Concentration Fluctuations in Ethanol/Dodecane Mixtures. A Light-Scattering and Ultrasonic Spectroscopy Study

Dynamic light-scattering measurements of liquid ethanol/dodecane mixtures have been performed using a self-beating digital photon correlation spectrometer. The binary system exhibits an upper critical consolute point at a mole fraction xc = 0.687 of ethanol. The critical temperature of our sample was Tc = 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 determine the mutual diffusion coefficient D, the fluctuation correlation length ξ, and the characteristic relaxation rate ωD of the order parameter fluctuations as a function of reduced temperature. For the mixture of critical composition these quantities follow power law behavior, as predicted theoretically. The amplitude of the relaxation rate of the ethanol/dodecane system is ω0 = (8.0 ± 0.2) × 109 s-1 from dynamic light scattering, which compares with ω0 = 2.8 × 109 s-1 and ω0 = 4.4 × 109 s-1 as resulting from broadband ultrasonic spectra. Fitting the light-scattering data at different scattering angles to the universal dynamic (Kawasaki) function yields ω0 = (7.7 ± 0.7) × 109 s-1. Critical mixtures with an amount of 0.2% (w/w) water added resulted in an enhancement of Tc by about 3.5 K and in a small increase (10%) in the amplitude D0 of the mutual diffusion coefficient. Light-scattering data for five ethanol/dodecane mixtures of noncritical composition are in conformity with the pseudospinodal conception. It remains unclear, however, whether noncritical exponents have to be used in the power laws. Using the critical exponents in the evaluation of data the amplitude ξ0 of the correlation length displays a relative maximum at the critical composition where D0 adopts a relative minimum.

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