Relaxation time 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. 

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 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. 

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.

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.

As mentioned above, the most important adjuvants are surface active agents of the anionic, nonionic or zwitterionic type. In some cases polymers are added as stickers or antidrift agents. The production of spray droplets (from a spray nozzle) is determined by the adsorption of surfactants under dynamic conditions (with time in the region of 1 ms). The droplet adhesion to the target surface and its wetting and spreading is also determined by the dynamic contact angle which is also determined by the rate of surfactant adsorption to the surfeice. Above the critical micelle concentration (cmc), the supply of monomers is determined by the relaxation time of micelle formation and its breakdown. The dynamics of surfactant adsorption is determined by the monomer concentration and the diffusion coefftcient of the surfactant molecules to the interface. 

Figure 4.13 Variations of the relaxation time with the concentration of the poly(a-methylstyrene)-poly(vinylphenethyl alcohol) copolymer. Left relaxation time for the micelle formation process at a temperature of 35°C, for a starting temperature of 60°C (o, the solution contains only unimers) and of 45°C ( , the solution contains unimers and micelles). Right relaxation times for micelle recovery Tr ( ) and breakdown Td (A) measured after a temperature jvunp from 35°C to 45°C (o) relaxation time for micelle formation measinred at 35°C following a temperature quench from 60 to 35°C. All relaxation times are expressed in hours. Reproduced from Reference 82 with permission of the American Chemical Society.

Dilute micellar solutions of surfactants are characterized by two well-separated relaxation times. The molecular origin of the fast relaxation time has been related to a monomer-micelle exchange [181-184]. It was realized later that the relaxation spectra of micellar solutions really exhibit two relaxation times. The theory of Aniansson and Wall [167,185] assumes a stepwise aggregation of surfactant monomers to form micelles [186]. The fast relaxation time is attributed to the exchange of monomeric surfactants between the micelles and the intermicel-lar solution. The slow relaxation time is attributed to micelle formation and breakdown. The theory and its modifications by Kahlweit and co-workers [170-174]... 

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