Hydrocarbon tail of the surfactant

The influence of surfactant structure on the nature of the microemulsion formed can also be predicted from the thermodynamic theory by Overbeek (17,18). According to this theory, the most stable microemulsion would be that in which the phase with the smaller volume fraction forms the droplets, since the osmotic term increases with increasing i. For w/o microemulsion prepared using an ionic surfactant, the hard sphere volume is only slightly larger than the water volume, since the hydrocarbon tails of the surfactant may interpenetrate to a certain extent, when two droplets come close together. For an oil in water microemulsion, on the other hand, the double layer may extend to a considerable extent, depending on the electrolyte concentration... 

By means of this apparatus, it is possible to vary the area of a spread monolayer and measure the corresponding film pressure directly. Many different variations of the film balance are available, and a number of instrumentational techniques can be combined with the Langmuir balance to obtain information on the microstructure of the films and the properties of the films. Figure 7.4b illustrates, for example, a laser optics arrangement to monitor the molecular orientation of the hydrocarbon tails of the surfactant molecules. Below in this... 

It can be seen from these data that the range of concentrations over which coagulation occurs is very narrow and that the c.c.c. is strongly dependent on the chain length of the hydrocarbon tail of the surfactant molecule. Studies of the electrophoretic mobility of the particles confirm that at the c.c.c the particle mobility becomes zero and studies of the adsorption suggest that up to this point the alkyl chains lie flat on the hydrocarbon-like surface of a polystyrene particle (36). 

The third contribution to the free energy comes from the steric repulsion in the oil layers between the hydrocarbon tails of the surfactant adsorbed on the interface. Extending the simple formula suggested by de Gennes18 for the steric force between long-chain polymer-covered surfaces, to relatively short chains, one can write... 

Alternatively, inverted micelles can also be formed in water-free medium. If no water is present in a two-component system, the difference between the solubility parameters of the hydrocarbon tail of the surfactant and the organic solvent contributes to inverted micelle formation. A large negative enthalpy change is the driving force to form spontaneous inverted micellization, in contrast with aqueous systems. 

Using the noncoulombic model, one can investigate the details of surfactant adsorption at the solid-water interface. The van der Waals interactions of the hydrocarbon tails oF the surfactant Ions lead to the abrupt condensation of tbe surfactant on the surface as the concentration of surfactant in solution increases. The formation of such a condensed film (called a hemimicelle) appeals to be neoessaty in tbe development... 

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