Concentrated Winsor microemulsions : a small angle X-ray scattering study

Abstract

By small angle X-ray scattering, we have studied the structure of Winsor microemulsions, with brine and oil (toluene) proportions comparable and much larger than the surfactant (sodium dodecyl sulfate, SdS) and co-surfactant (butanol 1) proportions. Three systems were made. In the first (already studied in detail), the microemulsions are in equilibrium with an oil and/or water phase in excess, the microemulsion composition varies as a fonction of the brine salinity. The two other systems are monophases made with a brine of salinity 6.5 %. In the second system, we vary the oil volume fraction Φo from 20 to 90 % at constant SdS concentration (5 x 10-2 g/ml). In the third, we chose the component proportions such that the quantity ΦoΦw/cs (Φw : brine volume fraction, cs : SdS concentration) remains constant as Φo varies from 20 to 80 %. Thus one fixes the theoretical microemulsion characteristic length according to the expression $$ predicted by Talmon-Prager and de Gennes (Σ is the area per SdS polar head). 1) The SdS polar head contribution to the contrast and to the intensity scattered at large angle in the asymptotic domain is large. From this, we deduce a direct proof of the existence of a soap film between the oil and water parts of the microemulsion and also a method of measurement of the area per polar head (Σ ~ 60 Å2); 2) We measure the characteristic microemulsion size as a function of cs and Φo . The various scans suggest that, for Winsor microemulsions, starting from the inversion point (Φo = 0.5) and increasing the water or oil proportion, a progressive change occurs from a random bicontinuous water and oil structure $$ to a dispersion made of distinct oil in water or water in oil droplets (Φo < 0.3 or Φo > 0.7, ξ ∼ $$ or $$; 3) At very small angles (q ∼ 10-2 Å-1), we observe correlations or interaction effects not predicted by the existing bicontinuous microemulsion models