Bespoke Diblock Copolymer Nanoparticles Enable the Production of Relatively Stable Oil-in-Water Pickering Nanoemulsions

Covalent Capture of Nanoparticle-Stabilized Oil Droplets via Acetal Chemistry Using a Hydrophilic Polymer Brush

We report the capture of nanosized oil droplets using a hydrophilic aldehyde-functional polymer brush. The brush was obtained via aqueous ARGET ATRP of a cis-diol-functional methacrylic monomer from a planar silicon wafer. This precursor was then selectively oxidized using an aqueous solution of NaIO4 to introduce aldehyde groups. The oil droplets were prepared by using excess sterically stabilized diblock copolymer nanoparticles to prepare a relatively coarse squalane-in-water Pickering emulsion (mean droplet diameter = 20 μm). This precursor was then further processed via high-pressure microfluidization to produce ∼200 nm squalane droplets. We demonstrate that adsorption of these nanosized oil droplets involves acetal bond formation between the cis-diol groups located on the steric stabilizer chains and the aldehyde groups on the brush. This interaction occurs under relatively mild conditions and can be tuned by adjusting the solution pH. Hence this is a useful model system for understanding oil droplet interactions with soft surfaces.

Sub-micron colloidosomes with tuneable cargo release prepared using epoxy-functional diblock copolymer nanoparticles

HYPOTHESIS

Pickering emulsions stabilized using epoxy-functional block copolymer nanoparticles should enable the formation of sub-micron colloidosomes that are stable with respect to Ostwald ripening and allow tuneable small-molecule cargo release.

EXPERIMENTS

Epoxy-functional diblock copolymer nanoparticles of 24 ± 4 nm were prepared via reversible addition-fragmentation chain transfer (RAFT)-mediated dispersion polymerization of methyl methacrylate (MMA) in n-dodecane. Sub-micron water-in-n-dodecane Pickering emulsions were prepared by high-pressure microfluidization. The epoxy groups were then ring-opened using 3-aminopropyltriethoxysilane (APTES) to prepare cross-linked colloidosomes. The colloidosomes survived removal of the aqueous phase using excess solvent. The silica shell thickness could be adjusted from 11 to 23 nm by varying the APTES/GlyMA molar ratio. The long-term stability of the colloidosomes was compared to precursor Pickering emulsions. Finally, the permeability of the colloidosomes was examined by encapsulation and release of a small molecule.

FINDINGS

The Pickering emulsion droplet diameter was reduced from 700 to 200 nm by increasing the salt concentration within the aqueous phase. In the absence of salt, emulsion droplets were unstable due to Ostwald ripening. However, emulsions prepared with 0.5 M NaCl are stable for at least one month. The cross-linked colloidosomes demonstrated much more stable than the precursor sub-micron emulsions prepared without salt. The precursor nanoemulsions exhibited complete release (>99 %) of an encapsulated dye, while higher APTES/GlyMA ratios resulted in much lower dye release, yielding nearly impermeable silica capsules that retained around 95 % of the dye.

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

A poly(glycerol monomethacrylate) (PGMA) precursor was chain-extended with 2,2,2-trifluoroethyl methacrylate (TFEMA) via reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization. Transmission electron microscopy (TEM) studies confirmed the formation of well-defined PGMA52-PTFEMA50 spherical nanoparticles, while dynamic light scattering (DLS) studies indicated a z-average diameter of 26 ± 6 nm. These sterically stabilized diblock copolymer nanoparticles were used as emulsifiers to prepare oil-in-water Pickering nanoemulsions: either n-dodecane or squalane was added to an aqueous dispersion of nanoparticles, followed by high-shear homogenization and high-pressure microfluidization. The Pickering nature of such nanoemulsion droplets was confirmed via cryo-transmission electron microscopy (cryo-TEM). The long-term stability of such Pickering nanoemulsions was evaluated by analytical centrifugation over a four-week period. The n-dodecane droplets grew in size significantly faster than squalane droplets: this is attributed to the higher aqueous solubility of the former oil, which promotes Ostwald ripening. The effect of adding various amounts of squalane to the n-dodecane droplet phase prior to emulsification was also explored. The addition of up to 40% (v/v) squalane led to more stable nanoemulsions, as judged by analytical centrifugation. The nanoparticle adsorption efficiency at the n-dodecane-water interface was assessed by gel permeation chromatography when using nanoparticle concentrations of 4.0, 7.0, or 10% w/w. Increasing the nanoparticle concentration not only produced smaller droplets but also reduced the adsorption efficiency, as confirmed by TEM studies. Furthermore, the effect of varying the nanoparticle concentration (2.5, 5.0, or 10% w/w) on the long-term stability of n-dodecane-in-water Pickering nanoemulsions was explored over a four-week period. Nanoemulsions prepared at higher nanoparticle concentrations were more unstable and exhibited a faster rate of Ostwald ripening. The nanoparticle adsorption efficiency was monitored for an aging nanoemulsion prepared at a copolymer concentration of 2.5% w/w. As the droplets ripened over time, the adsorption efficiency remained constant (∼97%). This suggests that nanoparticles desorbed from the shrinking smaller droplets and then readsorbed onto larger droplets over time. Finally, the effect of temperature on the stability of Pickering nanoemulsions was examined. Storing these Pickering nanoemulsions at elevated temperatures led to faster rates of Ostwald ripening, as expected.

Mucoadhesive pickering nanoemulsions via dynamic covalent chemistry

HYPOTHESIS

Submicron oil droplets stabilized using aldehyde-functionalized nanoparticles should adhere to the primary amine groups present at the surface of sheep nasal mucosal tissue via Schiff base chemistry.

EXPERIMENTS

Well-defined sterically-stabilized diblock copolymer nanoparticles of 20 nm diameter were prepared in the form of concentrated aqueous dispersions via reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate (TFEMA) using a water-soluble methacrylic precursor bearing cis-diol groups. Some of these hydroxyl-functional nanoparticles were then selectively oxidized using an aqueous solution of sodium periodate to form a second batch of nanoparticles bearing pendent aldehyde groups within the steric stabilizer chains. Subjecting either hydroxyl- or aldehyde-functional nanoparticles to high-shear homogenization with a model oil (squalane) produced oil-in-water Pickering macroemulsions of 20-30 µm diameter. High-pressure microfluidization of such macroemulsions led to formation of the corresponding Pickering nanoemulsions with a mean droplet diameter of around 200 nm. Quartz crystal microbalance (QCM) experiments were used to examine adsorption of both nanoparticles and oil droplets onto a model planar substrate bearing primary amine groups, while a fluorescence microscopy-based mucoadhesion assay was developed to assess adsorption of the oil droplets onto sheep nasal mucosal tissue.

FINDINGS

Squalane droplets coated with aldehyde-functional nanoparticles adhered significantly more strongly to sheep nasal mucosal tissue than those coated with the corresponding hydroxyl-functional nanoparticles. This difference was attributed to the formation of surface imine bonds via Schiff base chemistry and was also observed for the two types of nanoparticles alone in QCM studies. Preliminary biocompatibility studies using planaria indicated only mild toxicity for these new mucoadhesive Pickering nanoemulsions, suggesting potential applications for the localized delivery of hydrophobic drugs.

Recent Advances in Polymerization-Induced Self-Assembly (PISA) Syntheses in Non-Polar Media

It is well-known that polymerization-induced self-assembly (PISA) is a powerful and highly versatile technique for the rational synthesis of colloidal dispersions of diblock copolymer nanoparticles, including spheres, worms or vesicles. PISA can be conducted in water, polar solvents or non-polar media. In principle, the latter formulations offer a wide range of potential commercial applications. However, there has been just one review focused on PISA syntheses in non-polar media and this prior article was published in 2016. The purpose of the current review article is to summarize the various advances that have been reported since then. In particular, PISA syntheses conducted using reversible addition-fragmentation chain-transfer (RAFT) polymerization in various n-alkanes, poly(α-olefins), mineral oil, low-viscosity silicone oils or supercritical CO2 are discussed in detail. Selected formulations exhibit thermally induced worm-to-sphere or vesicle-to-worm morphological transitions and the rheological properties of various examples of worm gels in non-polar media are summarized. Finally, visible absorption spectroscopy and small-angle X-ray scattering (SAXS) enable in situ monitoring of nanoparticle formation, while small-angle neutron scattering (SANS) can be used to examine micelle fusion/fission and chain exchange mechanisms.

Efficient Synthesis of Hydrolytically Degradable Block Copolymer Nanoparticles via Reverse Sequence Polymerization-Induced Self-Assembly in Aqueous Media

Hydrolytically degradable block copolymer nanoparticles are prepared via reverse sequence polymerization-induced self-assembly (PISA) in aqueous media. This efficient protocol involves the reversible addition-fragmentation chain transfer (RAFT) polymerization of N,N'-dimethylacrylamide (DMAC) using a monofunctional or bifunctional trithiocarbonate-capped poly(ϵ-caprolactone) (PCL) precursor. DMAC monomer is employed as a co-solvent to solubilize the hydrophobic PCL chains. At an intermediate DMAC conversion of 20-60 %, the reaction mixture is diluted with water to 10-25 % w/w solids. The growing amphiphilic block copolymer chains undergo nucleation to form sterically-stabilized PCL-core nanoparticles with PDMAC coronas. 1 H NMR studies confirm more than 99 % DMAC conversion while gel permeation chromatography (GPC) studies indicate well-controlled RAFT polymerizations (Mw /Mn ≤1.30). Transmission electron microscopy (TEM) and dynamic light scattering (DLS) indicate spheres of 20-120 nm diameter. As expected, hydrolytic degradation occurs within days at 37 °C in either acidic or alkaline solution. Degradation is also observed in phosphate-buffered saline (PBS) (pH 7.4) at 37 °C. However, no degradation is detected over a three-month period when these nanoparticles are stored at 20 °C in deionized water (pH 6.7). Finally, PDMAC30 -PCL16 -PDMAC30 nanoparticles are briefly evaluated as a dispersant for an agrochemical formulation based on a broad-spectrum fungicide (azoxystrobin).

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

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.

Tartary Buckwheat Starch Modified with Octenyl Succinic Anhydride for Stabilization of Pickering Nanoemulsions

In this study, Tartary buckwheat starch was modified to different degrees of substitution (DS) with octenyl succinate anhydride (OS-TBS) in order to explore its potential for stabilizing Pickering nanoemulsions. OS-TBS was prepared by reacting Tartary buckwheat starch with 3, 5 or 7% (w/v) octenyl succinate in an alkaline aqueous solution at pH 8.5. Fourier-transform infrared spectroscopy gave peaks at 1726 cm−1 (C=O) and 1573 cm−1 (RCOO−), indicating the formation of OS-TBS. We further studied the physicochemical properties of the modified starch as well as its emulsification capacity. As the DS with octenyl succinate anhydride increased, the amylose content and gelatinization temperature of the OS-TBS decreased, while its solubility increased. In contrast to the original Tartary buckwheat starch, OS-TBS showed higher surface hydrophobicity, and its particles were more uniform in size and its emulsification stability was better. Higher DS with octenyl succinate led to better emulsification. OS-TBS efficiently stabilized O/W Pickering nanoemulsions and the average particle size of the emulsion was maintained at 300–400 nm for nanodroplets. Taken together, these results suggest that OS-TBS might serve as an excellent stabilizer for nanoscale Pickering emulsions. This study may suggest and expand the use of Tartary buckwheat starch in nanoscale Pickering emulsions in various industrial processes.

An investigation on pickering nano-emulsions stabilized by dihydromyricetin/high-amylose corn starch composite particles: Preparation conditions and carrier properties

With dihydromyricetin (DMY)/high-amylose corn starch (HCS) composite particles as the emulsifier, Pickering nano-emulsions were fabricated by combining high-speed shearing and high-pressure homogenization. The effect of particle properties and processing conditions on the formation and physicochemical properties of the Pickering nano-emulsions was then investigated systematically. The results showed that the DMY content of the composite particles, the oil phase volume fraction of the emulsion, and the homogenization conditions had obvious effects on the droplet size of the emulsion, where appropriate DMY content in the composite particles (5-20%) contributed to the formation of stable Pickering nano-emulsions. The oil phase of the obtained emulsions exhibited good stability during high-temperature storage, and their β-carotene protecting performance against UV irradiation was superior to the emulsion stabilized by Tween 20. The in vitro simulated digestion analysis indicated that the nano-emulsions developed by the composite particles could enhance the bioaccessibility of β-carotene and inhibit starch hydrolysis.

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

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 PDMAC77-PDAAM40 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 PDMAC77-PDAAM40 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.

Complex Polymeric Microstructures with Programmable Architecture via Pickering Emulsion-Templated In Situ Polymerization

Aside from smooth and spherical microcapsules, the concept of tailoring complex polymeric microstructures is being taken a step ahead due to their great demand in various applications and fundamental studies in the subjects of microfluidics and nanotechnology. Size, shape, and morphology are of paramount importance for their functional performance and various applications. However, simple, inexpensive, versatile, and high-throughput techniques for fabricating microcapsules with controlled morphology remain a bottleneck for discoveries in the subject of polymer colloids. In this paper, we directly fulfill this need by reporting a novel approach of Pickering emulsion-templated in situ polymerization for tailoring complex polymeric microstructures comprised of a composite shell of titanium dioxide nanoparticle (TiO₂ NP)-embedded poly(melamine-urea-formaldehyde) (polyMUF) and a core of hexadecane (HD, soft template). At first, we hydrophobize TiO₂ NPs by chemisorbing long-chain biobased myristic acid via a bidentate chelating complex and precisely tune their wettability by varying the grafting density of myristic acid to obtain highly stable oil-in-water (O/W) Pickering emulsion. Thereafter, we employ the optimized TiO₂ NPs in the intended encapsulation strategy that enables various microstructures and morphologies with the particle diameter ranging from 5 to 20 μm. Careful manipulation of reaction parameters and copolymer components leads to novel complex microstructures: smooth, raspberry-like, partially budded, hollow, filled, single-holed, and closed-cell-like microstructures. Particle properties such as morphology, size, shell thickness, and core content are governed by the TiO₂ NP content, core-to-shell ratio, copolymer component, conversion, and pH value. Based on the results of a series of control experiments, novel mechanisms for the formation of various such microstructures are proposed.

Supersmall Dendritic Mesoporous Silica Nanospheres as Antioxidant Nanocarriers for Pickering Emulsifiers

Encapsulation of flavor and aromatic compounds in emulsions holds great potential for development of novel formulations in food applications. In this paper, supersmall dendritic mesoporous silica nanospheres (DMSNs) were fabricated by the one-pot strategy. The morphologies of DMSNs were directly tuned in terms of diameter from 35 ± 2 to 85 ± 4 nm. The obtained DMSNs are nanocarriers for hydrophilic or hydrophobic antioxidants with superior loading performance. Both DMSNs and antioxidant-loaded ones can emulsify the flavor and aromatic compounds yielding stable Pickering emulsions with droplets of approximately 2 μm in diameter. The emulsions possess excellent physical stability for at least half a year. More importantly, gas chromatography-mass spectrometry-olfactometry (GC-MS-O) analysis shows that antioxidant-loaded DMSNs provide outstanding protective functionalities to the encapsulated flavoring oil. A universality study reveals that DMSNs are an ideal platform for stable Pickering emulsions for aromatic compounds. Our design could provide a new path for flavor and sensitive bioactives for codelivery with excellent stability in food, medicine, cosmetics, etc.

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?

Responsive Pickering emulsions exhibit promising application in industry owing to the integration of the high storage stability with on-demand demulsification. In this study, stimuli-responsive Pickering emulsions stabilized by poly[oligo(ethylene glycol) methyl ether methacrylate]₁₅-b-poly(diacetone acrylamide)₁₂₀ (E₁₅D₁₂₀) worms were indicated, in which E₁₅D₁₂₀ worms were prepared via reversible addition-fragmentation chain transfer-based aqueous dispersion polymerization using thermo-sensitive POEGMA₁₅ as both the stabilizer block and macro-chain transfer agent. The factors influencing the morphologies of copolymers during polymerization-induced self assembly have been investigated. A series of different morphological polymer nanoparticles including spheres, worms, and vesicles could be produced through rational synthesis. E₁₅D₁₂₀ worms demonstrated excellent emulsifying performances and could be used as emulsifiers to form n-dodecane-in-water Pickering emulsions at a low content. The formed n-dodecane-in-water Pickering emulsions revealed a slow demulsification at pH 10 or 70 °C or pH 10/70 °C combinations, and several hours were needed for the demulsification of Pickering emulsions. However, n-dodecane-in-water Pickering emulsions displayed a rapid demulsification (∼10 min) at an elevated temperature, such as 90 °C. The different demulsification rates were attributed to different sensitivities of E₁₅D₁₂₀ worms to external stimuli. Pickering emulsions integrating a rapid responsive demulsification with a slow one would be well satisfactory on different occasions.

Inter-droplet force between magnetically polarizable Pickering oil-in-water nanoemulsions stabilized with γ-Al2O3 nanoparticles: Role of electrostatic and electric dipolar interactions

HYPOTHESIS

The presence of nanoparticles at oil-water interface influences the interaction forces between Pickering emulsions. When charged nanoparticles are at the oil-water interface of an electrostatically stabilized emulsion, in addition to the screened Coulombic interaction, electric dipolar force also influences the total inter-droplet force profiles. An in-depth understanding of the effects of such electric dipolar forces is essential for designing colloidally stable Pickering nanoemulsions for various applications.

EXPERIMENTS

Inter-droplet forces between γ-Al₂O₃ nanoparticle stabilized oil-in-water nanoemulsion, containing superparamagnetic nanoparticles (magnetically polarizable) in the oil phase, are measured using the magnetic-chaining technique at different pH and salt concentrations. The role of mono-, di- and tri-valent salts on the inter-droplet force profiles are assessed.

FINDINGS

Force measurement studies reveal a lowering of inter-droplet spacing, within the linear chains, for higher salt concentrations due to an increased screening. Strong interfacial attachment of the charged nanoparticles results in the formation of an asymmetric charge cloud leading to an electric dipolar interaction. Incorporating the contributions of electric dipolar and screened Coulombic interactions, the theoretically estimated total repulsive force magnitudes are in good agreement with the experimental data. The obtained results offer better insights into the nature of colloidal force between charged particle stabilized nanoemulsions.

Block Copolymer Nanoparticles are Effective Dispersants for Micrometer-Sized Organic Crystalline Particles

Well-defined sterically stabilized diblock copolymer nanoparticles of 29 nm diameter are prepared by RAFT aqueous emulsion polymerization of methyl methacrylate using a dithiobenzoate-capped poly(glycerol monomethacrylate) precursor. These nanoparticles are evaluated as a dispersant for the preparation of organic crystalline microparticles via ball milling. This is exemplified for azoxystrobin, which is a broad-spectrum fungicide that is widely used to protect various food crops. Laser diffraction and optical microscopy studies indicate the formation of azoxystrobin microparticles of approximately 2 μm diameter after ball milling for 10 min at 400 rpm. Nanoparticle adsorption at the surface of these azoxystrobin microparticles is confirmed by electron microscopy studies. The extent of nanoparticle adsorption on the azoxystrobin microparticles can be quantified using a supernatant assay based on solution densitometry. This technique indicates an adsorbed amount of approximately 5.5 mg m-2, which is sufficient to significantly reduce the negative zeta potential exhibited by azoxystrobin. Moreover, this adsorbed amount appears to be essentially independent of the nature of the core-forming block, with similar data being obtained for both poly(methyl methacrylate)- and poly(2,2,2-trifluoroethyl methacrylate)-based nanoparticles. Finally, X-ray photoelectron spectroscopy studies confirm attenuation of the underlying N1s signal arising from the azoxystrobin microparticles by the former adsorbed nanoparticles, suggesting a fractional surface coverage of approximately 0.24. This value is consistent with a theoretical surface coverage of 0.25 calculated from the adsorption isotherm data. Overall, this study suggests that sterically stabilized diblock copolymer nanoparticles may offer a useful alternative approach to traditional soluble copolymer dispersants for the preparation of suspension concentrates affecting the context of agrochemical applications.

Limonene-in-water Pickering emulsion and on-demand separation using thermo-responsive biodegradable nanoparticles

In the last few decades, Pickering emulsions have regained attention due to the possibility of forming stable oil-in-water emulsions with interesting interfacial properties. As an example, the more and more stringent regulations on the products for home and personal care are pushing the market towards the use of biodegradable materials in order to reduce their environmental impact. In this scenario, an appealing opportunity is offered by the use of biodegradable polymeric nanoparticles (NPs) for the stabilization of fragrance oils in water. In this work, modular biodegradable NPs have been synthesized through a combination of ring opening polymerization and reversible addition-fragmentation chain transfer emulsion polymerization and used to produce limonene-in-water Pickering emulsions. This strategy allowed controlling independently the NP size, polymer molecular weight, and hydrophobicity acting on the microstructure of the constituting copolymers. Stable limonene-in-water Pickering emulsions could be obtained, with the size of the oil phase and the wetting by limonene that can be strictly controlled by tuning the NP physico-chemical properties. Finally, the adoption of thermo-responsive polymer chains within the shell of the Pickering emulsifiers enabled the on-demand destabilization of the emulsions and hence the selective dispensing of limonene by simply increasing the temperature.

Synthesis of Highly Transparent Diblock Copolymer Vesicles via RAFT Dispersion Polymerization of 2,2,2-Trifluoroethyl Methacrylate in n-Alkanes

RAFT dispersion polymerization of 2,2,2-trifluoroethyl methacrylate (TFEMA) is performed in n-dodecane at 90 °C using a relatively short poly(stearyl methacrylate) (PSMA) precursor and 2-cyano-2-propyl dithiobenzoate (CPDB). The growing insoluble poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA) block results in the formation of PSMA-PTFEMA diblock copolymer nano-objects via polymerization-induced self-assembly (PISA). GPC analysis indicated narrow molecular weight distributions (M w/M n ≤ 1.34) for all copolymers, with 19F NMR studies indicating high TFEMA conversions (≥95%) for all syntheses. A pseudo-phase diagram was constructed to enable reproducible targeting of pure spheres, worms, or vesicles by varying the target degree of polymerization of the PTFEMA block at 15-25% w/w solids. Nano-objects were characterized using dynamic light scattering, transmission electron microscopy, and small-angle X-ray scattering. Importantly, the near-identical refractive indices for PTFEMA (1.418) and n-dodecane (1.421) enable the first example of highly transparent vesicles to be prepared. The turbidity of such dispersions was examined between 20 and 90 °C. The highest transmittance (97% at 600 nm) was observed for PSMA9-PTFEMA294 vesicles (237 ± 24 nm diameter; prepared at 25% w/w solids) in n-dodecane at 20 °C. Interestingly, targeting the same diblock composition in n-hexadecane produced a vesicle dispersion with minimal turbidity at a synthesis temperature of 90 °C. This solvent enabled in situ visible absorption spectra to be recorded during the synthesis of PSMA16-PTFEMA86 spheres at 15% w/w solids, which allowed the relatively weak n→π* band at 515 nm assigned to the dithiobenzoate chain-ends to be monitored. Unfortunately, the premature loss of this RAFT chain-end occurred during the RAFT dispersion polymerization of TFEMA at 90 °C, so meaningful kinetic data could not be obtained. Furthermore, the dithiobenzoate chain-ends exhibited a λmax shift of 8 nm relative to that of the dithiobenzoate-capped PSMA9 precursor. This solvatochromatic effect suggests that the problem of thermally labile dithiobenzoate chain-ends cannot be addressed by performing the TFEMA polymerization at lower temperatures.

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

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n-alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.

Effect of Salt on the Formation and Stability of Water-in-Oil Pickering Nanoemulsions Stabilized by Diblock Copolymer Nanoparticles

Sterically stabilized diblock copolymer nanoparticles are prepared in n-dodecane using polymerization-induced self-assembly. Precursor Pickering macroemulsions are then prepared by the addition of water followed by high-shear homogenization. In the absence of any salt, high-pressure microfluidization of such precursor emulsions leads to the formation of relatively large aqueous droplets with DLS measurements indicating a mean diameter of more than 600 nm. However, systemically increasing the salt concentration produces significantly finer droplets after microfluidization, until a limiting diameter of around 250 nm is obtained at 0.11 M NaCl. The mean size of these aqueous droplets can also be tuned by systematically varying the nanoparticle concentration, applied pressure, and the number of passes through the microfluidizer. The mean number of nanoparticles adsorbed onto each aqueous droplet and their packing efficiency are calculated. SAXS studies conducted on a Pickering nanoemulsion prepared using 0.11 M NaCl confirms that the aqueous droplets are coated with a loosely packed monolayer of nanoparticles. The effect of varying the NaCl concentration within the droplets on their initial rate of Ostwald ripening is investigated using DLS. Finally, the long-term stability of these water-in-oil Pickering nanoemulsions is assessed using analytical centrifugation. The rate of droplet ripening can be substantially reduced by using 0.11 M NaCl instead of pure water. However, increasing the salt concentration up to 0.43 M provided no further improvement in the long-term stability of such nanoemulsions.

Pickering nano-emulsions stabilized by Eudragit RL100 nanoparticles as oral drug delivery system for poorly soluble drugs

The purpose of this study was to develop Pickering water-in-oil nano-emulsions only stabilized by Eudragit RL100 nanoparticles (NPs), in order to increase the nano-emulsion stability and create a barrier to improve the drug encapsulation and better control the drug release. The first part of this study was dedicated to investigating the nano-emulsion formulation by ultrasonication and understanding the interfacial behavior and role of NPs in the stabilization of the water/oil interface. The focus was on the surface coverage in the function of the formulation parameters (volume fractions) to disclose the extents and limitations of the process. The main physicochemical analysis of the Pickering nano-emulsions was performed by dynamic light scattering and transmission electron microscopy. On the other hand, the second experimental approach was dedicated to understanding the interfacial behavior of the Eudragit RL100 NPs toward a model water/oil interface, using a dynamic tensiometer with axisymmetric drop shape analysis. The study investigated the NPs' adsorption, as well as their rheological behavior. The aim of this part was to reveal the main phenomena that govern the interactions between NPs and the interface in order to understand the origin of Pickering nano-emulsions' stability. The last part of the study was concerned with the stability and in vitro release of a model encapsulated drug (ketoprofen) in a gastric and simulated intestinal environment. The results showed that Pickering nano-emulsions significantly improved the resistance to gastric pH, inducing a significantly slower drug release compared to classical nano-emulsions' stabilized surfactants. These Pickering nano-emulsions appear as a promising technology to modify the delivery of a therapeutic agent, in the function of the pH, and can be, for instance, applied to the oral drug delivery of poorly soluble drugs.

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?

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.

Self-Assembled Egg Yolk Peptide Micellar Nanoparticles as a Versatile Emulsifier for Food-Grade Oil-in-Water Pickering Nanoemulsions

Pickering emulsions stabilized by food-grade particles have garnered increasing interest in recent years due to their promising applications in biorelated fields such as foods, cosmetics, and drug delivery. However, it remains a big challenge to formulate nanoscale Pickering emulsions from these edible particles. Herein we show that a new Pickering nanoemulsion that is stable, monodisperse, and controllable can be produced by employing the spherical micellar nanoparticles (EYPNs), self-assembled from the food-derived, amphiphilic egg yolk peptides, as an edible particulate emulsifier. As natural peptide-based nanoparticles, the EYPNs have a small particle size, intermediate wettability, high surface activity, and deformability at the interface, which enable the formation of stable Pickering nanodroplets with a mean dynamic light scattering diameter below 200 nm and a polydispersity index below 0.2. This nanoparticle system is versatile for different oil phases with various polarities and demonstrates the easy control of nanodroplet size through tuning the microfluidization conditions or the ratio of EYPNs to oil phase. These food-grade Pickering nanoemulsions, obtained when the internal phase is an edible vegetable oil, have superior stability during long-term storage and spray-drying based on the irreversible and compact adsorption of intact EYPNs at the nanodroplet surface. This is the first finding of a natural edible nano-Pickering emulsifier that can be used solely to make stable food Pickering nanoemulsions with the qualities of simplicity, versatility, low cost, and the possibility of controllable and mass production, which make them viable for many sustainable applications.

Efficient occlusion of oil droplets within calcite crystals

It is well known that oil and water do not mix. Similarly, the incorporation of oil droplets within inorganic crystals is highly counter-intuitive because there is a large difference in surface energy for these two components. Nevertheless, herein we demonstrate the efficient occlusion of ∼250-500 nm oil droplets within 20-40 μm calcite crystals. These droplets are stabilized using various amphiphilic poly(methacrylic acid)-poly(n-alkyl methacrylate) diblock copolymer emulsifiers. Both copolymer concentration and diblock compositions affect the extent of occlusion, with optimized conditions producing calcite crystals containing up to 11% oil by mass. Moreover, compressive forces exerted by the growing crystals cause significant deformation of the oil droplets during occlusion. In principle, this protocol enables the incorporation of water-insoluble dyes or hydrophobic nanoparticles within calcite, which is a cheap, naturally-occurring and environmentally-benign mineral. The single crystal nature of this host lattice ensures efficient retention of such guests, while lowering the solution pH leads to triggered release via acid dissolution.

Individual and Collective Behavior of Emulsion Droplets Undergoing Ostwald Ripening

Ostwald ripening (OR) is the dominating phase separation mechanism in nanoemulsions consisting of the mass exchange between separated droplets by dissolution and absorption of molecules. Here, we propose a model based on a stochastic equation for the mass exchange coupled to a Brownian dynamics algorithm. Our model accounts for the simultaneous gain and loss of mass within a medium, where the presence of sources and sinks leads to a complex distribution of dissolved oil molecules. Also, a criterion for possible nucleation zones based on the definition of a saturation area around the droplets is found. The predictions of the collective behavior are constructed on the individual contributions of each droplet with its own environment. Individual droplets undergoing molecular exchange exhibited anomalous diffusion, whereas when performing the collective analysis, such a behavior was disguised. We used reported experiments under diverse conditions to validate and test the scope of our model, including the modification to the interfacial tension via Gibbs elasticity, finding close agreements. Our results imply that saturation is not conditional for the occurrence of OR. The ability of this model to extend the limitations imposed by traditional treatments to a broader number of physicochemical conditions makes it a useful complementary tool for predicting and understanding experimental results of emulsions experiencing OR.

An insight into nanocellulose as soft condensed matter: Challenge and future prospective toward environmental sustainability

Nanocellulose, a structural polysaccharide that has caught tremendous interests nowadays due to its renewability, inherent biocompatibility and biodegradability, abundance in resource, and environmental friendly nature. They are promising green nanomaterials derived from cellulosic biomass that can be disintegrated into cellulose nanofibrils (CNF) or cellulose nanocrystals (CNC), relying on their sensitivity to hydrolysis at the axial spacing of disordered domains. Owing to their unique mesoscopic characteristics at nanoscale, nanocellulose has been widely researched and incorporated as a reinforcement material in composite materials. The world has been consuming the natural resources at a much higher speed than the environment could regenerate. Today, as an uprising candidate in soft condensed matter physics, a growing interest was received owing to its unique self-assembly behaviour and quantum size effect in the formation of three-dimensional nanostructured material, could be utilised to address an increasing concern over global warming and environmental conservation. In spite of an emerging pool of knowledge on the nanocellulose downstream application, that was lacking of cross-disciplinary study of its role as a soft condensed matter for food, water and energy applications toward environmental sustainability. Here we aim to provide an insight for the latest development of cellulose nanotechnology arises from its fascinating physical and chemical characteristic for the interest of different technology holders.

Long-Term Stability of n-Alkane-in-Water Pickering Nanoemulsions: Effect of Aqueous Solubility of Droplet Phase on Ostwald Ripening

High-pressure microfluidization is used to prepare a series of oil-in-water Pickering nanoemulsions using sterically-stabilized diblock copolymer nanoparticles as the Pickering emulsifier. The droplet phase comprised either n-octane, n-decane, n-dodecane, or n-tetradecane. This series of oils enabled the effect of aqueous solubility on Ostwald ripening to be studied, which is the primary instability mechanism for such nanoemulsions. Analytical centrifugation (LUMiSizer instrument) was used to evaluate the long-term stability of these Pickering nanoemulsions over time scales of weeks/months. This technique enables convenient quantification of the fraction of growing oil droplets and confirmed that using n-octane (aqueous solubility = 0.66 mg dm^-3 at 20 °C) leads to instability even over relatively short time periods. However, using n-tetradecane (aqueous solubility = 0.386 μg dm^-3 at 20 °C) leads to significantly improved long-term stability with respect to Ostwald ripening, with all droplets remaining below 1 μm diameter after 6 weeks storage at 20 °C. In the case of n-dodecane, the long-term stability of these new copolymer-stabilized Pickering nanoemulsions is significantly better than the silica-stabilized Pickering nanoemulsions reported in the literature by Persson et al. ( Colloids Surf., A, 2014, 459, 48-57). This is attributed to a much greater interfacial yield stress for the former system, as recently described in the literature (see P. J. Betramo et al. Proc. Natl. Acad. Sci. U.S.A., 2017, 114, 10373-10378).

Influence of pH-Responsive Monomer Content on the Behavior of Di-Block Copolymers in Solution and as Stabilizers of Pickering Latex Particle Emulsifiers

In this study, diblock copolymers poly(methyl methacrylate)-block-poly (2-dimethylaminoethyl methacrylate) (pMMA-b-pDMAEMA) are investigated for the steric stabilization of latex particles and the subsequent use of these latex particles as Pickering emulsifiers. Solution properties of the diblock copolymers highlight that the pDMAEMA block length influences the critical micelle concentration (CMC) and micelle hydrodynamic diameter in response to changes in pH and the pK_a. The block length can also be used as a way to control the particle size of sterically stabilized polystyrene latex particles prepared via emulsion polymerization. The suspension properties of these latex particles are also presented. Emulsion studies using these latex particles as emulsifiers show that both continuous phase pH and electrolyte concentration affect emulsion stability to coalescence. At high pH, stable emulsions are formed due to the affinity of the particles to the interface. At low pH, protonation of the amine groups reduces the affinity and thus droplet coalescence is observed. Increasing the electrolyte concentration improves emulsion stability, but causes an increase in droplet size due to adsorption of flocculated/aggregated particles. Finally, it is shown that these latex particles can be used in conjunction with membrane emulsification techniques to produce emulsions with low polydispersity.

Synthesizing Pickering Nanoemulsions by Vapor Condensation

Nanoparticle-stabilized (Pickering) emulsions are widely used in applications such as cosmetics, drug delivery, membranes, and material synthesis. However, formulating Pickering nanoemulsions remains a significant challenge. Herein, we show that Pickering nanoemulsions can be obtained in a single step even at very low nanoparticle loadings (0.2 wt %) by condensing water vapor on a nanoparticle-infused subcooled oil that spreads on water. Droplet nuclei spontaneously submerge within the oil after nucleating at the oil-air interface, resulting in the suppression of droplet growth by diffusion, and subsequently coalesce to larger sizes until their growth is curtailed by nanoparticle adsorption. The average nanoemulsion size is governed by the competition between nanoparticle adsorption kinetics and droplet growth dynamics, which are in turn a function of nanoparticle size, concentration, and condensation time. Controlling such factors can lead to the formation of highly monodisperse nanoemulsions. Emulsion formation via condensation is a fast, scalable, energy-efficient process that can be adapted for a wide variety of emulsion-based applications in biology, chemistry, and materials science.