Abstract
A laser homodyne spectrometer with a measurable angular range of 20°-150° and a temperature control of better than f 0.001 OC was interfaced with an IBM 1800 digital computer for direct on-line data acquisition of the angular and the spestral distribution of the scattered intensity from a two-component critical liquid system, perfluoromethylcyclohexane-carbon tetrachloride, near its critical mixing point. Both components were purified by preparative gas chromatography and subsequent vacuum distillation. Special care was taken to select appropriate columns for the separation of impurities and to prevent contamination of the system from moisture. Scattering data were obtained by means of a cylindrical light-scattering cell of 8 mm i. d. and a flat cell with a 1 mm light path. The intensity measurements were obtained using an angular aperture of 1.40 while the linewidth measurements were obtained using an angular aperture of OSO. From our intensity studies we have observed that (1) the K2 dependence in the reciprocal scattered intensity due to concentration fluctuations (1~') is obeyed (2) the temperature dependence of the extrapolated zeroangle scattered intensity which is related to the osmotic coeffjcient (ap/aC)p,~, has the form : lim 1;' cc eY, where E = (T - Te)/Te ; and y is a critical exponent. We have obtained K+O y = 1.15 f 0.02 at the critical solution concentration. The Rayleigh linewidth of the critical mixture was measured at temperatures between 0.006O and 0.715 OC above Tc, for scattering angles 30° 1, we show that r approaches AK3. In the region 0.076 < Kts < 3.75, our data agree quite well with the Kawasaki extended mode-mode coupling theory. Our results show that a direct application of the Kadanoff-Swift-Kawasaki result y* (= y - Y) = v is invalid. However, if we take into account the temperature dependence and anomaly of the high-frequency shear viscosity, then y - Y approaches v and, in the case of isobu- tyric acid in water, y - Y z v. Scaling-law considerations also lead us to question the validity of the relation (sv = 2 - a) or its equivalents sv = y + 2 B and sv = p + v.
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