Surfactant critical association

Mixed micelles, comprising both polymerized and free surfactants, are formed once the critical association concentration (CAC) of the free am-phiphiles is reached. The CAC is typically much smaller than the CMC for the formation of free micelles. As the fraction of unpolymerized surfactants within the mixed micelles grows, their structure approaches that of free micelles. 

It has been well documented that surfactants self-associate in aqueous solution to minimize the are of contact between their hydrophobictails and the aqueous solution (Mukerjee, 1979 Tanford, 1980). This phenomenon occurs at a critical concentration of surfactant, the critical micelle concentration or CMC (see Figure 12.4) above where the surfactant molecules exist predominantly as monomeric units and above which micelles exist. The CMC can be measured by a variety of techniques, for example, surface tension, light scattering, osmometry, each of which shows a characteristic break point in the plot of the operative property as a function of concentration. Knowing the CMC of the particular surfactant system and understanding the conditions that may raise or lower that critical concentration is important to the design of a formulation based on micellar solubilization. 

Many techniques can be used for the determination of critical association concentrations (CACs), however, not all of them are sensitive enough to detect the onset of aggregation if this occm i at very low concentrations. Since the CACs of block copolymers are usually much lower than those of low molecular mass surfactants [54], we used pyrene as a fluorescent probe and calculated the effective CACs from the changes in the spectral characteristics of pyrene [55] as a function of surfactant concentration. If we represent the intensity of the emission spectra as a function of the block copolymer concentration, we directly obtain C4C, [56]. From the excitation spectra we obtain CACi by representing the ratio /340//335 vs. log C. 

Polymeric micelles have critical association concentrations that can be modified by the polymer structures. The critical concentrations are lower than those in the case of surfactant-based micelles enabling the stability of the polymeric micelles in the circulation [18]. Polymeric micelles have been used successfully for dmg targeting intravenously. 

Because the main driving force for surfactant self-association in polymer-surfactant mixed systems is the hydrophobic effect, the binding of surfactants to polyelectrolytes exhibits a similar dependence on the length of the alkyl chain as known for free micellization. Surfactants with longer hydrocarbon chains bind more strongly to polyions than those with shorter chains, and the binding starts a lower surfactant concentrations. In this context, a convenient parameter to characterize polyelectrolyte-surfactant systems is the critical aggregation concentration, cac, which is a counterpart of the well-known critical micellization concentration, cmc, but applies to solutions of surfactants in the presence of a polymer. It is defined as the... 

Several NMR parameters are markedly different in the unimeric and self-assembled states and are therefore possible candidates for characterization of surfactant self-association processes. NMR can be a convenient alternative for the determination of critical micelle concentrations (erne s), but an analysis of variable-concentration NMR data generally also provides additional information. We here describe only how self-diffusion can be used to measure the free unimer concentration in simple and complex surfactant systems. In practice it is an alternative to surfactant-selective electrodes to measure surfactant activities. With a two-state assumption, we have, under the normal rapid exchange conditions [cf Eq. (7)],... 

Surfactants are molecules that consist of a non-polar hydrophobic (e.g. alkyl-chains) and a polar hydrophilic part, which can be cationic, anionic, zwitter-ionic, or non-ionic. The relevance of the polar part for the molecule behaviour is quantified in form of the hydrophilic-lipophihc balance (HLB), for which values above 7 indicate dominance of the hydrophilic groups (Myers 1999, pp. 270-273). The heterogeneous composition leads to amphiphilic properties and interfacial activity. That means surfactants accumulate on interfaces and, thus, reduce the interfacial energy (cf. Sect. 3.1.3). Surfactant molecules associate to nanosized aggregates, so-caUed micelles, above a critical concentration (the critical micelle concentration—CMC). Such molecule associations do not only exist in the bulk solution but can also be formed by adsorbed siufactant molecules (Holmberg et al. 2002, pp. 39-66). 

Key words Polyelectrolyte-surfactant association - surface forces - steric forces - critical association concentration - adsorption... 

The critical association concentration between a polymer and a surfactant is defined as the free surfactant concentration at which the cooperative adsorption is initiated [16]. This concentration, that depends on ionic strength [3, 27] and polyelectrolyte concentration [28] can be determined from e.g. the adsorption isotherm. For the polyelectrolyte-surfactant mixtures studied here we have not yet determined the adsorption isotherm but instead we estimate an upper limit of the critical association concentration in bulk solution (caCb) as the total (i.e. bound -t- free) surfactant concentration needed to give a significant increase in turbidity due to formation of large floes in the polyelectrolyte-surfactant solution. The values obtained for the 100%, 30% and 10% charged polyelectrolyte (20 ppm polyelectrolyte solution, 0.1 mMKBr as background electrolyte) were about 0.005, 0.01 and 0.01 cmc. The free surfactant concentration at caCb is thus lower than these values. 

We have shown that surfactants associate with oppositely charged polyelectrolytes preadsorbed onto surfaces having the same charge as the surfactant. The association is, just like in bulk solution, a cooperative process driven by the hydrophobic interaction between the surfactant side chains and made more favorable by the presence of polyelectrolytes acting as counterions to the surfactant aggregates. It is possible to define a critical association... 

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