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Methods and Techniques This doctoral thesis was made out at the Institute of Physical and Theoretical Chemistry, University of Regensburg, between November 2012 and October 2015 and under the supervision of Prof. Dr. Werner Kunz. I would like to thank many people for their help and support throughout these years. First of all, I would like to thank Prof. Dr. Werner Kunz for welcoming me and giving me the opportunity to prepare my thesis at his institute. Thank you for the numerous ideas, advices and discussions and for the time you invested to follow my work. Thank you also for this interesting topic that I had the chance to present at many conferences. I also express my gratitude to Dr. Didier Touraud for his great help, his availability and his amazing ideas. Thank you for all the corrections and advices regarding the publications, and for taking the time to discuss any subject with me. Merci! I deeply thank Prof. Dr. Thomas Zemb and Dr. Olivier Diat at the Institut de Chimie Séparative de Marcoule, France, for their help, explanations and guidance regarding the small- and wide- angle x-ray scattering and the small-angle neutron scattering techniques. Many thanks for the interest you gave to my work. Thanks to PD Dr. Rainer Müller and Prof. Dr. Richard Buchner for letting me use their equipment. Special thanks to Rosi and Sonja for their help, their kindness, and their good mood every day! I would like to thank all my colleagues for all the scientific help and especially for the fantastic atmosphere at the institute. My thanks go to Auriane, Vroni, Theresa, Alex, Lydia, Andreas N., Stefan, Andreas S., Käthe, Claudia, Damian, Marcel, Tobi, Franzi, Manuel, Julian, Sebastian, and Gabriel. I really spent an amazing time with you, thank you for the fun and all the events! I thank also Michael, Vroni, Susanne and Andi for the warm welcome when I arrived at the institute. I would like to thank the members of the Kaffeerunde, i.e. Roland, Vroni, Evi, Manuel, Georg, Richard, Julian, Andreas N., Auriane, Hellmuth and Rainer for the good moments at lunchtime and for what I learned speaking with them, especially about the German culture. My thanks go also to the students Maximilian Pleines, Johannes Ramsauer, Johannes Mehringer, Michael Ludwig, Dea Zager, Sebastian Haumann, Alexandra Halemba, Christa Genslein, Elise Lambert, Marine Reinaud, and Erwan Le Goué for their help with some experiments of this work. Many thanks to all my friends that I met in Germany and that made me want to stay in Bavaria. I thank Sarah, Tom, Stefan, Vroni, Helene, Christian, Beate, and Michael, Andreas, Timm, Matthias, Vroni, Jörn, Andre, Christoph, Dino, Daniel, Robert and Kevin for all the fun, holidays and adventures over the past years that made this time possible to endure. My thanks go also to my WG, i.e. to Roxi, Ludwig, Maria and Laura, for the amazing atmosphere in the house and the best evenings. J’aimerais également remercier mes parents, Alain and Catherine Marcus, et ma sœur, Cécile. Je vous remercie infiniment pour toute votre aide, pour vos visites et pour m’avoir soutenu dans toutes les décisions que j’ai prises depuis le début de mes études. Je n’en serais certainement pas là sans vous. Finally, I owe my dearest thanks to my partner, Tobias Weigl, for his support and encouragement, for believing in me and making my new life in Germany so marvellous. This thesis deals with the study of microemulsions and is composed of two main parts. In the first part, surfactant-free microemulsions are studied, whereas in the second part microemulsions with surfactants and cosurfactants are investigated. Over the last few years, surfactant-free microemulsions became a major topic at our institute and were thoroughly studied using the reference system water/ethanol/1-octanol. As explained later in the Fundamentals part (see section 1.2), fluctuating nano-structures occur in such surfactant-free microemulsions in the pre-Ouzo region, i.e. just before enough water is added to reach the multiphasic domain. They are composed of an organic-rich core surrounded by a water-rich bulk phase with a slight accumulation of ethanol at the interface. The first work presented in this thesis concerns the influence of salts and of the surfactants sodium dodecylsulfate (SDS) and sodium diethylhexylphosphate (NaDEHP) on these nano- structures (developed in section 3.1.1). It was performed using dynamic light scattering (DLS), small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS). It was found that electrolytes either exhibit a salting-out effect towards ethanol, more or less strongly depending on the ion, or produce charge separation via the antagonistic ion effect described by Onuki et al. Amphiphilic electrolytes, such as SDS or NaDEHP, induce a gradual transition toward monodisperse ionic micelles with ethanol playing the role of a cosurfactant. Secondly, water-free and surfactant-free microemulsions were studied using DLS and SAXS (see section 3.1.2). The system glycerol/ethanol/1-octanol was studied, as well as the systems deep eutectic solvent (DES)/tetrahydrofurfuryl alcohol/diethyl adipate. Fluctuating nano-structures were also found in the pre-Ouzo region of these systems and existed in addition to molecular critical fluctuations which can appear near a critical point. This work proves that i) microemulsions can be formulated using DES instead of water, and ii) the amphiphilic character of 1-octanol is not responsible of these fluctuations. The last two studies are examples of formulations where such nano-structures occur. Using perfumery molecules and DLS, it was shown that these aggregates can also be found in perfume tinctures (see section 3.1.3). From the model formulations, nano-droplets can be predicted in Eau de Toilette, Eau de Parfum and possibly in perfumes in presence of very hydrophobic fragrance molecules. These nano-structures may have a significant influence on the performance of the perfume and on the fragrance evaporation. Using practically all the natural and synthetic repellent molecules that are commercially used to formulate mosquito repellents, it was shown that nano- structures can also be found in the pre-Ouzo region of hydro-alcoholic mosquito repellent formulations (developed in section 3.1.4). These structures can thus be found in products available on the market and may have an impact on the behaviour of such products on the skin. The two studies presented in the second main part deal with a specific oil-in-water microemulsion. Its composition is 75 wt% of water, 10 wt% of sodium oleate, 9.25 wt% of ethanol, 0.75 wt% of citronellol, and 5 wt% of limonene. The microemulsion is interesting because it is highly water dilutable and can be used as perfume or aroma carriers. The first work aims at investigating the influences of ethanol, citronellol, limonene, salts, and polyols on the temperature stability of the microemulsion and on the apparent pKA (apKA) of its surfactant (see section 3.2.1). Starting from the mixture water/sodium oleate, the apKA decreases with an increase of temperature or by adding ethanol. A small amount of citronellol does not influence the apKA, but the latter increases after the addition of limonene to form the final microemulsion, as uncharged oleic acid can be solubilised in the organic core. The anions have no specific influence on the evolution of the clearing temperature that follows electrostatic interactions, whereas the specificity of the cations is linked to their capacity to exchange with the original surfactant counterion. The second work aims at investigating the influences of artificial and natural high intensity sweeteners and sugar alcohols on the clearing temperature of the microemulsion and its apKA (see section 3.2.2). The high intensity sweeteners used in this study exhibit different behaviours: saccharin and acesulfame K act like electrolytes and dehydrate the interfacial film of the droplets, whereas neotame, stevia and neohesperidin DC act like (co)surfactants. Aspartame exhibits a precipitation behaviour, whereas cyclamate and sucralose interact with the interfacial film without penetrating it. Note that sucralose allows the formulation of a beverage microemulsion with a fatty acid salt as surfactant at nearly neutral pH. The sugar alcohols sucrose, sorbitol and isomalt seem to act like osmolytes and play indirectly on the interfacial film of the microemulsion nano- droplets by pushing the ethanol towards them. To sum up, the studies of the surfactant-free microemulsions help us to better understanding their structures and behaviours in presence of different additives. It is also interesting to note that they are already used in some applications. The studies of the microemulsions with a fatty acid salt as surfactant help us understanding the influences of salts and sweeteners on such a surfactant and the important role of ethanol. Diese Doktorarbeit handelt über Mikroemulsionen und besteht aus zwei Hauptteilen. Im ersten Teil werden tensidfreie Mikroemulsionen untersucht, während Mikroemulsionen mit Tensiden und Cotensiden im zweiten Teil betrachtet werden. In unserem Institut sind in den letzten Jahren tensidfreie Mikroemulsionen ein wichtiges Thema geworden und wurden eingehend mit dem Referenzsystem Wasser/Ethanol/1-Oktanol untersucht. Wie später im Fundamentals-Teil erklärt (siehe Absatz 1.2), befinden sich im pre-Ouzo Bereich dieser tensidfreien Mikroemulsionen fluktuierende Nanostrukturen. Dieser Bereich existiert bei einer geringen Wassermenge, bevor das mehrphasige System erreicht wird. Die Nanostrukturen bestehen aus einem von einer wasserreichen Phase umgegebenen Kern aus zum großen Teil organischen Molekülen mit einem Konzentrationspeak von Ethanolmolekülen an der Grenzfläche. Der erste Teil dieser Doktorarbeit behandelt die Einflüsse von Salzen und den Tensiden Natriumdodecylsulfat (SDS) und Natriumdiethylhexylphosphat (NaDEHP) auf diese Nanostrukturen (siehe Absatz 3.1.1). Die Einflüsse wurden mit Hilfe von dynamischer Lichtstreuung (DLS), Kleinwinkel-Röntgenstreuung (SAXS) und Kleinwinkel-Neutronenstreuung (SANS) untersucht.
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