Dimensionality of diffusion in lyotropic mesophases from fluorescence quenching

3 Dimensionality of diffusion in lyotropic mesophases from fluorescence quenching 

The aim of the present study was to demonstrate this concept within a single surfactant system forming both, one-dimensional aggregates (rod-like micelles) and a two-dimensional a-lamellar phase. The C14DMAO/C7H15OH/H2O system was chosen for this purpose because it forms in addition to the above-mentioned phases two distinct L3 mesophases of different viscosity [20]. The structure of these cubic phases has been the subject of intense investigations. Freeze fracture electron microscopy demonstrated their bicontinuous character [21], 

In our study, the C14DMAO concentration was held constant at 100 mM, while the heptanol concentration was varied between 0 and 200 mM in order to realize compositions that correspond to the mentioned phases. With view to comparability of earlier studies in rod-like micelles [22], pyrene (c = 10 M) was chosen as the fluorescent probe and benzo- 

Experimental results for a micellar phase consisting of rod-like micelles and of an a-lamellar phase in the same system are compared in Fig. 20.8. Fluorescence decay curves are presented as logarithmic plots. The deviations from exponentiality in the micellar system (Fig. 20.8 a) are evident as all traces are curved even for long times except for the system containing no quencher (top trace). Evidently, the decay is faster in the a-lamellar phase (Fig. 20.8 b) when comparing runs at the same quencher concentration. This result meets the intuitive expectation that the approach of quenchers from all sides within a lamellar plane should give rise to a faster decay as compared to one-dimensional random walk within a cylindrical or rod-like micelle. 

Corresponding representative results for the L3 phase (Fig. 20.9) show again a distinctly non-exponential decay. In view of the poorer signal-to-noise ratio, obtained with this sample, an extensive data set was acquired with this sample. 

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