Relaxation time surfactant exchange

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-... 

Telgmann and Kaatze studied the stmcture and dynamics of micelles using ultrasonic absorption in the 100-KHz to 2-GHz frequency range [100]. They detected several relaxation times in the long (ps), intermediate (10 ns), and fast (0.1-0.3 ns) time scale. The longest relaxation time has been attributed to the exchange of monomer between bulk and the micelles, and the fastest to the rotation of the alkyl chains of the surfactants in the core of the micelle. The intermediate relaxation time has not been assigned to any particular motion. We will discuss later that the intermediate relaxation times in the 10-ns time scale may well be due to solvent relaxation in the Stem layer. 

The second relaxation time T2, is related to a change in the number of micelles and is much slower because the surfactants have to be rearranged between the micelles in a cooperative fashion (formation/dissociation of micelles limited by unimer exchange). Again under the assumption of unimer exchange, this can be written as ... 

From fast kinetic measurements, two relaxation times are determined which characterize molecular processes in micellar solutions, i.e. t measures the rate at which surfactant molecules exchange between... 

The membranes of lipid bilayer vesicles are usually not in thermodynamie equilibrium. The exchange of lipids between the membranes of different vesicles is known to be very slow, with relaxation times up to hours or days. Rupture of lipid bilayers usually requires high lateral tension, and thermally excited pores seem to be very rare. Therefore, the number of vesicles is constant or changes very slowly. Equilibration including changes of topology appears to be much faster with the bilayers of ordinary surfactants, but the various relaxation times remain to be measured. 

Overall, these two techniques permit the determination of relaxation times in the range 0.5 ts to 0.3 ns. They have been used extensively at the early stage of kinetic studies of micellar solutions for the study of the exchange process of surfactants with a relatively short alkyl chain and, more recently, for the study of novel surfactants, including gemini (dimeric) surfactants. They have also been used for the study of phase transition in vesicle systems. 

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. 

In all instances where it could be measured, the relaxation time for the surfactant exchange process was found to obey Equation 3.9, that is, llx increased linearly with the... 

The relaxation time Tn associated with the exchange of the alcohol was found to be nearly independent of the nature of the surfactant when using fixed concentrations of alcohol and surfactant. The value of Ihn was found to increase nearly linearly with the cosurfactant concentration as theory predicts (see Figure 3.12). Unfortunately, many of the data reported in the early studies were analyzed using inadequate theories or the analysis did not yield values of the rate constants, Even in studies where the equilibrium data... 

Figure 3.13 Variation of the relaxation time for the TTAB exchange with the surfactant concentration in water (x) water + 0.2 M butanol (+) and water + 0.1 M pentanol (O) at 5°C. Reproduced from Reference 166 with permission of Elsevier/Academic Press.

The reciprocal of the relaxation time T12 characterizing the surfactant exchange varied linearly with the surfactant concentration at constant alcohol concentration (see Figure 3.13). The cosurfactant was found to have relatively little effect on the exchange rate constants and k" of the surfactant. Some examples are given in Table 3.3. 

There have been few investigations of the effect of aromatic or alkyl solubilizates on the micellar dynamics. Alkanes have been found to have almost no effect on the value of the relaxation time for the surfactant exchange in micellar solutions of sodium heptylsulfate and hexylammonium chloride. Likewise, cyclohexane has very little effect on the exchange in micellar solutions of sodium octylsulfate. In these studies the amount of solubilizate was relatively small. In contrast. 

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