Surfactant critical association concentration

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. 

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. 

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. 

Surfactants, not surprisingly, exert a highly significant influence on the fluorescence of FBAs in solution. This effect is associated with the critical micelle concentration of the surfactant and may be regarded as a special type of solvent effect. Anionic surfactants have almost no influence on the performance of anionic FBAs on cotton, but nonionic surfactants may exert either positive or negative effects on the whiteness of the treated substrate [33]. Cationic surfactants would be expected to have a negative influence, but this is not always so [34]. No general rule can be formulated and each case has to be considered separately. 

Aggregates generated in the spontaneous and dynamic association of ca. 50-100 surfactant molecules above a characteristic surfactant concentration, labeled the critical micelle concentration (CMC). 

Another aspect of polysorbates is that they are inherently susceptible to oxidative degradation. Often, as raw materials, they contain sufficient quantities of peroxides to cause oxidation of protein residue side chains, especially methionine (59). The potential for oxidative damage arising from the addition of stabilizer emphasizes the point that the lowest effective concentrations of excipients should be used in formulations. For surfactants, the effective concentration for a given protein will depend on the mechanism of stabilization. It has been postulated that if the mechanism of surfactant stabilization is related to preventing surface-denaturation, the effective concentration will be around the detergent s critical micellar concentration. Conversely, if the mechanism of stabilization is associated with specific protein-detergent interactions, the effective surfactant concentration will be related to the protein concentration and the stoichiometry of the interaction (39). 

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

The variation of n with concentration expresses the fact that in Region 2 complete removal of each CH2 group of the surfactant from water is only possible at a monolayer. In bulk systems the analogous processes are the pre-association into dimers, trimers, etc. just below the critical micelle concentration. 

Micelles. Surfactant molecules or ions at concentrations above a minimum value characteristic of each solvent-solute system associate into aggregates called micelles. The formation, structure, and behavior of micelles have been extensively investigated. The term critical micelle concentration (CMC) denotes the concentration at which micelles start to form in a system comprising solvent, surfactant, possibly other solutes, and a defined physical environment. 

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