xt xi'

In the coexistence region of vesicles and micelles in the solution, the mass balance relations for the total surfactants and surfactant 2 are respectively given by

and

mr = C'(X1-Xi1)/(X2 -Xi') (12) At the composition region Xi' < xrt < X2 < Xi, on the other hand, the total molality mn at which the vesicle disappears is obtained by putting mv=O,as

. Finally, curve 5 (X2 < X2 < Xi1) has only one break, corresponding to the eve. The shape of the curves at Xf < X2 is qualitatively similar to that at X2 < Xf. The qualitative behavior predicted by this simple model will be compared to the one obtained from the thermodynamic analysis of the experimental results.