Spherocylindrical model

In the spherical and spherocylindrical models represented by Figure 12.2a,b, the hydrocarbon chains that form the micellar interior are nonpolar and L uid. The representations of micelles in dilute solutions as perfect spheres or cylinders are idealized models (Lieberman et al., 1996). Examples of surfactants that form spherical micelles are sodium laureqt W( COOrNa+), sodium dodecyl sulfate (C F sSCfNa ), and CTAB (Q6H33N+(CH3)3Br ) (Mukerjee, 1979). 

In the spherocylindrical model, two distinct environments for SDS molecules are predicted. The hemispherical endcaps would approximate the environment found in spherical SDS micelles in the absence of electrolyte, while molecules in the second environment (cylindrical portion) would be characterized by closer packed headgroups, and as such might be expected to be "more ordered" than the molecules in the hemispherical endcaps. The model of Gruen (26) suggests, however, that there is no energy cost, as far as the methylene tails are concerned, in forming rod-like micelles (i.e. the tails are still able to pack at liquid hydrocarbon density throughout). ... 

The two distinct types of SDS headgroup packing indicated by the difference spectra are consistent with the spherocylindrical model of rod micelle formation discussed above. The increase in absorbance of the difference bands with increasing salt concentration indicates a continual increase in the number of relatively ordered SDS molecules packing into the cylindrical portion of the micelle, with the subtraction procedure simply eliminating the spectral contributions from the hemispherical endcaps. 

FIG. 9 Spherocylindrical micelle model with slightly swollen end cap where the Ws H term of Eq. (53) contributes (except at the very tip). 

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