Most of the time, cosmetic products come in the form of powders or emulsions made up of a mixture of liquids and powders. As the cosmetics industry uses particles to make makeup, lipsticks, moisturizers and many 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 mainly performed in university or industrial particle technology laboratories. It has applications in a range of industrial sectors from waste management to medicine to manufacturing (including cosmetics manufacturing). Chemical, food and beverage, forestry, agriculture, aggregates and energy companies also 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 motion of particles suspended in a medium, can be analyzed to determine particle size, as can the gravitational sedimentation of particles in air or another medium. High definition image processing has advanced enough in recent years to also be applicable to some particle size analysis applications.
Techniques based on light scattering
The most widely used particle size analysis techniques in most industrial sectors are based on light scattering technology, recording the Rayleigh and Mie scattering of photons that interact with the particulate matter at hand. Particle size analysis based on light scattering enables optical characterization of samples which can be enhanced by computational processes to improve speed and accuracy. This means industries like cosmetics can use particle size analysis for high-fidelity quality control processes.
Most size analyzes for particles larger than the nanometer scale are performed 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 scattered light onto a wide array of concentric optical sensor rings.
Smaller particles result in larger scatter angles of the laser beam, so measuring the intensity of scattered light as a function of angle allows technicians to infer the particle size distribution in the sample . Particle size analysis can use Fraunhofer or Mie diffusion models; Mie scattering requires prior knowledge of the refractive indices of the sample particles and the dispersant.
With wide dynamic range, fast measurement capability with high reproducibility, and remote operation capabilities, LD particle size analysis is widely adopted in industry. LD particle size analyzers are usually about the same size as a washing machine and cost between $50,000 and $200,000. Both size and price are dictated by the need for the distance between the sample and the 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 necessary for any cosmetic product that comes in powder or emulsion form. Powdered talc, kaolin, iron oxide, rice powder, zinc oxide, and titanium dioxide are all widely used in cosmetics to interact with light to create desired effects. The precise size and distribution of particles in the product affects product performance and customer satisfaction.
Detection of particle size and distribution at the nanoscale
Recent advances in light scattering techniques for particle characterization have enabled the size analysis of nanoparticles (with a diameter between 0.1 nm and 100 nm), and the dynamic light scattering technique (DLS) for nanoparticle size analysis is now an industry standard.
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In DLS, technicians combine analysis of Brownian motion with light scattering to discover the hydrodynamic sizes of particles (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 aiming to characterize the relationship between Brownian motion and light scattering and to determine the hydrodynamic sizes of nanoparticles.
DLS has a few drawbacks. It is a relatively low resolution technique and therefore 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 dissemination techniques continue to emerge. Nanoparticle tracking analysis (NTA), for example, tracks the movement of individual particles through their light scattering data using image processing. NTA can also measure hydrodynamic particle size and is capable of higher resolutions than DLS.
As a more mature technology, DLS remains 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 problems with microplastics and other particulate pollution grow. Small microparticles – specifically microplastics that are added to many cosmetic products – have been found in almost every body of water on the planet, even in high mountain lakes in the Alps and isolated islands in the Pacific Ocean. .
These particles enter the digestive systems of non-human animals and humans, causing and contributing to poor health and disease. They also disrupt natural chemical and mechanical balances in soil and water beds, altering delicate ecosystems around the world.
Over the past few decades, public attention has focused on microplastics in the cosmetics industry, and the industry has begun to move away from their use in its products. Reliable particle size analysis, capable of determining the distribution of nanoparticles in product samples, can ensure that polluting microparticles will not be included in cosmetics in the future.
References and further reading
Scientist Horiba (2018). Application note AN161: Scientific analysis of the particle size of cosmetic products. Scientist Horiba. [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, PG, et al. (2015). Detecting refractive index dispersion using a photonic crystal resonant reflector array. Letters of applied physics. Available at: https://doi.org/10.1063/1.4928548.
Hussain, R., M. Alican Noyan, G. Woyessa, et al. (2020). An ultra-compact particle size analyzer using a CMOS image sensor and machine learning. Light sciences and applications. Available at: https://doi.org/10.1038/s41377-020-0255-6.