DOI: 10.1002/cphc.201200667 Structure and Solubility in Surfactant-Free Microemulsions Michael L. Klossek, [a] Didier Touraud, [a] Thomas Zemb, [a, b] and Werner Kunz* [a] Microemulsions are transparent, macroscopically homogene- ous, low-viscous one-phase systems consisting of a surfactant component in the form of a molecular film separating a polar from an apolar fluid. [1, 2] Four types of microemulsions were identified by Schulmann (Winsor I, II, III, IV). [3–6] Often, also a co- surfactant is necessary. Microemulsions separated by a rigid or fluid interfacial film have profound differences. [7] All these sys- tems are microscopically structured in terms of domains of well-defined size, named “swollen micelles”. They can be con- nected and lead to bi-continuous structures, understood as co- alesced droplets, connected cylinders or bi-liquid foams. Al- though they are excellent solubilizers, the drawback is that usually a high amount of surfactant (> 10 weight %) is required to achieve a single Winsor IV phase (microemulsions). Solubili- sation is related to Winsor I and Winsor III equilibria, where oil- rich or water-rich phases separate. In contrast, over the last three decades and among the more than 10 000 papers dealing with microemulsions, only few papers deal with so-called surfactant-free microemul- sions. [8–22] However, to the best of our knowledge, there is no direct experimental proof of the presence of domains with well-defined size below the optical microscopy resolution. In- stead, indirect hints were taken as arguments, for example, the fact that enzymes that normally require interfaces work well in such systems, [17–20, 22] or an unexpected salt specific solubility when used in hydrometallurgy. [23] In the present study, we demonstrate that the combination of static (SLS) and dynamic light scattering (DLS) delivers an unambiguous proof that, indeed, well-defined domains of two fluids of clearly different composition can exist in surfactant- free ternary mixtures. The motivation for this study was initially different. We wanted to understand the so-called Ouzo effect. [24–28] This effect is related to the finding of remarkable fine and time- stable emulsions when water is added above a certain content to a mixture of ethanol and a longer-chain molecule, such as anethole. It was found that the necessary condition for such a phenomenon is the rapid addition of water (or any other sol- vent) to a second solvent (e.g. ethanol), which is highly or en- tirely miscible with the first solvent, and a third component (e.g. anethole), that is highly soluble in the second solvent, but not in the first. During our study of the phase diagrams of such systems, we discovered that even before the phase separation through the addition of water to a mixture of components 2 and 3 intrigu- ing phenomena occur. In particular, close to the phase separa- tion border, but still within the monophasic region, a remark- able light-scattering signal is observed in the single-phase domain, appearing as a clear solution to the naked eye. Since this is not the well-known Ouzo effect in the two-phase region, which appears as a stable milky emulsion, we propose to call this effect “pre-Ouzo” effect, expressing its occurrence before enough water is added to induce the phase separation. Figure 1 presents the phase diagram of the ternary system water/ethanol/n-octanol and selected self-correlation functions from DLS with increasing water content. As can be seen in Fig- ure 1 b the correlations become more and more pronounced Figure 1. a) Ternary phase diagram of water/ethanol/n-octanol mixtures at 25 8C. The white region represents compositions of monophasic, clear, mac- roscopically homogeneous mixtures (pre-Ouzo region), the dark region com- positions of two-phase systems (Ouzo region). Since the tie-lines point to- wards the water-corner, the grey area is a Winsor II equilibrium, if the pre- Ouzo region is a structured microemulsion. b) Time-dependent self-correla- tion functions obtained by DLS. The symbols correspond to the composi- tions in (a). [a] M.L. Klossek, Dr. D. Touraud, Prof. Dr. T. Zemb, Prof. Dr. W. Kunz Institute of Physical and Theoretical Chemistry University of Regensburg 93040 Regensburg (Germany) E-mail : werner.kunz@chemie.uni-regensburg.de [b] Prof. Dr. T. Zemb Commissariat à l’Ønergie atomique et aux Ønergies alternatives (CEA) L’Institut de Chimie SØparative de Marcoule (ICSM) UMR 5257 CEA/CNRS/UM2/ENSCM F-30207 Bagnols sur CØze (France) 4116  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2012, 13, 4116 – 4119