B Surfactant Structure and Micellar Shape

From Tanford (1980), Vh = 27.4 + 26.9n A3, where n is the number of carbon atoms of the chain embedded in the micellar core (the total number of carbon atoms in the chain, or one less) lc 1.5 + 1.265n A, depending upon the extension of the chain. For saturated, straight chains, lc may be 80% of the fully extended chain. 

The solubilization of hydrocarbons in the interior of the micelle (Chapter 4, Section I) increases the value of Vh- 

The value of o varies not only with the structure of the hydrophilic head group, but also with changes in the electrolyte content, temperature, pH, and the presence of additives in the solution. Additives, such as medium-chain alcohols that are solubilized in the vicinity of the head groups (Chapter 4, Section IIIA), increase the value of oq. With ionic surfactants, o decreases with increase in the electrolyte content of the solution, due to compression of the electrical double layer, and also with increase in the concentration of the ionic surfactant, since that increases the concentration of counterions in the solution. This decrease in the value of ao promotes change in the shape of the micelle from spherical to cylindrical. For POE nonionic surfactants, an increase in temperature may cause a change in shape if temperature increase results in increased dehydration of the POE chain. 

Some ionic surfactants form long, wormlike micelles in aqueous media, especially in the presence of electrolyte or other additives that decrease the repulsion between the ionic head groups (Raghavan, 2001). These giant, wormlike micelles give rise to unusually strong viscoelasticity because of the entanglement of these structures. 

When the value of the parameter Vh /lcao reaches a value of approximately 1, the surfactant can form either normal lamellar micelles in aqueous media or reversed micelles in nonpolar media. As the value of the parameter gets larger and larger than 1, the reverse micelles in nonpolar media tend to become less asymmetrical and more spherical in shape. 

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