Sterically Stabilized Diblock Copolymer Nanoparticles Enable Efficient Preparation of Non-Aqueous Pickering Nanoemulsions

Abstract

We report the first example of a non-aqueous Pickering nanoemulsion, which comprises glycerol droplets dispersed in mineral oil. The droplet phase is stabilized by hydrophobic sterically stabilized poly(lauryl methacrylate)-poly(benzyl methacrylate) nanoparticles which are prepared directly in mineral oil using polymerization-induced self-assembly. First, a glycerol-in-mineral oil Pickering macroemulsion with a mean droplet diameter of 2.1 ± 0.9 μm is prepared via high-shear homogenization using excess nanoparticles as an emulsifier. Then, this precursor macroemulsion is subjected to high-pressure microfluidization (a single pass at an applied pressure of 20,000 psi) to produce glycerol droplets of approximately 200−250 nm diameter. Transmission electron microscopy studies indicate preservation of the distinctive superstructure produced by nanoparticle adsorption at the glycerol/mineral oil interface, thus confirming the Pickering nature of the nanoemulsion. Glycerol is sparingly soluble in mineral oil, thus such nanoemulsions are rather susceptible to destabilization via Ostwald ripening. Indeed, substantial droplet growth occurs within 24 h at 20 °C, as judged by dynamic light scattering. However, this problem can be suppressed by dissolving a non-volatile solute (sodium iodide) in glycerol prior to formation of the nanoemulsion. This reduces diffusional loss of glycerol molecules from the droplets, with analytical centrifugation studies indicating much better long-term stability for such Pickering nanoemulsions (up to 21 weeks). Finally, the addition of just 5% water to the glycerol phase prior to emulsification enables the refractive index of the droplet phase to be matched to that of the continuous phase, leading to relatively transparent nanoemulsions.

Effect of Temperature, Oil Type, and Copolymer Concentration on the Long-Term Stability of Oil-in-Water Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles

“…However, their relatively high surface area makes them prone to droplet growth via Ostwald ripening. ,− For O/W nanoemulsions, This involves the diffusion of oil molecules from smaller droplets through the aqueous continuous phase to larger droplets over time. Although there are many literature reports on the formation of copolymer- or surfactant-stabilized nanoemulsions, there are surprisingly few studies focused on Pickering nanoemulsions. − ,− ,− This is no doubt because the Pickering emulsifier should be typically 5–10 times smaller than the mean droplet diameter . Thus, droplets of (say) 200 nm diameter require nanoparticles of 20–40 nm diameter.…”

“…Pickering nanoemulsions comprise either oil or water droplets of 50–500 nm diameter that are stabilized using nanoparticles. − They are readily prepared via high-pressure microfluidization of Pickering macroemulsions provided that sufficient excess nanoparticles are present to stabilize the additional surface area that is generated when producing much finer droplets. , The final droplet diameter typically depends on both the applied pressure and the number of passes through a commercial microfluidizer . Under optimized conditions, droplet diameters of less than 200 nm can be achieved. ,− Given their much higher surface area per unit mass, nanoemulsions offer more active formulations than conventional emulsions, which may be useful for applications in cosmetics, drug delivery, agrochemicals, , and food technology. , …”

“…Polymerization-induced self-assembly (PISA) offers an efficient synthetic route to a wide range of diblock copolymer nanoparticles of tunable surface wettability. − Typically, aqueous PISA formulations involve growing a water-insoluble polymer chain from one end of a water-soluble polymer precursor. ,,− The resulting amphiphilic diblock copolymer chains undergo in situ micellar nucleation to produce a concentrated colloidal dispersion of sterically stabilized nanoparticles. The most common copolymer morphology is spheres and the mean diameter can be as low as 10–20 nm diameter. ,,− Thus aqueous PISA enables the convenient synthesis of hydrophilic spherical nanoparticles that are suitable for the preparation of Pickering nanoemulsions. ,− ,, …”

Preparation of diblock copolymer nano-assemblies by ultrasonics assisted ethanol-phase polymerization-induced self-assembly

“…Nanoparticles of specific size, morphology and function have significant applications in biomedical [1] , [2] , [3] , [4] , [5] , batteries [6] , [7] , [8] , coatings [9] , [10] , Pickering emulsions [11] , [12] , [13] , [14] , [15] , hydrogel [16] , [17] , catalysis [18] , [19] , [20] , [21] , and so forth [22] , [23] . Traditional self-assembly requires the initial polymerization of diblock copolymers, followed by the preparation of assemblies by changing the solvent.…”

Efficient Synthesis of μ-A(BC)C Miktoarm Star Polymer Assemblies via Aqueous Photoinitiated Polymerization-Induced Self-Assembly

“…Recently, polymerization-induced self-assembly (PISA) via reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization has been explored as a robust technique for the efficient synthesis of block copolymer assemblies. − Currently, a majority of RAFT-PISA formulations have focused on the synthesis of diblock copolymer assemblies using a monofunctional macromolecular RAFT (macro-RAFT) agent. Morphologies of diblock copolymer assemblies could be controlled by changing packing parameter including monomer concentration, reaction temperature, initiation type etc. − In contrast, only a few RAFT-PISA formulations have focused on the synthesis of star block copolymer assemblies. − Compared with the linear analogues, star polymers may exhibit different self-assembled behaviors under RAFT-PISA conditions and therefore provide a new strategy to tune the morphology of polymer assemblies.…”