The determination of particle size has become an essential analysis method in many industries as it affects both the production process as well as the final properties of the product. Today several techniques for particle size measurements are available, allowing users to get a deep insight into their sample’s behavior.
What do the settling power of cement, the taste of chocolate, the effective targeting of cancer cells, and the hiding power of paint have in common? All are strongly affected by particle size, be it that of a cement granule, a droplet of cocoa butter, a liposomal drug formulation, or a pigment particle.
Micro- and nanotechnology are not modern inventions but affect the properties of a huge range of common man-made products. What has changed in the last decades is that we now have a variety of techniques for particle size determination in the nano- and micrometer range, enabling us to optimize manufacturing processes, transportation and storage, shelf-life, and, of course, the product’s final properties. Dynamic light scattering and laser diffraction are widely used as these techniques cover particle size analysis from the lower nanometer range up to several milimeters. Particle size measurements are employed in various industries, such as the food and beverage, pharamceutical, and chemical industries, but also in building materials manufacturing, serving a different purpose for each type of use:
Many food items are in particulate form at one time or another during their production process, either as powders, as suspensions, or as emulsions.
For powders, particle size affects bulk density and, thus, powder flow properties. Likewise, in suspensions, particle size strongly influences shear viscosity. This in turn critically affects pumping, mixing, and transportation of the raw materials.
The storage and stability of food are also strongly dependent on particle size. While powdered food items can undergo caking, emulsions (e.g. milk) can have stability issues if the size of lipid droplets is not tightly controlled.
Particle size also strongly affects the organoleptic properties of food items. Texture and mouthfeel play pivotal roles in the acceptability of food products. As the human tongue is capable of detecting particles of just a few microns in size, tweaking the size of particles can have a big impact on product acceptance.
Particle size is a critical process parameter in pharmaceutical production, for powdered active pharmaceutical ingredients (API) as well as for emulsions and for cell-based formulations.
For powdered APIs, the dissolution rate and, thus, the bioavailability of the drug is chiefly influenced by the particle size. This is particularly important for formulations requiring a controlled or delayed release in the organism.
The in vivo (in living organisms) distribution, deposition, and absorption rate of the drug upon administration also relates to particle size. This is particularly relevant when a particular cell type is targeted (e.g. cancer cells, endothelial cells…), as different target cells may have very different particle uptake abilities.
Liposomes, small vesicular particles consisting of phospholipid bilayers, are intensively used in pharmaceutical formulations for the targeting of cancer cells. Thereby, the sizes of the vesicles play a major role and are typically characterized by dynamic light scattering instruments. Within the last few years also other nanometer-range vesicles, called exosomes, have gained attention as candidates for cancer-targeting therapy.
Additionally, as for most other industries, particle size also affects the flow behavior of pharmaceutical powders, particle suspensions, and emulsions, affecting the transport and packaging properties of the formulations.
Paints and coatings
In terms of optical properties, the particle size of pigments strongly affects the color strength, i.e. the effect of a given pigment when mixed with others. In addition, it also affects the way the paint scatters light, which is instrumental to the paint’s hiding power as well as its finish (e.g. glossy or matt).
As paints are particle-bearing liquids, their rheological behavior depends not only on particle concentration, but also on particle shape and particle size. Therefore, particle sizing is important to predict the flow behavior of paint during transport, storage, and, crucially, when it is applied.
The interaction between pigment particles and their substrate is responsible for the stability of the paint emulsion. In this way, particle size also affects the shelf-life of paint.
Another application of particle size determination is in the manufacturing of building materials. For instance, in the production of cement the grinding is a very energy-consuming step and narrowing down the particle size distribution helps save energy and costs.
In addition to the economic aspects of cement production, particle size also has a major influence on the properties of the final product. Together with its chemical composition and specific surface area, the particle size distribution is a major factor affecting the hydration curve of cement as well as the strength of the hardened paste. Specifically, reduction in average particle size leads to decreased setting time and enhanced early strength. In contrast, the importance of coarser particles grows with the aging of the cement. The width of the particle size distribution also determines the packing density and water demand of cement and other building materials.
As these examples show, the determination of particle size is an important method in many industry groups and affects the production process as well as the final product. Among several measurement techniques dynamic light scattering as used in the Anton Paar Litesizer series and the laser diffraction technique of the PSA series help the users characterize their sample and understand its behavior.