Critical Opalescence of the Polystyrene—Cyclohexane System: Small‐Angle X‐Ray Scattering

The critical opalescence of polystyrene—cyclohexane mixtures has been studied by means of small‐angle x‐ray scattering for polystyrene samples of molecular weight 118 000 and 147 000. The upper limit of s/λ (s = 2 sin½θ, where θ is the scattering angle, and λ is the wavelength in the medium) was extended to 0.025 A−1, approximately 50 times greater than the upper limit accessible to visible light. At large values of s/λ, plots of the reciprocal of the intensity vs (s/λ)2 showed downward curvature. The experimental results can be represented by I = [A+B(s/λ)2]−1+C. The magnitude of the ratio of the two terms in the expression representing the intensity is consistent with the conclusion that the constant term C represents the scattering from density fluctuations, and the angular distribution of the scattering caused by concentration fluctuations is accurately represented by [A+B(s/λ)2]−1 over the range of s/λ investigated for the polystyrene—cyclohexane system. However, this is not true for mixtures of ordinary liquids for which the constant term is much too large to be interpreted as a density scattering term.