Halide Ions in Surfactant Systems

The linebroadening of chloride, bromide and iodide NMR signals effected by substituted ammonium ions was described in the preceding subsection. In addition to this phenomenon, which could be ascribed to an altered halide ion quadrupole relaxation rate close to nonpolar groups, another type of halide ion relaxation enhancement is obtained in the presence of monoalkylammonium ions with long alkyl chains. 

This can be referred to the amphiphilic nature of these cations leading to a considerable self-association in aqueous systems. 

In the case of ionic surfactants, a fraction of the counterions are more or less firmly attached to the different types of amphiphilic aggregates, and this counterion binding is of great importance in the association process. Thus it has been demonstrated that drastic changes in phase equilibria and micellar shape may accompany even small changes 

It appears that the quadrupole relaxation method is very well suited for the study of counterion binding in micellar solutions of different types as well as in liquid crystals. In contrast to some other methods, the NMR method is applicable irrespective of the macroscopic viscosity (or general rheological properties) of the sample. 

It should be noted that for anisotropic liquid crystals the NMR signal is split into 21 component signals due to first-order static quadrupole interactions but that, in the absence of second-order quadrupole effects, the width of the central line gives the transverse relaxation rate both for powder samples and for macroscopically aligned samples (see further Chapter 7). 

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