THERMODYNAMIC EVALUATION OF THE POLYDISPERSITY OF DROPLET MICROEMULSIONS

By exploiting our previously presented multiple chemical equilibrium approach to the theory of Winsor II microemulsions (Eriksson, J. C.; Ljunggren, S. Progr. Colloid Polym. Sci. 1990, 81, 41.) we derive relations which enable an evaluation from thermodynamic data of the polydispersity of Winsor II type microemulsions as well as of corresponding one-phase (ternary droplet) microemulsions. We find that, quite generally, a size polydispersity (sigma(Rmax)) in the range 0.1-0.2 is to be anticipated for such microemulsions and, moreover, that the absolute width of the size distribution to a large extent is determined simply by the value of the interfacial tension, gamma infinity, of the planar, surfactant-laden oil/water interface. The shape and surfactant film packing density fluctuations also influence the peak width but to a lesser extent. Furthermore, we show how the curvature-related bending constant k(2) = 2k(c) + (k) over bar(c)> can be evaluated from the overall volume fraction of droplets.