Micelle formation/breakdown

Ultrasonic relaxation spectroscopy (URS) is nothing but a special treatment of data from ultrasonic absorption measurements. Micelle dynamics involves characteristic relaxation processes, namely micelle-monomer exchange and micelle formation-breakdown. Ultrasonics can provide information about the kinetics of the latter, the fast relaxation process also, theoretical expressions for the relaxation time and relaxation strength such as those derived by Teubner [76] provide self-consistent estimates of both. 

J. Lang and R. Zana, Effect of Alcohols and Oils on the Kinetics of Micelle Formation-Breakdown in Aqueous Solutions of Ionic Surfactants, J. Phys. Chem., 90 5258 (1986). 

As has been shown above, the kinetics of micelle formation, breakdown, and associated dynamic processes has been documented. However, much less is known about the kinetic processes involved with transformations between other aggregate structures. 

Besides, micelles are dynamic objects. They constantly exchange surfactant with the bulk phase exchange process), and they form and break down by different pathways micelle formation / breakdown), which are reviewed in Chapter 3. 

The use of the relaxation amplitudes permits in some cases a choice between two possible reaction schemes or between two different sets of assumptions in the analysis of chemical relaxation data. For instance, a controversy arose about the validity of an assumption made in deriving the expression of the relaxation time associated with the micelle formation/breakdown process for surfactant solutions. This controversy was solved by fitting the expressions of the amplitudes derived with both sets of assumptions to the amplitude data and retaining the set giving the best fit. ... 

The stopped-flow method has been used to study the kinetics of micelle formation/breakdown in surfactant solutions (see Chapter 3), of the exchange process in micellar solutions of amphiphihc block copolymers (see Chapter 4, Sections IV and V), and also of colhsions between droplets in microemulsions (see Chapter 5, Section VI.F). It has been also used to study the kinetics of the vesicle-to-micelle transformation (see Chapter 6) and of various types of chemical reactions performed in micelles or microemulsion droplets (see Chapter 10). The stopped-flow method has also been used to study the rate of dissolution of oil or water in microemrdsions (see Chapter 5, Section VII.C). In such studies the syringe that contains the oil or water to be solubilized is of a much smaller diameter than that containing the microemulsion. 

The self-diffusion coefficients of the different chemical species present in a solution can be measured using the so-called pulsed field gradient Fourrier-transform NMR (PFG-FT NMR) spectroscopy. The diffusion coefficients do not give direct access to the dynamics of the processes of exchange or micelle formation-breakdown. They can nevertheless be use-... 

The chapter is organized as follows. Section II briefly recalls the theoretical aspects of micellar dynamics and the expressions of the relaxation times characterizing the main relaxation processes (surfactant exchange, micelle formation/breakdown). Section III reviews studies of micellar kinetics of various types of surfactants conventional surfactants with a hydrocarbon chain, surfactants with a fluorinated chain, and gemini (dimeric) surfactants. Section IV deals with mixed micellar solutions. Section V considers the d5mamics of solubilized systems. Section VI reviews the dynamics of sur-... 

This section recalls the main aspects of the derivation of the expressions of the relaxation times for the surfactant exchange process and for the micelle formation/breakdown as done by Aniansson and WalP in 1974 and 1975, and the main extensions of this theory by Kahlweit et al. and HalP in the years that followed. 

They also assign the slow process to the micelle formation/breakdown (global reaction (3.2)) and assume that this reaction takes place via a series of stepwise reactions (3.4). 

This section reviews the main experimental results for the exchange process and micelle formation/breakdown for solutions of pure surfactants, in the absence of additives other than salts. The surfactants considered are the conventional surfactants (one head group/one hydrocarbon chain and one head group/two hydrocarbon chains), the perfluoiinated surfactants (one head group/one perfluorocarbon chain), and the gemini (dimeric) surfactants (two head groups/two alkyl chains). 

C. Information Gained from Studies of the Micelle Formation/Breakdown... 

As pointed out in Subsection II.A.3, the values of Tg can be used to obtain information on the premicellar aggregates Ar at the minimum s = r of the micelle size distribution curve (see Figure 3.1), provided the measurements are performed on dilute solutions, where micelle formation/breakdown proceeds via stepwise reactions (4). The data were generally analyzed under the simplif3dng assumption that the concentration of the aggregates is much smaller than that of the other species in the narrow passage and kr k Equation 3.14 then reduces to... 

The amplitudes of the relaxation associated with the micelle formation/breakdown have been used to study the dependence of the aggregation number of several surfactants on temperature, pressure, and concentration of surfactant or added salt. The values of 61ogN/6p and of 61ogN/5T obtained in this manner agreed with reported values obtained from completely different measurements. 

Studies concerning the micelle formation/breakdown in mixed micellar solutions are few. Folger et al. showed that small amounts of STS (mole fraction 2-5%) significantly affected the value of T2 for SDS, particularly at C close to the cmc (see Figure 3.5). This is expected since micelle formation/breakdown is similar to a nucleation process. Patist et ai 160 reported that the slow relaxation process in solutions of SDS became considerably slower upon the addition of alkyl-trimethylammonium bromides. The largest effect was obtained with dodecyltrimethylammonium bromide, and the authors interpreted the results in terms of chain compatibility. Measurements of the slow relaxation time have been used to show that solutions of mixtures of some hydrocarbon and perfluorocarbon surfactants contain two types of mixed micelles, one rich in hydrocarbon surfactant, the other rich in perfluorocarbon surfactant. 

The length of the cosurfactant alkyl chain (carbon number nil) a very strong effect on the relaxation time T2 for tke micelle formation/breakdown. Indeed, the longer the cosurfactant (alcohol), the more it partitions in the micelles and the more it affects the micelle size distribution curve. Some representative resrdts are shown in Figure 3.14 for alcohol additions to CTAC. Similar results were reported for alcohol additions to tetradecyl and hexadecylpyridinium... 

Figure 3.14 Effect of alcohol on the relaxation time for the micelle formation/breakdown in 0.3 M CTAC solution (+) butanol ( ) pen-tanol (x) hexanol and ( ) heptanol. Reproduced from Reference 179 with permission of the American Chemical Society.

Most of these investigations were performed using chemical relaxation methods, ultrasonic absorption relaxation for the exchange process and T-jump, p-jump, or stopped-fiow for the micelle formation/breakdown. More recently, NMR methods have started to be used for the same purpose. 

D. Dynamic Surface Tension and Micelle Formation/Breakdown Kinetics... 

The behavior of mixed micelles conforms to existing theories. A remarkable effect of alcohols on the relaxation associated with micelle formation/breakdown has been shown and attributed to the effect of alcohol on the size distribution curve. The possibility of a coexistence of two populations of micelles, one alcohol-rich and the other surfactant-rich, may explain the observed kinetic behavior. Small alcohol-rich micelles may play the role of carrier between surfactant-rich micelles. 

Figure 4.9 Variation of the relaxation time for micelle formation/breakdown with the concentration of the P84 ( ) and P85 (A) copolymers at 36°C. Reproduced from Reference 39 with permission of Elsevier.

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