Concentration induced ordering of microemulsion droplets in bulk and near the liquid–air interface

Abstract

The structure of the air interface of water–AOT (sodium bis(2-ethyl hexyl) sulfosuccinate)–decane microemulsions in the droplet phase with ω = 20, 30, 40 is studied in a range of compositions with droplet volume fractions of ϕ = 0.2 to 0.7. A combination of small angle X-ray scattering (SAXS) and X-ray reflectometry (XR) reveals a detailed image of the morphology in the bulk and at the microemulsion–air interface. The structure of the volume is described in terms of effective structure factors to obtain the bulk droplet structure and correlations. With increasing droplet density ϕ the droplets remain unchanged in their size and show an increase in the local order. The reflectivity behaviour of the samples can be consistently explained by bulk small angle scattering and specular reflectivity at the surface. The microemulsion–air interface consists of droplets with the same ϕ-dependent local order as in the bulk, e.g. we found a droplet volume fraction induced ordering of droplets.