Raman spectroscopic studies of liquid phase ordering and dynamics for solutions of ME6N liquid crystal: the approach to simple molecule behavior at high dilutions.

We have measured the Raman isotropic profiles of the ν(C≡N) band at 2235 cm(-1) for five solutions of ME6N (4-cyanophenyl-4'-hexylbenzoate) liquid crystal dissolved in CCl(4) in the range from x = 0.12 to 0.007 (x, mole fraction of ME6N) and then obtained the corresponding vibrational correlation functions, C(v)(t), by time Fourier transformation. The increase with dilution of the dephasing times τ(v) complies with the behavior of the nonmonotonic concentration dependence predicted by the fluctuation concentration model for this concentration range (x < 0.5). The interpretation of C(v)(t) within the Kubo stochastic theory which, being based on the assumption that the environmental modulation arises from a single relaxation process, e(-t/τ(c)), applies to simple liquids, has proved to be inadequate except for the most diluted solutions where the environmental correlation time τ(c) amounted to 1.16 ps. On the other hand, we have found that the vibrational correlation functions for these ME6N/CCl(4) solutions comply, in the whole concentration range, with the approach proposed by Rothschild, which, being based on the assumption that the environmental modulation is described by a stretched exponential decay e(-(t/τ(0))(α)), is more appropriate for the interpretation of the vibrational correlation function arising from a distribution of relaxation processes caused, as in the present case, by the persistence of pseudonematic domains. With dilution the dispersion parameter α and the average correlation time τ(0) progressively increases and decreases, respectively, and tend to converge to the values appropriate for simple liquids, as given by the Kubo theory, i.e., α to unity and τ(0) to approximately 1.16 ps. The relaxation time probability distribution h(τ) progressively shrinks and shifts down with dilution. The evolution of α and τ(0) parameters and, contextually, of h(τ) with dilution offers a complete picture of the way a complex liquid attains the condition of a simple one.