Abstract

The assembly of surfactants in aqueous solution to form micelles was postulated by McBain more than a century ago [1] and ninety years have passed since the canonical structure of a spherical micelle was first proposed by Hartley [2]. The mechanism by which surfactant micelles form and break-up was addressed in 1974 in an elegant paper by Aniansson and Wall [3], who proposed a stepwise mechanism with two relaxation times, one characteristic of the entry or exit of a single surfactant molecule from a micelle and the other, slower, time with the complete disassembly of a micelle into surfactant monomers. The Aniansson and Wall model is limited to small deviations from equilibrium and, for some systems, gives poor predictions of the concentration dependence of relaxation times. In the intervening years, a consensus has begun to emerge that there are actually two distinct mechanisms by which micelles can break down (or form) [4]: one is via a stepwise loss of monomers from a micelle until it disappears [3], and the other is the collision of two micelles to form a ‘supermicelle’ that can then shed monomers to form a single micelle of normal size [5] [6]. Which mechanism dominates depends not only on the nature of the surfactant but also on the total concentration of surfactant and – critically – the monomer concentration. The rates of both mechanisms are strong functions of the deviation from equilibrium, for different reasons.

This talk will describe our current understanding of the assembly and disassembly of surfactant micelles, explain why the ‘rate constants’ of these processes vary strongly with the perturbation from equilibrium and discuss the implications of these observations for the many practical processes where surfactant solutions are far from equilibrium. The extent to which experimental evidence is consistent with these models will be discussed.

References

[1] J. W. McBain, Trans. Faraday Soc., 9, 99 (1913).

[2] G. S. Hartley, Kolloid Z., 88, 2 (1939).

[3] E. A. G. Aniansson and S. N. Wall, J. Phys. Chem. 78, 1024 (1974).

[4] For a clear recent exposition, see J. Mysona, A. McCormick and D. Morse, Phys. Rev. E, 105, 034602 (2022).

[5] M. Kahlweit and M. Teubner, Adv. Colloid Interface Sci. 13, 1 (1980).

[6] I. M. Griffiths, C. J. W. Breward, D. M. Colegate, P. J. Dellar, P. D. Howell, and C. D. Bain, Soft Matter, 9, 853 (2013).