Abstract
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 (η i) measurements confirmed that a stronger film was formed at the interface with quercetin particles (η i ≈ 25 N s m-1) rather than with curcumin particles (η i ≈ 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.
Highlights
Water-in-oil (W/O) emulsions are widely used for different soft matter applications in food, pharmaceutical, personal care, agriculture, and so forth
The overall particle distributions, which were quite broad, still remained largely in the micrometer range and were comparable with Figure 2, Figure 2 shows that the curcumin part of the distribution extends to smaller sizes compared to the quercetin part
We have demonstrated for the first time that W/O emulsions can be stabilized with curcumin or quercetin particles, which were both sufficiently hydrophobic according to the contact angle measurements
Summary
Water-in-oil (W/O) emulsions are widely used for different soft matter applications in food, pharmaceutical, personal care, agriculture, and so forth. The size of the particles dispersed in the continuous phase is an important parameter that determines the Pickering functionality. Simple geometrical consideration can be used to estimate the minimum amount of surface-active particles required for adequate surface coverage in order to form stable emulsions.[6] The particle contact angle (θ) represents the wettability of a particle at the interface, identifying whether the particle preferentially resides in the water or oil phase.[7] If θ, measured in the aqueous phase, is smaller than 90°, a larger fraction of the particle surface will reside in the aqueous rather than in the nonpolar phase; that is, the particle is more hydrophilic and will tend to stabilize an O/W emulsion
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