Measurements of interfacial curvatures and evidence for morphological transition in semifluorinated non-isometric microemulsions

We have studied the phase behavior and the associated microstructures of microemulsions formed by ternary mixtures of perfluorooctanoic acid (PFOA), water (H2O) and perfluorooctane (PFO). In this paper, we report the study of a series of non-isometric microemulsions at 20 wt% of PFOA with varying water to oil volume ratios. Analysis of small-angle neutron-scattering intensity distributions, with a clipped random wave (CRW) model having a specific spectral density function, containing three length scales, provides a detailed description of the microstructures of one-phase, non-isometric microemulsions. Specifically, we were able to deduce from the analysis the average mean, Gaussian, and square mean curvatures of the oil-water interface in these non-isometric bicontinuous microemulsions. The CRW model also gives a statistically consistent three-dimensional view of the interfacial morphology for each set of the three length scales. From the latter, we find evidence for morphological transformations (bicontinuous to cylindrical to globular) occurring within the one-phase channel, as the oil-to-water volume fraction ratio is varied.