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Current topics at the Department of Physical Chemistry II


    

Solvents and hydrotropes

To solubilise a component in a solvent, the solvent must be well characterised. We participated in the proposition of a new type of solvent classification based on the COSOM-RS theory [168]. Together with our colleagues in Lille we also considered new types of solvents that exhibit surfactant properties. Such a hybrid class of liquids we called solvo-surfactants. In particular we were interested in glycerol derivatives [109] and so-called dowanols©[69,72,116,136,68, 225] and related structures [134].

For a critical review of green solvents, see [262], also [242].

In this context it seemed of interest to illustrate the continuous cross-over of pure solvents to real surfactants and of mixtures between both [86,88,92,225,227]. Another interesting concept is the so-called facilitated hydrotropy [198, see also: M. Durand, A. Stoppa, V. Molininer, D. Touraud and J.-M. Aubry, J. Solution Chem., 2012, 41, 555–565]. Our present knowledge of hydrotropes is summarized in a review paper[238], see also [250].

The following picture is taken from paper[238].

   

    

    


    

Surfactants, Micelles and Classical Microemulsions

So many research groups and industrial companies work on surfactants. Are there still any open questions? We thought that some aspects are still worth being studied:

-       How to decrease the Krafft temperature of surfactants? Our idea was to use choline as a natural, biological cation that significantly reduces the solubilisation temperature of surfactants [130,144,195,208,224]. This is true for carboxylates and alkylsulfates, the latter being much less sensitive to water hardness [C5].

-        How to make olive oil and related fats miscible with water? To this purpose we studied so-called extended surfactants and indeed could make clear stable mini-emulsions with a minimum amount of surfactant [159,165,170,183].

-        How to achieve the spontaneous formation of vesicles as simply as possible and with biocompatible surfactants? This question was considered in several papers [110,113,135]. In this context we also studied mixtures of cationic and anionic surfactants [128,145,148,154].

-       We strongly believe that there is a future for soap-based surfactant systems [205] and that the inherent shortcomings (stable only at high pH, salt sensitivity, high Krafft points) can be overcome by appropriate formulations with other green ingredients. Currently, patents are filed to protect our efforts in this direction. A review paper on this subject can be found here [245]. In this context, we found that in particular the molecule rebaudioside A (the Stevia sweetener) is a very good co-surfactant and may be used as such in many formulations [245,231].

Micelles and microemulsions are also classical systems that have been studied thousands of times. We asked following questions:

-        How to characterise microemulsions by classical scattering techniques [43,62,58,97] and less common techniques such as dielectric relaxation spectroscopy [46,47,57,79,85,98] under the guidance of our specialist Prof. Richard Buchner, and some other techniques [56,59]?

-        What is the interplay between these structures and enzymatic reactions? How can such reactions be optimised by an appropriate design of nano-structuredsolutions [39,40,41,48,50,84,87,91,132]?

-        Can enzymatic reactions also be performed in microemulsions in supercritical CO2[51, 95]?

-        How to make green microemulsions [213,185,199,198,200,205,202]?

-        How to conceive microemulsions for well-defined purposes, for example for drug delivery [204] or for magnetic systems [190] or for food additives [231]?

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Penetration scan of ChC12 at 20°C acquired at 100x magnification between half-crossed polarizers, showing the following sequence of mesophases: L1, I1', I2'', H1, V1 and a gel+solid region.  [174]


    

Surfactant-free Microemulsions(SFME) – The pre-Ouzo Effect

In the last years, we focused on a fascinating new subject, which is the microemulsion-like structuring of mixtures of two immiscible solvents with a third one, which is miscible with both of the two other ones. Such a structuring always occurs when the mixture is not too far away from the dephasing boundary. A typical example is the mixture of water, anethole (anise camphor) and ethanol, the well-known Ouzo liquor. If too much water is added, the mixture gets turbid and a very stable emulsion is formed. This strange finding of a surfactant-free, stable emulsion is called the Ouzo-effect. We call the pre-Ouzo effect the phenomenon that even in the homogeneous monophasic region of the phase diagram well-defined microemulsions are formed without surfactants. In analogy to classical microemulsions, direct, inverse and bicontinuous structures can be found. It should be stressed that such structures have been postulated and discussed in literature since at least 40 years, but widely neglected by solution and colloidal chemists. We have started detailed studies on these systems [192,206,207,217,220,226,229,230,234,241,243], because the understanding of this effect has significant consequences for product formulation, but it is also essential to understand the onset of structuring caused by very subtle, week interactions. Our research on this topic is done in close collaboration with Prof. Thomas Zemb and Dr. Olivier Diat at ICSM, Marcoule, France and Prof. Dominik Horinek in our institute. Thomas Zemb and our Australian colleague Stjepan Marčelja from ANU, Canberra, developped with us a generalised DLVO theory to explain and quantitatively predict this pre-Ouzo structuring[237]. The following figure is taken from [243].

   BIC: bicontinuous critical point

First attempts of possible applications of SFME are presented in [249,256,261,267].


     

Plant Extraction

Having acquired a significant knowledge in solvent and solution chemistry and being interested in Green Chemistry, it is a natural step to apply our knowledge to the field of plant extraction processes. In the framework of several master and PhD theses, we considered  plant extraction with classical methods (steam and Soxhlet extraction, maceration etc.), but also extraction with supercritical CO2 and “green” Ionic Liquids. For the moment, we focus on rhizomes of Iris germanica and Iris pallida, where we consider not only optimized green extraction processes, but also the optimization of the conversion of Iridals to Irones, the valuable and very expensive perfume molecules in the rhizomes. We have a fruitful collaboration with the French company Phytotagante in Toulouges, further with a big perfume producer and with the laboratory "GREEN" under the direction of Prof. Farid Chemat at the University of Avignon[253].

We also consider Rosemary extraction[239] and its optimisation and formulation for cosmetic products as well as other plants such as Roses (where hexane is replaced by environmentally friendlier solvent systems) and plants containing promising and low toxic saponis.

Further, we are interested in stabilising Latex out of Caucasian dandelion[258]. During his six month stay at the University of Avignon, France, in the lab GREEN of Professor Farid Chemat, Prof. Kunz also participated in several other projects concerning plant extraction.

Several new inventions are summerized in pending patents in the domain of plant extraction and valorization through green processing.


    

Formulation Chemistry

We take profit of our knowledge on complex solutions to conceive finished products ready to use. Examples are high-performance shampoos, environmentally friendly shaving foams, green anti-graffiti solutions, deodorants[151,152,175, 220], green high performance oil dissolvers, but also food formulations. The latter is part of our effort to use plant extracts and to convert them to formulated products. Recently we succeeded in dissolving the very hydrophobic essential oil of the Arab Nanah mint tea in a drinkable microemulsion concentrate that can be diluted with water to get a clear stable tee of excellent taste.

For our anti-graffiti product, see http://www.skh-gmbh.de/graffiti-entfernen.html

We also start to formulate new types of biofuels with a significant part of unmodified plant oils mixed with diesel and biodiesel and optimized with respect to the required conditions for fuels by using new types of green additives [235,241,251,266].

We also manage to stabilize biofuels with fully sustainable and price-competitive antioxidants. Related patents are filed.


    

Aqueous and Non-queous Electrolyte solutions

As a continuation of Josef Barthel’s work (Josef Barthel is the founder of the chair of physical chemistry II at Regensburg university), we pursued the study of the structure and thermodynamics of non-aqueous electrolyte solutions[11,12,13,19,65,70,73,81,82,83,89,99,136,146], among others with Pierre Turq and his group in Paris. We also have a long-standing collaboration with our Ukrainian colleague Elena Tsurko from Charkiw University [117,126,147,191,221,236,248,263] and more recently with the group of Prof. Sadowski at TU Dortmund [211,247,248]. The subject is the determination of thermodynamic properties of aqueous amino acid-salt solutions and specific-salting in and out behaviour of ingredients in aqueous systems

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Experimental osmotic coefficients of (LiBr + DMAc) at different temperatures. The theoretical curves are generated using the extended Pitzer model of Archer. [82]


    

Specific Ion Effects