Van der Waals Emulsions: Emulsions Stabilized by Surface-Inactive, Hydrophilic Particles via van der Waals Attraction

Marina, Paula Facal; Cheng, Chongling; Sedev, Rossen; Stocco, Antonio; Binks, Bernard P.; Wang, Dayang

Abstract

Surface-inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil-in-water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil-water interfaces when the repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by the van der Waals attraction between the particle monolayer shells and the oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by the van der Waals attraction between single particles and oil droplets which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion-based technologies in practice. Myriads of natural and industrial products are in the form of emulsions, in which fine oil droplets are well dispersed in water or vice versa by mechanical means with the aid of stabilizing agents. [1,2] In the context of nanotechnology, nowadays emulsions are increasingly appreciated as soft, flexible, versatile toolboxes to produce smart, hierarchically structured materials. [3-6] In almost all cases, emulsions need to be stabilized by surface-active agents: surfactants, amphiphilic polymers, solid particles or their combinations. By " surface-active " , it is meant that the agents are able not only to adsorb onto oil-water interfaces but also straddle the interface by projecting themselves partially into oil and partially into water. [1,7] Up to date, the partial partitioning of surface-active agents at oil-water interfaces is universally accepted as the key to emulsion stabilization, which enables them to self-assemble into topologically ordered films and thereby stabilize the dispersed droplets by, for instance, electrical double layer (EDL) repulsion against coalescence. [1,7] Counter-intuitively, here we show effective stabilization of oil-in-water (o/w) emulsions with surface-inactive agents–solid particles with exceedingly hydrophilic surfaces hardly wetted by oil at oil-water interfaces. Our results reveal that provided the EDL repulsion between the particles and oil droplets is suppressed, the van der Waals (VW) attraction between them is sufficiently strong to confine the particles in close proximity to oil droplet surfaces to form densely packed shells which efficaciously envelop the oil droplets for emulsion stabilization. In this new process, intriguingly, the emulsifying power of the highly hydrophilic particles is governed preferentially by the VW attraction between the densely packed particle shell as a whole and the oil droplets enclosed therein, while their stabilizing power is governed by the VW attraction between individual particles and oil droplets. To highlight the contribution of VW attraction, here the present o/w emulsions are referred to as " VW emulsions ". Figure 1. (a) Formation of VW emulsions by homogenization of toluene and aqueous dispersions of 15 nm H-SiO2 particles at pH 2.5, evidenced by milky upper phase (emulsion) and transparent lower phase (water) in the glass vial and negligible recovery of free oil after emulsification. No emulsions can be stabilized by 40 and 80 nm H-SiO2 particles, evidenced by well-separated oil and water phases in the glass vials and fast and complete recovery of free oil phase emulsification. The concentrations of 15,40,and 80 nm H-SiO2 particles are 5.4×10 15 , 8.8×10 14 , and 9.8×10 13 particles/mL. (b) Dependence of the VW emulsion stabilization on the concentration of 150 nm H 2-SiO2 particles in the aqueous phase at pH 2.5. The particle concentrations are marked on the photos. (c) Reversible stabilization of VW emulsions in response to pH. The inset indicates that the VW emulsions, stabilized by 15 nm H-SiO2 particle at a concentration of 5.4×10 15 particles/mL at pH 2.5, significantly destabilize upon increasing the pH to 4.0 and completely collapse at pH above 6.0.The pH adjustment is executed by dropwise addition of 0.1 M NaOH or 0.1 M HCl into the emulsions, followed by incubation for ca. 12 h under ambient condition. (d) Stabilization of VW emulsions (the upper phase in the photo) by citrate-stabilized gold particles of diameters of 16 nm at pH 2.5. The oil phase is toluene in all experiments. According to thermodynamics, the depth (∆E) of the potential well that traps solid particles at an oil-water interface with tension (í µí»¾ "#) is correlated with the contact angle (í µí¼ƒ) of the particles at the [*] Dr.

Citation

Marina, P. F., Cheng, C., Sedev, R., Stocco, A., Binks, B. P., & Wang, D. (2018). Van der Waals Emulsions: Emulsions Stabilized by Surface-Inactive, Hydrophilic Particles via van der Waals Attraction. Angewandte Chemie, 57(30), 9510-9514. https://doi.org/10.1002/anie.201805410

• Journal Article Type: Article
• Acceptance Date: Jun 1, 2018
• Online Publication Date: Jun 20, 2018
• Publication Date: Jul 20, 2018
• Deposit Date: Jun 22, 2018
• Publicly Available Date: Jun 21, 2019
• Print ISSN: 1433-7851
• Publisher: Wiley
• Peer Reviewed: Peer Reviewed
• Volume: 57
• Issue: 30
• Pages: 9510-9514
• DOI: https://doi.org/10.1002/anie.201805410
• Keywords: Colloids; Emulsions; Interactions; Interfaces; Particles
• Public URL: https://hull-repository.worktribe.com/output/889352
• Publisher URL: https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201805410
• Contract Date: Jun 22, 2018