Institut für Festkörperforschung
  Forschungszentrum Jülich GmbH
  D-52425 Jülich
  Deutschland

Curriculum Vitae of P. Lang

Peter Lang who joined the group in October 2000 studied Chemistry at the Albert Ludwigs Universität in Freiburg, Germany and got his diploma degree in 1987. He was member of the group of Prof. W. Burchard at the Institut für Makromolekulare Chemie during the work for his diploma thesis. For two years he was a research fellow of the European Community at Chemistry Department of the Unisversità "La Sapienza" in Rome, Italy, where he was working on the characterization of the solution structure of polysaccharides. Returning to the Burchard group in 1989 he completed this work with light scattering studies on polysaccharide solutions. He received his Ph. D. in 1991 for a thesis with the title "'Aggregation and thermoreversible gelation of Tamrind Samen Polysaccharide"'. With a Feodor-Lynen grant of the Alexander von Humboldt Stiftung he went to the Kyoto Institute of Technology in Kyoto, Japan for one year. After his return to Germany he joined the group of Prof. G. H. Findenegg at the physical chemistry department of the Technische Universität Berlin. There he studied the structure formation of amphiphilic systems in bulk solutions and at interfaces using scattering techniques. He received his Habilitation in 1999 and was offered his current position in 2000.

Research Interests of P. Lang

Interfaces represent regions in which the symmetry of the interparticular force field is severely perturbed. In solid state physics it is well established that minimization of the surface free energy may cause a spontaneous reconstruction of the structure in the outermost layers of a crystal. It was only in the last decade that similar effects have been observed in ordered soft condensed matter systems. Lang et al have investigated this problem for the particular case of lyotropic liquid crystalline systems consisting of nonionic surfactants and water. One example is discussed here (pdf) in more detail. In the future we will investigate this effect in suspensions of rod-like colloidal particles which undergoe isotropic to nematic transitions.

Phase behavior at interfaces

If the surface particles are less tightly bound than those in the interior of a crystal, one may expect that melting will start at the surface and propagate into the interior of the crystal. Such a behavior is called surface melting and has been observed for various metals and molecular crystals including ice. The opposite effect, i.e. when an ordered surface layer coexists with a bulk liquid at temperatures above the melting point of the bulk phase, is called surface freezing and has been observed so far for mesogenic substances forming thermotropic liquid crystals and for long-chain n-alkanes and alkanols. Lang et al have shown that such surface-induced shifts of first-order phase transitions also occur in lyotropic systems of amphiphilic compounds. We will investigate in the future as to whether similar phenomena will also occur in suspensions of colloidal particles which undergoe order/disorder transitions.

Interaction of colloidal particles with a wall

In collaboration with Dr. C. Bechinger (Universität Konstanz)

In order to understand the phenomena described in the sections "Phase behaviour at interfaces" and "Structure formation at interfaces" on a microscopic basis it is necessary to obtain a quantitative picture of the interaction potential of the colloidal particles with the interface. An elegant method, Total Internal Reflection Microscopy (TIRM) has been developed over the last 10 years [1], which allows to measure interaction potentials between particles and appropriately designed solid surfaces. With this technique it is in principle possible to measure electrostatic interaction potentials, van der Waals potentials and depletion forces as well as attraction potentials between receptor-ligand pairs of proteins. We will in future apply TIRM to measure the interaction potential of rod like colloids against solid walls and attempt to expand the features of the technique to measure potentials against liquid/liquid interfaces as well.

[1]. D. C. Prieve Adv. Colloid Interf. Sci. 82, 93 (1999).

Dynamics at interfaces

The dynamics of colloids in solution will be affected by the presence of an interface. It is rather obvious that the translational diffusion coefficient of spheres parallel to a nearby interface will be different from that in the direction perpendicular to the interface. Accordingly, the rotational diffusion of rod like colloids will be hindered, if they come close to an interface. We will investigate these effects by comparing results from bulk dynamic light scattering and DLS using evanescent wave techniques.

Synthesis and characterization of soft rod colloids

In collaboration with Dr. L. Willner, Institut für Festkörperforschung, FZJ-Jülich

We are synthesizing soft rod colloids on the basis of cross linked polymeric networks. The formation of mesoscopically anisometric particles is mediated by the use of micellar assemblies as templates for the structures to be formed. As Förster and Bates have shown recently, it is possible to form rubber like micro gels of cylindrical shape, if the core of a micelle consisting of poly(butadiene)-b-poly(ethyleneoxide) in water is cross linked either by g-irradiation or by chemical means. We use two different approaches to synthesize polymeric rods, one with high and one with low glass transition temperature

• Crosslinking of Blockcopolymer micelles

• Polymerization of oil-swollen cylindrical micelles

Selected Publications of P. Lang

1. Influence of temperature and oil-to-surfactant ratio on micellar growth in aqueous solutions of C₁₂E₅ with decane
   U. Menge, P. Lang, G. H. Findenegg; Coll. Surf. A 163, 81 (2000)
2. Surface effects of lyotropic liquid crystalline phases of nonionic surfactants
   P. Lang, Chr. Braun, R. Steitz; Coll. Surf. A 163, 91.(2000)
3. Bulk structure and amphiphilicity of semifluorinated alkanes
   P. Marczuk, P. Lang, M. Möller; Coll. Surf. A 163, 103 (2000)
4. Shape transformation of Poly(butadiene)-b-poly(ethyleneoxide) plus DTAB compound micelles in aqueous solutions
   H. Egger, A. Nordskog, P. Lang; Macromolecules 162, 291-306 (2000)
5. Stratification in Monolayers of a Bidisperse Melt Polymer Brush as Revealed by Neutron Reflectivity
   W.A. Goedel, C. Luap, R. Oeser, P. Lang, Chr. Braun, R. Steitz; Macromol. 32, 7599 (1999)
6. The surface phase diagram of the hexagonal phase of the C₁₂E₅ / water system
   P. Lang; J. Phys. Chem. B 103, 5100 (1999)
7. From oil-swollen wormlike micelles to microemulsion droplets: A static light scattering study of the L1-phase of the system water+C₁₂E₅+decane
   U. Menge, P. Lang, G. H. Findenegg; J. Phys. Chem. B 103, 5768 (1999)
8. Surface effects accompanying the -to- transition of the amphiphile C₁₂E₄ in water as studied by neutron reflectivity
   R. Steitz, Chr. Braun, J. Bowers, P. Lang, G. H. Findenegg; Ber. Bunsen-Ges. Phys. Chem. 102, 1615 (1998)
9. Surface relaxation of a hexagonal lyotropic mesophase
   P. Lang, Chr. Braun, R. Steitz, G. H. Findengg, H. Rhan; J. Phys. Chem. B 102, 7590 (1998)
10. A structural x-ray study on semifluorinated alkanes (SFA): SFA revisited
    P. Marczuk, P. Lang; Macromolecules 31 9013 (1998)
11. Gels of semifluorinated alkanes: structural investigations by small-angle x-ray scattering
    P. Lang, P. Marczuk, E. Lermann, M. Möller; Ber. Bunsen-Ges. Phys. Chem. 102, 1644 (1998)
12. Micellar solutions of octyl monoglucoside in the presence of butanol: a small angle and light scattering study
    A. Möller, P. Lang, G. H. Findenegg; Ber. Bunsen-Ges. Phys. Chem. 101, 1121 (1997)
13. Preordering phenomena of complex fluids at solid/liquid interfaces
    R. Steitz, Chr. Braun, P. Lang, G. Reiss, G. H. Findenegg; Physica B 234-236, 377 (1997)
14. Small-angle x-ray scattering from aqueous solutions of tetraoxyethylene-n-octyl ether
    P. Lang, O. Glatter; Langmuir 12, 1193 (1996)
15. Phase behavior and scattering experiments of an oligo(ethylene oxide)-n-perfluorooctyl ether/water system
    P. Marczuk, P. Lang, H.-N. Huang; J. Phys. Chem. 100, 13822 (1996)
16. Surface induced shift of the hexagonal-to-isotropic phase transition in a lyotropic system studied by x-ray reflectivity
    Chr. Braun, P. Lang, G. H. Findengg; Langmuir 11, 764 (1995)
17. Clustering of micelles in aqueous solutions of tetraoxyethylene-n-octyl ether C₈E₄ as monitored by static and dynamic light scattering
    H. Strunk, P. Lang, G. H. Findenegg; J. Phys. Chem. 98, 11557 (1994)
18. Structure of PMMA/EGDMA star-branched microgels
    P. Lang, W. Burchard, M. S. Wolfe, J. Spinelli, L. Page; Macromolecules 24, 1306 (1991)

Chapters in books

1. Amphiphiles at interfaces studied by surface sensitive x-ray scattering P. Lang, "Modern Characterization Methods of Surfactant Systems", B. P. Binks (Ed.), "Surfactant Science Series 83", Marcel Dekker Inc., New York, 1999
2. Interfacial Effects of Dilute Solutions and Lyotropic Liquid Cristalline Phases of Nonionic Surfactants G.H. Findenegg, Chr. Braun, P. Lang, R. Steitz in "ACS Sympossum Series 736 Supramolecular Structure in Confined Geometries" S. Manne, G.G. Warr (Eds.) 1999