Light scattering applied to self‐assembling systems
Uncharged Systems
Equilibrium
Static and dynamic light scattering provide a unique tool to study the shape and size of systems on a length scale ranging from 1 nm to several µm. Light scattering techniques are non‐
evasive and can be used to learn about the in situ structure of a system as it forms in solvent, contrary to for example electron microscopy or atomic force microscopy. We applied light scattering to two newly developed molecular systems that were designed to spontaneously self‐organize into a one‐dimensional structure: Ring‐coil triblock copolymers [1] and dendron rodcoil molecules [2]. Ring‐coil triblock copolymers consist of a rigid ring with two polymer coils attached. When dissolved in a solvent that is a poor solvent for the ring but a good solvent for the coils rodlike aggregates are supposed to form, as shown in Figure 1. Static light scattering on solutions of ring‐coil triblock copolymers in cyclohexane
indeed shows the presence of rigid rods whose length increases with time and decreases with increasing molecular weight of the coils. Dynamic light scattering gives a hydrodynamic size of the aggregates and shows the presence of unaggregated ring‐coil triblock copolymers.
Dendron rodcoil molecules, on the other hand, comprise a dendritic unit capable of forming hydrogen bonds, attached to a rodcoil molecule. It is supposed that four dendron rodcoils self‐assemble into a tetramer via hydrogen bonds. The tetramers then form a columnar structure with the tetramers stitched along the column axis via additional hydrogen bonds.
In this case, static light scattering shows the presence of wormlike chains, with a persistence length of 45 nm [2].
For both systems an unexpected discrepancy appears:
Dynamic light scattering indicates an aggregate size which is a factor two larger than found with static light scattering. This problem is the subject of ongoing research.
Non‐Equilibrium
Micellar and microemulsions systems change their shape with temperature and concentration.
Therefore, they are also suitable model systems to study systematically non‐equilibrium effects, such as the Soret effect. Sugar surfactants have frequently been used to study the dissolution and for‐
mation of biological membranes and the stabilization of proteins [3]. Among the nonionic sugar sur‐
factants n–Octyl –D–glucopyranoside has a fairly high critical micelle concentration (cmc). This makes measurements below the cmc possible. The sugar surfactant molecules form spherical micelles above the cmc, while below the cmc, the surfactant molecules in solution are in equilibrium with those adsorbed at the water/air interface. Therefore, below the cmc we will observe the thermal diffusion behavior of individual surfactant molecules, and above the cmc we additionally
Fig.1: Schematic view of the Ring coil triblock copolymer which are in equilibrium with formed aggregates
Fig. 2: Schematic view of the aggregates formed by the dendron‐rod‐coil tirblocks.
Fig.1: Schematic view of the Ring coil triblock copolymer which are in equilibrium with formed aggregates
have a thermophoretic motion of the micelles.
This might lead to a pronounced change of the thermal diffusion or Soret coefficient, because above the cmc the interface between micelles and solvent is mainly determined by the polar headgroups of the sugar surfactant molecules.
Figure 3 shows the measured Soret coefficient as function of the concentration in the vicinity of the cmc at 40 °C [3]. The slope of the concentration dependence of the Soret coefficient becomes much steeper for concentrations above the cmc.
Two physical reasons are responsible for the observed behavior. The micelles are larger than the single sugar surfactant molecules, which leads to a slower diffusion and therefore to an increase of the Soret coefficient. Additionally, the interface interactions between the solvent molecules and the micelles are mainly determined by the hydrophilic glucopyranoside headgroups and not anymore by the alkyl chains as below the cmc. The obtained cmc values are in good agreement with the results from surface tension measurements.
We also investigated the thermal diffusion behavior for higher surfactant concentrations [3]. For high concentrations above w = 1.0 wt% the Soret coefficient decays almost linearly. By decreasing the temperature this decay becomes steeper and the Soret coefficient becomes negative indicating that the micelles enrich at the warm side. For the two highest temperatures of 30 °C and 40 °C we did not observe a sign change in the investigated concentration range, but it is expected that it will occur at higher concentrations.
The decay of the Soret coefficient at high concentrations seems to be a typical phenomenon and has also been found for polymer solutions and colloidal dispersions. For high concentrations the Soret coefficient of the polymeric system shows an asymptotic scaling law with concentration ST = C0 C‐0.65, whereas the exponent changes from ‐0.65 to ‐1 approaching the concentrated regime. For the investigated sugar surfactant system the exponent is not temperature independent but decreases from ‐0.42 to ‐1.44 with decreasing temperature.
With this contact free method we are able to study the transport of biocompatibel materials in a nonequilibrium environment caused by a temperature gradient.
References:
[1] S. Rosselli, A.‐D‐ Rammminger, T. Wagner, B. Silier, S.Wiegand, W.Häußler, G.Lieser, V.Scheumann and S. Höger, Coil‐ring block copolymers as building blocks of hollow supramolecular cylindrical brushes, Angew. Chemie Int. Ed., 40 (2001), p.3138‐3141.
[2] E.R. Zubarev, S.I. Stupp, Journal of Physical Chemistry B, A light scattering study of the self‐
assembly of dendron rodcoil molecules, B.J. de Gans, S. Wiegand, 106 (2002), p.9730‐9736.
[3] Arlt, B., S. Datta, T. Sottmann, and S. Wiegand, Soret Effect of n-Octyl beta-D-Glucopyranoside C8G1 in Water around the Critical Micelle Concentration J. Phys. Chem. B 114(2010) 2118-2123.
Fig. 3: The Soret coefficient ST at at 40 °C versus concentration. All measurements have been performed with the IR‐TDFRS without dye. The vertical line marks the cmc. The solid lines are guides to the eye
Charged Systems ‐Polyelectrolytes
Recent numerical modelling and analytical theory support earlier claims that attractive interactions between rodlike macroions of the same charge give rise to complex superstructures in aqueous solutions of polyelectrolytes. The structures arise from mutual attractions mediated by counterions which remain nonuniformly condensed near the rod surface. Double‐helical DNA is a system frequently addressed in this context. The implications of attractive forces between negatively charged DNA strands themselves and other charged biosystems such as proteins or phospholipid membranes have been amply discussed (e. g.
superfolding, creation of DNA vectors).
The goal is to use a simple synthetic model system based on poly(p‐phenylene)s which mimics some of the principle phenomena governing interactions between biopolymeric systems. The primary structure consists of a rigid poly(p‐
phenylene) backbone with sulfonate and n‐alkyl side‐groups which together render the polyelectrolyte amphiphilic. In water it forms cylindrical micelles of defined radial aggregation number with a negatively charged
surface on account of the sulfonate groups. These micelles undergo further association to form lyotropic objects of internal nematic order on the 500 nm length scale. The actual length depends on the chain length and also on the counterion.
Future research includes the investigation of the influence of parameters as concentration, main chain length and counterion valency as well as the length of the aliphatic side chains on the structure formation. By comparison with simulation results a better physical understanding of aggregation behavior should be achieved.
References:
[1] J. Belack, S. Wiegand, D. Vlassopoulos, G. Fytas, G. Wegner, Structure and Dynamics of a shape persistent polyelectrolyte of the poly(para‐phenylene) type, manuscript prepared for submission to Macromolecules.
[2] J. Belack, A. delCampo, S. Wiegand, G. Wegner, A. Janshoff, Observation of Secondary Structure Formation of Amphiphilic Polyelectrolytes on Modified Silica Surfaces by Atomic Force Microscopy in Aqueous Solution, manuscript prepared for submission to Langmuir.
Fig. 5: Schematic view of the cylindrical micelle formed by the poly(p‐phenylene)s.
Fig. 6: AFM image of poly(p‐
phenylene) micelle.
