Long-Term Stability of Pickering Nanoemulsions Prepared Using Diblock Copolymer Nanoparticles: Effect of Nanoparticle Core Crosslinking, Oil Type, and the Role Played by Excess Copolymers

Abstract

A poly( N , N ′-dimethylacrylamide) (PDMAC) precursor is chain-extended via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of diacetone acrylamide (PDAAM) to produce PDMAC 77 -PDAAM 40 spherical nanoparticles. Post-polymerization core-crosslinking of such nanoparticles was performed at 20 °C, and the resulting covalently stabilized nanoparticles survive exposure to methanol. The linear and core-crosslinked nanoparticles were subjected to high-shear homogenization in turn in the presence of n -dodecane to form macroemulsions. Subsequent processing of these macroemulsions via high-pressure microfluidization produced nanoemulsions. When using the core crosslinked nanoparticles, the droplet diameter was strongly dependent on the copolymer concentration. This indicates that such nanoparticles remain intact under the processing conditions, leading to formation of genuine Pickering nanoemulsions with a z -average diameter of 244 ± 60 nm. In contrast, the linear nanoparticles undergo disassembly to afford molecularly dissolved diblock copolymer chains, which stabilize oil droplets of 170 ± 59 nm diameter. The long-term stability of these two types of n -dodecane-in-water nanoemulsions with respect to Ostwald ripening was examined using analytical centrifugation. When prepared at the same copolymer concentration, Pickering nanoemulsions stabilized by core-crosslinked nanoparticles proved to be significantly more stable than the nanoemulsion stabilized by the amphiphilic PDMAC 77 -PDAAM 40 chains. Moreover, higher copolymer concentrations led to a significantly faster rate of droplet growth. This is attributed to excess copolymer facilitating the diffusion of n -dodecane through the aqueous phase. Finally, analytical centrifugation is used to assess the long-term stability of the analogous squalane-in-water nanoemulsions. These systems are much more stable than the corresponding n -dodecane-in-water nanoemulsions, regardless of whether the copolymer is adsorbed as sterically stabilized nanoparticles or surface-active chains.

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

“…Moreover, our prior studies suggested that nanoparticle disintegration can occur at higher applied pressures, leading to the generation of individual diblock copolymer chains. , Thus such conditions are best avoided if genuine Pickering nanoemulsions are desired. In view of these preliminary observations, a nanoparticle concentration of 5% w/w and a single pass at an applied pressure of 20,000 psi were used for the remaining experiments.…”

“…Analytical centrifugation was used to determine the volume-average size distribution for both fresh and aged Pickering nanoemulsions prepared using various NaI concentrations. We have shown that analytical centrifugation is well-suited for determining the long-term stability of both oil-in-water and water-in-oil Pickering nanoemulsions. − Unlike DLS, analytical centrifugation subjects the nanoemulsions to droplet fractionation prior to detection and therefore offers higher resolution.…”

“…Polymerization-induced self-assembly (PISA) has become widely recognized as an efficient and versatile route to a wide range of sterically stabilized diblock copolymer nanoparticles. − Importantly, PISA can be conducted in either polar or non-polar solvents, which enables the rational design of either hydrophilic or hydrophobic nanoparticles. Over the past seven years, we have reported that such nanoparticles can be used to stabilize either oil-in-water − or water-in-oil Pickering nanoemulsions, respectively.…”

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

“…56,60,63−65 Thus aqueous PISA enables the convenient synthesis of hydrophilic spherical nanoparticles that are suitable for the preparation of Pickering nanoemulsions. 13,[16][17][18]27,43 For example, Thompson and co-workers examined the relative stability of a series of n-alkane-in-water Pickering nanoemulsions prepared using sterically stabilized nanoparticles comprising a hydrophilic poly(glycerol monomethacrylate) (PGMA) block and a hydrophobic poly(2,2,2trifluoroethyl methacrylate) (PTFEMA) block. 16,27 Analytical centrifugation studies confirmed that nanoemulsions prepared using oils with higher aqueous solubility (e.g., n-octane or ndecane) were more susceptible toward Ostwald ripening during storage for up to 7 days at 20 °C.…”

“…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.…”