Demonstration of bicontinuous structures in microemulsions using automatic‐mode NMR (self‐)diffusion measurements

Abstract

AbstractThe specifications of an NMR instrument dedicated to frequency‐resolved automated Fourier transform pulsed‐field‐gradient proton NMR measurements are reported. In order to be able also to monitor accurately the slow diffusion of surfactant aggregates and polymers in automatic mode, an option for matching of the gradient pulses was developed and implemented. In a first application, the spectrometer was used to carry out NMR (self‐) diffusion measurements of microemulsions of the surfactant sodium p‐(7‐tetradecyl)benzenesulphonate in benzene and water. They indicate drastic changes in the morphology of the aggregates in the microemulsion on increasing the surfactant concentration. The aggregate morphology changes from a bicontinuous structure to hard, noninteracting spheres by increasing the surfactant concentration in the microemulsion. The bicontinuous structure, i.e. a solution which is continuous in both water and oil, is the result of a strong interaction between the microemulsion droplets (‘percolation phenomenon’). It appears that the water molecules inside the microemulsion droplets with the hard sphere character exchange rapidly, on the (NMR) time scale of 10–100 ms, with water which is dissolved in the benzene. The droplets have an apparent diameter of 56 Å.