EMULSIFICATION FAILURE IN A TERNARY MICROEMULSION

The microstructure of the water rich microemulsion phase of the C12E5, water and decane system has been studied using surfactant H-2-NMR rel axation and H-1 Fourier transform pulsed gradient spin-echo (FTPGSE) s elf-diffusion experiments. The surfactant-to-oil ratio is kept constan t, and the system is investigated as a function of water dilution and temperature. Particular attention is focused at the phase boundary whe re the microemulsion, consisting of normal oil swollen micelles is in equilibrium with excess oil. On this phase boundary, which occurs at l ower temperatures, the oil-swollen micelles adopt a minimum size, and it is argued that this corresponds to a spherical shape. Increasing th e temperature, the micelles grow in size. However, the results indicat e that the micellar growth is only minor, in particular at lower conce ntrations. 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 beh aviour of the system is consistent with a monotonic variation of the s pontaneous mean curvature of the surfactant monolayer with temperature . The phase equilibrium with excess oil can be identified with a so-ca lled emulsification failure. Over a large dilution range, PHI less-tha n-or-equal-to 0.35, the emulsification failure boundary occurs at cons tant temperature and surfactant-to-oil ratio, which, within the model, imposes a lower limit to the possible values of the monolayer bending rigidity.