Browsing by Author "Laupheimer, Michaela"

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Open Access
Gelled bicontinuous microemulsions : a new type of orthogonal self-assembled systems
(2013) Laupheimer, Michaela; Stubenrauch, Cosima (Prof. Dr.)
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.
Open Access
Surfactant monolayer bending elasticity in lipase containing bicontinuous microemulsions
(2021) Engelskirchen, Sandra; Wellert, Stefan; Holderer, Olaf; Frielinghaus, Henrich; Laupheimer, Michaela; Richter, Sven; Nestl, Bettina; Nebel, Bernd; Hauer, Bernhard
Lipase-catalyzed reactions offer many advantages among which a high degree of selectivity combined with the possibility to convert even non-natural substrates are of particular interest. A major drawback in the applicability of lipases in the conversion of synthetically interesting, non-natural substrates is the substantial insolubility of such substrates in water. The conversion of substrates, natural or non-natural, by lipases generally involves the presence of a water-oil interface. In the present paper, we exploit the fact that the presence of lipases, in particular the lipase from Candida antarctica B (CalB), changes the bending elastic properties of a surfactant monolayer in a bicontinuous microemulsion consisting of D2O/NaCl -n-(d)-octane-pentaethylene glycol monodecyl ether (C10E5) in a similar manner as previously observed for amphiphilic block-copolymers. To determine the bending elastic constant, we have used two approaches, small angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy. The time-averaged structure from SANS showed a slight decrease in bending elasticity, while on nanosecond time scales as probed with NSE, a stiffening has been observed, which was attributed to adsorption/desorption mechanisms of CalB at the surfactant monolayer. The results allow to derive further information on the influence of CalB on the composition and bending elasticity of the surfactant monolayer itself as well as the underlying adsorption/desorption mechanism.