Complex Self-Assembled Systems

Research Activities of the Sottmann Group

Nanostructured self-assembled systems in microemulsions

Overview

This topic deals with complex self-assembled systems which are formed either by addition of amphiphilic block copolymers or by application of flow to nano-structured systems such as microemulsions. Methods used to characterize the structure of these systems are scattering methods, e.g. static or dynamic light scattering (SLS or DLS) and small-angle neutron scattering (SANS), as well as NMR self-diffusion.

In previous studies, it was found that polymers of the type poly(ethylene propylene)-co-poly(ethylene oxide) can induce an enormous efficiency boosting effect in microemulsion systems [1]. Therefore, in one of our studies we examined the efficiency boosting effect of a new type of polyether-polycarbonate block-co-polymers (mPEG-b-PBC) in microemulsion systems. These novel block copolymers, which are accessible from inexpensive materials via a simple and solvent-free polymerization using CO2, were synthesized by the group of Prof. Dr. H. Frey (Department of Chemistry, Johannes Gutenberg University Mainz). We were able to show that these block copolymers boost the efficiency of medium-chain surfactants when technically relevant polar oils, e.g. ether and ester oils, have to be solubilized in an emulsion or a microemulsion [2].

mPEG-b-PBC block copolymers and their boosting effect with SANS curves
Figure 1: (Left, top) Synthesis of mPEG-𝑏-PBC block copolymers. (Left, bottom) Large parts of the oil and water excess phases (left test tube) are sucked up by the surfactant-rich middle phase upon adding copolymers. Therefore, the volume of the middle phase is increased (right test tube). (Right) SANS curves of the system D₂O − hexyl methacrylate − C₁₀E₆/mPEG₄₄-𝑏-PBC₁₀ with increasing polymer to surfactant plus polymer ratio (increasing ẟ values) recorded at ϕ = 0.50 (identical volumes of water and oil) in bulk contrast. The shift of the correlation peak indicates a considerable increase of the length scale of the microstructure [2].

Similarly, the Frey group synthesized new diblock copolymers of the type poly(ethylene oxide)-poly(alkyl glycidyl ether), namely mPEO-b-PAlkGE and the technical component-based, CO2-containing mPEO-b-PCO2AlkGE. These polymers exhibit a strong boosting effect in different H2O/NaCl – oil – non-ionic surfactant systems. The boosting can not only be observed with medium-chain surfactants/oils, but also in systems containing long-chain oils and technical-grade waxes and surfactants. The so-called membrane model [3, 4] can be used to scale the efficiency boosting of different polymers based on the number density of polymers in the membrane and the polymer blocks’ end-to-end distances [5]. For microemulsions at room temperature containing medium-chain n-alkanes, our data are in good agreement with PEO-PEP literature data [6]. However, for long-chain oil microemulsions stable at elevated temperatures, altered polymer-oil-surfactant interactions seem to influence the scaling.

mPEO-P(CO2)AlkGE block copolymers and scaling of efficiency boosting depending on polymer and system
Figure 2: (Left) Efficiency boosting effect in a H₂O/NaCl – 𝑛-decane – C₁₀E₄ at different polymer concentrations δ. An increase in δ increases the efficiency strongly. (Middle) Schematic drawing of the amphiphilic film with adsorbed block copolymers. (Right) Scaling of efficiency boosting with polymer coverage for different polymers and microemulsion systems and compared to literature data [5,6].

In another project with Prof. Dr. João T. Cabral (Imperial College London, Department of Chemical Engineering) and Lionel Porcar (Institut Laue-Langevin, Grenoble), we study the structural influence of flow on bicontinuously structured microemulsions via the Microfluidic-SANS method [7]. The chosen standard microemulsion system shows a bicontinuous-to-lamellar transition with increasing surfactant concentration at the phase inversion temperature located near room temperature, i.e. PIT = 22.5 °C. Furthermore, the scattering experiments reveal the emergence of an anisotropic scattering pattern under certain flow conditions (Fig. 3). Our main goal is trying to understand the physical reasons for this transition as well as the morphology of the induced structure.

Microfluidics: SANS data and scattering patterns
Figure 3: (Left) SANS curve of the bicontinuously structured microemulsion D₂O – 𝑛-octane – C₁₀E₄ at 𝑇 = 22.5 °C and γ = 0.150, fitted with the Teubner-Strey model [8] (Right) 2D-scattering patterns of the same system under flow before, inside, and after the constriction of a Microfluidic-SANS chip.

References

  1. Jakobs, B., Sottmann, T., Strey, R., Allgaier, J., and Richter, D.: Amphiphilic Block Copolymers as Efficiency Boosters for Microemulsions. Langmuir. 15, 6707–6711 (1999). https://doi.org/10.1021/la9900876
  2. Kunze, L., Tseng, S.-Y., Schweins, R., Sottmann, T., Frey, H.: Nonionic Aliphatic Polycarbonate Diblock Copolymers Based on CO2, 1,2-Butylene Oxide, and mPEG: Synthesis, Micellization, and Solubilization. Langmuir. 35, 5221–5231 (2019). https://doi.org/10.1021/acs.langmuir.8b04265
  3. Hiergeist, C., Lipowsky, R.: Elastic properties of polymer-decorated membranes. Journal de Physique II. 6. 1455-1481 (1996). https://doi.org/10.1051/jp2:1996142
  4. Endo, H., Mihailescu, M., Monkenbusch, M., Allgaier, J., Gompper, G., Richter, D., Jakobs, B., Sottmann, T., Strey, R., Grillo, I.: Effect of amphiphilic block copolymers on the structure and phase behavior of oil–water-surfactant mixtures. The Journal of Chemical Physics. 115, 580–600 (2001). https://doi.org/10.1063/1.1377881
  5. Jakobs, B.: Amphiphile Blockcopolymere als "Efficiency Booster" für Tenside: Entdeckung und Aufklärung des Effektes. Dissertation, Universität zu Köln, Cuvillier Göttingen (2002). ISBN: 9783898734370
  6. Schneider, K., Verkoyen, P., Krappel, M., Gardiner, C., Schweins, R., Frey, H., Sottmann, T.: Efficiency Boosting of Surfactants with Poly(ethylene oxide)-Poly(alkyl glycidyl ether)s: A New Class of Amphiphilic Polymers. Langmuir. 36, 9849--9866 (2020). https://doi.org/10.1021/acs.langmuir.0c01491
  7. Lopez, C., Watanabe, T., Martel, A., Porcar, L. and Cabral, J.T.: Microfluidic-SANS: flow processing of complex fluids. Scientific Reports. 5, 7727 (2015). https://doi.org/10.1038/srep07727
  8. Teubner, M. and Strey, R.: Origin of the scattering peak in microemulsions. The Journal of Chemical Physics. 87, 3195-3200 (1987). https://doi.org/10.1063/1.453006

Cooperations

This image shows Maximilian Krappel

Maximilian Krappel

 

Doctoral Researcher

This image shows Julian Fischer

Julian Fischer

 

Doctoral Researcher

This image shows Thomas Sottmann

Thomas Sottmann

Prof. Dr.

Professor

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