Prof. Dr. Werner Kunz
- Important information: At the Chair of Physical Chemistry of Prof. Kunz, research is focussed on Solution Chemistry (conception, understanding and application of solvent media) and Formulation of Products based on liquids and colloidal chemistry. Sustainable chemistry and the quest for "greener" products and processes in industry is another key aspect. These points are also the basis of our teaching, further to the basic teaching in thermodynamics, electrochemistry, kinetics, colloidal chemistry, and technical chemistry.
News from our working group
Trainee (m/f/d) – Functional coatings or processing of textile cellulose waste.
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The best surfactant is not a surfactant at all.
Nowadays, industrial chemical reactions are predominantly carried out in organic solvents. In view of growing environmental requirements and the search for sustainable processes, intensive research is being conducted into reactions that can take place in aqueous solutions. The low solubility of hydrophobic reactants presents the greatest challenge here. To solve this problem, surfactants are usually added to the water. This concept is known as ‘micellar catalysis’.
In a series of publications in renowned specialist journals, an American colleague was able to convince the scientific community and industrial companies that micelles are indispensable for optimal reactivity in aqueous systems. Furthermore, it was claimed that the reactions yield the best results, particularly when using the specialised surfactants developed by his research group.
As we questioned these conclusions, we re-examined some of the published reactions on micellar catalysis – this time, however, without surfactants. Instead, mass transfer took place in simple water-alcohol mixtures. This revealed that the reactions not only yielded higher yields but also proceeded under milder conditions.
The decisive factor is therefore not the encapsulation of the reactants in micelles, but rather their sufficient and appropriate solubility. On this basis, chemical reactions of industrial significance can be significantly simplified in future.
The results were published in the high-impact journal *Chemical Engineering Journal* (Impact Factor 16.74): https://doi.org/10.1016/j.cej.2023.144599 (external link, opens in a new window).
Award for sustainable products and processes
At this year’s Formula XI international conference in Lille, France, Prof. Kunz was honoured with the Pierre Fillet Prize by the French Chemical Society for his decades of contributions to the development of more sustainable products, particularly in the fields of cosmetics and cleaning products, and to the more environmentally friendly production of chemicals. The award is presented every three years to individuals who have made outstanding contributions to improving the formulation of ready-to-use products. Following the 2019 Steinkopff Prize from the German Colloid Society, this is already the second award for sustainable chemistry that Prof. Kunz has received. In addition to this award, Prof. Kunz had the opportunity to present the key findings of his research group over recent years to an international audience during a 60-minute keynote lecture. In doing so, he placed particular emphasis on the sustainability of the products and processes developed, addressing not only scientific aspects (environment, energy, toxicity) but also performance, price, customer acceptance and the social dimension. It is precisely this holistic approach to implementing sustainable ideas that is particularly close to Prof. Kunz’s heart.
A new strategy for dissolving long-chain surfactants in water at low temperatures
Surfactants are used in a wide variety of everyday applications as cleaning agents and are an indispensable component of numerous industrial formulations. Until now, however, their use has been limited to amphiphilic structures with comparatively short alkyl chains (typically up to C14 or C16), as longer-chain homologues exhibit only low water solubility under practical conditions.
In this work, we present a concept that, for the first time, enables the effective solubilisation of octadecyl sulphates and octadecyl carboxylates in water at room temperature. Through the synthesis of alkoxylated choline derivatives – starting from choline, a naturally occurring and abundantly available molecule – we have developed a new class of counter-ions that prevent the precipitation of long-chain surfactants, as is typically observed with alkali metal ions or unmodified choline.
The amphiphilic systems obtained in this way exhibit excellent surfactant properties, which are reflected in significantly improved cleaning performance in laboratory washing tests. Studies on the cytotoxicity and biodegradability of the alkoxylated choline derivatives also highlight their potential for the development of sustainable surfactants.
Overall, our approach opens up the possibility of harnessing previously untapped raw material resources for tailor-made surfactant formulations – not only for cleaning applications, but also for numerous other areas of use.
DOI: doi.org/10.1039/D2GC02460H (external link, opens in a new window)
Physicochemical and toxicological properties of osmolite-based cationic surfactants and the low-toxicity cat-anionic vesicles formed spontaneously from them
In the context of efficient drug delivery systems, cationic vesicles offer numerous advantages, including their spontaneous formation and high long-term stability. However, the cationic component of such vesicles, in particular, often exhibits significant cytotoxicity. It is therefore of great importance to develop less toxic and biocompatible amphiphilic compounds that simultaneously retain the desired aggregation properties.
In this work, we investigate the cytotoxicity towards the human HeLa cell line, as well as the biodegradability of various cationic surfactants based on the natural osmolytes choline and ectoine. Furthermore, the synthesis, aggregation and solubility behaviour, as well as the stability of these compounds in aqueous solution, are described.
To facilitate the spontaneous formation of vesicles, selected cationic surfactants were combined with choline carboxylates of varying chain lengths in different mixing ratios and characterised using cryo-transmission electron microscopy (cryo-TEM). Furthermore, the cytotoxicity of these novel cation-anion systems was investigated as a function of the ratio between cationic and anionic components and compared with conventional combinations of sodium dodecyl sulphate and dodecyltrimethylammonium bromide.
DOI: doi.org/10.1016/j.molliq.2022.119549 (external link, opens in a new window)
Nanoscale microheterogeneities or pseudo-phase separations in unconventional liquids
This article discusses new concepts in macroscopically single-phase colloidal systems, in which entropy acts as a key driving force for very weak interactions and the formation of structures. Firstly, we demonstrate how microemulsion-like structures can be generated using hydrotropes. These aggregates exhibit a less well-defined structure and a shorter lifetime than classical micelles, but represent potentially useful microheterogeneities with internal interfaces.
The other limiting case involving strong interactions occurs when cationic and anionic surfactants are mixed in equimolar ratios. It is known that the solubility of surfactants can be increased by the introduction of ethylene oxide groups. This strategy is applied to so-called ‘katanionics’ to prevent the precipitation of the surfactants.
Finally, we consider the fact that ethylene oxide groups increase the size of the hydrophilic head groups of carboxylates, so that geometric constraints force a direct spherical structure even in the absence of water. This makes ‘anhydrous direct microemulsions’, consisting exclusively of charged surfactants and oil, a conceivable concept.
DOI: https://doi.org/10.1016/j.cocis.2021.101535 (external link, opens in a new window)
Development of a mint concentrate that is fully water-soluble, based on a food-grade microemulsion
Mentha spicata L. disappears in winter. The lack of fresh mint during the cold season can limit the preparation of mint tea. One source of fresh flavour that can also be stored over the winter is mint essential oil. However, as the oil is not water-soluble, a food-grade, water-soluble microemulsion of the essential oil was investigated, with various surfactants, in particular Tween® 60, being considered.
The challenge was to dissolve a highly hydrophobic essential oil in a homogeneous, stable, transparent and spontaneously forming solution consisting exclusively of edible additives, without altering the original fresh flavour of the mint. By utilising the water and oil pseudophases of the microemulsions, hydrophilic sweeteners and hydrophobic colourings were incorporated to mimic mint leaf infusions both in terms of flavour and appearance.
The resulting formulation was a concentrate consisting of approximately 90% ‘green’ ingredients, which could be diluted with water or tea to produce a drink with a pleasant minty flavour.
DOI: https://doi.org/10.1016/j.foodchem.2021.131230 (external link, opens in a new window)
Ionic liquids based on the concept of melting point lowering through ethoxylation
Most commonly used ionic liquids (ILs) contain bulky organic cations in combination with suitable anions. With our COMPLET concept (Concept of Melting Point Lowering due to Ethoxylation), we are taking a different approach. We use simple, low-toxicity, cost-effective and commercially available anions of the type Cx(EO)yCH₂COO⁻ to liquefy virtually any simple metal ion – regardless of its charge.
In the simplest case, the cation may be sodium or lithium; however, it is also possible to synthesise ionic liquids using higher-valent cations such as transition metals or rare-earth metals. Anions with longer alkyl chains exhibit surfactant properties and form surfactant ionic liquids (SAILs), which combine the properties of ionic and non-ionic surfactants at room temperature. Even in their pure state – that is, in the absence of water or other added solvents – they exhibit a distinct structural organisation.
This approach opens up new areas of application that go far beyond the classic real or hypothetical uses of conventional ionic liquids. Potential applications include the separation of rare-earth metals, their use as versatile media for metal catalysis, and the synthesis of entirely new materials (for example, analogous to metal-organic frameworks, or MOFs).
DOI: https://doi.org/10.3390/molecules26134034 (external link, opens in a new window)
Promising ‘green’ solvent, producible from wood and grasses
The molecule ‘γ-valerolactone (GVL)’ can be easily produced from cellulose-rich biomass and could be used in a wide range of commercial products, whilst also having the potential for application in large-scale chemical processes. Furthermore, it exhibits very low toxicity to aquatic organisms and is readily biodegradable.
These findings were recently published by Prof. Kunz’s department, in collaboration with researchers from TU Dresden, in Green Chemistry, the leading journal for sustainable chemistry. The article appeared on the cover of the relevant issue and was included in both the ‘2021 Hot Green Chemistry Articles’ collection and the ‘Green Chemistry Editor’s Choice’ section.
The article demonstrated that the ecotoxicity of GVL towards aquatic plants, bacteria, invertebrates and a vertebrate cell line is very low. Furthermore, it was shown that the lactone is completely biodegraded within one month.
Furthermore, the solvent properties of GVL were modelled using the Hansen solubility parameters, COSMO-RS calculations and existing literature data on GVL, in order to evaluate potential applications of this ‘green solvent’. This revealed that GVL could be a promising substitute for various highly polar, aprotic solvents, such as the reproductive toxins N-methyl-2-pyrrolidone (NMP) and dimethylformamide (DMF).
GVL was classified as a sustainable and less toxic solvent with great potential for the production of certain polymers or active pharmaceutical ingredients, as a cleaning agent in various paint and coating formulations, and as a solubilising agent in cosmetics, pharmaceuticals or agrochemicals.
In light of these promising results, Prof. Kunz’s department is currently working with an industrial partner to set up a pilot plant with a capacity of 2,000 tonnes per year. Should demand increase, production could subsequently be scaled up to several thousand tonnes per year.
Chemical photocatalysis takes a dip
Light, water and soap are sufficient to break stable chemical bonds between carbon and chlorine atoms and activate them for reactions. This has been discovered by chemists in Regensburg. Their findings were published in the journal *Nature Catalysis *.
