Dispersions are heterogeneous systems that consist of at least two immiscible phases. They tend to become unstable over time. The MultiScan Stability Index provides an easy interpretation of their destabilisation. The Stability Index consists of a single number that describes the global sample stability within a defined period of time.

What are dispersions?

The MultiScan Stability Index describes the stability of dispersions. 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.

Over time, the position and ratio of the particles of the two phases to each other change. These changes are also referred to as destabilisation processes. 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. The stability index is a simple method of comparing these destabilisation mechanisms with each other.

Stability index for the classification of dispersions

To calculate the MultiScan Stability Index of dispersions, the stability must first be analysed optically. Such investigations can be carried out with a dispersion stability analysis system such as the MultiScan MS 20 from DataPhysics Instruments.

The measuring method used for this 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. 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 position and time resolution.

In practical applications, it is very important to quantitatively classify and compare the stability of different formulations. Comparing the stability of formulations based solely on the values of transmitted and backscattered light requires the use of complicated calculation techniques.

To facilitate the comparison of the stability of different formulations, DataPhysics Instruments has introduced the MultiScan Stability Index. The MultiScan Stability Index consists of a single number that describes the global sample stability. Specifically, the MultiScan Stability Index is a numerical value calculated at a given point in time by summarising all temporal and spatial variations of transmitted and backscattered light. The interpretation of the MultiScan Stability Index is very simple: the closer the value is to zero, the more stable a sample is. The higher the MultiScan Stability Index, the less stable a sample is.

To simplify the interpretation of the MultiScan Stability Index, the MultiScan software has a scale that categorises the value. The scale is coloured, which makes it easy to interpret the results. This scale is based on an empirical analysis of a database with hundreds of measurements.