Current research topics at the Department of Physical Chemistry II

Solvents and hydrotropes

To solubilise a component in a solvent, the solvent must be well characterised. Recently, 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 in part show also 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] and related structures [134].

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].

Temperature-composition diagram for PnP/water system at 20 kPa [136].

Surfactants, Micelles and Classical Microemulsions

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

-    How do 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]. 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].

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-structured solutions? [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? [185].

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]

(Non-)aqueous 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], essentially with Jean-Pierre Simonin in Paris and some aqueous ones [117,126,127] with our Ukrainian colleague Elena Tsurko. 

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

Theory of electrolytes

A long time ago I learnt to model aqueous and non-aqueous electrolyte solutions [10,12,16,17,22,29,30,32] both in Regensburg and under the guidance of Pierre Turq in Paris. Whereas first a relatively complex solvent-averaged modelling was intended, we turned later on to efficient working equations for engineers [49,53,67,100,104,123]. We acknowledge the tremendous work done by Jean-Pierre Simonin at the University Pierre et Marie in Paris.

Distribution of NaNO3 ions at the air-aqueous solution interface with and without dispersion forces at 1M conzentration. [61]

Specific ion effects

Inspired by Barry Ninham during his six month stay in Regensburg in 2004, we began to get interested in specific ion effects. At first we followed the idea of dispersion forces to describe these effects [61,74,75,77,80]. However, we soon realized that the problem is much more complex. Therefore, we went back to experiments and considered specific ion effects in a multitude of systems, e.g. in “simple” electrolyte solutions [90,94,103,108], surfactants systems (association colloids) [125,141,143,187], enzymatic systems [76,96] and on biological membranes [114]. The collaboration with Pavel Jungwirth and his group as well as with Roland Netz and his group (especially Dominik Horinek, and Joachim Dzubiella) and finally discussions with Kim D. Collins have been of great importance for us to understand these effects [101,105,106,118,124,127,129, 131,141,153,155,157]. We could even use them to specifically design new surfactants [130,144].

Ordering of anionic surfactant headgroups and the corresponding counterions regarding their capabilities to form close pairs. The green arrows mean strong interactions (close ion pairs). [141]

Ionic Liquids

Having worked for a long time with electrolytes, solvents and surfactants, it was natural to get interested in Ionic Liquids, i.e. salts that are liquid below 100°C or even at room temperature. In contrast to many other groups we concentrated on two queuestions:

1. How can we conceive Ionic Liquids that are not based on the classical imidazolium or other organic cations? In this context we had two major achievements: the discovery that short-chain alkyl-oligoether carboxylates are liquid at low temperatures even with sodium as counterion and second that choline as counterion can also lead to Ionic liquids when combined with convenient middle- to long chain carboxylates [C3,142,164,169,172,174].
2. Can we incorporate Ionic Liquids in micellar and microemulsion systems [112] to make aqueous systems over wide temperature ranges? This question we answered in details in the following papers: [139,156,160,161,162,163,167,173,176,184].

Sketch of the postulated crosslinked structure of liquid [Na][TOTO] with strongly bound Na+–––carbocxylate ion pairs and weaker Na+–––ether-oxygen interactions. [164]

Mixtures of carbonates and silicates – from nanocrystals to biomorphs and silica gardens

Having heard fascinating lectures by Stephen Hyde, Canberra, and Juan-Manuel García-Ruiz, Granada, I got interested in the world of complex structures made of simple ions in aqueous solutions such as silicates and carbonates. I wanted to understand the origin of the phenomenon of biomorphs and the origin of curvature that simple ions create by an amazing self-aggregation process during crystallisation. Several papers came out of this research[115,119,121,133,137,140, 149,166,179,186,188]. A side-effect of this subject was the characterisation of silica-gardens, an old subject that even found entrance in school presentations [181].

In a somewhat related research topic collaboration with the German Papiertechnische Stiftung (Foundation of Paper Industry) led to the fabrication of interesting hollow oxide microspheres [122,150].

Chemical garden....are beautiful structures that show fascinating membrane and diaphragm properties. [181]

Structured aqueous alginate gels

Following a fascinating idea of our colleague Prof. Klaus Heckmann in Regensburg, Rainer Müller and I were interested in alginate gels that, under certain conditions, make highly ordered gels when in contact with bivalent cations. Driven by a dissipative process the gels undergo a self-organising process thus forming pores of up to 2 cm length that are parallel und uniform. These pores can be used as guidelines for nerve axon growth. The whole material can also be used as templates for well-defined structures [102,111,171].

Ultrastucture of alginate-based anisotropic capillary gels (ACH), (a) cross - and (b) longitudinal section. [102]

Formulation chemistry and the development of a natural water-soluble insect repellent

Formulation is one of our central activities. We develop products based on solutions and surfactants for many international companies well-known in the field of cosmetics, household products and pharmaceuticals. In the SKH GmbH we also conceive products against bad odours in industrial plants, for better solvent management etc. A particularly interesting product came out of a collaboration with the local company BioGents that is specialised in insect attractants. Further to formulations of their products we managed to create a natural insect repellent. What is more, we could transform it in a simple and environmentally friendly way to a highly active water-soluble repellent [151,152,175,C6].

Cemical structures of cis- and trans-para-menthane-3,8-diol isomers. (+)-cis and (-)-trans-p-menthane-3,8-diols are mainly present in the solution according to Takasago (Paris, France).Traces of (-) -cis and (+)-trans isomers can also be detected by gas chromatography (Drapeau). [151]

Reactions under hydrothermal conditions

Water and many aqueous solutions are naturally green solvents. Under hydrothermal conditions (between 150 and 250°C and under its own pressure) water has properties that significantly differ from its usual behaviour at room temperature. With increasing temperatures the dielectric constant and hence the polarity of water decreases rapidly and in parallel the self-dissociation of water increases. As a result, water under hydrothermal conditions can be interesting as solvent for many organic reactions [178,182].

This is a study that is complementary to our former investigations of enzymatic reactions performed in microemulsions in supercritical CO2 [51, 95], see before.

An artist's view on hydrothermal reactions showing Denis Papin, one of the inventors of the autoclave (by Ekaterina Shilova). [184]