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How to choose the best particles to improve emulsion stability? Understand the impact of contact angle on stability!
With the rise of environmental awareness, traditional emulsifiers have been increasingly questioned, prompting scientists to re-explore the stability of liquids. A technology called "Ramsden emulsion" or "Pickering emulsion" has gained attention because it uses solid particles to stabilize the emulsion. This technology can not only improve the stability of emulsion, but also take into account environmental protection and cost, making it an emerging research hotspot.
Ramsden emulsion is an emulsion that uses solid particles to stabilize the water-oil interface. By adsorbing the particles between the two phases, the particles can effectively prevent the aggregation of oil droplets, thereby improving the stability of the emulsion.
Traditional emulsions are composed of water and oil. When oil is mixed into water to form a water-in-oil (O/W) emulsion, the oil droplets will eventually aggregate due to the reduction of energy. However, when solid particles are added, these particles are able to bind to the surface of the oil droplets, preventing them from aggregating with each other, thus making the emulsion more stable.
Particle characteristics such as hydrophilicity, shape and size, as well as the electrolyte concentration in the continuous phase and the volume ratio of the two phases, all affect the stability of the emulsion. The contact angle at the interface between particles and oil droplets is an important indicator to describe the hydrophilicity of particles. If the contact angle between the particle and the interface is low, the particle will be mostly wetted by the oil droplets, making it difficult to prevent the aggregation of the oil droplets. Therefore, particles that are partially hydrophilic are more desirable because such particles are able to be partially wetted by both liquids at the same time, thereby better binding to the oil droplet surface.
Optimum emulsion stability is achieved when the contact angle between the particles and the two phases is 90°, which requires the lowest stabilization energy.
In most cases, the particle's preferred wetting phase will be the continuous phase in an emulsion system. Taking milk as an example, it is a type of Ramsden emulsion. Milk protein (casein) units are adsorbed on the surface of milk fat globules and act as surfactants. During the emulsification process, casein replaces damaged milk fat globule membranes, thereby enhancing the stability of the emulsion.
In the past 20 years, with the focus on the problems of traditional emulsifiers, Ramsden emulsions have gained new attention in research. Synthetic nanoparticles are regarded as the main research object of Ramsden emulsion stabilizers, but recently natural organic particles have also gradually received attention. Not only are these natural particles cost-effective, they are also degradable and can be sourced from renewable resources.
Pickering emulsions are widely used in areas such as oil recovery and water restoration. Certain types of Pickering emulsions remain stable in the gastrointestinal tract and display surprising resistance to lipases, which facilitates their use in oral delivery systems to control lipid digestion and meet demand.
In fact, the stability of Ramsden emulsion can also be improved by using "Janus particles" with different hydrophilicity and hydrophobicity on both sides. This method is more effective because the particles have higher adsorption energy at the liquid-liquid interface. In addition, latex particles can also be used to stabilize Ramsden emulsions and be fused and molded to form permeable shells or capsules. This form of entrapment can also be used in water-in-water emulsions.
These Ramsden emulsion particles can be used not only as templates for microencapsulation, forming closed, impermeable capsules, but also for making water-in-water emulsions (i.e., a dispersion of phase-separated aqueous polymer solutions). Such encapsulation techniques can be designed to be reversible in different environments. Pickering's stable microbubbles may also show potential value in the application of ultrasound contrast agents.
With the deepening of research on emulsion stability, scientists are increasingly discovering the importance of factors such as particle selection, contact angle control and surface characteristics in improving emulsion stability. So in the future, in this technology, What further breakthroughs and applications will people get?
