Acidic microemulsions of dodecane/ p-dodecylbenzenesulfonic acid/water/HCl part 2. structure by proton NMR self-diffusion and relaxation studies

Abstract Two isotropic regions, S (surfactant phase) and L 2 , exist in the ternary phase diagrams for the title system formed with 10 and 11 % (w/w) HCl. The microemulsions present within these regions were investigated by proton Fourier transform pulsed-gradient spin-echo NMR spectroscopy and aromatic linewidth measurements. Self-diffusion coefficients of dodecane and water reveal a constant bicontinous microstructure across the S region with oil effectively diffusing in one dimension. Transverse relaxation rates of surfactant aromatic protons increase significantly with an increase in oil content, which is accompanied by a decrease in HCl content, across the S region. This is discussed in terms of changes in monomer packing. A water droplet-in-oil micro-structure is present in L 2 solutions with low amounts of surfactant while an extended layered structure occurs at higher concentrations of surfactant.