The combination of polyelectrolytes and ionic surfactants in precise proportions presents the possibility of producing a new class of emulsifiers with tunable emulsification properties. We use chitosan along with dioctyl sulfosuccinate sodium, also known as aerosol-OT (AOT), to demonstrate that emulsion types can be varied, and phase inversion emulsification (PIE) can be induced via changes in the water-phase pH and the molar ratio of the surfactant to the repeat unit of the polyelectrolyte. Confocal microscopy of the emulsions shows that the morphology can be changed from O/W to O/W/O to W/O by varying the surfactant to polyelectrolyte molar ratio at a fixed aqueous-phase pH while maintaining droplet sizes in the range of micrometers to tens of micrometers. Measurements of the oil (toluene)–water partition coefficient suggest that controlling the emulsion type relies on the ability of the surfactants to partition from the bulk oil to the bulk water phase and to induce polyelectrolyte–surfactant aggregation. We confirm this hypothesis using different combinations of polyelectrolytes and surfactants. Changes in the water-phase pH in situ induce phase inversion only in a particular direction, which suggests that the complexes at the interface are in a kinetically trapped state. Changes in the molar ratio in situ by addition of an oppositely charged surfactant also can induce phase inversion.
Colloidal colorimetric microsensors enable the in‐situ detection of mechanical strains within materials. Enhancing the sensitivity of these sensors to small scale deformation while enabling reversibility of the sensing capability would expand their utility in applications including biosensing and chemical sensing. In this study, we introduce the synthesis of colloidal colorimetric nano‐sensors using a simple and readily scalable fabrication method. Colloidal nano sensors are prepared by emulsion‐templated assembly of polymer‐grafted gold nanoparticles (AuNP). To direct the adsorption of AuNP to the oil‐water interface of emulsion droplets, AuNP (≈11nm) are functionalized with thiol‐terminated polystyrene (PS, Mn = 11k). These PS‐grafted gold nanoparticles are suspended in toluene and subsequently emulsified to form droplets with a diameter of ≈30µm. By evaporating the solvent of the oil‐inwater emulsion, we form nanocapsules (AuNC) (diameter < 1µm) decorated by PS‐grafted AuNP. To test mechanical sensing, the AuNC are embedded in an elastomer matrix. The addition of a plasticizer reduces the glass transition temperature of the PS brushes, and in turn imparts reversible deformability to the AuNC. The plasmonic peak of the AuNC shifts towards lower wavelengths upon application of uniaxial tensile tension, indicating increased inter‐nanoparticle distance, and reverts back as the tension is released.
Flow-based nanoprecipitation of different solutes via rapid mixing of two miscible liquids is a scalable strategy for manufacturing nanoparticles with various shapes and morphologies. Controlling the size of nanoparticles in flow-based nanoprecipitation, however, is often left to empirical variations in the flow rate ratios or the total flow rate of the two streams. In this work, we investigate the coprecipitations of oil and polymer to form nanocapsules via the Ouzo effect using glass capillary microfluidics across a range of mixing conditions. In the range of flow rates studied, the two streams mix convectively in microvortices formed at the junction of the two stream inlets. Using computational fluid dynamics simulations and glass capillary microfluidic nanoprecipitation, we establish a relationship between the precipitation conditions occurring experimentally in situ and the location on the ternary Ouzo phase diagram where precipitation is taking place. We find that a key variable in the resulting average diameter of the fabricated capsules is the degree of supersaturation experienced by both the oil and the polymer in the vortex zone of the device, showing a strong correlation between the two values. The control over the nanocapsule size by varying the extent of supersaturation of both precipitants is demonstrated by using two oils having distinct phase diagrams. This work provides a systematic approach to controlling the size of nanoparticles fabricated via continuous nanoprecipitation by linking the in situ flow conditions to ternary phase diagram behavior, enabling accurate control over nanocapsule size.
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