Effect of water content on structural and phase behavior of water-in-oil (n-decane) microemulsion system stabilized by mixed nonionic surfactants SPAN 80/TWEEN 80
2017
Koneva, A.S. | Safonova, E.A. | Kondrakhina, P.S. | Vovk, M.A. | Lezov, A.A. | Chernyshev, Yu. S. | Smirnova, N.A.
For the search of effective microemulsion systems with the properties demanded for various applications the knowledge of structural properties of the reverse microemulsions for various compositions and temperatures is of high value. In the present study, the microstructure and diffusion of the components of water-in-n-decane Winsor IV microemulsions stabilized by a mixture of nonionic surfactants Span 80 (sorbitane monooleate, HLB 4.9) and Tween 80 (polyoxyethylene sorbitan monooleate, HLB 15.0) at the weight ratios 49:51 (i.e. at the maximal ability to solubilize water) were investigated by viscosimetry, PGSTE NMR and DLS. The system phase diagram obtained at 298.2K includes the intermediate heterogeneous region between the reverse micelles region and the microemulsions. The water-to-surfactant molar ratio was changed up to 15.5 (13wt.% of water) at the weight ratios n-decane: surfactants 65:35 and 85:15. The microemulsion droplets are spherical (that is corresponded also to the cryo-TEM data) and grow with the increase of water content. The temperature effect on the aggregates diffusion (and on the aggregates size) in the range 298.2–318.2 K is not pronounced. The bimodal distributions of the diffusion coefficients determined using both DLS and PGSTE NMR methods are specific for the system. The slower diffusion mode relates to the diffusion of mixed microemulsion droplets with the radii Rh ∼10–20nm. The second stable form of nanoaggregates are “dry” micelles with Rh ∼1–2nm. The data on the diffusion coefficients are of special interest in particular for the molecular dynamic simulation of the reverse micelles or of the microemulsion droplets.
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