Micelles, Microemulsions, and Monolayers: Quarter Century Progress at the University of Florida

It is well recognized that a surfactant solution has three components: surfactant monomers in the aqueous solution, micellar aggregates, and monomers adsorbed as a film at the interface. The surfactant is in dynamic equilibrium among all these components. From various theoretical considerations, as well as experimental results, it can be said that micelles are dynamic structures whose stability is in the range of milliseconds to seconds. Thus, in an aqueous surfactant solution, micelles break and reform at a fairly rapid rate, in the range of milliseconds (1–3). Figure 1 shows the two characteristic relaxation times, τ ₁ and τ ₂, associated with micellar solutions. The shorter relaxation time, τ ₁, generally of the order of microseconds, relates to the exchange of surfactant monomers between the bulk solution and micelles, whereas the longer relaxation time, τ ₂, generally of the order of milliseconds to seconds, relates to the dissolution of a micelle after several molecular exchanges (4,5). It has been proposed that the lifetime of a micelle can be given by nτ ₂ where n is the aggregation number of a micelle (6). Thus, relaxation time τ ₂ is proportional to the lifetime of the micelle. A large value of τ ₂ represents a high stability of the micellar structure. Figure 1 Two relaxation times of micelles, <italic>τ</italic> ₁, and <italic>τ</italic> ₂, and related molecular processes. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203747339/a5e879a5-4097-4b69-8fb2-cf2424a3734f/content/fig1_1.tif"/>