An attempt to detect bicontinuity from SANS data

Abstract

SANS is a powerful tool to characterise microemulsions, which can have a discontinuous droplet-like structure (oil in water (O/W), water in oil (W/O)) or a bicontinuous one. In the present study, we try to distinguish O/W, W/O and bicontinuous microemulsions by SANS measurements under practical conditions and by a certain evaluation technique. For this reason we chose the well characterised ternary system water-non-ionic surfactant (C12E5)-oil (n-octane), at a fixed surfactant concentration and performed SANS measurements throughout its one-phase channel where droplet-like phases as well as bicontinuous phases are well established. We evaluated the scattering data via the 'Generalised Indirect Fourier Transformation' method (GIFT) which is based on a particulate picture. It should therefore give good results in the droplet domains while a poor fit could be expected for the bicontinuous regime. For comparison we also applied the model of Teubner and Strey (TS) which was developed especially for bicontinuous phases, here a bad fit can be expected for the particulate regime. The data evaluation via GIFT leads to relatively good fits throughout the one-phase channel. The results are physically meaningful and are comparable to those of the TS model. We show that the scattering pattern of a bicontinuous microemulsion can be represented by that of a polydisperse particulate system. This is in clear contradiction to the expectation that the particle picture used in the GIFT method must fail when the bicontinuous regime is reached.