Physical Sunscreens
Physical Sunscreens
by Jasmine Chan
INTRODUCTION
Elaborating on my article on ‘Sunscreens and Ultraviolet Light’, I will be looking into the science of how physical sunscreens work and their properties.
INTRODUCTION TO PHYSICAL SUNSCREENS
Physical sunscreens are commonly inorganic substances that sit on top of the skin after application and are able to reflect or scatter UV light. This includes chemicals such as zinc oxide (ZnO) and titanium dioxide (TiO2). These chemicals in sunscreen are 1/20th smaller than conventional pigments which are known as microfine pigments. They are then dispersed and spread evenly into a base. Combinations of these chemicals with other substances can potentially reduce UV transmission, which means that it provides a good protection for the skin against UVA and some wavelengths of UVB. Both titanium dioxide and zinc oxide are able to reflect and scatter UV and visible light, as well as absorb UV light. [1] These chemicals are semiconductors (substances that conduct electricity under specific conditions) that can absorb light and generate reactive species, meaning that they are photocatalysts. [2, 3] They are able to promote the transformation of organic molecules when absorbing UV radiation. [3]
PHOTOCATALYSIS OF PHYSICAL SUNSCREENS
When the photocatalyst absorbs UV radiation, it produces pairs of electrons and holes. The electron of the valence band (the band of electron orbitals that electrons can jump out of) of the photocatalyst becomes excited when is illuminated with light which promotes the electron to the conduction band (the band of electron orbital that electrons can jump up into from the valence when excited) of the photocatalyst. [4, 5, 6] The excitability of the chemical depends on its crystalline structure and the band gap - the difference in energy between the highest occupied energy state of the valence band and the lowest unoccupied state of the conduction band. [1, 5] This creates pairs of negative electrons (e-) and positive holes (h+). A redox reaction then occurs. The positive holes break water molecules, which forms hydrogen gas (H2) and hydroxyl radicals (●OH) - this is the oxidation reaction. The negative electrons react with the oxygen molecule to form superoxide anions (O2-●) - this is the reduction reaction. This photocatalyst cycle repeats once light is available. [4]
Fig.1 Visual diagram showing the different sizes of band gaps for conductors, semiconductors, and insulators.
Source: “Band Gap.” Energy Education, energyeducation.ca/encyclopedia/Band_gap.
Fig.2 Visual Diagram of How a Photocatalyst Works
Source: Nguyen, Thao Thi, et al. “Tungsten Trioxide (WO3) - Assisted Photocatalytic Degradation of Amoxicillin by Simulated Solar Irradiation.” Nature News, 27 June 2019, www.nature.com/articles/s41598-019-45644-8.
PROPERTIES OF SUBSTANCES IN PHYSICAL SUNSCREENS
The ability of a substance to block light in physical sunscreens are determined by several physical properties: the substance’s opacity and particle size. [1]
OPACITY
The opacity of a physical sunscreen is calculated by the Snell’s Law of Refraction: Np sin i / Nm sin r where Np is the refractive index of the pigment in the physical sunscreen, Nm is the refractive index of the adjacent medium which is air in this case, i is the angle of incidence (the angle between the incident light ray and the normal), and r is the angle of refraction (the angle between the emergent light ray and the normal). [7, 8]
Fig.3 Visual Diagram of Snell’s Law of Refraction.
Source: “Refraction, Snell's Law, and Total Internal Reflection.” Boston University, buphy.bu.edu/py106/notes/Refraction.html.
Fig.4 Table of Refractive Indexes of Different Inorganic Pigments.
Source: Murphy, G. M. “Sunblocks: Mechanisms of Action.” Photodermatology Photoimmunology & Photomedicine, 15 Sept. 1998, onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0781.1999.tb00051.x.
Refraction occurs when light meets a boundary between two media, and because there is a change in the refractive index (usually entering a medium with a higher refractive index), the velocity of light travelling in this new medium will be different (if higher refractive index, then its velocity will decrease). Molecules with a high refractive index can increase the reflectiveness of the sunscreen. [1]
As the refractive index of the pigment increases, opacity increases since more light is scattered. [9] As the sunscreen is more opaque, the sunscreen has a white tint when applied to the face. This is known as white casting.
Fig.5 Example of White Casting.
Source: “Chemical vs Physical Sunscreens: The Science (with Video).” Lab Muffin Beauty Science, 2 Apr. 2018, labmuffin.com/chemical-vs-physical-sunscreens-the-science-with-video/.
Nowadays, cosmetic chemists have incorporated more brown pigments in sunscreen such as iron oxide (Fe2O3) which reduces the white-casting effect, making the sunscreen seem more natural on the face. This type of sunscreen is known as tinted sunscreens. The additional use of pigments can also enhance the scattering effect of the physical sunscreens, making the sunscreen overall more effective as different pigments have different relative opacities. [1]
Fig. 6 Comparison of the Relative Opacities of Different Microfine Pigments.
Source: Murphy, G. M. “Sunblocks: Mechanisms of Action.” Photodermatology Photoimmunology & Photomedicine, 15 Sept. 1998, onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0781.1999.tb00051.x.
PARTICLE SIZE
The particle size of a pigment is the average size of the particles in the pigment. [10] The best pigments used in physical sunscreen is when the diameter of the particle is half of the wavelength of visible light. [1] One of the most common pigments used in physical sunscreen is titanium dioxide. As the size of titanium dioxide is relatively small (200-500nm in size), they generally have a greater ability to reflect light. [10] Despite particles being small which can lead to transparency, the ability of the particle to reflect and scatter UV radiation retains.
As the particle size varies, the type of scattering changes. In titanium dioxide, two types of scattering can occur: Mie scattering and Rayleigh scattering. [1] For particle sizes larger than a wavelength of light, Mie scattering occurs which is a type of scattering that produces a pattern similar to an antenna lobe. For larger particles, the antenna lobe like shape would have a sharper and more intense forward lobe. [11] For particle sizes around a tenth of the wavelength of light, Rayleigh scattering occurs where the patterns for forward and backward scattering are symmetrical. [12]
Fig. 7 Visual Diagram of Mie and Rayleigh Scattering.
BIBLIOGRAPHY
[1] Murphy, G. M. “Sunblocks: Mechanisms of Action.” Photodermatology Photoimmunology & Photomedicine, 15 Sept. 1998, onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0781.1999.tb00051.x.
[2] Hanania, Jordan, et al. “Semiconductor.” Energy Education, 31 Jan. 2020, energyeducation.ca/encyclopedia/Semiconductor.
[3] Picatonotto, Tatiana, et al. Photocatalytic Activity of Inorganic Sunscreens. Journal of Dispersion Science and Technology, 2001.
[4] “What Is Photocatalyst?” Abolin, www.abolinco.com/downloads/downloads/What_is_Photocatalyst.pdf.
[5] Hanania, Jordan, et al. “Valence Band.” Energy Education, 4 June 2018, energyeducation.ca/encyclopedia/Valence_band.
[6] Dharan, Gokul, et al. “Conduction Band.” Energy Education, 4 June 2018, energyeducation.ca/encyclopedia/Conduction_band.
[7] “Refractive Index.” Encyclopædia Britannica, 23 Dec. 2019, www.britannica.com/science/refractive-index.
[8] “The Law of Refraction.” Mathematics Department, University of British Columbia, www.math.ubc.ca/~cass/courses/m309-01a/chu/Fundamentals/snell.htm.
[9] O'Hanlon, George. “Why Some Paints Are Transparent and Others Opaque.” Natural Pigments Inc., 6 Dec. 2013, www.naturalpigments.com/artist-materials/transparent-opaque-paints.
[10] MacEvoy, Bruce. “The Material Attributes of Paints.” Handprint, 8 Jan. 2015, www.handprint.com/HP/WCL/pigmt3.html.
[11] Nave, R. “Blue Sky and Rayleigh Scattering.” HyperPhysics, hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html.
[12] “Rayleigh Scattering.” Encyclopædia Britannica, 12 Feb. 2018, www.britannica.com/science/Rayleigh-scattering.
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