Multicomponent Diffusion in Systems Containing Molecules of Different Size. 3. Mutual Diffusion in the Ternary System Hexa(ethylene glycol)-Di(ethylene glycol)-Water

The diffusion of a model system describing a polydisperse sample has been investigated experimentally. Precise mutual diffusion coefficients were measured, with a Gouy interferometer, for the ternary system hexa(ethylene glycol)(6)-di(ethylene glycol)(2)-water at four values of the concentration ratio β6 = C6/(C6 + C2), and at a constant total concentration C6 + C2 = 0.3000 mol dm-3. The main-term diffusion coefficient, D22, shows a substantial decrease with β6, while D66 varies modestly. Both cross-term diffusion coefficients are positive, the largest, D26, reaches a value that is about 15% of the corresponding main-term. The experimental diffusion coefficients of this system have been interpreted through an extension of the common hard sphere hydrodynamic theory. Good agreement between the experimental diffusion coefficients and those predicted by a set of equations recently proposed by the authors for systems of non-ionic and slightly interacting solutes has been found. These equations, accounting only for the "excluded volume" effect, succeed in the evaluation of the value of the diffusion coefficients in ternary systems with solute size mismatch. A comparative analysis, with analogous systems investigated previously confirms the efficiency of our predictive procedure.