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

Abstract

The specifications of an NMR instrument dedicated to frequency-resolved automated Fourier transform pulsedfield-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 A.

Organization of Amphiphiles, Part VII: Effect of Variation of Composition and Salt on the Phase Behavior of Some Ethoxylated Surfactants

“…Transient water channels are formed when the surfactant interface breaks through merging droplets (Feldman et al 1996;Pitzalis et al 2000;Plaza et al 2002). The formation of random and momentary continuous water channels, therefore, explains the large increase in electrical conductivity (Datema et al 1992;Vollmer et al 1994;Lindman et al 1989). The three arms of the electrical conductivity plot are suggested due to structural transition from W/O microemulsion to O/W microemulsion through bicontinuous structure.…”

Organization of Amphiphiles, Part IX: Effect of Molecular Structure of Cosurfactants and Oils on the Phase Behavior of Tween-80: Alkanol-Oil-Water Systems

“…[34,[39][40][41] The formation of these random and momentary continuous water channels, results in the large increase in electrical conductivity. [42][43][44] With further increase of water an o=w microemulsion is formed due to encroachment of oil by large volume of water. Three arms of the electrical conductivity versus water plots are therefore suggested to be due to structural transition from w=o microemulsion to o=w microemulsion via bicontinuous structure.…”

Electrical Conductivity, Near-Infrared Absorption, and Thermal Lens Spectroscopic Studies of Percolation of Microemulsions

“…The formation of random and momentary continuous water channels would therefore explain the large increase in electrical conductivity. [7][8][9][10][11][12] Clearly, electrical conductivity measurements do not provide sufficient evidence to unambiguously characterize the structures of the microemulsions. 12,13 It is important, therefore, to develop new methods that can provide structural information of microemulsions.…”

“…For instance, ion hopping between micelles has been suggested as the cause for these phenomena . Fast material and water exchange when micelles coalesce may also be responsible for these. , Another possibility is the formation of bicontinuous structures in solution. − Bicontinuous structures are understood as having a dynamic nature where both water and oil are continuous phases and are separated by a small curvature surfactant film. The formation of random and momentary continuous water channels would therefore explain the large increase in electrical conductivity. − …”

“…5,6 Another possibility is the formation of bicontinuous structures in solution. [7][8][9][10][11] Bicontinuous structures are understood as having a dynamic nature where both water and oil are continuous phases and are separated by a small curvature surfactant film. The formation of random and momentary continuous water channels would therefore explain the large increase in electrical conductivity.…”