Micelles aggregation

Van Paassen [57] describes the CMC of some polyether carboxylates with different fatty chains and EO degrees (Fig. 2). In an extensive study, Binana-Limbele et al. [59] investigated the micellar properties of the alkylpolyether carboxylates of the general formula CnH + OCF CH OCI COONa with n = 8, x = 5, and n = 12 and x = 5,1, and 9, by means of electrical conductivity (CMC, apparent micellar ionization degree) and time-resolved fluorescence probing (micelle aggregation number A7) as a function of temperature and surfactant concentration (Table 1). 

It has been found that the CMC values are higher and the micelle aggregation numbers smaller than those of the corresponding nonionic surfactants. The CMC increases with increasing EO chain, which is, according to the authors, opposite to the results for sodium alkyl ether sulfate. 

From the apparent ionization degree it was concluded that the EO chain probably behaves as part of the headgroup. As with Aalbers [49], a low surface charge of the sodium alkyl ether carboxylate micelles was mentioned. The micelle aggregation number N increases with the C chain much more than for the corresponding nonionic surfactants. In the case of C8 there was no influence of temperature. A small decrease was found with increasing EO, but much smaller than in the case of nonionics. 

Based on studies on the influence of NaCl and pentanol on N for n = 12, x = 7 and n = 12, x = 9 it has been found that NaCl increases and pentanol decreases the micelle aggregation number N. Qualitatively this is similar to the ionic surfactants however, changes are smaller in the case of the classical ionic surfactants. 

Monomer/Micelle Equilibrium Mixtures of surfactants, like any surfactant species in an aqueous solution, give rise to monomer or micelle aggregates provided that the concentration reaches a minimum value, called the critical micellar concentration (CMC). The micelles thus formed are mixed, i.e. made up of the different surfactant species in solution. 

Although the micelle particle is an aggregate, it behaves like a liquid indeed, it is often convenient to regard these micelle aggregates as a separate phase. For this reason, we usually class micelles as a liquid-in-liquid colloid. 

At CMC, micelles (aggregates of SD" with some counterions, such as Na+) are formed, and some Na+ ions are bound to these, which is also observed from conductivity data. In fact, these data analyses have shown that approximately 70% Na+ ions are bound to SD" ions in the micelle. The surface charge was estimated from conductivity measurements (Birdi, 2002). Therefore, the concentration of Na+ will be higher than SD" ions after CMC. A large number of reports are found in the literature, in which the transition from the monomer phase (before CMC) to the micellar phase (after CMC) have been analyzed. 

As mentioned earlier, surfactants aggregate to form micelles, which may vary in size (i.e., number of monomers per micelle) from a few to over a thousand monomers. However, surfactants can form, besides simple micellar aggregates (i.e., spherical or ellipsoidal), many other structures also when mixed with other substances. The curved micelle aggregates are known to change to planar interfaces when additives, the so-called cosurfactants, are added. A reported recipe consists of... 

The appearance of the excimer peak has been attributed to the formation of a premicelle or nascent micelle aggregate, which because of its small size and hydrophobic nature allows the solubilized excited pyrene to interact with the ground-state pyrene to form an excimer. The peak vanishes in the plateau due to the difficulties for the ground-state pyrene to encounter the excited species in the relatively large microdomain of the micelle. This observation led Sahoo et al. [123] to propose a step-wise formation for the micelles (Scheme 2, Fig. 16)... 

The mass action model (MAM) for binary ionic or nonionic surfactants and the pseudo-phase separation model (PSM) which were developed earlier (I EC Fundamentals 1983, 22, 230 J. Phys. Chem. 1984, 88, 1642) have been extended. The new models include a micelle aggregation number and counterion binding parameter which depend on the mixed micelle composition. Thus, the models can describe mixtures of ionic/nonionic surfactants more realistically. These models generally predict no azeotropic micellization. For the PSM, calculated mixed erne s and especially monomer concentrations can differ significantly from those of the previous models. The results are used to estimate the Redlich-Kister parameters of monomer mixing in the mixed micelles from data on mixed erne s of Lange and Beck (1973), Funasaki and Hada (1979), and others. 

Many proteinases catalyse the hydrolysis of a specific bond in K-casein, as a consequence of which the micelles aggregate or gel in the presence of Ca2 + or other divalent ions. This is the key step in the manufacture of most cheese varieties (Chapter 10). 

Association and shattering of micelles. Electron microscopy shows that the casein micelles aggregate initially, then disintegrate and finally aggregate into a three-dimensional network. 

The probe molecule pyrene (-10"6 M) was used in time-resolved fluorescence quenching experiments using a single photon counting apparatus, cetylpiridinium chloride (CpyC, 10"3 M) being introduced as a quencher of the pyrene fluorescence[ll-13]. All the experiments were performed at 303K. From these fluorescence studies the micelle aggregation number (N) and the pyrene fluorescence lifetime (x) were obtained [14]. 

Systems CTAB/SKVNaOH, CTAC/Si02/NaOH, and CTAB/SiO/TMAOH/MeOH Values of the pyrene fluorescence lifetime (t) and micelle aggregation number (N). ... 

In the build-up from surfactant monomers to micelles, the existence, albeit transitorily, of intermediate levels of aggregation is to be expected. Close examination of experimental evidence suggests that there may be some smoothness of property change at the c.m.c., but that sub-micelle aggregates exist only in trace amounts. 

Although a number of infrared bands can be used to establish that a micellar shape change has occurred, it is difficult to determine the actual shape unambiguously from the spectroscopic data alone. We therefore make use of micelle aggregation numbers and solution rheological properties, which depend on micelle size and shape, for correlation with the structural information (packing) provided by the FTIR spectra. 

Figure 9 Plot of the frequency dependence of the composite symmetric CH2 stretching band and micelle aggregation numbers (19) vs. mixed micelle composition in 0.3 M DTAC/SDS mixed micelles (T = 23°C). Reprinted from ref. 47. Copyright 1990 American Chemical Society.

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