Headgroup bands

The frequency of the (CH3)3N band has been found to be sensitive to the extent of hydration of the headgroups, exhibiting shifts upon the formation of ice (41). and with incorporation of cholesterol into the bilayer (44). 

The monomer to inverse micelle transition of dodecylammonium propionate (DAP) in CH2G2 was monitored by tire shifts in the COO bands. Addition of water caused further decreases in the COO- band frequency due to headgroup hydration. Micelle formation in hexane yielded similar spectroscopic changes (83). Frequency shifts induced by hydrogen bonding to water in the inverse micelle core were illustrated for phosphonate, carboxylate, and sulfate headgroups (84). 

An early paper on the use of ATR for the in - situ monitoring of the adsorption of AOT from heptane and water solutions utilized the intensity of the 0=0 band (near 1740 cm1) and the sulfonate band (1045 cm1) for construction of adsorption isotherms. In addition, the utility of detailed examination of the shifts in the sulfonate band frequency for assessing the headgroup environment in adsorbed layers was discussed (115). 

Headgroup Modes. Table II summarizes the bands due to headgroup vibrations of the surfactants to be discussed in this chapter, i.e. it is not an exhaustive list. As discussed further below, the combination of spectroscopic data from both the headgroups and tails of micellar surfactants of different size and shape can provide information about molecular packing. 

The driving force for formation of rod shaped SDS micelles is the elimination of water from die micellar core/water interface (31). The reduction in average headgroup area reflects the removal of water molecules between the SDS headgroups, and should affect the bands due to the asymmetric S-O stretching vibrations, as indicated in the discussion of the transition moment vectors above. 

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. 

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