Aggregates concentration

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 distinction between pairwise and bulk hydrophobic interactions is often made, although some authors doubt the existence of an intrinsic difference between the two ". Pairwise hydrophobic interactions denote the interactions behveen two isolated nonpolar solutes in aqueous solution. They occur in the regime where no aggregation takes place, hence below the critical aggregation concentration or solubility limit of the particular solute. If any breakdown of the hydrophobic hydration shell occurs, it will be only transient. 

De Minimis Limitation. A listed toxic chemical does not have to be considered if it Is present in a mixture at a concentration below a specified de nvnimis level. The de minimis level is 1.0%, or 0.1% if the chemical meets the OSHA carcinogen standard. See Table II for the de minimis value associated with each listed toxic chemical. For mixtures that contain more than one member of a listed chemical category, the de minimis level applies to the aggregate concentration of all such members and not to each individually. EPA included the de minimis exemption In the njle as a burden reducing step, primarily because facilities are not likely to have information on the presence of a chemical in a mixture or trade name product beyond that available in the product s MSDS. The de minimis levels are consistent with OSHA requirements lor development of MSDS information concerning composition. 

FIGURE 3.3 Surface tension y plotted against the concentration c of lysine derivative 3.20. Critical aggregate concentration (O) cM=2430mg/L=2.43mM, yc =58mN/m. (Reprinted from Nalum Naess, S. et al., Chem. Phys. Lipids, 148, 63, 2007. With permission.)M... 

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]. 

Self-aggregating amphiphiles can broadly be divided into hydrotropes and surfactants. The main difference between hydrotropes and surfactants lies in the fact that hydrotropes are typically not sufficiently hydrophobic to cooperatively self-aggregate and form organized structures, whereas surfactants form distinct aggregates such as micelles and vesicles above their critical aggregation concentrations. 

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...

Fig. 22 Comparison of critical aggregate concentration, determined by excimer formation of covalently labeled polymer and micropolarity response of free pyrene...

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). 

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. 

This is a fortunate result, since the paramagnetic resonance spectra of phospholipid spin labels such as (V), (VI), (IX), and (X) are sensitive to their state of aggregation, concentration, and rates of lateral motion. This... 

To estimate the precision of the analysis, five of the field-sample extracts were analyzed by GC-MS in duplicate at laboratory A. The results, gathered by compound classes, are shown in Table XI. The table also includes the results of a single analysis from laboratory B. The average percent deviation of the duplicate analyses from the mean value was 16, whereas the average percent deviation between the results from laboratory B and the mean value from laboratory A was 22. A comparison of the results from a single analysis of the treated water extracts (summer and winter) for class aggregate concentration in laboratories A and B (Table XII) showed an average percent deviation of 20. Hence, the intra- and interlaboratory uncertainties in the results are comparable. 

Table XI. Aggregate Concentrations (ng/L) in Great Lakes Water by GC-MS...

FIGURE 13.7 Effect of polymer concentration and temperature on the critical aggregation concentration of nystatin, as interpreted from dynamic light scattering data. (Reproduced from Croy, S. R. and G. S. Kwon. 2004. J. Control. Rel.95 161-171. With permission from Elsevier Ltd.)... 

Y. Li, S. Zhang, Q. Wang, and J. Yang, Study on surface activity and critical aggregation concentration of sucrose esters containing different isomers of mono-, di- and polyesters, Ten-side Surfactants Deterg., 41 (2004) 26-30. 

G. Garofalakis, B. S. Murray, and D. B. Sarney, Surface activity and critical aggregation concentration of pure sugar esters with different sugar headgroups, J. Colloid Interface Sci., 229 (2000) 391-398. 

The transition from stable dispersion to aggregation usually occurs over a fairly small range of electrolyte concentration. This makes it possible to determine aggregation concentrations, often referred to as critical coagulation concentrations (CCC). The Schulze-Hardy rule summarizes the general tendency of the CCC to vary inversely with the sixth power of the counter-ion charge number (for indifferent electrolyte). 

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