Water-In-Oil Pickering Emulsions Stabilized by Water-Insoluble Polyphenol Crystals

In recent years, there has been a resurgence of interest in Pickering emulsions because of the recognition of the unique high steric stabilization provided by particles at interfaces. This interest is particularly keen for water-in-oil (W/O) emulsions because of the limited range of suitable Pickering stabilizers available. We demonstrate for the first time that W/O emulsions can be stabilized by using crystals from naturally occurring polyphenols (curcumin and quercetin particles). These particles were assessed based on their size, microstructure, contact angle, interfacial tension, and ζ-potential measurements in an attempt to predict the way that they act as Pickering stabilizers. Static light-scattering results and microstructural analysis at various length scales [optical microscopy, confocal laser scanning microscopy (CLSM), and scanning electron microscopy (SEM)] confirmed that the quercetin particles has a nearly perfect crystalline rod shape with a high aspect ratio; that is, the ratio of length to diameter (L/D) was ca. 2.5:1–7:1. On the other hand, the curcumin particles (d₃,₂ = 0.2 μm) had a polyhedral shape. Droplet sizing and CLSM revealed that there was an optimum concentration (0.14 and 0.25 wt % for quercetin and curcumin, respectively) where smaller water droplets were formed (d₃,₂ ≈ 6 μm). Interfacial shear viscosity (ηi) measurements confirmed that a stronger film was formed at the interface with quercetin particles (ηi ≈ 25 N s m⁻¹) rather than with curcumin particles (ηi ≈ 1.2 N s m⁻¹) possibly because of the difference in the shape and size of the two crystals. This study provides new insights into the creation of Pickering W/O emulsions with polyphenol crystals and may lead to various soft matter applications where Pickering stabilization using biocompatible particles is a necessity

Water-in-oil Pickering emulsions stabilized by an interfacial complex of water-insoluble polyphenol crystals and protein

Long term stabilization of water-in-oil (W/O) emulsions remains a particularly challenging problem in colloid science. Recent studies have shown that polyphenols act as Pickering stabilizers at the water-oil interface. In this work we propose a novel way to stabilize water droplets via interfacial complex formation. It was observed that polyphenol crystals (curcumin or quercetin) absorb at the interface and provide stabilization of water droplets for several days; however formation of a polyphenol- whey protein (WPI) complex at the water-oil interface revealed a pronounced improvement in the stabilization. The mechanism of complex formation was tested by subjecting the systems to different environmental conditions, such as ionic strength and temperature. The evidence suggests that the complex is probably stabilized by electrostatic attraction between the oppositely-charged polyphenol particles and protein at the interface, although hydrogen bonding between the two components may also contribute. The resulting stable water droplets have a Sauter mean diameter (D3,2) of approximately ∼22 and ∼27 μm for curcumin and quercetin systems, respectively. Emulsions were more stable at pH 3 than at pH 7, due to either weaker complex formation at pH 7 and/or chemical degradation of the polyphenols at this more alkaline pH. Interfacial shear viscosity measurements confirmed that there was strong interfacial complex formation with aqueous WPI concentrations of ∼0.5 wt.%

Water-in-Oil Pickering Emulsions Stabilized by Synergistic Particle-Particle Interactions

Here, we report a novel ‘double Pickering stabilization’ of W/O emulsions, where complex formation at the interface between Pickering polyphenol particles adsorbing from the oil side and WPM particles co-adsorbing from the aqueous side of the interface is investigated. The interfacial complex formation was strongly dependent on the concentration of WPM particles. At low WPM concentrations, both polyphenol crystals and WPM particles are present at the interface and the water droplets were stabilized through their synergistic action, whilst at higher concentrations, the WPM particles acted as ‘colloidal glue’ between the water droplets and polyphenol crystals, enhancing the water droplet stability for more than 90 days and prevented coalescence. Via this mechanism, addition of WPM up to 1 wt% gave a significant improvement in the stability of the W/O emulsions, allowing increasing to 20 wt% water droplet fraction. The evidence suggests that the complex was probably formed due to electrostatic attraction between oppositely-charged polyphenol Pickering particles on the oil side of the interface and WPM ‘Pickering’ particles mainly on the aqueous side of the interface. Interfacial shear viscosity measurements and monolayer (Langmuir trough) experiments at the air-water interface provided further evidence of this strengthening of the film due to the synergistic particle-particle complex formation at the interface

Friction between soft contacts at nanoscale on uncoated and protein-coated surfaces

The understanding of friction on soft sliding biological surfaces at the nanoscale is poorly understood as hard interfaces are frequently used as model systems. Herein, we studied the influence of elastic modulus on the frictional properties of model surfaces at the nanoscale for the first time. We prepared model silicone-based elastomer surfaces with tuneable modulus ranging from hundreds of kPa to a few MPa, similar to those found in real biological surfaces, and employed atomic force microscopy to characterize their modulus, adhesion, and surface morphology. Consequently, we used friction force microscopy to investigate nanoscale friction in hard-soft and soft-soft contacts using spherical colloidal probes covered by adsorbed protein films. Unprecedented results from this study reveal that modulus of a surface can have a significant impact on the frictional properties of protein-coated surfaces with higher deformability leading to lower contact pressure and, consequently, decreased friction. These important results pave the way forward for designing new functional surfaces for serving as models of appropriate deformability to replicate the mechanical properties of the biological structures and processes for accurate friction measurements at nanoscale

Surface adsorption and lubrication properties of plant and dairy proteins: A comparative study

The aim of this work was to compare the surface adsorption and lubrication properties of plant and dairy proteins. Whey protein isolate (WPI) and pea protein isolate (PPI) were chosen as model animal and plant proteins, respectively, and various protein concentrations (0.1–100 mg/mL) were studied with/without heat treatment (90 °C/60 min). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments were performed on hydrophilic (gold) and hydrophobic polydimethylsiloxane (PDMS) sensors, with or without a mucin coating, latter was used to mimic the oral surface. Soft tribology using PDMS tribopairs in addition to wettability measurements, physicochemical characterization (size, charge, solubility) and gel electrophoresis were performed. Soluble fractions of PPI adsorbed to significantly larger extent on PDMS surfaces, forming more viscous films as compared to WPI regardless of heat treatment. Introducing a mucin coating on a PDMS surface led to a decrease in binding of the subsequent dietary protein layers, with PPI still adsorbing to a larger extent than WPI. Such large hydrated mass of PPI resulted in superior lubrication performance at lower protein concentration (≤10 mg/mL) as compared to WPI. However, at 100 mg/mL, WPI was a better lubricant than PPI, with the former showing the onset of elastohydrodynamic lubrication. Enhanced lubricity upon heat treatment was attributed to the increase in apparent viscosity. Fundamental insights from this study reveal that pea protein at higher concentrations demonstrates inferior lubricity than whey protein and could result in unpleasant mouthfeel, and thus may inform future replacement strategies when designing sustainable food products

Stability of water-in-oil emulsions co-stabilized by polyphenol crystal-protein complexes as a function of shear rate and temperature

The process stability of water-in-oil (W/O) Pickering emulsions (10 or 20 wt% water), co-stabilized by 0.14 wt% of polyphenol crystals (curcumin or quercetin) dispersed in a soybean oil phase, plus 2.0 wt% whey protein isolate (WPI) or 0.1–2.0 wt% whey protein microgel (WPM) particles present in the inner aqueous phase, was assessed by measuring the apparent viscosity (η), water droplet size (via light scattering) and microstructural changes (via confocal laser scanning microscopy, CLSM). Stability was measured as a function of temperature (25–50 °C), using a shear rate cycle between 0.1 and 100 s−1 to highlight shear- and time-dependent hysteresis of η. All the emulsions showed shear thinning to some extent, but those without added WPI or WPM particles in the aqueous phase exhibited coalescence at increasing shear rate, that was more pronounced at higher temperatures. Emulsions containing WPI in the dispersed phase were stable, whilst those containing WPM particles showed a decrease in mean droplet size (D4,3) on shearing due to the disruption of the aggregates of droplets, polyphenol crystals and/or WPM particles in the continuous oil phase, but with no droplet coalescence. The low shear rate (0.1 s−1) viscosity showed an increase with increasing WPM particle concentration. This increase, plus CLSM of the emulsions, suggested that the WPM particles increased W/O emulsion stability not only via their adsorption to the inner surface of the water droplets, but possibly also due to them promoting the formation of mixed weak flocs of polyphenol crystals + WPM particles + small water droplets in the oil phase attached to the surface of the main population of water droplets

Pickering emulsions stabilised by hydrophobically modified cellulose nanocrystals: responsiveness to pH and ionic strength

The aims of this study were to hydrophobically modify cellulose nanocrystals (CNCs), investigate the ability of such modified CNCs (MCNCs) to stabilise Pickering oil-in-water (O/W) emulsions and understand their stability at different pHs (2.0–7.0) and ionic strengths (0–150 mM NaCl). Structural changes that resulted from esterifying CNCs with octenyl succinic anhydride (OSA) were determined using Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD), transmission electron microscopy (TEM) and wettability analysis. The stability of the Pickering O/W emulsions (20 wt% oil, 0.05–1.00 wt% MCNCs) was assessed using droplet sizing, microscopy, ζ-potential, apparent viscosity and oscillatory rheological measurements. FTIR spectroscopy confirmed a decrease in the intensity of the –OH-associated band because of reaction of the hydroxyl group with OSA. XRD indicated a lower (11.5%) crystallinity index in MCNCs. TEM revealed that there was no change in morphology of the needle-shaped CNCs upon OSA modification (length/diameter = 40–100 nm/2–4 nm). Hydrophobic modification of CNCs with OSA was evidenced by an increase in static water contact angle from 56° (untreated CNCs) to 80.2° (MCNCs) which allowed the MCNCs to be partially wetted by both the phases and stabilise O/W emulsions. The Pickering emulsions showed droplet flocculation at pH < 4.0 (without addition of NaCl) or ionic strength ≥ 20 mM NaCl (pH 7.0), with a predominant elastic gel-like behaviour observed at ≥ 20 mM NaCl. Resistance of MCNC-based Pickering emulsions to coalescence and responsiveness to flocculation at bio-relevant pHs and ionic strengths show promise in the design of delivery vehicles