Choosing between wet and dry dispersion

Choosing the most appropriate dispersion method when characterizing materials using laser diffraction should depend on both the physical and chemical characteristics of your sample and the goal of the dispersion procedure. Two methods are available – wet dispersion and dry dispersion. Both methods have their own strengths, and both should be applied with care and consideration to ensure valid results.

So, before going any further, let’s just clarify what we are discussing here. During dry analysis, the sample being analyzed is injected into a clean compressed air stream within a venturi. The pressure drop across the venturi is adjusted to apply just enough energy to cause dispersion without smashing the particles to pieces. In contrast, wet analysis requires the particles to be dispersed in a suitable liquid, with the processes of wetting and gentle shear during stirring causing deagglomeration. In addition, ultrasonics can be used to aid agglomerate separation – this process involves careful optimization.

Influencing factors when choosing between wet and dry dispersion

When making the choice between wet and dry dispersion, influencing factors include the natural state of the sample, the ease with which it can be dispersed and the volume of sample to be measured.
Sample types which benefit from dry dispersion include:

• materials which dissolve in water or the liquid medium
• samples, such as magnetized particles, that agglomerate easily in liquid medium
• products designed to be dispersed in  air e.g. sprays and inhaled drugs

Wet measurement on the other hand may be preferred when handling:

• cohesive and or very fine particles (below 10 microns), which can be difficult to disperse completely using dry techniques
• hygroscopic materials, at risk of collecting moisture when exposed to the atmosphere
• fragile particles
• toxic substances that are more easily controlled in the wet state – reduced risk of inadvertent inhalation

Where the sample is a powder, dry dispersion may present the simplest option. Dry methods are conceptually easier to develop, as the sample feed and the applied dispersion energy are the only measurement variables which need careful attention. Dry dispersers also require minimal cleaning between samples, and can therefore be a lot quicker to apply. However, care must be taken to balance the requirements for dispersion against the risk of particle break-up. For this reason, many materials are unable to be characterised in the dry state and wet dispersion is required.

Referencing dry with wet results

 
Pressure titration for a pharmaceutical powder. Comparison with a reference wet measurement shows that this material should be characterised at low pressures.

ISO13320 (2009) notes that if dry dispersion is applied a “pressure/particle size” titration should, in the ideal case, identify a region where particle size is nearly constant over a range of pressures. This indicates that agglomerate dispersion has occurred without particle break-up. However, in the many cases where this is isn’t the case, it becomes important to reference dry results against wet measurements, to optimise dry dispersion conditions.

If you wish to read up on advances in dry powder dispersion I would point you to the article “Investigating the dispersion of dry powders” published in the Spring 2011 edition of Pharmaceutical Online Magazine.