Most of the time, cosmetic products either come in powders or in emulsions made of a mix of liquids and powders. As the cosmetics industry uses particulate matter to manufacture makeup, lipsticks, moisturizers, and numerous other products, particle size analysis is important.
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Particle Size Analysis: The Basics
Particle size analysis, an activity of particle science, is mostly carried out in academic or industrial particle technology laboratories. It has applications in a range of industrial sectors varying from waste management to medicine to manufacturing (including cosmetics manufacturing.) Chemistry, food and drink, forestry, agriculture, aggregates, and energy companies also all apply particle size analysis in their workflows.
There are a few different techniques available to technicians, based on different technologies and approaches. Brownian motion, the movement of particles suspended in a medium, can be analyzed to determine particle sizes, as can the gravitational settling of particles in the air or some other medium. High definition image processing has progressed enough in recent years to be applicable for some particle size analysis applications as well.
Light Scattering-Based Techniques
The most widely used particle size analysis techniques in most industrial sectors are based on light scattering technology, recording Rayleigh and Mie scattering of photons that interact with the particulate matter at hand. Light scattering-based particle size analysis enables optical sample characterization which can be enhanced by computer processes to improve speed and accuracy. This means that industries like cosmetics can use particle size analysis for high-fidelity quality control processes.
Most size analysis for bigger-than-nanoscale particles is carried out with static light scattering or laser diffraction (LD) techniques. LD particle size analysis irradiates a dilute suspension of particles with a laser beam. A lens focuses the scattered light onto a large array of concentric optical sensor rings.
Smaller particles result in larger scattering angles from the laser beam, so measuring the angle-dependent scattered intensity of light enables technicians to infer particle size distribution in the sample. Particle size analysis can use Fraunhofer or Mie scattering models; Mie scattering requires prior knowledge of the sample particles’ and dispersant’s refractive indices.
With a broad dynamic range, ability to measure quickly with high reproducibility, and remote operating capabilities, LD particle size analysis is widely adopted in industry. LD particle size analyzers are typically around the same size as a washing machine and cost anywhere from $50,000 to $200,000. The size and price are both dictated by the need for the distance between the sample and laser source and up to twenty sensors carefully placed at different scattering angles.
In cosmetics, particle size analysis is an important quality control process. The particulate material is needed for any cosmetic product that comes in a powder or emulsion. Powdered talc, kaolin, iron oxide, rice powder, zinc oxide, and titanium dioxide are all used extensively in cosmetics to interact with light to create desired effects. The precise size and distribution of particles in the product affect the product’s performance and customer satisfaction.
Detecting Nanoscale Particle Size and Distribution
Recent advances in light scattering techniques for particle characterization have enabled size analysis for nanoparticles (with diameters between 0.1 nm and 100 nm), and the dynamic light scattering (DLS) technique for nanoparticle size analysis is now an industry standard.
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In DLS, technicians combine Brownian motion analysis with light scattering to find out particles’ hydrodynamic sizes (which can be used to determine their absolute sizes.) We still use the Stokes-Einstein equation derived by Albert Einstein as part of his Ph.D. thesis to characterize the relationship between Brownian motion and light scattering and to determine nanoparticles’ hydrodynamic sizes.
DLS does have some disadvantages. It is a relatively low-resolution technique and so is not suitable for measuring polydisperse samples, as large particles in the sample would affect the accuracy of the overall size analysis. As a result, new scattering techniques are still emerging. Nanoparticle tracking analysis (NTA), for example, tracks the movement of individual particles through their light scattering data using image processing. NTA can also measure particles’ hydrodynamic size, and it is capable of higher resolutions than DLS.
As a more mature technology, DLS is still the industry standard for nanoscale particle size analysis, while acoustic spectrometry and laser diffraction are also sometimes used.
Nanoscale particle size analysis is becoming increasingly important in the cosmetics industry as the problems of microplastics and other particulate pollution grow more severe. Small microparticles – especially microplastics that are added to many cosmetics products – have been found in nearly every body of water on the planet, even in high mountain lakes in the Alps and remote islands in the Pacific Ocean.
These particles enter non-human and human animals’ digestive systems alike, causing and contributing to poor health and illness. They also disrupt natural chemical and mechanical balances in soil and water beds, altering delicate ecosystems worldwide.
In the last few decades, public attention has focused on microplastics in the cosmetics industry, and the industry has started to move away from using them in its products. Reliable particle size analysis that can determine the distribution of nanoparticles in sample products can ensure that microparticle pollutants are not included in cosmetics in the future.
References and Further Reading
Horiba Scientific (2018). Application Note AN161: Scientific Particle Size Analysis of Cosmetics. Horiba Scientific. [Online] .pdf available at: https://static.horiba.com/fileadmin/Horiba/Application/Health_Care/Pharmaceuticals_and_Medicine_Manufacturing/Cosmetics/AN161__Particle_Size_Analysis_of_Cosmetics.pdf.
Hermannsson, P. G., et al. (2015). Refractive index dispersion sensing using an array of photonic crystal resonant reflectors. Applied Physics Letters. Available at: https://doi.org/10.1063/1.4928548.
Hussain, R., M. Alican Noyan, G. Woyessa, et al. (2020). An ultra-compact particle size analyser using a CMOS image sensor and machine learning. Light Science & Applications. Available at: https://doi.org/10.1038/s41377-020-0255-6.