Premicellar aggregates

The issue of water in reverse micellar cores is important because water swollen reverse micelles (reverse microemulsions) provide means for carrying almost any water-soluble component into a predominantly oil-continuous solution (see discussions of microemulsions and micellar catalysis below). In tire absence of water it appears tliat premicellar aggregates (pairs, trimers etc.) are commonly found in surfactant-in-oil solutions [47]. Critical micelle concentrations do exist (witli some exceptions). 

This localization phenomenon has also been shown to be important in a case of catalysis by premicellar aggregates. In such a case [ ] premicellar aggregates of cetylpyridinium chloride (CPC) were shown to enhance tire rate of tire Fe(III) catalysed oxidation of sulphanilic acid by potassium periodate in tire presence of 1,10-phenantliroline as activator. This chemistry provides a lowering of tire detection limit for Fe(III) by seven orders of magnitude. It must also be appreciated, however, tliat such premicellar aggregates of CPC actually constitute mixed micelles of CPC and 1,10-phenantliroline tliat are smaller tlian conventional CPC micelles. 

Alexiev A, Rubio S, Deyanova M, Stoyanova A, Sicilia D and Perez-Bendito D 1994 Improved catalytic photometric determination of iron (III) in cetylpyridinium premicellar aggregates Anal. Chim. Acta 295 211-19... 

A study of the effects of surfactants on the reaction of 2-(4-cyanophenoxy)quinoxa-line with hydroxide ions has shown that rate accelerations caused by cetyltrialkylam-monium chlorides may be due to reaction in premicellar aggregates or in micelles.54... 

This article will, in addition to a short description of the essential features of surfactant systems in general, concentrate on the energy conditions in premicellar aggregates, the transition premicellar aggregates/inverse micellar structures and the direct transition premicellar aggregates/lyotropic liquid crystals. 

It should be emphasized that these structural changes within a one-phase region may change the kinetics of a chemical reaction in a pronounced manner. As an example may be mentioned the catalytic effect of Inverse micelles on ester hydrolysis. Fig. 5 is from the first publication — on this subject. It clearly shows the lack of catalytic effect by the premicellar aggregates and the sudden increase of hydrolysis rate in the concentration range where the Inverse micelles begin being formed. 

Figure 5. The premicellar aggregates (< 12% water) do not catalyze the hydrolysis of an ester (22).

An approximate thermodynamic evaluationbased on liquid water and crystalline sodium octanoate as reference states has recently evaluated the energy conditions in the premicellar aggregate. The calculations essentially were concerned with the free energy of a gaseous soap/water complex. No attempt was made to evaluate the chemical potential changes in any of the components when dissolved in the pentanol. 

Stability of Premicellar Aggregates in Water-in-Oil Microemulsion Systems... 

Micelles of cationic surfactants have been found to form both in glycerol [44] and in ethylene glycol [18], The micelle formation of Ci6PyBr in ethylene glycol and glycerol was studied with surfactant-selective electrodes [45,46], The monomer concentration could in this way be measured at different total surfactant concentrations, and it was concluded that there is some premicellar aggregation... 

The relation (5.149) holds for the current concentrations cj in the course of the slow process in the regions of premicellar aggregates and full micelles because we assumed a quasi-equilibrium size distributions of aggregates in these regions. Then we can rewrite this relation for the concentration perturbations in the following form... 

After summation (5.174) over the region of premicellar aggregates we find... 

These processes are effective if q and r are outside the minimum region of the size distribution of aggregates. This means that q and r belong to the range of premicellar aggregates and j to the range of full micelles (Fig. 12). The linearised kinetic equation has the following form... 

Let us assume that the condition for the aggregation equilibrium (5.149) holds for the local concentrations cj of aggregates, which belong to the regions of premicellar aggregates and full micelles (j < 1, s < j) in an arbitrary point of the system. Then Eq. (5.174) holds also locally for the perturbations 5 Cj ... 

In many instances, it is found that rate enhancements in surfactant systems appear to begin below the cmc (Fig. 1). This seemingly premature behavior is usually ascribed to premicellar aggregates of various types [12-18] although hard evidence for their existence is too difficult to obtain. The fact that this is found with some experimental methods while... 

Premicellar aggregates The dimers or small aggregates of surfactant formed in the vicinity of the CMC of the surfactant, often observed just below the surfactant concentration at which full micelles form. 

For the formation of micelles in these solutions, such a model would allow faster relaxation times than the pure surfactant because the dissociation-association process has been restricted to the one dimension along the polymer chain these micellar aggregates are formed by diffusion of monomers along the polymer chain. In this model the polymer acts as a nucleus for surfactant aggregates by promoting clusters of monomers in the form of premicellar aggregates or sub-micelles. 

---