On the application of forced Rayleigh light scattering to mass diffusion measurements

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Abstract
The newly developed forced Rayleigh light scattering (FRS) method has been applied to the measurement of anisotropic mass diffusion in well-aligned nematic samples of p-azoxy anisol (PAA) and p-methoxy benzilidene-p-n-butyl aniline (MBBA). Since both materials are photochromic and can thus be photochemically labeled, self-diffusion can be studied as well as impurity diffusion. In the latter case, the diffusing species was chosen to be methyl red (MR), which is photochromic but had also been used as a regular dye in optical tracer diffusion experiments. The FRS data for self-diffusion are shown to agree well with this earlier determination; a satisfactory result if one considers that MR and both PAA and MBBA have very similar molecular structures. They also agree to within 10% with the Franklin theory of self-diffusion in liquid crystals. On the other hand, the FRS measurements of impurity diffusion with photochemically labeled MR give diffusion coefficients that are considerably smaller. This discrepancy can, in principle, be explained through nonequilibrium thermodynamic considerations which show that the diffusion coefficient actually measured by FRS is different in the two cases of self- and impurity diffusion. However, the observed variation seems anomalously large. We tentatively suggest that photoactivated MR molecules tend to form aggregates, a problem apparently not encountered with photoexcited MBBA or PAA molecules. This demonstrates that, although FRS is undoubtedly a powerful technique to measure mass transport, the possibility of photoinduced spurious intermolecular interactions between the diffusing species must always be considered with care.