What is the Mixing Stability of Dispersions?

Figure 1: In practice, the investigation and determination of mixing stability is important if dispersions are to be stored over a longer period of time without changing their properties.

The mixing stability of a liquid dispersion describes how long the dispersion remains stable in its mixture. A quantifiable method for analysing the mixing stability is the multiple light scattering method. It can be used to calculate the stability index, a simple parameter for determining and comparing different mixture stabilities.

How can dispersions be categorised?

Dispersions are heterogeneous systems that consist of at least two immiscible phases. A distinction is made between the disperse and the continuous phase. The disperse phase consists of smaller particles or droplets that are distributed in the continuous phase. The continuous phase is usually present in larger quantities and surrounds the dispersed components.

Dispersions can be categorised according to the aggregate states of the disperse and continuous phases. Liquid dispersions, in which the continuous phase is liquid, can be divided into three classes: An emulsion is formed when both the continuous and disperse phases are liquid. A suspension is formed when the continuous phase is liquid and the disperse phase is solid. A foam is formed when the continuous phase is liquid and the disperse phase is gaseous.

The mixing stability of these systems describes whether and in what form the continuous and disperse phases mix. This is also referred to as the storage stability or shelf life of the liquid dispersion.

Which processes can influence the mixing stability of dispersions?

As mentioned above, dispersions consist of two immiscible phases. Over time, the position and ratio of the molecules and particles of the two phases change. This is also referred to as destabilisation processes. Various such destabilisation processes can occur in dispersions. They lead to changes in the dispersion and thus influence its stability. Important destabilisation processes are sedimentation, creaming, coalescence, agglomeration and aggregation as well as Ostwald ripening.

									
Figure 2: Schematic representation of the measuring principle of the MultiScan MS 20 for analysing mixture stability.
Investigation of mixture stability using the static multiple light scattering methodOne way to quantify the stability of mixtures is to analyse the mixing stability optically. Such tests can be carried out with a Dispersion Stability Analysis System such as the MultiScan MS 20 from DataPhysics Instruments. Even the smallest changes in a dispersion can be recognised with such analysis systems.
The measuring method used to do so is called the static multiple light scattering method (SMLS). The liquid sample is filled into a round container and placed in a measuring tower. There are two light sources and a light detector in the measuring tower. One light source is located opposite the detector and shines through the sample - the detector records the transmitted light. The second light source is located next to the detector - the detector detects the backscattered light (see Fig. 2). The detector and light sources scan the sample from bottom to top at regular intervals. This allows changes in the transmitted and backscattered light to be determined with positional and temporal resolution.
Classification of mixing stability with the stability indexIn many applications, it is necessary to compare the mixing stability of different dispersions. To simplify this comparison in practice, the so-called MultiScan Stability Index can be used. The MultiScan Stability Index consists of a single number that describes the global mixing stability of the sample. The MultiScan Stability Index represents an aggregation of all temporal and spatial fluctuations of the signals for the transmitted and backscattered light. If the stability index is close to zero, there is virtually no destabilisation on the measured time scale. If the index is higher, the sample is less stable.
Mixing stability in practiceIn practice, the investigation and determination of mixing stability is important if dispersions are to be stored over a longer period of time without changing their properties. In other applications, the determination is important if a sample is to separate as quickly as possible.
In the food industry, for example, mixing stability is important for juices, milk or smoothies. Here, sedimentation or creaming must be prevented in order to ensure a homogeneous product with a consistent texture and flavour over the entire storage period.
The stability of creams, lotions and make-up products is also essential in the cosmetics industry, as phase separation or lump formation would impair application, shelf life and visual properties. In the pharmaceutical industry, on the other hand, the quality of syrups, injectables or vaccine formulations depends heavily on stability, as destabilisation processes can lead not only to a loss of efficacy but also to safety risks. 
The coatings and paints industry also relies on stable pigment dispersions to ensure colour, opacity and uniform processing. Instabilities here quickly manifest themselves in flocculation or uneven colour results.
Finally, the measurement of destabilisation processes also plays an important role in the oil and energy industry, as the stability of oil-water emulsions, for example, is crucial to the efficiency of separation processes and lubricants or fuels only retain their full functionality and shelf life if they remain stable.

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