Micelles and Critical Micelle Concentration CMC

Both ionic and non-ionic surfactants associate to form micelles. However, there are several property differences between the micelles formed. The CMC value depends on the length of the hydrophobic groups for both types, but it also depends on the state of charge for ionic surfactants and on the length of the hydrophilic part for non-ionic surfactants. The core of a micelle is assumed to be filled with liquid hydrocarbon, and the maximum size of the micelle radius is equal to the length of the fully extended hydrocarbon tail. In 

If we have the estimated values of Ah, Vt and Lc, then it is possible to define a surfactant packing parameter 

Micelles are used in many applications. Their largest industrial use is in emulsion polymerization, as detailed in Section 5.9 below. On the other hand, micelles made of ionic surfactants can trap hydrocarbon wastes in polluted water, since these hydrocarbon molecules prefer to be in the hydrocarbon interior of the micelle in an aqueous environment. In addition, ionic wastes dissolved in water adsorb onto the polar heads of these micelles. The resulting waste-filled micelles may be removed by simple ultrafiltration. As an example of another application, micelles can affect the rate of several chemical reactions and are used in micellar catalysis, similar to enzyme catalysis, in biochemistry. The rate of the chemical reaction increases with increasing micelle concentration, eventually leveling off. Nevertheless, micellar catalysts are less specific than enzymes. 

8 Bilayers, Vesicles, Liposomes, Biological Cell Membranes and Inverted Micelles 

Let us further consider micelles. Although not always correct, we often picture micelles as spherical or semi-spherical aggregates (Eigure 5.2). They are almost spherical at low concentrations, close to CMC, but can have different forms at higher concentrations (hexagonal or lamellar phases). We believe that they are more or less hquid-hke in a very dynamic state. There are many surfactant molecules in each micelle, often about 50-100. This number is called the aggregate (or aggregation) number (see below). 

The CMC can be measured by observing the change of several properties, e.g. surface tension with concentration (Eigure 5.4). Other properties like osmotic pressure and electrical conductivity also change at CMC. The conductivity increases after CMC and this is a useful CMC estimation method for ionic surfactants. Turbidity and detergency also increase up to the CMC. Solubilization also increases above CMC, as many compounds are readily soluble in the micelles. AU of these observations further verify the existence of micelles after CMC and indicate the importance of micelles in cleaning (detergency). 

As mentioned, CMC is not, strictly speaking, a thermodynamic transition state its value roughly depends on the estimation method. We observe in most cases that there is a small range of concentrations (range 

1/3-1/2 Cylindrical micelles Single-chained surfactants with small head group areas, e.g. SDS and CTAB in high salt concentrations, non-ionic surfactants 

1/2-1 Flexible bilayers Double-chain surfactants with large head group areas, e.g. phosphatidyl choline (lecithin), dihexadecyl phosphate 

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