Light-Scattering Studies of Critical Opalescence in Binary Liquid Mixtures. IV Paraffins in β,β′-Dichlorodiethyl Ether

Abstract

A specially designed light-scattering photometer with a measurable angular range of 15°–140° and with a temperature control of better than 0.001°C was used to study the angular dependence of the scattered intensity from binary critical liquid mixtures, consisting of a homologous series of alkanes as one of the two components, and β,β′-dichlorodiethyl ether (or chlorex) as the other component. Both components were purified by preparative gas chromatography and subsequent vacuum distillation. Special care was taken to prevent contamination of the systems from moisture. Scattering data were obtained by means of cylindrical light-scattering cells of different diameters (6 and 8 mm i.d.), detectors of different slit-widths (one set consisting of two slits: 1 × 10 mm each and 20 mm apart, and the other set: 0.5 × 10 mm each and 20 mm apart) and incident light of different wavelengths (λ0 = 365, 436, and 578 mμ). Phase diagrams for n-decane–chlorex and n-dodecane–chlorex were carefully determined in order to properly locate their critical mixing points. From our studies we have observed that (1) the classical theory of Ornstein–Zernicke and Debye provides a very good representation of experimental facts in the intermediate-temperature distances over large ranges of s / λ; (2) deviations seem to exist at small temperature distances (say ΔT < 0.03°C) for the n-dodecane–chlorex system. Estimates on the exponent η show qualitative agreement with Fisher's theoretical prediction of 0.059; and (3) the reciprocal scattered intensity extrapolated to zero angle has the form lim K → 0 Ic− 1 = f[T − Tc]γ, where f and γ are constants with respect to temperature. We estimate the true value of γ to be between 1.1 and 1.3 for our systems.