pH-Responsive Pickering Emulsions Synergistically Stabilized by Maleopimaric Acid and Alumina Nanoparticles

doi:10.6023/A21120567

Abstract

Using organic particles to stabilize Pickering emulsions has attracted more and more attention. However, Pickering emulsions synergistically stabilized by organic particles with adjustable wettability combined with inorganic particles is rarely reported. Herein, maleopimaric acid (MPA) with multi-carboxylic acid groups and alumina nanoparticles are used to stabilize Pickering emulsions, and the transitions of the type of Pickering emulsions induced by pH and related mechanisms are systematically studied. With the increase of pH, resulting in the change of the type of emulsions from a W/O Pickering emulsion to an O/W Pickering emulsion to an O/W emulsion when only MPA is used. The increase of the hydrophilicity of MPA is the reason for the transformation of the emulsion. When alumina nanoparticles are added to the system, W/O Pickering emulsions can be transformed into O/W Pickering emulsions (pH=1), which is attributed to the increase of the hydrophilicity of MPA particles caused by the adsorption of alumina nanoparticles on the surface of MPA particles. When pH=6, both MPA particles and alumina nanoparticles have strong hydrophilicity and cannot form stable emulsions respectively, but the mixture of the two can form stable W/O Pickering emulsions. It is because that MPA particles and alumina nanoparticles can form hydrophobic complexes in aqueous solution. In addition, the influence of MPA concentration and volume fraction of oil phase on the appearance and droplet size of Pickering emulsion is studied. It is found that the droplet size of Pickering emulsion gradually decreases with the increase of MPA concentration. However, the increase of volume fraction of oil phase will cause the increase of droplet size. Finally, the stability mechanism of Pickering emulsion is studied by the measurement of Zeta potential, adsorption rate of particles at the oil-water interface, three phase contact angle and surface/interface tensions. The armor-like particles layer adsorbed on the oil-water interface and the network structure formed between the particles are the reason for the stability of emulsion droplets. This study provides a new avenue for the green preparation of Pickering emulsions, which will be important applications in the fields of cosmetics, medicine and new materials.

Key words: maleopimaric acid, pickering emulsions, alumina nanoparticles, phase transition, synergistic stable

Cite this article

Xufa He, Kangle Jia, Longfei Yu, Mingjie Liu, Xiaoshan Zheng, Huanling Li, Jinlan Xin, Linjia Huang. pH-Responsive Pickering Emulsions Synergistically Stabilized by Maleopimaric Acid and Alumina Nanoparticles[J]. Acta Chimica Sinica, 2022, 80(6): 765-771.

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Fig. & Tab. 7

Figure 1. Schematic illustration of phase transition of emulsions induced by pH (a), appearance (b) and optical photographs (c) of emulsions stabilized by 0.5% (w) MPA at different pH

Figure 2. Appearance and optical pictures of Pickering emulsions stabilized by 0.5% (w) MPA (a) and 0.5% (w) Al2O3 nanoparticles (b) at different pH

Figure 3. Appearance and optical pictures of Pickering emulsions stabilized by 0.5% (w) Al2O3 nanoparticles and different concentrations of MPA at pH=7: (a) 0.0001% (w), (b) 0.001% (w), (c) 0.01% (w) and (d) 0.1% (w)

Figure 4. Appearance and microscopy pictures of Pickering emulsions stabilized by 0.1% (w) MPA and 0.5% (w) Al2O3 nanoparticles at pH=7 with different oil volume fractions

Figure 5. (a) Zeta potential of 0.5% (w) Al2O3 nanoparticles with different concentrations of MPA aqueous solutions. (b) Percentage of adsorbed Al2O3 nanoparticles at the decane-water interface in the Pickering emulsions stabilized by 0.5% (w) Al2O3 nanoparticles with different concentrations of MPA

Figure 6. Three-phase contact angles of oil droplets in different concentrations of MPA aqueous solution on aluminum oxide plate at pH=7

Figure 7. Surface tensions (a and b) and interfacial tensions (c and d) of decane-water at different concentration ratios of MPA and Al2O3 nanoparticles at pH=7

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