The microgel synthesis was done by standard precipitation polymerization with surfactant and was already described in detail elsewhere.[26] A detailed description of the experimental procedures can be found in the Supporting Information. In brief, compression isotherms were recorded on a Langmuir trough equipped with two movable barriers. A platinum plate was placed parallel to the barriers and the change in surface pressure was recorded with a Wilhelmy balance. A defined amount of aqueous microgel dispersion was placed directly at the decane-water interface and the compression was performed after an equilibration time of 1 h with a speed of 10 mm/min. Isopropylalcohol was added to the microgel dispersion to facilitate spreading at the interface.
6.6 Supporting Information
Materials. N-decane (Merck, > 94%) was filtered three times over a column of basic aluminium oxide (Merck) to remove any polar contaminants. Doubly distilled Milli-Q water was used for synthesis and characterization of the microgels and preparation of microgel dispersions. 0.1M HCl and 0.1M NaOH was used to adjust the pH of the Milli-Q water that was used as subphase in the Langmuir trough measurements to pH 3 or pH 9.
The microgels were placed at the interface in a mixture of aqueous 1 wt% microgel dispersion (pH 3 or pH 9) with isopropyl alcohol (IPA, 99.8%, Merck) in a ratio of 5:1 (v:v).
Microgel Synthesis and Characterization. The microgel synthesis was done by standard precipitation polymerization with surfactant and was already described in detail elsewhere.[26] An ALV-5000 instrument with light of 633 nm wavelength was used to determine the hydrodynamic diameter of the microgels in bulk with dynamic light scattering (DLS). Electrophoretic mobility was measured with a NANO ZS Zetasizer (Malvern Instruments, UK). The content of MAA in the microgels was determined by pH titration to 6.3 ± 0.6 wt% for the P(NiPAm-co-MAA) microgel and 2.8 ± 0.5 wt% for the second microgel. The diameter of the core at the interface determined from FreSCa cryo-SEM imaging is di,c = 358 ± 23 nm (pH 3) and di,c = 352 ± 10 nm (pH 9) for the P(NiPAm-co-MAA) microgel and di,c = 436 ± 43 (pH 3) and di,c = 438 ± 15 nm (pH 9) for the second microgel. The diameter of the corona at the interface is di = 559 ± 49 nm
COMPRESSION ISOTHERMS
(pH 3) and di = 534 ± 14 nm (pH 9) for the P(NiPAm-co-MAA) microgel and di = 642 ± 66 (pH 3) and di = 671 ± 22 nm (pH 9) for the second microgel.[26]
FreSCa Cryo-SEM. The sample preparation for FreSCA cryo-SEM is analogous to the one already described.[26] 0.5 µL of an aqueous microgel suspension at 0.1 wt% were placed inside a custom-made copper holder with a 200 µm deep central cavity. Prior to filling, the sample holders were roughened, ultrasonicated in 95% sulphuric acid and ethanol for several minutes and finally exposed to a negative glow discharge to improve hydrophilicity and adhesion during freezing. Successively, a 3.0 µL droplet of heptane was carefully placed on top to create the liquid-liquid interface and then the holder was closed with a flat copper plate (also roughened and cleaned but not exposed to the glow discharge). The closed holder was vitrified in a liquid propane jet freezer (Bal-Tec/Leica JFD 030, Balzers/Vienna) with a cooling rate of 30000 Ks-1 to avoid water crystallization.
After freezing, the samples were mounted under liquid nitrogen onto a double fracture cryo-stage and transferred under inert gas in a cryo-high vacuum airlock (< 5×10-7 mbar Bal-Tec/Leica VCT010) to a pre-cooled freeze-fracture device at -140°C (Bal-Tec/Leica BAF060 device). The samples were then fractured and partially freeze-dried at -100°C for 1 min to remove deposited residual water condensation and ice crystals, followed by unidirectional tungsten deposition at an elevation angle α = 30° to a total thickness δ = 2 nm at -120°C and by additional 2 nm with a continuously varying angle between 90° and 30°. The second deposition is needed in order to avoid charging of the shadow during imaging which may compromise image stability at high magnifications. The presence of a macroscopic flat oil-water interface covered by particles promotes the fracture at the interface itself and allows for inspection of the particle arrangement. Freeze-fractured and metal-coated samples were then transferred for imaging under high vacuum (< 5×10-7 mbar) at -120°C to a pre-cooled (-120°C) cryo-SEM (Zeiss Gemini 1530, Oberkochen) for imaging either with an in-lens or secondary electron detector.
Langmuir Trough. Compression isotherms were recorded at 20°C with a Langmuir trough for liquid-liquid interfaces (KSV NIMA). The trough is made from polyoxymethylene (Delrin) and is equipped with two movable Delrin barriers and a Wilhelmy balance with a Pt-plate. The compressible area between the barriers equals 398 cm2 and the plate is hanging parallel to the barriers. In a typical measurement, the lower part of the trough is filled with water at pH 3 or pH 9. The water-air interface is cleaned with a suction pump and checked for impurities prior to addition of the n-decane. After addition of the n-decane, the interface is checked for impurities again and the Wilhelmy balance is set to zero. The microgel-IPA solution is placed directly at the interface with a Hamilton syringe and the interface is left to equilibrate for 60 min before the compression starts. A velocity of 10 mm/min is used for compression and subsequent expansion of the barriers. The Wilhelmy balance records the change in surface pressure π, which is defined as the difference between the interfacial tension of the clean interface γ0 and the
COMPRESSION ISOTHERMS
Figure 26. Surface pressure/ trough area compression isotherms of different amounts of P(NiPAm-co-MAA) microgels at the decane-water interface at pH 3 in the uncharged state (left) and at pH 9 in the charged state (right). The filled lines correspond to the compression of the interfacial layer and the dashed lines represent the subsequent expansion.
0 50 100 150 200 250 300 350 400 second microgels (P(NiPAm-co-MAA) plus PNiPAm shell) at the decane-water interface at pH 3 in the uncharged state (left) and at pH 9 in the charged state (right). The filled lines correspond to the compression of the interfacial layer and the dashed lines represent the subsequent expansion.
The compression and expansion isotherms of different amounts of the P(NiPAm-co-MAA) microgel in the uncharged and the charged state are shown in Figure 26. Figure 27 presents the corresponding isotherms of the second microgel where the P(NiPAm-co-MAA) core is surrounded by a pure P(NiPAm) shell.
COMPRESSION ISOTHERMS
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ORDERED MICROGEL ARRAYS