MULTICOMPONENT SELF-DIFFUSION NMR-STUDY OF ACIDIC MICROEMULSIONS AND AQUEOUS-SOLUTIONS OF DODECYLBENZENESULFONIC ACID

Molecular self-diffusion coefficients of oil, water, and surfactant in isotropic solutions from an acidic microemulsion system composed of dodecane, dodecylbenzenesulfonic acid, water, and HCl were determined by using Fourier transform pulsed-gradient spin-echo H-1 NMR spectroscopy. The large microstructural changes which occur to oil-in-water micelles upon addition of HCl are rationalized in terms of micellar growth in the low acid content region and in the intermediate acid content region intermicellar dynamics dominate diffusion behavior leading to bicontinuous solutions at high acid content. The effect of HCl on the acidic microemulsion is completely analogous to that of salt on ionic microemulsion systems, and microstructure studies are simplified since no cosurfactant is necessary. Bicontinuous solutions occur above an HCl/DBSA mole ratio of 2.7 even at low oil content. Line widths of DBSA phenyl H-1 NMR resonances reveal significant changes occurring at the bicontinuous micellar interface, such as a decrease in the mean curvature of the surfactant film, when the oil content is increased. Addition of oil to oil-in-water micelles causes a greater confinement of oil consistent with mediation of intermicellar oil migration by small micellar fragments. Water diffusion in micellar solutions clearly shows the micelles to have small obstructing volumes (i.e., ellipsoids shaped as long prolates or oblates with small axial ratio). Large micellar growth occurs, reaching a maximum apparent spherical radius of 70 nm, as surfactant or HCl is added to aqueous surfactant solutions. The relative amounts of configurational isomers comprising this double alkyl chain surfactant are determined by C-13 NMR spectroscopy.