How long are Pickering emulsions stable?
A new study, published in Nature Materials in December 2011, investigated the time necessary for a Pickering emulsion to reach equilibrium conditions. The results showed that the process is much slower than expected, as the equilibrium timescale can be of the order of months. The slow kinetics of the process can be important for applications such as medicine or nanomaterials.
Emulsion: What It Is and How It Is Formed
An emulsion is a mixture of two or more fluids which are immiscible, or do not form a solution when combined. Oil and water, for instance, can form an emulsion.
When two immiscible fluids are placed in contact, they tend to separate into two distinct phases. In the case of oil and water, the oil will form a layer above the water, as it is lighter.
For the two liquids to be mixed, and the emulsion to be formed, some energy has to be given to the system. For oil and water, this can be done by stirring. In the emulsion, small droplets of one fluid will be dispersed into the other, the two fluids being named the dispersed and continuous phases, respectively.
Stability of an Emulsion
Once the emulsion is formed, it will be stable for a certain length of time, which depends on the characteristics of the two fluids. Eventually, however, the two fluids will separate again, as the droplets of the dispersed fluid will aggregate together.
To avoid this, and stabilize the emulsion for longer periods of time, some chemical compounds – emulsifiers – can be used. Emulsifiers form a very thin film on the surface of the droplets of the dispersed fluid; this stabilizes the dispersed droplets and stops their aggregation. In these conditions, the emulsion is in equilibrium, with the two fluids homogeneously mixed. Surfactants such as detergents are examples of emulsifiers.
An emulsion can also be stabilized by a solid, which can be added to the emulsion in the form of small particles. This is called a Pickering emulsion.
The solid particles prevent the aggregation of the dispersed droplets by binding to their surface; this process is affected by many parameters, the principal being the hydrophobicity of the solid material.
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