Concentration fluctuations in ethanol/dodecane mixtures. A light-scattering and ultrasonic spectroscopy study

Dynamic light-scattering measurements of liquid ethanol/dodecane mixtures h ave been performed using a self-beating digital photon correlation spectrom eter. The binary system exhibits an upper critical consolute point at a mol e fraction x(c) = 0.687 of ethanol. The critical temperature of our sample was T-c = 285.64 +/- 0.05 K. Along with shear viscosity data the decay rate of the autocorrelation function of the scattered light has been used to de termine the mutual diffusion coefficient D, the fluctuation correlation len gth xi, and the characteristic relaxation rate omega(D) of the order parame ter fluctuations as a function of reduced temperature. For the mixture of c ritical composition these quantities follow power law behavior, as predicte d theoretically. The amplitude of the relaxation rate of the ethanol/dodeca ne system is omega(0) = (8.0 +/- 0.2) x 10(9) s(-1) from dynamic light scat tering, which compares with omega(0) = 2.8 x 10(9) s(-1) and omega(0) = 4.4 x 10(9) s(-1) as resulting from broadband ultrasonic spectra. Fitting the light-scattering data at different scattering angles to the universal dynam ic (Kawasaki) function yields omega(0) = (7.7 +/- 0.7) x 10(9) s(-1). Criti cal mixtures with an amount of 0.2% (w/w) water added resulted in an enhanc ement of T-c by about 3.5 K and in a small increase (10%) in the amplitude D-0 of the mutual diffusion coefficient. Light-scattering data for five eth anol/dodecane mixtures of noncritical composition are in conformity with th e pseudospinodal conception. It remains unclear, however, whether noncritic al exponents have to be used in the power laws. Using the critical exponent s in the evaluation of data the amplitude xi(0) of the correlation length d isplays a relative maximum at the critical composition where D-0 adopts a r elative minimum.