Chapter 2 – Overview of the Thesis
2.1 Janus Cylinders at Liquid-Liquid Interfaces
Colloidal objects in the range from nanometer to micrometer are one of the main building blocks for modern materials. Therefore, Janus structures have been the subject of inten-sive research due to their special architecture. The lack of centrosymmetry in Janus sys-tems has led to the discovery of new properties as well as very interesting aggregation behavior into superstructures.1,2 Until now, simulations and calculations demonstrated that the adsorption energy of Janus particles can be higher compared to classic surfactants by several orders of magnitude. As a consequence, a considerable increase in surface ac-tivity is expected. These predictions render Janus structures as an extremely interesting new class of future surfactants.1,3
In this chapter, adsorption studies of moderately amphiphilic Janus cylinders at liquid-liquid interfaces are presented. The focus was laid on the effect of the Janus cylinder length and the adsorption times on the resulting structures at the interface. This was char-acterized via TEM and time-dependent evolution measurements of the interfacial tension.
Figure 2-1. (A) Schematic synthesis of Janus cylinders. (B) Time dependence of the number-average length for different sonication times at 30% amplitude. Error bars represent the standard deviations of the length distribution. (C) Cryo-TEM image of Janus cylinder dispersion in THF with a number-average length of 2300 nm.
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The synthetic pathway to obtain cylindrical Janus structures was based on a template-assisted synthesis, involving crosslinking of a microphase-segregated lamellar-cylinder morphology of a bulk film of a polystyrene-b-polybutadiene-b-poly(methyl methacrylate) (S41B14M45110), followed by sonication (Figure 2-1 A). This process resulted in the for-mation of core-crosslinked cylinders, possessing a PB core and two hemicylinders of PS and PMMA (Figure 2-1 C). It was possible to adjust the cylinder length via variation of the sonication time. A detailed look at the evolution of cylinder length with sonication time showed a rapid decay with increasing sonication time (Figure 2-1 B). Here, the length of the cylinders followed a non linear decay from 2300 nm to 350 nm with increas-ing sonication times from 0 to 35 minutes.
Figure 2-2. Influence of the Janus cylinder length on the interfacial tension. Interfacial tension isotherms of solutions of Janus cylinders in dioxane at the PFO/dioxane interface (c = 1 g/L). Interfacial tension iso-therms for uncrosslinked SBM and homogeneous BS core-shell cylinders are included.
An elegant way for the determination of the influence of particles at liquid-liquid inter-faces is to analyze the interfacial tension of a dispersion of the desired material via the pendant-drop method. From the time dependent measurements of the interfacial tension, it was possible to specify the characteristics of different stages of the adsorption process.
In Figure 2-2 the adsorption behavior of Janus cylinders with different lengths, uncross-linked SBM and polystyrene-b-polybutadiene (BS) core-shell cylinders at a perfluoroc-tane (PFO)/dioxane interface was investigated. Due to their slightly amphiphilic charac-ter, the Janus cylinders demonstrated a significantly enhanced surface activity compared to cylindrical particles with a homogeneous PS corona (made from the diblock copolymer SB (polystyrene-b-polybutadiene)) or the block terpolymer precursor SBM. Therefore,
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these particles are ideal systems for a new class of superior surfactant. A series of pendant drop tensiometer measurements for cylinders with number-average lengths of 2300 nm to 350 nm, dissolved in dioxane at a concentration of 1 g/L, is shown in Figure 2-3. The interfacial tension decreased with time and approached quasi-equilibrium after 3 h. At the beginning of the adsorption process, the interfacial tension decreased fast. Subsequently, the decrease slowed down, and finally, it approached a plateau, where the maximum cov-erage of the interface with cylinders was obtained. After reaching the plateau value, the Janus cylinders were located and arranged at the interface. An increase of the average length of the Janus cylinders led to an enhanced adsorption at the interface and the plat-eau value was reached earlier. The interfacial tension decreased with increasing Janus cylinder length and concentration.
For a better understanding of the structures formed at the interface and the adsorption mechanism, the absorption of 2300 nm Janus cylinder was investigated using pendant drop tensiometry (Figure 2-3). Simultaneously, TEM images of the particle assemblies at the interface were taken at the adsorption times indicated in the interfacial tension iso-therms. In combination with the logarithmic representation of the interfacial tension (Fig-ure 2-3 inset), the three different adsorption stages can be identified. At first, free diffu-sion to the interface occurred (I), followed by continuous adsorption of cylinders includ-ing an orderinclud-ing and domain formation at the interface (II). Finally, additional packinclud-ing led to a rearrangement of the domains and to the formation of a multilayer system (III). The cylinders showed a liquid crystalline like short-range correlation at the fluid interfaces.
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Figure 2-3. Left top: Adsorption curves in linear and logarithmic presentation. The red points A-H indi-cate: Series of TEM images (obtained from lacey grids) of 2300 nm Janus cylinders (1 g/L) adsorbing at the PFO/dioxane interface at different times as noted in the interfacial isotherms. (scale bars: 1 µm).
As a further result it was found that the adsorption behavior is independent of the solvent combination. This was confirmed by two different systems, PFO/Dioxane and PFO/DMSO, and the obtained relative change in interfacial tension showed similar char-acteristics. Hence, these results showed the high potential of Janus particles as a new class of superior surfactants and as a novel tool for the nanostructuring of interfaces.
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