Surfactant intermicellar exchange

Some investigations have emphasized the importance of micellar size as a control parameter of nanoparticle size [224]. It has been suggested that other factors also influence the nanoparticle size, such as the concentration of the reagents, hydration of the surfactant head group, intermicellar interactions, and the intermicellar exchange rate [198,225-228],... 

For instance, nanoparticles of silver chloride have been synthesized by mixing two mi-croemnlsions, one containing silver ions and the other containing chloride ions. It was shown that the average particle size, the polydispersity and the number of particles formed depend on the intermicellar exchange rate and/or the rigidity of the surfactant shell [228],... 

The effect of supercritical solvent continuous phase properties on particle growth behavior was investigated by Roberts et al. throngh comparison of Cu and Ag particles growth rates in snpercritical alkan and in normal liquid solvents at the same conditions. Favorable properties of SCFs solvent, such as lower density and solvent power, decreased solvent interaction with the surfactant tails and led to smaller nanocrystals and faster particle growth rate, due to the increased kinetics of the intermicellar exchange mechanism [42]. 

Abstract Monte Carlo simulations were carried out to study the different stmctures showed by bimetallic nanoparticles synthesized in microemulsions. It is observed that the difference in reduction rates of both metals is not the only parameter to determine the metals segregation, playing the interdroplet channel size a relevant role. The reduction rates difference determines nanoparticle structure only in two extreme cases when both reactions take place at the same rate a nanoalloy structure is always obtained if both reactions have very different rates, the nanoparticle shows a core-shell structure. But in the large interval between both extreme cases, the nanoparticle structure is strongly dependent on the intermicellar exchange, which is mainly determined by the surfactant film flexibility, and on reactants concentration. This result is very promising for the preparation of bimetallic nanoparticles with a given structure. 

The main conclusion is that nanoparticle structure is determined by the chemical reaction rates ratio, as in homogeneous media, only if both reactions take place at the same rate and if the reaction rates are very different. The first case leads to a nanoalloy and the second one to a core-shell structure. But in the common case of a nanoparticle composed by two metals with a moderate difference in reduction potentials, the dynamics of the intermicellar exchange causes that the synthesis variables, such as surfactant film flexibility or initial reactants concentrations, become more relevant, modifying the nanoparticle structure An increase of flexibility gives rise to alloys, and an increase of concentration implies a higher degree of mixture in the inner layers and an enrichment in the slower product in the outer layers. 

The influence of the surfactant concentration upon the quenching process may be caused by the change of the micelles shape, rather than intermicellar exchange of quencher molecules, as it was supposed for pyrene fluorescence quenching by copper ions in SDS micelles [49]. 

FIGURE 1.3 Intermicellar exchange of a surfactant monomer and of the micelle formation to breakup. 

Micelles are not frozen objects. They are in dynamic equilibrium with the free (nomnicellized) surfactant. Surfactants are constantly exchanged between micelles and the intermicellar solution (exchange process), and the residence time of a surfactant in a micelle is fmite. Besides, micelles have a finite lifetime. They constantly form and break up via two identified pathways by a series of stepwise entry/exit of one surfactant A at a time into/from a micelle (Reaction 1) or by a series of frag-mentation/coagulation reactions involving aggregates A, and Aj (Reaction... 

The numerical simulation of the intermicellar counterion exchange proved [55] that this process is controlled by the Couloumb factor and its rate increases drastically with the growth of the surfactant concentration. 

The kinetics of surfactant exchange between gemini surfactant micelles and intermicellar solution was investigated. Gemini surfactants with short alkyl chains, 8-6-8, 2Br" and 8-3-8, 2Br", were found to behave similarly to their monomeric counterparts [35]. Gemini surfactants associate to, and dissociate from, their micelles in a single step (the two chains at a time). The association reaction is nearly diffusion controlled, whereas the rate of dissociation (exit) depends strongly on the surfactant hydrophobicity. 

Thus far only processes involving motion of the surfactant as a whole have been mentioned. Other processes may occur in micellar solutions internal motion of the surfactant alkyl chains within the micelles exchange of cormterions between free and micelle-bound states and fast changes of micelle shape, among others. Also in the case of solubilized systems, i.e., micellar solutions that have solubilized compounds that are sparingly soluble in water, the solubilizate may exchange between micelles and the intermicellar solution. The dynamics of the exchange of counterions and of solubilizates are reviewed later. The dynamics of internal motions of the surfactant alkyl chains are not dealt with in this chapter, but some information and references can be found in Chapter 5, Section V. Some information on the fluctuations of micelle shapes can be found in Chapter 1, Section III.B. 

Micelles are not frozen objects. They are in equilibrium with free surfactants. In a micellar solution, surfactants are constantly exchanged between micelles and surrounding (intermicellar) solution. This implies processes of entry (or incorporation or association) of surfactants into micelles. Conversely, surfactants can exit (or dissociate) from micelles. The entry/exit processes are usually referred to as exchange processes. Owing to these processes a given surfactant resides in a micelle a finite time, which is the surfactant residence time. Likewise, one can define the residence time of a solubilizate in a micelle. 

Micellar colloids are in a dynamic association-dissociation equilibrium, and the kinetics of micelle formation have been investigated for a long time. " In 1974, a reasonable explanation of the experimental results was proposed by Aniansson and Wall, " and this conception has been accepted and used ever since. The rate of micelle dissociation can be studied by several techniques, such as stopped flow, pressure jump, temperature jump, ultrasonic absorption, NMR, and ESR. The first three methods depend on tracing the process from a nonequilibrium state brought about by a sudden perturbation to a new equilibrium state— the relaxation process. The last two methods, on the other hand, make use of the spectral change caused by changes in the exchange rate of surfactant molecules between micelle and intermicellar bulk phase. 

The fast process involves an exchange of monomeric surfactants between the micelles and the intermicellar solution. A monomer or several monomers dissociate from or associate to existing micelles. As a result, the micellar distribution curve shifts without a change in the number of micelles [61,62,67, 68]. 

---