I-SURF

New possibilities with new surfactants

Surfactants are not only widely used in everyday life, they’re also an important part of nanoscience research. Most existing surfactants are pure organic compounds, now researchers in the I-SURF project aim to develop a new class of surfactants containing inorganic constituents, which could pave the way towards new technological applications, as Professor Sebastian Polarz explains

A type of compound widely used in detergents and emulsifiers, surfactants are also an important tool in nanoscience research, allowing researchers to prepare and develop new nanoparticles. A surfactant molecule is made of two contrasting parts, as Professor Sebastian Polarz explains. “One part is the oilloving, or hydrophobic part – and this is typically an alkyl chain. The second part, which is called the head group of the surfactant, is the group which is compatible with water, it’s hydrophilic, and it gives you water solubility,” he says. Based at the University of Konstantz in Germany, Professor Polarz is the Principal Investigator of the I-Surf project, an ECbacked initiative which aims to synthesise a new class of inorganic surfactants. “Our idea was to ask; can we produce an entirely new class of surfactants?” he outlines.

The vast majority of the surfactants are pure organic compounds, with an alkyl chain and a head group which are both purely organic. These surfactants are very interesting from a research point of view, because they show some fascinating self-organisation behaviour, now Professor Polarz and his colleagues aim to add additional functionality. “These surfactants should still have the interesting self-organisation properties we see in organic surfactants, but at the same time we want to add some additional functionality, which will be due to the inorganic constituents,” he explains.

This research centres around modifying the head group of the surfactant, while leaving the alkyl chain unchanged. There are several different strategies for changing the head group. “One strategy is to have a head group ISURF Project Team (from left to right): Professor Sebastian Polarz, Alexander Klaiber, Dr James A. Odendal, Sebastian Sutter, Adrian Donner, Marius Kunkel, Stefanie Hermann.

which is purely inorganic. We do this with metal oxide entities,” outlines Professor Polarz. Another approach is to have a head-group which is able to bind metals, a so-called ligand. “This ligand coordinates to a metal, but then typically the head group will only contain one metal centre, rather than many,” continues Professor Polarz. “In our research we are investigating molecules with many metal centres and purely inorganic head-groups. But also, we are looking at surfactants where we have a ligand at the head group, which then coordinates the metal.”

Surfactant functionalities

A core aim of the project is to develop molecules with specific redox, catalytic or magnetic functionalities. Professor Polarz and his colleagues have a wide variety of inorganic compounds which can be used as head groups in this research. “If you have an inorganic compound which is known for its catalytic properties, then you will eventually have a surfactant with a catalytically active head group,” he explains. These compounds would not normally show any self-organisation behaviour, but circumstances change when they are part of a surfactant. “We now see some self-organisation,” says Professor Polarz. “For example, one structure that is well-known from studies of surfactants are micelles, spherical objects which form in solution.”

These micelles are formed by several surfactant molecules coalescing, now researchers are looking to modify the properties of the surfactants. The head group of the surfactant can be precisely positioned on the surface of the micellar aggregates, which could hold important implications in terms of catalysis. “For catalysis, it’s very important that you have a high surface area of the catalytically active species,” explains Professor Polarz. Researchers aim to precisely position the catalytically active centre of these micellar aggregates, which are just a few nanometres in length; Professor Polarz says this will increase the surface area of these catalytically active species. “This is one of the ideas that we are working on, and we have already published some interesting results,” he outlines.

The project is also investigating redox and magnetic functionalities. A major challenge in terms of the latter area is developing surfactant systems that can be controlled by an external stimulus, namely a magnetic field. “We have created a molecule which can form self-organised structures, like micelles and liquid crystals, but these structures need to

sense the magnetic field. We have developed surfactants with magnetic head-groups, which can be manipulated and controlled by an external magnetic field. Unlike when an electrical field is used as a trigger, the advantage is that this works in water,” says Professor Polarz. Hybrid surfactants may also have a much higher charge than ordinary surfactants and so will behave very differently, which is an important issue with respect to their redox functions.“We know that metalcontaining compounds can change their redox state,” outlines Professor Polarz.

This means that electrons can be transferred into the compound, increasing or reducing the charge. This holds clear potential in terms of the development of smart materials, which are capable of adapting to the specific circumstances in which they are being used. “A smart material has different properties if you change its state. In this case, you change a charge and have different self-organisation properties, and then you can change the state as often as you like,” explains Professor Polarz. Research in this area is largely fundamental in nature, but Professor Polarz and his colleagues are looking towards the potential applications of the surfactants with magnetic functionalities. “After we have studied the fundamental properties of these systems, our goal is to use them for certain applications,” he outlines.

Applications

A number of potential applications have been identified, including optics and drug delivery. Surfactants with magnetic functionalities could be used to deliver drugs to the precise location in the body affected by disease, helping improve treatment effectiveness. “There could be some active compounds inside the emulsion droplets for example. Then when you apply a magnetic field, the emulsion droplet will change and it will release the substance,” says Professor Polarz. With respect to catalytic surfactants, the goal is to develop what Professor Polarz calls catalytic relay systems.

“You have not one catalytic step but several, following after each other, like a chain of catalytic steps,” he explains. “This can be organised, because we can precisely position the catalytic centre.”

The wider goal in this research is to explore cooperative effects in materials chemistry. Cooperative effects are often seen in biological systems, for example in enzymes. “There is a catalytically active species, which is surrounded by groups which help the catalytic sensor to do its job. The idea is – can we do something similar in materials chemistry?” outlines Professor Polarz. Researchers are investigating the potential to develop fundamental surfactant systems with these cooperative effects. “Now we can combine functionality with selforganisation properties,” says Professor Polarz. “So we introduce some extra properties and some extra features to the system, and we are looking at how this can be optimised.”

These surfactants should still have the interesting selforganisation properties we see in organic surfactants, but at the same time we want to add some additional functionality, which will be due to the inorganic constituents

Full Project Title

Inorganic surfactants with multifunctinal heads (ISURF)

Project Objectives

The current project aims at the synthesis of unique inorganic surfactants (I-SURFs), which contain multinuclear, charged metal-oxo entities as heads, and their exploration with regard to additional redox, catalytic or magnetic functionalities. A particular challenge is the creation of smart surfactant systems that can be controlled via external stimuli. While thermotropic liquid crystals and their adjustment in electric fields (enabling LCDs) have been studied in depth, very limited research concerns the control of self-assembled amphiphilic structures by use of magnetic fields.

Project Funding

Funded under: FP7-IDEAS-ERC (ERC-CG - ERC Consolidator Grants).

Project Partners

• Please see website for full partner details.

Contact Details

Sebastian Polarz, Professor for Functional Inorganic Materials at University of Konstanz, Department of Chemistry Universitaetsstrasse 10 78457 Konstanz, GERMANY T: + 49 (0)7531 884415 E: sebastian.polarz@uni-konstanz.de W: https://cms.uni-konstanz.de/polarz/

Professor Sebastian Polarz

Professor Sebastian Polarz has headed the ‘Functional Inorganic Materials’ department at University of Konstanz since 2007. Before that, he worked with A. Mueller (Bielefeld; 1999), M. Antonietti (Max-Planck-Institute for Colloids; 2001), G. Ozin (Toronto; 2002) and M. Driess (Emmy-Noether-group, Berlin; 2006). Current research interests involve ’selfassembly’, ’mesoporous materials’ and ‘semiconductors nanoparticles’.