Turbidity very near the critical point of methanol-cyclohexane mixtures

We have measured the turbidity of a critical mixture of methanol and cyclohexane extremely close to the consolute point. The data span the reduced-temperature range ${10}^{-7\mathrm{}}<t<\mathrm{}{10}^{-3\mathrm{}}$, which is two decades closer to $T_c$ than previous measurements. In this temperature range the turbidity varies approximately as $\ln t$, as expected from the integrated form for Ornstein-Zernike scattering. A thin cell (200-μm optical path) with a very small volume (0.08 ml) was used to avoid multiple scattering. A carefully controlled temperature history was used to mix the sample and to minimize the effects of critical wetting layers. The data are consistent with a correlation-length amplitude ${\xi }_0=3.9\mathrm{}\pm 1.0$ Å, in agreement with the value 3.5 Å calculated from two-scale-factor universality and heat-capacity data from the literature.