Emulsification failure in a ternary microemulsion

The microstructure of the water rich microemulsion phase of the C$_{12}$E$_5$, water and decane system has been studied using surfactant $^2$H-NMR relaxation and $^1$H Fourier transform pulsed gradient spin-echo (FTPGSE) self-diffusion experiments. The surfactant-to-oil ratio is kept constant, and the system is investigated as a function of water dilution and temperature. Particular attention is focused at the phase boundary where the microemulsion, consisting of normal oil swollen micelles is in equilibrium with excess oil. On this phase boundary, which occurs at lower temperatures, the oil-swollen micelles adopt a minimum size, and it is argued that this corresponds to a spherical shape. Increasing the temperature, the micelles grow in size. However, the results indicate that the micellar growth is only minor, in particular at lower concentrations. The implications of the experimental results and the phase equilibria are discussed within the frame work of the flexible surface model, associating a curvature energy to the surfactant film. The behaviour of the system is consistent with a monotonic variation of the spontaneous mean curvature of the surfactant monolayer with temperature. The phase equilibrium with excess oil can be identified with a so-called emulsification failure. Over a large dilution range, $\phi \leq 0.35$, the emulsification failure boundary occurs at constant temperature and surfactant-to-oil ratio, which, within the model, imposes a lower limit to the possible values of the monolayer bending rigidity