Michaela Laupheimer

Studying orthogonal self-assembled systems: phase behaviour and rheology of gelled microemulsions

Microemulsions in a gelled form are desirable for applications like topical and transdermal drug delivery as they enable local application and enhanced residence times of the drug. One method for gelling microemulsions consists of adding an appropriate low molecular weight gelator. Although our group has reported this method before, a systematic study on the nature and properties of such gelled microemulsions has been missing to date. In this paper we present phase studies and rheology results which demonstrate that microemulsions gelled by a low molecular weight gelator are orthogonal self-assembled systems. We studied the gelled microemulsion H2O–n-decane/12-hydroxyoctadecanoic acid (12-HOA)–tetraethylene glycol monodecyl ether (C10E4) with 1.5 wt%, 2.5 wt% and 5.0 wt% of the low molecular weight organogelator 12-HOA. We found that the phase boundaries of the gelled microemulsion are about 6 K below those of the non-gelled “base” microemulsion H2O–n-decane–C10E4, irrespective of the gelator concentration. Moreover, we detected by differential scanning calorimetry and rheological measurements a sol–gel boundary about 20 K below that of the respective binary gel n-decane/12-HOA. Both temperature shifts are not surprising considering that (a) 12-HOA is surface active, thus influencing the microemulsions phase behaviour and (b) that the microemulsion can be treated as a solvent in a 12-HOA gel which influences the gel properties. The general phase behaviour of both base systems, however, is indeed maintained in the gelled microemulsion. For the rheological properties we found, accordingly, perfect agreement with the respective binary gel. Thus, our data clearly substantiate that gelled microemulsions are orthogonal self-assembled systems.

Bicontinuous Microemulsion as Reaction Medium for ω-Transaminase Catalysed Biotransformations

Abstract Due to their high enantioselectivity biotransformations, i.e. enzyme-catalysed conversion of organic compounds, are extremely attractive reactions. However, a limiting factor for choosing substrates is the enzyme-substrate incompatibility. This occurs when a hydrophilic enzyme which naturally resides in the aqueous cell cytoplasm is supposed to convert a hydrophobic substrate. In this context bicontinuous microemulsions appear to be a beneficial reaction medium for biotransformations, particularly due to their large interfacial area between a hydrophilic and a hydrophobic compound. As a “proof of concept” we performed ω-transaminase (EC 2.6.1.18) catalysed model reactions in a bicontinuous microemulsion of the type phosphate buffer/NaCl – n-octane – pentaethylene glycol monodecyl ether.

Gelled Lyotropic Liquid Crystals

In our previous work we were able to prove that gelled bicontinuous microemulsions are a novel type of orthogonal self-assembled system. The study at hand aims at complementing our previous work by answering the question of whether gelled lyotropic liquid crystals are also orthogonal self-assembled systems. For this purpose we studied the same system, namely, water-n-decane/12-hydroxyoctadecanoic acid (12-HOA)-n-decyl tetraoxyethylene glycol ether (C10E4). The phase boundaries of the nongelled and the gelled lyotropic liquid crystals were determined visually and with (2)H NMR spectroscopy. Oscillating shear measurements revealed that the absolute values of the storage and loss moduli of the gelled liquid crystalline (LC) phases do not differ very much from those of the binary organogel. While both the phase behavior and the rheological properties of the LC phases support the hypothesis that gelled lyotropic liquid crystals are orthogonal self-assembled systems, freeze-fracture electron microscopy (FFEM) seems to indicate an influence of the gel network on the structure of the Lα phase and vice versa.

Gelled Bicontinuous Microemulsions

In this work a new type of orthogonal self-assembled systems, namely a gelled bicontinuous microemulsion, was investigated with a set of complementary physico-chemical methods. Orthogonal self-assembly means that different structures self-assemble simultaneously in a system and coexist independently. In the chosen model system H2O – n-decane / 12-hydroxyoctadecanoic acid (12-HOA) – tetraethylene glycol monodecyl ether (C10E4) the organogelator 12-HOA forms a network which is surrounded by bicontinuous microemulsion domains. This was proved by comparing characteristic properties and the microstructure of the gelled bicontinuous microemulsion with those of the two ‘base systems’, i.e. the non-gelled bicontinuous microemulsion H2O – n-decane – C10E4 and the binary gel n-decane / 12-HOA. Firstly, phase studies were carried out which showed that the microemulsion phase boundaries are maintained upon gelation, merely shifted by about 6 K to lower temperatures. Likewise, a sol-gel transition occurs in the gelled microemulsion just as in the binary gel. Differential scanning calorimetry and temperature-dependent oscillating shear rheometry measurements revealed that the sol-gel transition temperature is about 20 K lower when a microemulsion, instead of pure n-decane, surrounds the gelator network. This reflects that part of the surface-active 12-HOA molecules adsorb at the water-oil interface instead of forming gelator fibers when a microemulsion is present. Accordingly, studying the linear viscoelastic range and the frequency-dependence of the storage and the loss modulus it was found that the gelator network is somewhat weaker in the gelled bicontinuous microemulsion than in the binary gel, although both systems are strong gels. In the following the focus turned to the microstructure of the gelled bicontinuous microemulsion. To begin with, the bicontinuity of the microemulsion domains in the middle of the one-phase region was verified determining the relative self-diffusion coefficients of water and n-decane with Fourier transform pulsed-gradient spin-echo 1H-NMR measurements. Subsequently, the coexistence of the bicontinuous microemulsion domains and the gelator network in the gelled bicontinuous microemulsion was evidenced by means of small angle neutron scattering. Finally, a visualization of the coexisting microstructures with freeze-fracture transmission electron microscopy complemented the work. In dieser Arbeit wurde ein neuer Typ Orthogonal-Selbstorganisierter Systeme, namlich eine gelierte bikontinuierliche Mikroemulsion, mit einer Reihe komplementarer physikochemischer Methoden untersucht. Orthogonale Selbstorganisation bedeutet, dass sich verschiedene Strukturen in einem System in simultanen Selbstorganisationsprozessen bilden und unabhangig voneinander koexistieren. Im gewahlten Modellsystem H2O – n-Dekan / 12-Hydroxyoktadekansaure (12-HOA) – Tetraethylenglycolmonodecylether (C10E4) bildet der Organogelator 12-HOA ein Netzwerk, das von bikontinuierlichen Mikroemulsionsdomanen umgeben ist. Dies wurde nachgewiesen, indem charakteristische Eigenschaften und die Mikrostruktur der gelierten bikontinuierlichen Mikroemulsion mit denen der zwei „Basissysteme“, der ungelierten bikontinuierlichen Mikroemulsion H2O – n-Dekan – C10E4 und des binaren Gels n-Dekan / 12-HOA, verglichen wurden. Zunachst zeigte eine Phasenstudie, dass die Mikroemulsionsphasengrenzen bei der Gelierung erhalten bleiben. Sie verschieben sich lediglich um etwa 6 K zu tieferen Temperaturen. Ebenso findet in der gelierten Mikroemulsion, ganz wie im binaren Gel, ein Sol-Gel-Ubergang statt. Laut Ergebnissen der dynamischen Differenzkalorimetrie und temperaturabhangiger Oszillationsmessungen am Scherrheometer ist die Sol-Gel-Temperatur um etwa 20 K erniedrigt, wenn das Gelatornetzwerk von einer Mikroemulsion anstatt von reinem n-Dekan umgeben ist. Dies lasst sich darauf zuruckfuhren, dass ein Teil der grenzflachenaktiven 12-HOA-Molekule in Anwesenheit einer Mikroemulsion an die Wasser-Ol-Grenzflache adsorbiert anstatt Gelatorfibrillen zu bilden. Entsprechend zeigte sich auch bei der Untersuchung des linear-viskoelastischen Bereichs und der Frequenzabhangigkeit von Speicher- und Verlustmodul, dass das Gelatornetzwerk in der bikontinuierlichen Mikroemulsion etwas schwacher ist als im binaren Gel, obgleich beide Systeme starke Gele sind. Im Folgenden wurde der Fokus auf die Mikrostruktur der gelierten bikontinuierlichen Mikroemulsion gelenkt. Als Erstes wurde die Bikontinuitat der Mikroemulsionsdomanen in der Mitte des Einphasengebiets bestatigt, indem die relativen Selbstdiffusionskoeffizienten von Wasser und n-Dekan durch Fourier-transformierte Spinecho-1H-NMR-Messungen mit Magnetfeldgradientenpulsen bestimmt wurden. Anschliesend wurde die Koexistenz der bikontinuierlichen Mikroemulsionsdomanen und des Gelatornetzwerks in der gelierten bikontinuierlichen Mikroemulsion durch Kleinwinkelneutronenstreuexperimente nachgewiesen. Die Visualisierung der koexistierenden Mikrostrukturen durch Gefrierbruch-Transmissionselektronenmikroskopie komplettierte schlieslich die Arbeit.

Transmission Measurements as Tool to Study Phase Transitions of Liquid Mixtures

Abstract We present a quantitative method to determine the phase transition temperatures between one-phase and two-phase regions of multi-component liquid mixtures via temperature-dependent transmission measurements with an UV/Vis spectrometer. The method is based on the fact that multi-phase samples are turbid, while one-phase samples are transparent. We describe the method in detail and discuss the choice of the experimental parameters (wavelength, sample layer thickness), a suitable temperature program as well as the data analysis. We prove the validity of our method by measuring the phase diagrams of two model systems, namely a liquid and a gelled microemulsion. The results are in good agreement with those obtained with the conventional visual method used for phase studies.

Microstructure of Gelled Bicontinuous Microemulsions

To verify the orthogonal self-assembled character of gelled bicontinuous microemulsions it is important to study not only their phase behaviour and rheological properties (see Sect.4.3) but also their microstructure. In orthogonal self-assembled systems the microstructures of the base systems are maintained and coexist with each other. Thus in case of gelled bicontinuous microemulsions one expects bicontinuous microemulsion domains to coexist with a three-dimensional gelator network. In order to prove this assumption for the system H2O–n-decane/12-HOA–C10E4 the microstructure was investigated with different methods. Those methods were also applied to the base systems, i.e. the non-gelled bicontinuous microemulsion H2O–n-decane–C10E4 and the binary gel n-decane/12-HOA, to be able to disclose the structural similarities. First, the gelled and the non-gelled microemulsion were studied with 1H-NMR self-diffusion measurements. Those revealed that the microemulsion domains are indeed bicontinuous at the \(\widetilde{T}\) temperature, irrespective of the presence of the gelator network (Sect. 4.1). The coexistence of the bicontinuous microemulsion with the network of 12-HOA fibers was subsequently demonstrated with small angle neutron scattering (SANS, Sect. 4.2) and freeze-fracture transmission electron microscopy (FFEM, Sect. 4.3). The scattering curves of the gelled bicontinuous microemulsion comprise the characteristic scattering patterns of both base systems and the FFEM pictures visualize the coexisting microstructures.

Phase Behaviour and Rheology of Gelled Microemulsions

In order to investigate the orthogonal self-assembled character of gelled bicontinuous microemulsions the first and fundamental step was to identify under which conditions a mixture of water, n-decane, C10E4 and 12-HOA forms the desired system. Thus it was crucial to carry out phase studies. On the one hand, a microemulsion is known to be bicontinuous at its \(\widetilde{T}\) temperature.

Colloid and Interfacial Chemistry at Stuttgart University

Abstract The research work carried out in our group can be referred to as “Colloid and Interfacial Chemistry”. We subdivide this rather broad research area into four main topics which are covered by the projects presented in this overview. The surfaces we study are surfactant-loaden water-air surfaces, the films are mainly free-standing thin foam films of less than 100 nm thickness, and the foams are 3D aqueous foams whose stability and drainage we investigate. As regards the topic “Complex Fluids” we study lyotropic liquid crystalline phases and microemulsions. In the past, we were able to establish two new tuning parameters for the formation and destruction of lyotropic liquid crystals, while current research focuses on the lyotropic mesomorphism of new surfactants and of surfactant mixtures. Apart from lyotropic liquid crystals microemulsions are a central theme in the group. Due to their unique properties and fascinating structure variety microemulsions offer a great potential as templates for the synthesis of new functional materials, which is a further research topic in our group. These studies involve the gelation of and the polymerisation in microemulsions preserving their nanostructure to create high surface area polymers. Currently, we also use microemulsions as tailor-made nano-compartmented reaction media. The studied reactions are either enzyme-catalysed conversions of substrates or the reduction of metal salts to synthesize mono- or bimetallic nanoparticles. In this context we focus on bicontinuous and water-in-oil droplet microemulsions. Last but not least we also synthesize new surfactant structures such as inositol-based surfactants and explore the properties.