How Do Charged End-Groups on the Steric Stabilizer Block Influence the Formation and Long-Term Stability of Pickering Nanoemulsions Prepared Using Sterically Stabilized Diblock Copolymer Nanoparticles?

Abstract

Reversible addition−fragmentation chain transfer (RAFT) solution polymerization is used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing carboxylic acid, tertiary amine, or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate to form spherical nanoparticles as confirmed by transmission electron microscopy (TEM) analysis. Dynamic light scattering studies indicated an intensity-average diameter of approximately 25 nm. Aqueous electrophoresis measurements confirmed that the amine-functional nanoparticles became cationic at low pH owing to end-group protonation. In contrast, carboxylic acid-functional nanoparticles became appreciably anionic at pH 10 owing to end-group ionization. Finally, nanoparticles bearing neutral end-groups exhibited zeta potentials close to zero over a range of solution pH. High-shear homogenization of n-dodecane in the presence of such sterically stabilized nanoparticles led to the formation of oil-in-water Pickering macroemulsions with volume-average diameters of 20− 30 μm. High-pressure microfluidization was then used to prepare the three corresponding Pickering nanoemulsions. Each Pickering nanoemulsion was characterized by analytical centrifugation and TEM studies of the dried nanoemulsion droplets confirmed their original nanoparticle superstructure. The nanoparticle adsorption efficiency at the oil−water interface was assessed by gel permeation chromatography (using a UV detector) for each nanoparticle type at both pH 3 and 7. Nanoparticles with charged end-groups exhibited relatively low adsorption efficiency, whereas up to 90% of the neutral nanoparticles were adsorbed onto the oil droplets. This observation was supported by small-angle X-ray scattering experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Analytical centrifugation was used to evaluate the colloidal stability of the aged Pickering nanoemulsions. Pickering nanoemulsions stabilized with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups.

Effect of Ionic Strength, Nanoparticle Surface Charge Density, and Template Diameter on Self-Limiting Single-Particle Placement: A Numerical Study

“…Their relationship is obtained by solving the PBE in the diffuse layer. The OHP is located very close to the slip plane and we assume that the Stern potential ψ d at the OHP is equal to the zeta potential ζ at the shear plane. ,,,− Various techniques can be used to experimentally measure the zeta potentials ζ, ,,− from which the values of the surface charge density σ are obtained from the ψ d –σ relationship (with ψ d = ζ).…”

Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly

“…Nanoemulsions comprise oil or water droplets for which the mean droplet diameter is less than 200 nm. , There are various reports of copolymer- or surfactant-stabilized nanoemulsions in the literature . In contrast, there have been remarkably few examples of Pickering nanoemulsions in which the droplets are solely stabilized by solid particles. ,,− No doubt one reason for the paucity of such studies is the rule-of-thumb requirement that the Pickering emulsifier should be at least 5–10 times smaller than the mean droplet diameter. However, the recent development of polymerization-induced self-assembly now enables the highly convenient synthesis of sterically stabilized diblock copolymer spheres of 20–25 nm diameter directly in the form of concentrated aqueous dispersions. , In principle, such nanoparticles should constitute model Pickering emulsifiers for the stabilization of oil-in-water nanoemulsions.…”

“…Microfluidizer conditions: 20 000 psi, 10 passes. Reproduced from ref (copyright 2020 American Chemical Society).…”

Pickering Emulsions Stabilized by Diblock Copolymer Worms Prepared via Reversible Addition–Fragmentation Chain Transfer Aqueous Dispersion Polymerization: How Does the Stimulus Sensitivity Affect the Rate of Demulsification?

“…have been prepared via PISA formulations mediated by controlled radical polymerization techniques, anionic living polymerization, and ring-opening metathesis polymerization, mainly reversible addition–fragmentation chain transfer (RAFT) polymerization. − For RAFT-mediated PISA, soluble macromolecular chain transfer agents (CTAs) are chain-extended with soluble monomers to form an insoluble second block in a suitable solvent, driving the as-formed amphiphilic diblock copolymer in situ self-assembly during the chain growth . Such sterically stabilized nanoparticles demonstrated promising applications as Pickering emulsifiers. − Compared with other methods for the synthesis of polymer-based Pickering emulsifiers, PISA technology exhibited several outstanding advantages. First, anisotropic worm-like polymer nanoparticles possessed excellent emulsifying performances. , Worm-like polymer nanoparticles are difficult to be accessible by other methods, whereas they could be massively synthesized by PISA.…”