DIFFUSION MEASUREMENTS NEAR CRITICAL MIXING POINTS IN BINARY AND TERNARY LIQUID SYSTEMS
CLARK, WILLIAM MCDONALD
Doctor of Philosophy
A single lens convergent light Gouy diffusiometer was constructed and used to measure diffusion coefficients near the critical mixing point of a binary and a ternary liquid mixture. Liquid-liquid equilibria measurements were made on each system to determine the binodal curves and critical mixing points. The composition and temperature dependence of the diffusion coefficient near the critical point of the methanol-hexane system was determined. Since the binary diffusion coefficient is expected to vanish not only at the critical point but everywhere on the spinodal curve, the results of these studies allowed location of spinodal compositions by extrapolation to D = 0. It is suggested that spinodal locations could be used in developing accurate models of the liquid state. Gouy interferometric diffusion studies employing a temperature jump technique yielded a critical exponent along the critical line for the methanol-hexane system consistent with established results. Stability conditions require that the thermodynamic factor determinant vanish as the ternary plait point is approached, indicating that the determinant of the four independent diffusion coefficients may also vanish as the plait point is approached. The usually small cross diffusion terms would, therefore, become equal in magnitude to the main terms at this point. Temperature jump diffusion studies on the water - 2-propanol - cyclohexane system showed that the determinant of the diffusion coefficient matrix rapidly goes to zero with a 1.31 (+OR-) 0.2 critical exponent. Near the plait point, negative main diffusion coefficients have been observed which are consistent with positive semidefinite thermodynamic factor stability conditions. Although the results are not entirely conclusive, it appears that each term of the diffusion coefficient matrix goes to zero as the plait point is approached; the average value of the critical exponent for the individual terms was determined to be 0.55 (+OR-) 0.2.