Relaxation times micellization-dissolution

When the system has reached its quasi-equilibrium state a slower process, involving the relaxation to the true equilibrium, becomes measureable. This process involves a change in the number of micelles. The formation or dissolution of a micelle involves according to scheme (5.1) the appearence of aggregates of size at the minimum of the size distribution curve, and since these aggregates occur with low probability the process can be a very slow one. Aniansson and Wall showed that this process is also characterized by an exponential decay with a relaxation time r2,... 

We have also shown in a recent study (41) that the addition of a short chain alcohol (propanol) increases the rate of micelle formation-dissolution significantly. The amplitude of Xgi, process in the water-rich region is very small and hence the resolution of relaxation time xsiow is poor. Within the range of experimental accuracy, Xgioj, was found to be independent of the microemulsion composition. 

Fig. 6, Variation of the reciprocal of the relaxation time T2 for the micelle formation-dissolution process upon addition of pentanol to O.IM TTAB (+) and O.lM tetradecylpyridinium bromide ( ) solutions in water at 25°. (The measurements were performed by means of T-jump Eosine was used as a probe for the relaxation process in TTAB solutions (see C. Tondre, J. Lang and R. Zana, J. Colloid Interface Sci., 52, 372 (1975).)...

Using a pressure-jump apparatus the dissolution time for sodium dodecyl sulphate micelles has been measured in solutions which also contain polyvinyl pyrrolidone. The relaxation time associated with the association/dissolution of the micelles depends on the amount of polymer present and also its molecular weight. In addition the activation energy of the dissociation/ association process decreases in the presence of the polymer. These data suggest that the micelles are incorporated along the polymer chain and a simple mechanism for the dissolution of micelles on the polymer is proposed. 

A longer relaxation time Z2 (of the order of 10 -1 s), which is a measure of the micellization-dissolution process. Both ti and xi depend on the surfactant structure, its chain length, and these relaxation times determine some of the important factors in selecting adjuvants, such as the dynamic surface tension (discussed below). 

Many modem technologies depend on the optimum use of surfactants. The applied concentrations are often above the critical micelle concentration (CMC) and special effects are direcdy related to the presence of micelles. This is tme for example in cleaning and detergency [1], encapsulation of drugs in micelles (2, 3] or microemulsions [4], and many others [5]. The important parameters of micellar solutions are the CMC and the aggregation number n. The formation and dissolution of aggregates or the release or incorporation of single molecules are controlled by the relaxation times of slow and fast processes. Their values, however, depend on the models applied. 

The first models for describing micelle break-up from experimental studies were published by Aniansson et al. [9,10]. Their model assumed essentially the release of single molecules from a micelle, given by the relaxation time Ti. The complete aggregate dissolution was assumed as a second relaxation process, characterized by %2 [9,10]. The relaxation mechanisms were discussed in many reviews, for example in a book chapter [11], and are not covered here. 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

Surfactants form micelles above the critical micelle concentration (c.m.c.) of different sizes and shapes, depending on the nature of the molecule, temperature, electrolyte concentration, etc. (see Chapter 2). The dynamic nature of micellisation can be described by two main relaxation processes, ti (the life time of a monomer in a micelle) and t2 (the life time of the micelle, i.e. complete dissolution into monomers). 

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