A.S. Koneva, E.A. Safonova, P.S. Kondrakhina, M.A. Vovk, A.A. Lezov, Yu.S. Chernyshev, N.A. Smirnova
“Effect of water content on structural and phase behavior ofwater-in-oil (n-decane) microemulsion system stabilized by mixednonionic surfactants SPAN 80/TWEEN 80”
For the search of effective microemulsion systems with the properties demanded for various applica-tions the knowledge of structural properties of the reverse microemulsions for various compositionsand temperatures is of high value. In the present study, the microstructure and diffusion of the compo-nents of water-in-n-decane Winsor IV microemulsions stabilized by a mixture of nonionic surfactantsSpan 80 (sorbitane monooleate, HLB 4.9) and Tween 80 (polyoxyethylene sorbitan monooleate, HLB15.0) at the weight ratios 49:51 (i.e. at the maximal ability to solubilize water) were investigatedby viscosimetry, PGSTE NMR and DLS. The system phase diagram obtained at 298.2 K includes theintermediate heterogeneous region between the reverse micelles region and the microemulsions. Thewater-to-surfactant molar ratio was changed up to 15.5 (13 wt.% of water) at the weight ratios n-decane:surfactants 65:35 and 85:15. The microemulsion droplets are spherical (that is corresponded also to thecryo-TEM data) and grow with the increase of water content. The temperature effect on the aggregatesdiffusion (and on the aggregates size) in the range 298.2–318.2 K is not pronounced. The bimodal distri-butions 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 theradii Rh ∼10–20 nm. The second stable form of nanoaggregates are “dry” micelles with Rh ∼ 1–2 nm. Thedata on the diffusion coefficients are of special interest in particular for the molecular dynamic simulationof the reverse micelles or of the microemulsion droplets.