High-resolution Imaging of Double Emulsion

What are emulsions?

An emulsion is a solution where two different liquids that do not dissolve into each other, such as oil and water, separate into two layers due to the interfacial tension effect. One liquid forms droplets and is scattered within the other liquid. The process of forming an emulsion from two liquids is called emulsification. In the case of water and oil, the emulsion takes one of two compositions: Oil-in-Water (O/W) or Water-in-Oil (W/O). An example of an O/W emulsion is milk. An example of a W/O emulsion is butter.

In recent years, attention has been given to how emulsions are used in various fields such as cosmetics, food, and medicine. In the cosmetics industry, the characteristic of emulsions of being able to control factors such as skin affinity, appearance, and texture has been applied to moisturizing creams and serums for the skin. Furthermore, in the area of food processing, research is progressing in order to control factors such as the flavor of and number of calories in foods. On top of that, there is active research in the pharmaceutical industry aimed at the use of emulsions in drug delivery systems (DDS), which precisely control the movement of drugs within the body so as to deliver them to the target location through the use of emulsions as drug carriers.

Amid such trends, the methods for evaluating emulsions are also being researched. Particle size measurement and condition assessment of emulsions have garnered much attention and interest, thereby contributing to the identification of the issues in actual evaluations.

In this section, we will use a concrete example in the food industry to introduce the double emulsion, which has come into the spotlight.

Advantages of double emulsion

Larger emulsifying particles tend to strengthen certain characteristics, such as richness and aroma, of constituents. Therefore, the flavor, including taste, can be controlled by adjusting the size of the emulsifying particles. On the other hand, in the case of an O/W emulsion, since emulsifying particles are lipids, making them larger increases the costs proportionally. It also increases the caloric content of these particles, which may make the products less desirable as the current trend favors low-calorie products.

Double emulsion is a method that resolves the above issues. Encapsulating water inside emulsifying particles makes it possible to increase the size of the emulsifying particles but still keeps the total use of lipids down. This makes it possible to reduce the total amount of lipids used, further contributing to reductions in both costs and calories without having to sacrifice taste. Because the structure is such that more water is contained within an O/W emulsion, this is known as a W/O/W (Water-in-Oil-in-Water) emulsion. However, the bar is still high for realizing stable production and for maintaining the emulsion until consumption. Examples of issues making it difficult to realize stable production include the difficulty of adjusting production conditions and the loss of double emulsion due to instability arising from heat treatment.

A

Water phase

B

Oil phase

C

Outer water phase

D

Inner water phase

Observing double emulsions

To derive stable production conditions or to accurately determine the status when various treatments have been applied, a microscope is used to directly observe the status of an emulsion.

Nevertheless, such observations were difficult due to the minuscule size of the water capsules (the inner water phase) in double emulsions. These capsules have diameters of only several hundred nanometers, and ordinary optical microscopes may not have the resolution to enable such observations. While a scanning electron microscope (SEM) is capable of observation of this size, double emulsions are composed of water and lipids, so it is difficult to observe them with an SEM, which requires the observed target to be in a vacuum.

The BZ-X800 has a high NA (numerical aperture) and can use a plan apochromat oil-immersion lens having minimal aberration. As such, the BZ-X800 enables high-resolution observation like that shown below.

A

Outer water phase

B

Oil droplet

C

Oil phase

D

Inner water phase

E

Water phase