Dispersions explained

Figure 1: To make them easier to take, medicines are made in the form of dissolvable tablets. Once dissolved, these form a suspension of water and solid active ingredient.

Dispersions consist of at least two different phases. These phases can be divided into a disperse phase and a continuous phase. Many industrial products are dispersions, including milk, emulsion paints, and creams. Detailed studies of the stability of dispersions are essential for their further development.

What are dispersions?

Dispersions are heterogeneous systems consisting of at least two immiscible phases. They are divided into a disperse phase and a continuous phase. The dispersed phase consists of smaller particles or droplets distributed within the continuous phase. The continuous phase is usually present in a larger quantity and surrounds the dispersed components (see Figure 2).

The following table classifies dispersions based on the state of matter of the dispersed and continuous phases:

dispersion type	continuous phase	dispersed phase
emulsion	liquid	liquid
foam	liquid	gaseous
suspension	liquid	solid
fog	gaseous	liquid
smoke	gaseous	solid
wet, porous solid	solid	liquid
dry, porous solid	solid	gaseous
solid mixture	solid	solid

Dispersions can also be classified according to their particle size. Systems with smaller particles are generally more stable.

Molecularly dispersed systems contain particles smaller than 1 nm.
Colloidal dispersions contain particles between 1 µm and 1 nm.
Coarse dispersions contain particles larger than 1 µm.

Dispersions in practical applicationsWe encounter dispersions every day in products ranging from salad dressings, mixed drinks, and beer in the food sector to emulsions and creams in cosmetics and pharmaceuticals, as well as multiphase cleaning agents, dispersion paints, and sealing slurries in construction. For all these products, dispersion stability is a critical parameter that must be carefully analysed and optimised during product development.
Why is dispersion stability important?Stability describes the ability of a dispersion to maintain its original structure and properties over a certain time period. A stable dispersion will show little or no change in terms of particle size, phase separation, and other properties. Unstable dispersions, on the other hand, show destabilisation processes such as sedimentation, creaming, coalescence, agglomeration, and aggregation.
How can the stability of emulsions, suspensions, and foams be characterised?The stability and aging behavior of liquid dispersions such as emulsions, suspensions, and foams can be optically analysed. For example, the MultiScan MS 20 from DataPhysics Instruments can be used for dispersion stability analysis. This involves measuring how the transmission and backscattering behaviour of a light source directed at the sample changes over time.

Figure 2: A dispersion consists of a continuous phase (blue), usually present in larger amounts, and a dispersed phase (red).

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