Ultrasonic Investigation of the Lower Consolute Point of the 2,6-Lutidine: Water System

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Abstract
Sound absorption coefficients and velocities have been measured over ranges in frequency, temperature, and composition for 2, 6-lutidine/water near its critical mixing point. Large excess absorption coupled to critical fluctuation dynamics is found, as in other binary mixtures. A strong background of ordinary thermal relaxation is also apparent. Interpretation of the details of the absorption and velocity, as well as of auxiliary data on the shear viscosity and density, in terms of the Fixman mechanism of critical region absorption combined with a chemical association model cannot be attempted quantitatively. The reason lies in defects in Fixman theory for the composition dependence of the critical absorption and in the inherent inability of the chemical association model to account for phase separation behavior. Nevertheless, Fixman's form for the absorption as a function of frequency and temperature at the critical composition is satisfactory, and some important qualitative aspects of binary fluid structure come to light, including inherent differences of upper and lower consolute points.