Chapter 6
Micellar Sphere to Rod Transitions Jeffry G. Weers and David R. Scheuing
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Micellar growth induced by changes in electrolyte concentration, micelle composition, or temperature, can be monitored by changes in the frequency and shape of C H stretching and deformation bands. Decreases in the gauche conformer content of the methylene chains of surfactant molecules with increasing aggregation number are observed. Simultaneous changes in bands due to surfactant headgroups can distinguish differing mechanisms of micelle growth because of their sensitivity to the location and type of counterion species present. The spectroscopic data support the spherocylindrical model of micelle growth, and show that surfactant molecules in the cylindrical portion exhibit methylene chain ordering similar to the rotator phase of alkanes. 2
The utility of FTIR spectroscopy in studies of phase transitions involving micellar surfactant solutions has been increasingly demonstrated in recent years (i-7). The familiar concentration-dependent monomer to micelle transition (cmc), the thermally induced hydrated solid to micelle transition (cmt), and the micelle to coagel transition achieved at high pressure (ccp) have all been investigated (Figure 1). These transitions can be monitored by shifts in frequency (« 5 cm"*) of the C H stretching bands which appear in the spectra. Only relatively small shifts of these bands (» 1 cm" ) as a function of concentration or temperature are observed in the spectra of micellar surfactant solutions in the absence of the transitions mentioned above. It has therefore been postulated that the methylene chains comprising the tails of surfactants are in a highly disordered "liquid-like" state which does not vary greatly with micelle structure or shape. For example, Mantsch et al. reported no differences in the spectra of micellar alkali hexadecylsulfates with temperature or counterion variations (4). Cameron et al concluded that the methylene chain packing was the same in spherical sodium laurate and rod-shaped sodium oleate micelles (5). An early Raman spectroscopic study of a micelle sphere to rod transition did, however, suggest that there may indeed be changes in methylene chain packing accompanying the shape change (8). As we demonstrate in this chapter, FTIR spectroscopy can be an ideal technique for studying the molecular packing in micellar aggregates. The major 2
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0097-6156/91/0447-0087$10.00/0 © 1991 American Chemical Society In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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