As is generally known, the solar radiation spectrum extends from wavelengths of 200 nm to 3,000 nm. Within this spectrum, the different types of radiation can be classified by wavelength and energy content. Ultraviolet (UV) radiation, from 200 nm to 380 nm, produces burning and erythema, in the case of UVB, or skin aging and photocarcinogenesis possibly leading to skin cancer, in the case of UVA. Visible light falls in the range of 380 nm to 700 nm; and infrared (IR) radiation, from 700 nm to 3,000 nm, is responsible for heat and also appears to be involved in skin aging and cancer.
Solar radiation that reaches the earth’s surface is composed of approximately 7% UV. The remaining 93% is roughly divided between visible and IR radiation. While 7% is seemingly small in comparison, this level of UV radiation is sufficient to cause skin damage. Moreover, the ozone layer depletion during the past few decades has enhanced the levels of UV radiation that reach the Earth’ s surface. Therefore, efforts have been devoted to the development of organic and inorganic UV filters, and the sunscreen industry has benefited from the introduction of new active ingredients to enhance UV protection.
Organic chemical molecules including salicylates, cinnamates, camphor, triazone derivatives (UVB) or benzophenones, avobenzone, bemotrizimol (UVA), etc. absorb UV radiation, whereas inorganic particulates like titanium dioxide (TiO2) and zinc oxide (ZnO) reflect and scatter UV rays. Micronized particles of these latter compounds have been used to improve protection against UVA since they scatter light efficiently in the 320–400 nm range, provided they are present in sufficient quantities. Cosmetic manufacturers currently use these materials in conjunction with organic UV-absorbing chemicals to boost sun protection in the UVA region or to broaden the spectral coverage. However, while these particles effectively scatter UV, their use in sun care formulas poses a challenge since they appear white on skin. This is due to the attenuation of visible light, which is particle-size dependent, and is generally aesthetically unacceptable.
Lab Practical: Using Silicon Microspheres
- Silicon microspheres are dispersible in aqueous phases; further processing enables their inclusion in oil phases.
- The microspheres are slightly acidic with a pH of 5-6, which ensures the stability of the dispersions and minimizes particle aggregation.
- The microparticles are obtained as aggregates of many colloids, i.e., a photonic sponge; previous grinding is required to improve the efficiency of the dispersions in emulsions as well as their optical properties.