molecules [16], but its overall polarity is much lower than that of water. The micelle core can be, thus, compared to a nonaqueous phase. However, due to the dynamic micelle fluctuations, it is not exactly a phase. The micellar phase is often called a pseudo-phase, because it is not possible to separate it from the aqueous phase. When the aqueous phase is removed by drying or evaporation, the crystalline form of the surfactant is obtained. This form is not identical to the micellar pseudo-phase, which is a hydrated form of surfactant molecules. 

Amphiphilic block copolymers are polymers that can self-assemble into diverse structures in aqneons media above a critical micellization concentration (CMC) or critical aggregation concentration (CAC). Snch block copolymers contain hydrophilic and hydrophobic segments. The amphi-philicity of these block copolymers may be tnned by incorporating stimuli-responsive blocks, which allow a dynamic micellization process (Fig. 3.6). 

Figure 4.7 Overbased lead octanoate structure. The dynamic micelles (B) have a well-ordered core (C) (deduced from EXAFS) with a diameter of 1 nm (deduced from light scattering and cryo microscopy). Deposited on a carbon support (A), micelles will rearrange during the solvent evaporation and lead to lamellar aggregates (D)...

This experiment demonstrates the static state of the OMeABS, commonly called overbased micelles . Effectively in the case of true dynamic micelles in equilibrium with single molecules, the continuous extraction with pure solvent would have completely extracted the molecules and the rebuild micelles would then be present in the boiler. 

In order to complete the understanding of the action mechanisms of colloidal additives the nature and structure of antiwear films obtained in the presence of two representative systems (Sr octanoate dynamic micelles and OCABS particles) are investigated and results are presented in Figures 4.20 and 4.21 [38, 43 5]. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Forbes M D E, Schulz G R and Avdievich N I 1996 Unusual dynamics of micellized radical pairs generated from photochemically active amphiphiles J. Am. Chem. Soc. 118 10 652-3... 

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. 

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

Figure C2.3.7. Snapshot of micelle of sodium octanoate obtained during molecular dynamics simulation. The darkest shading is for sodium counter-ions, the lightest shading is for oxygens and the medium shading is for carbon atoms. Reproduced by pennission from figure 2 of [36].

One of the most important characteristics of micelles is their ability to take up all kinds of substances. Binding of these compounds to micelles is generally driven by hydrophobic and electrostatic interactions. The dynamics of solubilisation into micelles are similar to those observed for entrance and exit of individual surfactant molecules. Their uptake into micelles is close to diffusion controlled, whereas the residence time depends on the sttucture of the molecule and the solubilisate, and is usually in the order of 10 to 10" seconds . Hence, these processes are fast on the NMR time scale. 

Mesoscale simulations model a material as a collection of units, called beads. Each bead might represent a substructure, molecule, monomer, micelle, micro-crystalline domain, solid particle, or an arbitrary region of a fluid. Multiple beads might be connected, typically by a harmonic potential, in order to model a polymer. A simulation is then conducted in which there is an interaction potential between beads and sometimes dynamical equations of motion. This is very hard to do with extremely large molecular dynamics calculations because they would have to be very accurate to correctly reflect the small free energy differences between microstates. There are algorithms for determining an appropriate bead size from molecular dynamics and Monte Carlo simulations. 

A number of examples have been studied in recent years, including liquid sulfur [1-3,8] and selenium [4], poly(o -methylstyrene) [5-7], polymer-like micelles [9,11], and protein filaments [12]. Besides their importance for applications, EP pose a number of basic questions concerning phase transformations, conformational and relaxational properties, dynamics, etc. which distinguish them from conventional dead polymers in which the reaction of polymerization has been terminated. EP motivate intensive research activity in this field at present. 

M. E. Cates, S. J. Candau. Statics and dynamics of worm-like surfactant micelles. J Phys Condens Matter 2 6869-6892, 1990. 

M. Kroger, R. Makhloufi. Wormlike micelles under shear flow A microscopic model studied by nonequihbrium molecular dynamics computer simulations. Phys Rev E 55 2531-2536, 1996. 

E. Faetibold, G. Waton. Dynamical properties of wormlike micelles in the vicinity of the crossover between dilute and semidilute regimes. Langmuir 77 1972-1979, 1995. 

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