CAC critical aggregation

Figure 5.3 Self-assembly of a vesicle. Water-soluble molecules can be entrapped inside, ionic molecules on the polar head groups of the surface, amphiphatic molecules in the hydrophobic bilayer, (cac critical aggregate concentration).

The cac (critical aggregate concentration) values for oleate are in the millimolar range, which means that at the operational concentration of 10-50 mM there will be a signihcant concentration of monomer in equilibrium with the aggregate. This consideration allows us to go back to the question of whether vesicles are chemical equilibrium systems. Oleate vesicles cannot be considered proper chemical equilibrium systems, however they behave in a mixed way, with some features that are typical of micelles in equilibrium (Luisi, 2001). 

Recently, the competitive adsorption dynamics of phospholipid/protein mixed system at the chloroform/water interface was investigated by using the drop volume technique. The three proteins P-Lactoglobulin, P-Casein, and Human Serum Albumin were used in this study. To investigate the influence of the phospholipid structure at concentrations close to the CAC (critic aggregation concentration) the four lipids dipalmitoyl phosphatidyl choline (DPPC), dimyristoyl phosphatidyl choline (DMPC), dimyristoyl phosphatidyl ethanolamine (DMPE)... 

It must be mentioned that although the surface tension method is facile and readily applicable to the study of a mixture of surfactant/polymer, it also has its limitation. Generally, the polymer used should be less surface active than the surfactant itself. Otherwise, interpretation of the data may be difficult in terms of Tj (cac, critical aggregation concentration) and T2 (apparent cmc) in spite of a clear indication from the curve of the interaction in such a system. For example, the surface activity of polypropylene glycol makes it difficult to interpret the surface-tension results [15]. Similar to a system of mixed surfactants, the composition of polymer is also a factor in the surfactant/polymer system. The case of poly(vinyl alcohol) (PVA) is more complicated than that of other synthetic polymers, since PVA is known to be prepared by hydrolysis of the acetate group of poly(vinyl acetate). The hydrolysis is only partially done hence acetate and hydroxyl groups are present. Therefore, consideration of the compositions of polymer is important for a reliable interpretation of experimental data. 

CAC Critical aggregation concentration CMC Critical micelle concentration FTT Film trapping technique... 

Additives, whether hydrophobic solutes, other surfactants or polymers, tend to nucleate micelles at concentrations lower than in the absence of additive. Due to this nucleating effect of polymers on micellization there is often a measurable erne, usually called a critical aggregation concentration or cac, below the regular erne observed in the absence of added polymer. This cac is usually independent of polymer concentration. The size of these aggregates is usually smaller than that of free micelles, and this size tends to be small even in the presence of added salt (conditions where free micelles tend to grow in size). 

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]. 

Our group [124] has used pyrene and pyrene labeled poly(acrylic acid) as a fluorescent probe to investigate the interaction between poly(acrylic acid) (PAA) and dodecyltrimethylammonium bromide (DTAB) in water. We have measured the Is /1 ratio of pyrene as a function of DTAB in the presence of 1 g/L PAA. A sharp decrease in polarity is found well below the CMC of DTAB (1.3 X 10 M, Fig. 17). The onset of this polymer-induced association is referred to as the critical aggregation concentration. The CAC has been measured at various pH and NaCl levels (Fig. 18). It was observed that the... 

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 number-average diameter, for the HAP particles increases with the electrolyte KNO3 concentration until the critical aggregation concentration is reached, where the value remains approximately constant. The starting point of the maximum d corresponds to the electrolyte concentration called CAC. The last value, which depends on the electrolyte used, was found to be 1.27 x lO M for the electrolyte KNO3. 

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. 

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