Surfactant micelle dynamics

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

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. 

This work also shows that the time constants for the ionic surfactant micelle solutions are twice as fast as the TX solution time constant. Differences between the Stern layers of the micelles appear to be the charge of the surfactant polar headgroups and the presence of counterions. However, these differences do not account for the observed dynamics. Since the polar headgroups and counterions should interfact more strongly with the water molecules, the water motion at the interface should be slower. This view is supported by recent investigations where systematic variation of surfactant counter-... 

The peptide chain in globular proteins is folded into fairly compact conformations. Water-soluble enzymes are typical globular proteins which have most of the hydrophobic amino acid residues located in the interior and the hydrophilic residues located mainly at the surface in contact with solvent water. The average radii are 20-40 A (Boyer, 1970). It is clear that there are common morphological features between surfactant micelles and enzyme molecules. This fact has prompted many chemists to use micelles as enzyme models. However, it must be emphasized that micelles exist in dynamic equilibria with monomeric surfactant and their hydrophobic core is quite fluid, whereas enzyme molecules have precisely fixed three-dimensional structures. 

We also describe the spreading of a thin surfactant laden aqueous film on a hydrophilic solid, i.e., one in which the dynamic contact angle is small. In such a case, the osmotic pressure gradient generated by the nonuniform distribution of surfactant micelles in the liquid film can drive fhe spreading process. The mofivation for this study comes from the need to understand the detergent action involved in the removal of an oily soil from a soiled surface. This paper presents an overview of our recent work. 

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

At room temperature, these molecules occupy well-defined locations in their respective crystal lattices. However, they tumble freely and isotropically (equally in all directions) in place at their lattice positions. As a result, their solid phase NMR spectra show features highly reminiscent of liquids. We will see an illustration of this point shortly. Other molecules may reorient anisotropically (as in solid benzene). Polymer segmental motions in the melt may cause rapid reorientation about the chain axis but only relatively slow reorientation of the chain axes themselves. Large molecular aggregates in solution (such as surfactant micelles or protein complexes or nucleic acids) may appear to have solidlike spectra if their tumbling rates are sufficiently slow. There are numerous other instances in which our macroscopic motions of solid and liquid may be at odds with the molecular dynamics. Nuclear magnetic resonance is one of the foremost ways of investigating these situations. 

R. Wagner, Y. Wu, L. Richter, S. Siegel, J. Weissmuller, J. Reiners, Appl. Organometal. Chem., 1998, 12(12), 843-853. Silicon-modified carbohydrate surfactants IX dynamic wetting of a perfluorinated solid surface by solutions of a siloxane surfactant above and below the critical micelle concentration. ... 

K. Suga, K. Maemura, M. Fujihira, and S. Aoyagui, ESR studies ofthe dynamic properties of ion radicals captured by surfactant micelles, Bull Chem. Soc. Jpn. 60, 2221-2226 (1987). 

Micelle formation is briefly discussed in Section 2.2.5, item 4 see especially Figure 2.8. Soap micelles typically contain 50 to 100 molecules, and the radius is roughly 2nm (about the length of a surfactant molecule). The core of a micelle contains a little water, at most one molecule per surfactant molecule. The size and shape of the micelles closely depend on the molecular configuration of the surfactant. Micelles are dynamic structures. They are not precisely spherical, and surfactant molecules move in and out. Characteristic times for these processes are a matter of debate, but they seem to be of the order of 10 ps. Presumably, a micelle can disappear in 10-100 ms upon dilution. 

Polymer-surfactant interactions are the basis for the rheological behavior of MHAPs. Other surfactant-polymer systems have previously been investigated. One example is the interaction of surfactants with polymers such as poly(ethylene oxide), which results in greater solution viscosities than with the polymer alone (e.g., ref. 25 and references therein). The interaction of surfactants or latexes with hydrophobically modified water-soluble polymers has also been shown to produce unique rheology (2, 5, 26, 27). In these systems, the latex particles or the surfactant micelles serve as reversible cross-link points with a hydrophobic region of a polymer molecule in dynamic association with a latex particle or surfactant micelle (27). 

Theoretical studies of the dynamics of self-assemblies of wormlike surfactant micelles have been reported by a number of investigators, such as Cates and coworkers [Turner and Cates, 1991 Marques et al 1994]. Since they are subject to reversible scission and recombination, they are called living polymers. The continuous breaking and repair of the micellar chains provides more complex solution behavior than do reptating polymer chains that is, their stress relaxation mechanisms are a combination of reptation and breaking followed by reassembly. At low frequencies, linear viscoelastic (Maxwell) behavior is predicted and observed for some surfactant systems. However, non-Maxwell behavior was observed in Cole-Cole plots of a number of cationic surfactant systems [Lu, 1997 Lin, 2000]. 

Finally, Mattice and coworkers have used lattice Monte Carlo simulations for various studies of micellization of block copolymers in a solvent, including micellization of triblock copolymers [43], steric stabilization of polymer colloids by diblock copolymers [44], and the dynamics of chain interchange between micelles [45]. Their studies of the self-assembly of diblock copolymers [46-48] are roughly equivalent to those of surfactant micellization, as a surfactant can in essence be considered a short-chain diblock copolymer and vice versa. In fact, Wijmans and Linse [49,50] have also studied nonionic surfactant micelles using the same model that Mattice and coworkers used for a diblock copolymer. Thus, it is interesting to compare whether the micellization properties and theories of long-chain diblock copolymers also hold true for surfactants. 

Nuclear magnetic resonance relaxation is a useful experimental technique to study surfactant aggregation in liquid solutions and liquid crystals [2,50,51]. It yields information on the local dynamics and the conformational state of the surfactant hydrocarbon chain and has, for example, demonstrated the liquidlike interior of surfactant micelles. However, the aim of NMR relaxation studies of microemulsions is often to study properties such as the surfactant aggregate (droplet) size. 

Dynamics of Hydrogels With and Without Free Surfactant Micelles. 113... 

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