Micelles. Physical chemistry of surfactant association

The association of many classes of surface-active molecules into micellar aggregates is a well-known phenomenon. Micelles are in dynamic equilibrium, constantly disintegrating and reforming. This relaxation process is characterized by the slow micellar relaxation time constant, tau(2), which is directly related to the micellar stability. Theories of the kinetics of micelle formation and disintegration have been discussed to identify the gaps in our complete understanding of this kinetic process. The micellar stability of sodium dodecyl sulfate micelles has been shown to significantly influence technological processes involving a rapid increase in interfacial area, such as foaming, wetting, emulsification, solubilization, and detergency. First, the available monomers adsorb onto the freshly created interface. Then, additional monomers must be provided by the breakup of micelles. Especially when the free monomer concentration is low, which is the case for many nonionic surfactant solutions, the micellar breakup time is a rate-limiting step in the supply of monomers. The Center for Surface Science & Engineering at the University of Florida has developed methods using stopped flow and pressure jump with optical detection to determine the slow relaxation time of micelles of nonionic surfactants. The results showed that the ionic surfactants such as SDS exhibit slow relaxation times in the range from milliseconds to seconds, whereas nonionic surfactants exhibit slow relaxation times in the range from seconds (for Triton X-100) to minutes (for polyoxyethylene alkyl ethers). The slow relaxation times are much longer for nonionic surfactants than for ionic surfactants, because of the absence of ionic repulsion between the head groups. The observed relaxation times showed a direct correlation with dynamic surface tension and foaming experiments. In conclusion, relaxation time data of surfactant solutions correlate with the dynamic properties of the micellar solutions. Moreover, the results suggest that appropriate micelles with specific stability or tau(2) can be designed by controlling the surfactant structure, concentration, and physicochemical conditions (e.g., salt concentration, temperature, and pressure). One can also tailor micelles by mixing anionic/cationic or ionic/nonionic surfactants for a desired stability to control various technological processes.

Importance of micellar kinetics in relation to technological processes.

Publisher Summary This chapter reviews the importance of micellar catalysis in relation to mechanistic physical–organic and bio-organic chemistry, to enable to design meaningful experiments in the area, and to guide one through the abundant literature of surfactants and solubilization. Although solubilization by surface active agents has found considerable utility in pharmaceutical and industrial processes, the attention is primarily focussed on the mechanistic aspects of micellar catalyzed organic reactions. The subject is in its rapidly growing infancy, and is scattered and involves interdisciplinary collaboration among colloid, electro, structural, biological, pharmaceutical and organic chemists. The effects of many charged macromolecules, such as polymers, on ionic reactions can be qualitatively predicted on the basis of simple electrostatic considerations analogous to those involved in Hartley's rule.

Micellar Catalysis in Organic Reactions: Kinetic and Mechanistic Implications

Micelles of non-functional surfactants (detergents) can catalyze bimolecular reactions by bringing reactants together in an environment conducive to reaction and they inhibit reactions by keeping reactants apart but they affect rates of unimolecular reactions by providing a submicroscopic medium. The relation between rate and surfactant concentration can be explained in terms of the distribution of reactants between the aqueous and micellar pseudophases which can also be perturbed by added solutes. Catalysis depends upon the charge type of the reaction and reactant hydrophobicity. Functional micelles are often highly effective as nucleophilic or general base catalysts and can give chiral recognition. Substrate micellization can control reaction stereochemistry.

Micellar catalysis and inhibition

Dynamic surface tension (DST) measurements have been carried out with a series of CnE6-type surfactants with varying alkyl chain length (n = 10−16). Major emphasis has been placed on the DST relaxation in complex solutions containing both the micellar and monomer forms of surfactant. This paper also provides a step-by-step guidance to the theoretical interpretation of DST data for micellar solutions. Distributions of monomers and micelles in the vicinity of the gas/liquid interface, adsorption kinetics, and DST relaxation curves have been simulated in the framework of the Fainerman theory (Fainerman, V. B. Kolloid Z. 1981, 43, 94) explicitly allowing for monomer/micelle interconversation. A thorough numerical analysis has shown that although the Fainerman theory can follow the general experimental trends, it clearly lacks the flexibility needed to provide a self-consistent and quantitatively correct description of the process. Possible reasons for this disagreement are discussed.

Dynamic Surface Tension in Concentrated Solutions of CnEm Surfactants: A Comparison between the Theory and Experiment

Determination of the Dissociation Rate of Dodecylpyridinium Iodide Micelles by a Temperature-Jump Technique1a,b

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Determination of the Dissociation Rate of Dodecylpyridinium Iodide Micelles by a Temperature-Jump Technique1a,b