Lifetime of a micelle

Aniansson, G.E.A. The mean lifetime of a micelle. Prog. Colloid Interface Sci. 1985, 70, 2-5. 

In kinetic measurements of micellization processes one generally obtains two relaxation times from which rate constants, the width of the micelle size distribution and information on the rarest intermediate single micelles can be deduced [1, 2]. From such measurements it is also possible, as we shall see, to calculate the mean lifetime of a micelle. Such a quantity is of course of interest in itself but further interest arises in connection with the role of the micelle as host for one or more solubilized or adsorbed molecules. 

In calculating the lifetime of a micelle we will be interested in S " values larger than Sz and s values smaller than s. Regarding the first sum in equation (17) we see that R(s, s) will be negligible for s s and will therefore stay practically constant equal to R. Equation (17) can therefore be written... 

Except close to the c.m.c. a/[ + o /n)a] is the order of one so that generally the order of magnitude of <0 is determined by the product nxz- Since n is typically of the order of one hundred the lifetime of a micelle is generally much longer than the relaxation time of the slow process. 

For a series of anionic surfactants with the same ionic head group, the lifetime of a micelle decreases with decreasing alkyl chain length of the hydrophobic component. Branching of the alkyl chain could also play lui important role in the lifetime of a micelle. It is, therefore, important to carry out dynamic surface tension measurements when selecting a surfactant as an adjuvant as this may play an important role in spray retention. However, these above measurements should not be taken in isolation as other factors may also play an important role, e.g. solubilization which may require larger micelles. The selection of a surfactant as an adjuvant requires knowledge of the factors involved. 

The termination constants kt found previously (see Table XVII, p. 158) are of the order of 3 X10 1. mole sec. Conversion to the specific reaction rate constant expressed in units of cc. molecule" sec. yields A f=5X10". At the radical concentration calculated above, 10 per cc., the rate of termination should therefore be only 10 radicals cc. sec., which is many orders of magnitude less than the rate of generation of radicals. Hence termination in the aqueous phase is utterly negligible, and it may be assumed with confidence that virtually every primary radical enters a polymer particle (or micelle). Moreover the average lifetime of a chain radical in the aqueous phase (i.e., 10 sec.) is too short for an appreciable expectation of addition of a dissolved monomer molecule by the primary radical prior to its entrance into a polymer particle. 

The process of self-assembly is a dynamic one. With some single-chained ionic surfactants, e.g. sodium dodecyl sulphate), the residence time of a monomer in a micelle is of the order of 10 sec, and the lifetime of the micelle as a whole is of the order of a millisecond. At the other extreme, for highly insoluble phospholipids that form the bulk of bilayer membranes, the... 

Micelles made of soaps break up and reform within milliseconds. The individual lifetime of a charged micelle is very short. Nevertheless, eicosane sulfate micelles have been trapped by rapid cryo-fixation at liquid nitrogen temperature and freeze etching. They can then be seen under the transmission electron microscope (TEM) (Figure 2.5.3). 

Another point wMch needs amplification is the entry of a z-(or higher-)mer into a micelle. The lifetime of a surface-active species in a given micelle is voy short, typically of the ordo- of microseconds. Hence a z-mer might enter a micelle, or surfEKtant species might migrate to a z-mw to form a micelle these evoits are so on the time-scale of interest that they will not be... 

The ability of micelles, or similar aggregates, to increase the lifetime of a potentially useful energetic intermediate is illustrated by micellar effects upon the... 

Micelles are dynamic aggregates and exchange surfactant molecules with the surrounding bulk solution. The lifetime of a surfactant molecule in a micelle is in the order of 10 -10 s and depends upon various factors. The solubilized substances can also transfer from the micelles to the bulk phase and vice versa. The rate of such a process can be comparable with the rate of the excited molecule s deactivation [15-17]. The usual implication is that the photoexcitation does not lead to any considerable change of the binding constant. The binding constant for the products of the photoreaction may differ markedly from the ones of the reactants. So the reaction can result in some photoinduced flows of substance from the micellar phase to the bulk phase [18,19]. Such photoinduced transport processes can also be noticed where the... 

The site of incorporation of solubUisate is closely related to its chemical stmcture (Eigure 13.21). Although in many cases a particular location is preferred, the lifetime of a solubilisate within the micelle is long enough for a rapid interchange between different locations. 

As with surfactant monomers, the solubilisate molecules are not rigidly fixed in the micelle, but have a freedom of motion that depends on the solubilisation site. The lifetime of a solubilisate in the micelle is very short, usually less than 1 ms. These short relaxation times have been determined using NMR and ultrasonic techniques. 

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