Critical aggregate concentration, CAC

The SC nanogels had 70 nm mean diameter with narrow size distribution, significantly lower critical aggregation concentration (CAC), and stronger thermodynamic stability compared with those of the corresponding l- or D-isomer nanogels [163]. 

Fig. 18 Critical aggregate concentration, CAC of DTAB in 1 g/L sodium polyacrylate in 0 and 0.03 M added NaCl...

Micelles and vesicles can be formed above a certain concentration. For instance, small micelles are formed above critical micellar concentration, cmc. (The latter abbreviation is often used for critical vesicle concentration, too. However, sometimes a more general term critical aggregate concentration, cac is also applied.) Bilayers of specific amphiphiles with two tails are typical of the central part of cell membranes discussed in some detail in the next chapter. Studying artificial mono- and bilayers (uniform or with built in pores) is indispensable for gaining information about the structure and functioning of cell membranes involving the transport through them. 

Silicone surfactants in aqueous solutions show the same general behavior as conventional hydrocarbon surfactants - the surface tension decreases with increasing concentration until a densely packed film is formed at the surface. Above this concentration, the surface tension becomes constant. The concentration at the transition is called the critical micelle concentration (CMC) or critical aggregation concentration (CAC). The surface and interfacial activity of silicone surfactants was reviewed by Hoffmann and Ulbricht [27]. Useful discussions of the dependence of the surface activity of polymeric silicone surfactants on molecular weight and structure are given by Vick [28] and for the trisiloxane surfactants by Gentle and Snow [29]. 

The values obtained by dynamic light scattering (DLS) for the diameter of these vesicles are larger due to aggregation. The critical aggregation concentration (CAC) was determined with pyrene as probe molecule [56] and were for all generations... 

The variation of the surface tension with the surfactant concentration was determined for several sur tant molecules, and the critical aggregation concentration (CAC) was calculated fix>m the... 

Like other amphiphilic molecules, LPS aggregate and build up clusters in aqueous solutions. This process occurs only above a critical aggregate concentration (CAC) which has been established for a few lipids, but not, however, for LPS owing to extreme experimental difficulties. Estimations based on comparisons to values of other lipids led to the assumption of a CAC of < 10 M for hexaacyl-lipid A (O Fig. 16). For the lipid A precursor IVa (O Fig. 16) which represents a tetraacyl-lipid A, a CAC of < 10 has been reported [78]. 

Equations (2)-(4) show that the total potential energy of interaction between two colloidal spherical particles depends on the surface potential of the particles, the effective Hamaker constant, and the ionic strength of the suspending medium. It is known that the addition of an indifferent electrolyte can cause a colloid to undergo aggregation. Furthermore, for a particular salt, a fairly sharply defined concentration, called critical aggregation concentration (CAC), is needed to induce aggregation. 

The nonionic surfactant Ci2G2 is used in the experiments at a fixed concentration of 10 5 mol L 1 (O.lxcmc). The cationic hexadecyltrimethylammoni-um bromide (CieTAB) was recrystallized four times from 10 1 ethyl acetate-ethyl alcohol and used at a fixed concentration of 10-4 mol L-1 (O.lxcmc). Therefore, the surfactant concentration is always below the critical micelliza-tion concentration (cmc) and the critical aggregation concentration (cac) of the respective polymer/surfactant systems. 

Aggregation of amphiphilic molecules occurs specifically in the presence of water (see Figure 1) (recently, though, aggregation has been observed in other solvents [12]) above the critical aggregate concentration (cac), dubbed the critical micelle concentration (cmc) in the case of the most simple of these aggregates the micelle. 

It is noted that the ratio of I3/I1 in PMA - C gTAB solutions reaches a steady value ( 0.70) at a C gTAB concentration, which is called a Critical Aggregate Concentration, CAC. At the CAC, the hydrophobic aggregates of C g formed which host... 

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... 

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