Soft Matter at Interfaces – Research Areas
Structure and dynamics of polyelectrolyte multilayers
The properties of these systems are studied by e.g. ellipsometry, x-ray and neutron reflectometry and atomic force microscopy.
Moreover, the extent of polyelectrolyte adsorption on the substrate is followed by quartz-crystal microbalance with dissipation monitoring (QCM-D), which gives information about the viscoelastic properties of the growing multilayer. They are compared with AFM indentation experiments. Parameters of current studies are molecular weight and substrate properties.
The general question addresses the correlation between structure, dynamics and swelling behavior of these multilayers.
1. Microgels at surfaces
The synthesis and characterisation of microgels based on NIPAM is the main topic of the present research. Not only pure NIPAM but also microgels with different co-monomers are in the focus. The project addresses the comparison of the swelling/shrinking behavior in bulk (DLS) and after adsorption on substrates (AFM, contact angle). The characterization (size, elasticity, …) of the synthesized particles is done by atomic force microscopy (AFM) in air and in liquid medium. Special focus is the on the viscoelastic properties studied by AFM indendation.
- Burmistrova, A.; Richter, M.; Üzüm, C. & von Klitzing, R.: Effect of cross-linker density of p(NIPAM-co-AAc) microgels at solid surfaces on the swelling/shrinking behaviour and the Young's modulus. Colloid Polym. Sci., 2011, 289, 613-624
- Burmistrova, A.; Richter, M.; Eisele, M.; Üzüm, C. & von Klitzing, R.: The Effect of Co-Monomer Content on the Swelling/Shrinking and Mechanical Behaviour of Individual Adsorbed pNIPAM Microgel Particles. Polymers, 2011, 3, 1575-1590
2. Microgels and proteins
Enzymes are very important as biocatalysts. For the application in industrial processes it is necessary that the used enzymes are stable at high temperatures, at different pH values and in the presence of organic solvents. One method to achieve such stable systems is the embedding of enzymes into polymer matrices. Microgels made of poly-N-Isopropylacrylamide (p-NIPAM) show a reversible shrinking above the volume phase transition temperature (VPTT). This behavior makes them a promising system as matrix for the embedding of organic particles. In order to get the possibility to make the microgels visible with a confocal laser scanning microscopy (CLSM) we synthesize p-NIPAM microgels up to a diameter of 1.7 µm using a temperature ramp during the polymerization.
- Gawlitza, K.; Wu, C.; Georgieva, R.; Wang, D.; Ansorge-Schumacher, M.B.; von Klitzing, R.: Immobilization of lipase B within micron-sized poly-N-isopropylacrylamide hydrogel particles by solvent exchange Phys. Chem. Chem. Phys. 2012, 26, 12980-12987.
- Gawlitza, K.; Wu, C.; Georgieva, R.; Ansorge-Schumacher, M.B.; von Klitzing, R.: Temperature Controlled Activity of Lipase B from Candida Antarctica after Immobilization within p-NIPAM Microgel Particles Z. Phys. Chem. 2012, 226, 749-759.
- Gawlitza, K.; Georgieva, R.; Tavraz, N.; Keller, J.; von Klitzing, R.: Immobilization of Water-Soluble HRP within Poly-N-isopropylacrylamide Microgel Particles for Use in Organic Media Langmuir 2013, 29, 16002-16009.
2. Biocompatible, thermoresponsive microgels based on Poly(Ethylene glycol) (PEG)
Due to their reversible temperature induced volume phase transition (VPT), microgels made of poly-N-Isopropylacrylamide (p-NIPAM) served as model systems for basic research and have been intensively studied, e.g. their swelling behavior and internal structure. Although this microgel system is a useful model system and seems to be promising for pulsatile drug delivery, the fact that the NIPAM monomer is carcinogenic or teratogenic limits its application in biomedical applications. The presented project deals with the synthesis and characterization of biocompatible PEG based microgels. Fundamental properties are studied in bulk as well as after deposition on Silicon wafers using Dynamic Light Scattering (DLS), Small Angle Neutron Scattering (SANS) and Atomic Force Microscopy (AFM). The synthesis was done by precipitation polymerization using 2-(2-methoxyethoxy)ethyl methacrylate (MeO2MA) as monomer, Poly(ethylene glycol) methyl ether methacrylate (OEGMA) as comonomer and Ethylene glycol dimethacrylate (EGDMA) as cross-linker.
Interactions in thin liquid films
1. Foam films
Mixtures of cationic and anionic surfactants are investigated focusing on the adsorption at aqueous surfaces. Especially the synergistic effect (also due to a variation in chain length), their aggregation behavior and the effect on foam films, foam film stability and foam lifetime are of main interest. Anther project addresses ion specific effects (Hofmeister series) and the effect of polyelectrolytes as additives.
Methods to identify are mainly surface tension (dynamic/static), thin film pressure balance and neutron scattering.
- Kristen-Hochrein, N.; Laschewsky, A.; Miller, R. & von Klitzing, R.: Stability of Foam Films of Oppositely Charged Polyelectrolyte/Surfactant Mixtures: Effect of Isoelectric Point. J. Phys. Chem. B, 2011, 115, 14475-14483
- Kristen-Hochrein, N.; Schelero, N. & von Klitzing, R.: Effects of oppositely charged surfactants on the stability of foam films. Colloids Surf., A, 2011, 382, 165-173
- Schelero, N. & von Klitzing, R.: Correlation between specific ion adsorption at the air/water interface and long-range interactions in colloidal systems. Soft Matter, 2011, 7, 2936-2942
2. Wetting films
During flotation solid particles are attached to air bubbles. In order to adsorb at the air bubble the particle is hydrophobized by the adsorption of a suitable surfactant (collector). At the same time the collector adsorbs at the air/ solution interface. Therefore, it affects both lifetime of the thin liquid film and the contact line around the particle. Up to now the correlation between the properties of the collector and its effect on the floatation mechanism are not fully understood and the choice of the collector is more or less empirical. The main problems deal with the stability of wetting films related to the adsorption of surfactant and/ or amphiphilic polymers at the solid/ liquid and air/ liquid interface.
To get a better understanding of the correlation between the simultaneous adsorption of surface active compound at a solid and a fluid interface and the stability of a wetting film formed between these two interfaces, planar wetting films as a suitable model system are studied by TFPB, ellipsometry and tensiometric methods.
- Haenni-Ciunel, K.; Schelero, N. & von Klitzing, R.: Negative charges at the air/water interface and their consequences for aqueous wetting films containing surfactants. Faraday Discuss., 2009, 141, 41-53
3. Films between two solid surfaces
The origin of the oscillatory force is due to the entropic excluded volume effect in confined thin films. The confinement induces a layered ordering of colloidal particles or polyelectrolyte chains in the vicinity of the confining surfaces. Therefore, studying the oscillatory force is a direct way to understand the interaction between colloidal particles or polyelectrolyte chains and control the stability and other properties of the system.
We perform force measurements by using CP-AFM, in which the colloidal probe on the cantilever and the subtract act as the confining surfaces. The characteristic lengths obtained from the oscillatory force in the confined thin film are in good agreement with those obtained from the corresponding free volume (bulk) case using SAXS.
- Zeng, Y. & von Klitzing, R.: Structuring of colloidal suspensions confined between a silica microsphere and an air bubble. Soft Matter, 2011, 7, 5329-5338
- Üzüm, C.; Christau, S. & von Klitzing, R.: Structuring of Polyelectrolyte (NaPSS) Solutions in Bulk and under Confinement as a Function of Concentration and Molecular Weight. Macromolecules, 2011, 44, 7782-7791
- Üzüm, C.; Makuska, R. & von Klitzing, R.: Effect of molecular architecture on the polyelectrolyte structuring under confinement. Macromolecules, 2012, 45, 3168-3176
Surface induced ordering of colloidal particles and complex fluids
1. Surface induced ordering of colloidal particles
When dealing with structures or particles on the nanoscale, forces that are relevant in the macroscopic world like gravity lose their importance when compared to electrostatic or surface forces. The latter ones usually are stronger than their, for example gravitational, counterpart by orders of magnitude. A decrease of the feature size goes along with an increase of the surface to volume ratio. Therefore, chemistry and physics on the nanoscale are closely related to the surface properties of the materials studied. The research is aimed at understanding how nanoparticles interact at interfaces which is relevant for material science (e.g. foams and surface patterning) as well as subject of fundamental research.
- Carl, A. & von Klitzing, R.: Smart Foams: New Perspectives Towards Responsive Composite Materials. Angew. Chem. Int. Ed., 2011, 50, 11290-11292
2. Complex fluids as decontamination media for hydrophobic solid surfaces
The implementation of new and biocompatible decontamination media is desired to efficiently decontaminate urban and natural environment in case of intentional or accidental release of highly toxic compounds. These decontamination media have to extract the contaminant out of sorptive surfaces, solubilize and convert the contaminant into non-toxic products. At the same time, these decontamination media must be non-corrosive and environmentally compatible.
Dynamics in polymer brush-nanoparticle systems
Polymer brushes are long polymer chains tethered to a surface by one end. They have applications in chemistry, engineering, medicine, microelectronics, etc. Many of these applications involve interactions between the brush and nanoparticles which are near or within the brush (for instance, when used to arrest toxic substances or to arrange nanoparticles in the patterned surface).