Observation of Mutual Diffusion Coefficients and Cooperative Motions in Binary Solutions of t-Butyl Alcohol–Water and 2-Butoxyethanol–Water

Rayleigh scattering spectra were observed for the binary solutions of t-butyl alcohol(TBA)–water and 2-butoxyethanol (BE)–water. The observed line-widths were reduced to mutual diffusion coefficients. The observed concentration dependencies of the mutual diffusion coefficients were well explained by postulating the existence of “moving units”— a group of molecules which move together for a time much longer than the velocity autocorrelation time. The structure of the moving units which are formed in the TBA–water solution was (H2O)20TBA for XTBA0.05, and 4(H2O)20TBA for XTBA0.05, where XTBA is the mole fraction of TBA. For the BE–water solution, the structure was found to be (H2O)50BE for XBE0.02 and 3(H2O)50BE for XBE0.02. These results were compared with the results which had been obtained from the concentration dependencies of the mean-square concentration fluctuation values.