Aggregates curvature

A full discussion of water penetration into micelles is beyond the scope of this chapter. The results described above, and others employing longer chain keto -surfactants in other micelles and bilayers, indicated a trend toward less water penetration to the core of aggregates as the surfactant tail length increased, and as aggregate curvature decreased (bilayer formation). More data from FT-IR studies... 

As we have seen above, the Ii, Hi and phases have structures based on ordered globular (spherical), rod and disc micelles. Intermediate and V i phases have an aggregate curvature between that of rods and discs. Hence, the mesophases formed, and their sequences, can... 

Atomic force microscopy (AFM) [19-21] provides a more detailed molecular picture of adsorbates at concentrations above the CMC. Adsorbed cationic surfactant C14TAB (tetradecyltrimethylammo-nium bromide) was arranged in surface micelles on silica and in long cylinders on mica. Double-chained DDAB was organized in bdayers on mica. These structures result from the balance of adsorbate-surface interactions and aggregate curvature controlled by intermolecular interactions. 

Fig. 11 Left Relative order parameters in a series of volume aggregates, in which the aggregate curvature is decreasing with increasing surfactant concentration (micelles, hexagonal, lamellar phase). Right Relative order parameters in adsorption layers at different surface coverage. Figures taken from [38] with permission.

Surfactants, when dissolved in water, can form aggregates called micelles. The micelles are generally spherical at small surfactant concentration and become rod-like at higher concentrations. The evolution is faster when salt is added to an aqueous solution of an ionic surfactant the salt screens the electrostatic interactions between the polar parts of the surfactant molecules, and lowers the aggregates curvature. In some cases, very long aggregates can form.2> Because of thermal fluctuations, they adopt, like polymers, worm-like structures. These structures have also been observed in microemulsions (oil-water-surfactant mixtures), where their occurrence was predicted earlier... 

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