SPATIAL AND TIME-DEPENDENT CONCENTRATION FLUCTUATIONS OF ISOBUTYRIC ACID-WATER SYSTEM IN NEIGHBORHOOD OF ITS CRITICAL MIXING POINT

The coexistence curve and critical opalescence of a binary liquid mixture, isobutyric acid-water, have been reinvestigated. Experiments are reported on the angular and the spectral distribution of scattered light for this system at critical solution concentration over a range of temperatures near the critical solution temperature in the one-phase region. The results are discussed in terms of the effects of long-range correlations. We observe that the temperature dependence of the extrapolated zero-angle scattered intensity obeys a relation limK→0( 1/Ic*) ∝ (∂π/∂c)T ∝ (T - TC)γ with γ ≈ 5/4 and that of the extrapolated line width from quasi-elastic scattering studies obeys a relation limK→0Γ/K2 = α*(∂π/∂c)T ∝ (T - Tc)γ* with γ* = 2/3. The rate at which the diffusion coefficient D (= α*(∂π/∂c)T) decreases in the neighborhood of the critical mixing point, as the temperature T approaches the critical solution temperature TC, is therefore not entirely dictated by the decrease in the inverse susceptibility, (∂π/∂c)T ∝ (T - Tc)γ. The discrepancy (γ ≠ γ*) implies that the transport coefficient α* diverges as (T - TC)γ within the temperature range of our investigation. © 1968, American Chemical Society. All rights reserved.