Determining aggregation numbers

Luminescence and electronic energy transfer have been used frequently tq determine aggregation numbers of hydrocarbon type surfactants [3]. In contrast, a few luminescence studies have been made for fluorinated surfactants [16-19]. 

On simple inspection, it appears that the risks presented by the cafeteria exceed the company criteria. However, F-N curves are not always the most effective tools for identifying the major contributors to the risk. More detailed analysis, or use of other aggregate risk methodologies, such as determining expected numbers of fatalities per year, may reveal the risk to be more (or less) tolerable than that shown by the F-N curves. 

Section 4.2.2 discussed the use of methods other than F-N curves for determining aggregate risk. An aggregate risk measure can be calculated for Example 10 by multiplying each incident frequency by the expected number of serious injuries or fatalities. This results in a parameter of "fatalities per year."... 

An interesting application of static quenching is the determination of micellar aggregation numbers (see Box 4.2). 

Box 4.2 Determination of micellar aggregation numbers by means of fluorescence quenching3 ... 

Quenching of pyrene by excimer formation (Py + Py —> (PyPy) —> 2Py) (see Section 4.4.1) is widely used for the determination of micellar aggregation numbers for new surfactant systems. An example is given in Figure B4.2.1. 

Fig. 48 Surfactant aggregation numbers determined at various adsorption densities (average number at each adsorption density shown along the adsorption isotherm)...

The aggregation number rt and radius of sodium dodecyl sulfate micelles (by light scattering) and the zeta potential (from electrophoresis, by an accurate formula) were determined in the presence of various concentrations of NaCl. ... 

For the same amount of sulfide formation per unit area, significant differences in the number and sizes of aggregates could occur for different emulsion preparations. The number and types of lattice imperfections could be important in determining the number and distribution. Direct experimental evidence on the distribution is lacking. The gold treatment that can make reduction centers developable does not cause the silver sulfide centers to become developable (140). 

With monomeric molecules, the aggregation number of micelles is determined by equilibrium thermodynamics. In polymeric molecules, however, topological constraints are imposed on the system. If the degree of polymerization exceeds the aggregation number of the monomeric micelle, unsaturated sites of the polymeric molecules become available (directed to the aqueous phase) and inter-molecular interactions (agglomeration) occur. In the case of polymer with Mw= 6.23x105, typical surfactant behavior was found. 

The aggregation numbers Nagg is determined as 27 for C1-(EO)53-C4-VB and 38 for Cr(EO)53-C7-VB micelles by analysis of fluorescence curves. A micelle formation mechanism is proposed for nonionic polymeric surfactants with weakly hydrophobic groups. At low concentrations of PEO macromonomers, large loosely aggregated structures involving the PEO chains are formed. At higher concentrations normal micelles form. These are star-shaped, with a hydrophobic core surrounded by a corona of PEO chains. 

In contrast to the ammonium compounds cited above, the 12-(10 -pheno-thiazinyl)-dodecy1-1-sulfonate (Q) is a stable surfactant molecule forming aggregates over a wide pH range by itself or with SLS. Ij s CMC has been determined by different methods to be approx. 5x10 M with an aggregation number of 4.2x10 (38). Various experiments imply... 

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