Critical micelle concentration surfactant-polymer systems

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

The temperature (or salinity) at which optimal temperature (or optimal salinity), because at that temperature (or salinity) the oil—water interfacial tension is a minimum, which is optimum for oil recovery. For historical reasons, the optimal temperature is also known as the HLB (hydrophilic—lipophilic balance) temperature (42,43) or phase inversion temperature (PIT) (44). For most systems, all three tensions are very low for Tlc < T < Tuc, and the tensions of the middle-phase microemulsion with the other two phases can be in the range 10 5—10 7 N/m. These values are about three orders of magnitude smaller than the interfacial tensions produced by nonmicroemulsion surfactant solutions near the critical micelle concentration. Indeed, it is this huge reduction of interfacial tension which makes micellar-polymer EOR and its SEAR counterpart physically possible. 

When a polymer is added in a surfactant system, there are two critical concentrations CAC and CMC2. CAC is the critical adsorption concentration at which surfactant starts to adsorb on the polymer chains it is lower than the critical micelle concentration (CMC). CMC2 is the surfactant concentration at which micelles are formed when polymer is present it is higher than CMC (Li et al 2002). Both CAC and CMC2 are on the order of magnitude of CMC. 

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... 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

Figure 13.5 shows the variations of surface tension versus surfactant concentration for pure surfactant system and the mixture of polymer/surfactant where the polymer concentration is constant. In the case of pure surfactant solution, a sharp decrease in the surface tension occurs with the increase in surfactant concentration up to the critical micelle concentration (CMC). For surfactant concentrations higher than the CMC, the surface tension remains constant. In the mixture of polymer and surfactant, the surface tension plot shows two break points. The first point is the CAC point where the interaction between the polymer and the surfactant begins. The second point is the PSP point where the polymer chains become saturated with the surfactant. When the interaction between the polymer and surfactant is weak, CAC and PSP values are close to the CMC of pure surfactant (Mohsenipour 2011). 

In the case of oppositely charged polymer and surfactant, it has been generally observed that the critical aggregation concentration (CAC) is an order of magnitude lower than the critical micelle concentration (CMC) of surfactant. As an example. Figure 13.15 shows the comparison of CAC and CMC for a system consisting of oppositely charged surfactant and polymer. 

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