- [Show abstract] [Hide abstract] ABSTRACT: Microemulsions (μe) with water and a hydrophobic ionic liquid (IL) usually require 45-60 wt% surfactant to solubilize equal amounts of water and IL. To increase the efficiency we designed a new class of surfactants by combining a hydrophilic but IL-ophobic carbohydrate-based part with a hydrophobic but IL-ophilic IL-based part. These surfactants allow formulating microemulsions with 20 wt% surfactant only which opens up a new arena for efficient water-IL μes.
- [Show abstract] [Hide abstract] ABSTRACT: The properties of polymer foams are tightly linked to the porous structure of the material: two of the most decisive parameters are the interconnectivity of the pores and the thickness of the pore walls. Despite the vital importance of these parameters, a deep understanding of the processes that control the wall thickness and the pore opening is still lacking. We tackle these questions by studying monodisperse, highly ordered polymer foams which are generated via emulsion templating using microfluidic lab-on-a-chip techniques. We explore the influence of different processing parameters, and we show that the most crucial parameter is the locus where the polymerization is initiated. If initiation starts within the continuous monomer matrix, the morphology of the liquid template is “frozen in” with pore openings arising where neighboring drops are separated by thin films. However, if the locus of initiation is at the interface, not only do the pores remain closed, but we evidence a hitherto unexplained mechanism which leads to an osmotically driven redistribution of monomer in the walls during polymerization. This changes dramatically the pore morphology (polyhedral pores with thick walls) and therefore the final material properties—opening the pathway to new applications of low weight or acoustic bandgap materials.
- [Show abstract] [Hide abstract] ABSTRACT: Lactic acid is a promising biogenic platform chemical which can be produced by fermentation of cellulose and hemicellulose. However, separating lactic acid from the fermentation broth is extremely costly and technically complex. We therefore investigated whether liquid/liquid extraction of lactic acid with tri-n-octylamine is a cost-effective alternative to the existing downstream processing method. In order to find an answer to this question, the structure of the middle phase of the occurring three-phase region, which is enriched with up to 20 wt. % lactic acid, was explored. The results of our IR, small-angle X-ray scattering and NMR measurements show that this phase is ionic and has a bicontinuous structure. Due to the analogy with bicontinuous microemulsions, it should be possible to further enrich the lactic acid, which could lead to a rethink regarding the design of extraction processes.
- [Show abstract] [Hide abstract] ABSTRACT: This review summarizes everything we currently know about the nonionic surfactants alkyl dimethyl (CnDMPO) and alkyl diethyl (CnDEPO) phosphine oxide (PO surfactants). The review starts with the synthesis and the general properties (Section 2) of these compounds and continues with their interfacial properties (Section 3) such as surface tension, surface rheology, interfacial tension and adsorption at solid surfaces. We discuss studies on thin liquid films and foams stabilized by PO surfactants (Section 4) as well as studies on their self-assembly into lyotropic liquid crystals and microemulsions, respectively (Section 5). We aim at encouraging colleagues from both academia and industry to take on board PO surfactants whenever possible and feasible because of their broad variety of excellent properties.
- [Show abstract] [Hide abstract] ABSTRACT: Gelierte komplexe Fluide sind weiche Materialien, in denen die Mikrostruktur des komplexen Fluids mit der mechanischen Stabilität eines Gels verbunden wird. Um ein geliertes komplexes Fluid zu erhalten, fügt man entweder einen Gelator zu einem komplexen Fluid hinzu, oder man ersetzt das in einem Gel enthaltene Lösungsmittel durch ein komplexes Fluid. Das bekannteste Beispiel eines “natürlichen” gelierten komplexen Fluids ist die Zelle. Es gibt verschiedene Strategien, mit denen man ein geliertes komplexes Fluid herstellen kann; eine dieser Strategien ist die orthogonale Selbstorganisation, also die unabhängige, aber gleichzeitige Bildung von zwei koexistierenden, selbstorganisierten Strukturen innerhalb eines Systems. Der vorliegende Aufsatz zielt darauf ab, die Struktur sowie potenzielle Anwendungen verschiedener gelierter komplexer Fluide zu beschreiben und abzuklären, ob das entsprechende System durch orthogonale Selbstorganisation entsteht.
- [Show abstract] [Hide abstract] ABSTRACT: Gelled complex fluids are soft materials in which the microstructure of the complex fluid is combined with the mechanical stability of a gel. To obtain a gelled complex fluid one either adds a gelator to a complex fluid or replaces the solvent in a gel by a complex fluid. The most prominent example of a “natural” gelled complex fluid is the cell. There are various strategies by which one can form a gelled complex fluid; one such strategy is orthogonal self-assembly, that is, the independent but simultaneous formation of two coexisting self-assembled structures within one system. The aim of this Review is to describe the structure and potential applications of various man-made gelled complex fluids and to clarify whether or not the respective system is formed by orthogonal self-assembly.
- [Show abstract] [Hide abstract] ABSTRACT: With an appropriate tuning parameter a microemulsion can be forced to undergo a phase inversion, i.e. to invert the curvature of the amphiphilic film. In microemulsions consisting of water, an n-alkane and a nonionic alkyl polyglycolether (CiEj) temperature is an appropriate parameter since the surfactant head groups dehydrate with increasing temperature. Recently we were able to show that this also holds true for ethylammonium nitrate (EAN) – n-alkane – CiEj systems. However, the temperature sensitivity is weaker which is why we decided to use a hydrophobic cosurfactant to induce a phase inversion in the EAN-containing microemulsion. We studied the quaternary system EAN − n-octane − C12E3 − 1-octanol as a function of the alcohol concentration at T = 15 °C and found the same trends as for the aqueous counterparts. (1) Studying the phase behavior as a function of the alcohol content (δ) and of the total surfactant concentration (γ), one finds that the phase boundaries resemble the shape of a fish. (2) Measuring the interfacial tension σ between the IL- and oil-rich phase, one observes a minimum in the center of the three-phase region. (3) NMR self-diffusion measurements reveal a transition from an oil-in-EAN microemulsion to a bicontinuous microemulsion and finally to an EAN-in-oil microemulsion.
- [Show abstract] [Hide abstract] ABSTRACT: Micelles are increasingly used as carrier systems. In previous studies, we have shown that the blockcopolymers PEO13-PPO30-PEO13 and αs-casein form micelles at distinct temperature and concentration ranges. The aim of the study at hand was to determine the thermodynamics of this micelle formation by isothermal titration calorimetry (ITC). For αs-casein, micelle formation was investigated as a function of temperature from 15-30 °C at pH = 6.6, while micelle formation of PE6400 and of mixtures of PE6400 with αs-casein was studied from 29-31 °C. In addition, the influence of increasing αs-casein mole fraction from 0.135 × 10-3 to 3.6 × 10-3 was examined at 30°C. In case of αs-casein, the standard experimental approach could be used while we developed a new experimental approach for PE6400 and for the mixture. The results show that the enthalpy of demicellization increases for αs-casein and decreases for PE6400 with increasing temperature. In case of the mixtures, the enthalpy of demicellization was not influenced by either the temperature or the αs-casein mole fraction. The results obtained for αs-casein suggest that the micellization follows a "shell-model". In the case of the mixtures, αs-casein is likely to form the core of the micelle while the outer part appears to be composed of PE6400. The mixing properties were antagonistic, which was shown by comparing the enthalpies of PE6400 with those of the mixtures. The results show that ITC is a powerful tool to precisely determine micellization properties not only of the single but also of mixed surfactant systems.
- [Show abstract] [Hide abstract] ABSTRACT: The paper at hand deals with biocatalysis in bicontinuous microemulsions. The latter consist of a dynamic network of oil and water domains separated by a monolayer of surfactant molecules, i.e. the interfacial layer. A microemulsion with the composition buffer - n-octane - nonionic surfactant was tested as reaction medium for an enzyme-catalysed reaction with a focus on the conversion of hydrophobic substrates, which are difficult to convert in aqueous buffer solutions. For the study at hand, we chose to investigate the activity of the squalene-hopene cyclase from Alicyclobacillus acidocaldarius (AacSHC) towards its natural substrate squalene in bicontinuous microemulsions. Firstly, the study revealed that the activity of AacSHC depends linearly on the enzyme concentration. Secondly, a hyperbolic curve was found for the dependence of the activity on the substrate concentration and a saturation of the AacSHC at substrate concentrations above 20mM was observed. Thirdly, the composition of the interfacial layer was found to have no significant influence on the activity or on the conformation of AacSHC. Surprisingly and unexpectedly, a distinctly enhanced selectivity towards hopene was discovered in the microemulsion. To conclude, bicontinuous microemulsions were found to be a suitable reaction medium for biocatalytic reactions with the enzyme AacSHC. Copyright © 2015 Elsevier B.V. All rights reserved.
Article: Gelled Lyotropic Liquid Crystals[Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: The paper at hand deals with the influence of the pH-value on the conformation and activity of the lipase B from Candida antarctica (CalB) which is incorporated in a bicontinuous microemulsion. The microemulsion used for this purpose consists of water/NaCl, n-octane, and the non-ionic surfactant penthaethylene glycol monodecylether (C10E5). The conformational study clearly shows (1) that CalB molecules are partitioned between the interfacial monolayer and the water domains and (2) that the pH-value of the microemulsion's water domains strongly influences the conformation of CalB at the interfacial monolayer. From these observations we conclude that there is a continuous exchange between the CalB molecules, which are located at the interfacial monolayer and those which are located in the water domains of the microemulsion. This exchange strongly influences the CalB conformation in both regions. In addition to the conformation, we also studied the catalytic activity of CalB. The catalytic measurements revealed a bell-shaped dependence between the CalB activity and the pH-value. The maximum catalytic activity of CalB in bicontinuous microemulsions was observed at pH=5.5. At this pH we observed the highest amount of α-helix conformation of the CalB molecules that are located at the interfacial monolayer, which, in turn, allows connecting the activity with the conformation. Copyright © 2015 Elsevier B.V. All rights reserved.
- [Show abstract] [Hide abstract] ABSTRACT: The paper at hand deals with the properties of a particular organogel, namely the binary system n-decane/12-hydroxyoctadecanoic acid (12-HOA). To provide a complete characterisation we firstly determined the sol–gel transition temperatures as a function of the gelator concentration with DSC and rheometry. Moreover, we calculated thermodynamic parameters from the DSC measurements such as the sol–gel transition enthalpy. Secondly, we studied the rheological properties with an oscillating shear rheometer. Thirdly, we investigated the structure of the gel network and of the gelator fibers. For this purpose, we carried out SANS measurements from which we obtained structural parameters such as the mean radius of the fibers and the mean thickness of the fiber nodes, but also the mole fraction of gelator involved in building up the gelator network. Additionally we present electron microscopy (EM) pictures of a 12-HOA gel prepared with the freeze-fracture method (FFEM). FFEM allows imaging a sample in its original state due to cryofixation while other EM methods require modifications like evaporation of the gel solvent. From our pictures we were able to determine the sense of the helical twist of the gelator fibers and we also observed a layered fine structure, which – to the best of our knowledge – has not been seen in EM pictures of 12-HOA gels yet. Our study allows an in-depth understanding of the gelator network in the model system n-decane/12-HOA. Moreover, our results are in perfect agreement with all data published for binary organogels consisting of an oil and 12-HOA, which, in turn, will help to understand many of the unanswered questions that still exist in relation to gels.
- [Show abstract] [Hide abstract] ABSTRACT: This study deals with the synthesis of Pt nanoparticles via oil-inwater microemulsions formulated with the technical grade surfactant BIODAC (R) 510. For this purpose we studied the influence of the Platinum precursor dimethyl(cyclooctadiene) platinum (Pt(COD) Me-2) on the phase behavior of the base microemulsion. It was found that the addition of Pt(COD) Me-2 has nearly no effect on the phase boundaries even at relatively high concentrations. Small angle X-ray scattering data confirmed that the addition of the metal organyl also does not influence the size of the microemulsion droplets. According to transmission electron microscopy (TEM) the size of the resulting platinum nanoparticles is independent on the amount of Pt(COD) Me-2 in the templating microemulsion. This result was rather surprising since it indicates that a greater amount of precursor in the o/w-microemulsion leads to more rather than to larger nanoparticles.
- [Show abstract] [Hide abstract] ABSTRACT: We have developed a templating route for the synthesis of monodisperse and highly ordered polystyrene foams. The route is based on the generation of monodisperse templates, namely of foamed emulsions and emulsions, respectively, via microfluidic techniques. The polymerization of the templates leads to polystyrene foams with controllable pore size, pore size distribution and connectivity and thus opens up a new arena for the synthesis of tailor‐made polymer foams. Microfluidic techniques can be used to generate highly monodisperse polystyrene foams with periodic pore organization. For this purpose, monodisperse foamed emulsions and emulsions, respectively, are to be used as templates. The polymerization of the templates leads to tailor‐made polystyrene foams with controllable pore size, pore size distribution, and connectivity.
- [Show abstract] [Hide abstract] ABSTRACT: This paper deals with the synthesis of Pt and Pt/Pb nanoparticles via oil-in-water microemulsions. Studying systematically the phase behavior of microemulsions which contain metal precursors, it was possible to determine the influence of the metal precursors on the phase behavior of the base microemulsion. It was found that the addition of Pb(ethylhexanoate)2 to the microemulsion shifts the phase boundaries to slightly lower temperatures, while adding dimethyl-(cyclooctadiene) platinum has no effect on the phase boundaries. SAXS measurements confirmed that the addition of the metal precursors does not influence the size of the microemulsion droplets along the emulsification failure boundary. Using transmission electron microscopy (TEM) we found that by increasing the oil content wB in the microemulsion from 0.04 to 0.08 the size of the resulting mono- and bimetallic Pt and Pt/Pb nanoparticles changes from 3 to 4.4 nm.
- [Show abstract] [Hide abstract] ABSTRACT: The effect of sodium halides on the surface structure of foam films stabilized by the nonionic surfactant dodecyldimethyl phosphine oxide (C12DMPO) was investigated using a combination of neutral impact collision ion scattering spectroscopy and thin film pressure balance. The former technique was used to determine the concentration depth profiles of the elements at the liquid surface, while the latter technique was used to provide film thickness information. An excess of iodide was found at the liquid surface in the presence of C12DMPO. Upon formation of the foam film, iodide was shifting closer to the surface showing that the foam film surface is more negatively charged than the surface of the respective bulk liquid. In contrast, an excess of chloride was found at the surface only in the presence of the surfactant and formation of a foam film. The iodide-containing films were slightly thicker than the chloride-containing films, which is concluded to be due to the increased surface charge of the iodide-containing films.
- [Show abstract] [Hide abstract] ABSTRACT: Selective enzyme-catalysed biotransformations offer great potential in organic chemistry. However, special requirements are needed to achieve optimum enzyme activity and stability. A bicontinuous microemulsion is proposed as reaction medium because of its large connected interface between oil and water domains at which a lipase can adsorb and convert substrates in the oil phase of the microemulsion. Herein, a microemulsion consisting of buffer–n-octane–nonionic surfactant CiEj was used to investigate the key factors that determine hydrolyses of p-nitrophenyl esters catalysed by the lipase B from Candida antarctica (CalB). The highest CalB activity was found around 44 °C in the absence of NaCl and substrates with larger alkyl chains were better hydrolysed than their short-chained homologues. The CalB activity was determined using two different co-surfactants, namely the phospholipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and the sugar surfactant decyl β-D-glucopyranoside (β-C10G1). The results show the CalB activity as linear function of both enzyme and substrate concentration with an enhanced activity when the sugar surfactant is used as co-surfactant.
- [Show abstract] [Hide abstract] ABSTRACT: The critical micelle concentration (cmc), the surface excess (Γ), as well as the micelle aggregation number (m) of the surfactant dodecyldimethylamine oxide (C12DMAO) have been reported to strongly depend on the pH-value of the aqueous surfactant solution. At high ionic strength, the cmc displays a minimum, while both Γ and m have a maximum at a pH-value close to the pKa of the surfactant. These experimental observations have been explained as being due to specific hydrogen bonds between the head groups, which are formed once the surfactant is partly or fully protonated. This investigation addresses the question of whether the pH also affects the foaming properties of C12DMAO solutions. To answer this question we measured the foamability and the foam stability of C12DMAO solutions at a fixed C12DMAO concentration of 5 cmc for five different pH-values, namely pH = 2, 3, 5, 8, and 10. We found that the foamability is hardly affected by the pH-value, while the foam stability strongly depends on the pH. As is the case for the above mentioned properties, the foam stability also displays an extremum in the studied pH-range, namely a maximum at pH = 5. We discuss our results in terms of the hydrogen bond hypothesis and show that this hypothesis indeed is in line with the observed trend for the foam stability. Moreover, we discuss that hydrogen bond formation may rationalize how the molecular structure of a surfactant affects foam stability.
- [Show abstract] [Hide abstract] ABSTRACT: An orthogonal self-assembled system consists of different structures which coexist independently. Furthermore, the characteristic properties of the respective “base systems”, i.e. of the systems which contain only one of the structures, are retained in the orthogonal self-assembled system. We have identified gelled bicontinuous microemulsions as orthogonal self-assembled systems and reported in a preceding paper (Soft Matter, 2013, 9, 3661) that their phase behaviour and rheological properties are in perfect agreement with those of the two base systems, namely a non-gelled bicontinuous microemulsion and a binary gel. In the paper at hand we present the results of structural investigations. With FT-PGSE 1H-NMR measurements we verified the bicontinuity of our gelled model system H2O–n-decane/12-hydroxyoctadecanoic acid (12-HOA)–tetraethylene glycol monodecyl ether (C10E4) at appropriate temperatures. Apart from that, we proved the coexistence of the bicontinuous microemulsion domains with the gelator network in a small angle neutron scattering (SANS) study. A qualitative comparison between the SANS data of the gelled bicontinuous microemulsion and those of its base systems reveals that the characteristic scattering features of both base systems are present. Moreover, we were able to quantitatively describe and interpret the SANS data, yielding at the same time the structural parameters of the bicontinuous microemulsion and the gelator network.
- [Show abstract] [Hide abstract] ABSTRACT: Water-in-oil (w/o) microemulsions were used as a template for the synthesis of mono- and bi-metallic nanoparticles. For that purpose, w/o-microemulsions containing H2PtCl6, H2PtCl6 + Pb(NO3)(2) and H2PtCl6 + Bi(NO)(3), respectively, were mixed with a w/o-microemulsion containing the reducing agent, NaBH4. The results revealed that it is possible to synthesize Pt, PtPb and PtBi nanoparticles of similar to 3-8 nm in diameter at temperatures of about 30 degrees C. The catalytic properties of the bimetallic PtBi and PtPb nanoparticles were studied and compared with monometallic platinum nanoparticles. Firstly, the electrochemical oxidation of formic acid to carbon monoxide was investigated, and it was found that the resistance of the PtBi and PtPb nanoparticles against the catalyst-poisoning carbon monoxide was significantly higher compared to the Pt nanoparticles. Secondly, investigating the reduction of 4-nitrophenol to 4-aminophenol, we found that the bimetallic NPs are most active at 23 degrees C, while the order of the activity changes at higher temperatures, i.e., that the Pt nanoparticles are the most active ones at 36 and 49 degrees C. Furthermore, we observed a strong influence of the support, which was either a polymer or Al2O3. Thirdly, for the hydrogenation of allylbenzene to propylbenzene, the monometallic Pt NPs turned out to be the most active catalysts, followed by the PtPb and PtBi NPs. Comparing the two bimetallic nanoparticles, one sees that the PtPb NPs are significantly more active than the respective PtBi NPs.
University of Cologne
Köln, North Rhine-Westphalia, Germany
- • Institute of Physical Chemistry
- • Institute of Organic Chemistry
University College Dublin
Dublin, L, Ireland
- School of Chemical and Bioprocess Engineering
Stuttgart, Baden-Württemberg, Germany
- Institute of Physical Chemistry
Technische Universität BerlinBerlín, Berlin, Germany