An attempt to detect bicontinuity from SANS data

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 (C 12E 5)–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. Scattering experiments (SANS) cannot differentiate between particulate and bicontinuous microemulsions using GIFT evaluation technique.