Dielectric Constant to Develop Clear Sunscreens for Wet Skin

Feb 21, 2014 | Contact Author | By: Donald Prettypaul, Diane Kennedy, Anna Gripp and Hani Fares, Ashland Specialty Ingredients
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Title: Dielectric Constant to Develop Clear Sunscreens for Wet Skin
UV radiationx estersx dielectric constantx alcoholx miscibilityx polarityx spraysx
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Keywords: UV radiation | esters | dielectric constant | alcohol | miscibility | polarity | sprays

Abstract: Sunscreen sprays have gained momentum due to their ease of application. However, while they appear clear on dry skin, they often turn cloudy and white when applied to wet skin. Here, the authors present a formulating approach based on the dielectric constant of esters and level of alcohol in formulations to produce anhydrous sprays that appear clear on wet skin.

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D Prettypaul, D Kennedy, A Gripp and H Fares, Dielectric Constant to Develop Clear Sunscreens for Wet Skin, Cosm & Toil 129(2) 86 (2014)

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It is widely known that prolonged exposure to UV radiation can cause severe skin damage.1 This radiation can be divided into three subregions: UV-A, B and C. UV-A rays, 320 nm to 400 nm, induce skin tanning. Long-term exposure to UV-A causes the loss of skin elasticity and the appearance of wrinkles. UV-B rays, 290 nm to 320 nm, can cause erythema and sunburn. UV-C rays, 200 nm to 290 nm, are mostly blocked by the ozone layer. To reduce skin damage, an effective sunscreen therefore must absorb in the UV-A and UV-B regions.2 Additionally, one that works on wet skin would be advantageous since outdoor activities such as swimming as well as perspiration can reduce sunscreen retention on the skin.

The composition of anhydrous sunscreen sprays typically includes ethanol, UV filters, esters and a polymer to increase water resistance. These formulations are usually clear and appear clear when applied to dry skin. However, when the skin is wet, these products appear milky. This is a result of the emulsification process. The UV filter oils and the water on the skin are not miscible; therefore, one gets dispersed into the other, forming many oil and water interfaces that scatter light, thus appearing white.

This problem can be addressed by considering the polarity of each ingredient. Figure 1 shows a schematic of the polarity of the major components in a typical anhydrous sunscreen spray formulation. Water is on the opposite side of the polarity spectrum from the sunscreen and esters. The addition of alcohol to water makes the mixture less polar, i.e., less hydrophobic, whereas adding alcohol to esters or sunscreens makes the mixture more polar, i.e., more hydrophilic.

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Table 1. Measured values of dielectric constants for some common materials

Table 1. Measured values of dielectric constants for some common materials

Glycerin and methanol fall in the middle of the range with a dielectric constant of 42.5 and 32.6, respectively. These values are displayed in Table 1.

Table 2. Dielectric constant values of esters and appearance wet skin

Table 2. Dielectric constant values of esters and appearance wet skin

The appearance of the spray was then noted, as shown in Table 2, along with the esters’ dielectric constants.

Figure 1. Schematic of a polarity scale showing reference points for water, alcohol, esters and sunscreen filters

Figure 1. Schematic of a polarity scale showing reference points for water, alcohol, esters and sunscreen filters

Figure 1 shows a schematic of the polarity of the major components in a typical anhydrous sunscreen spray formulation.

Footnotes (CT1403 Prettypaul)

a BI-870 dielectric constant meter, Brookhaven

b-e Escalol UV filters, Ceraphyl emollient esters, X-Tend 226 solubilizer/emollient ester and Advantage Plus polymer; Ashland

f HydroSheer, Ashland

Formula 1. Anhydrous sunscreen formulations (SPF 25)

Ingredient a (% w/w) b (% w/w)
Ethanol 61.0 41.0
VA/Butyl Maleate/Isobornyl Acrylate Copolymer 2.0 2.0
Avobenzone 3.0 3.0
Oxybenzone 6.0 6.0
Homosalate 15.0 15.0
Octisalate 5.0 5.0
Octocrylene 8.0 8.0
Diisopropyl Adipate  — 10.0
Phenethyl Benzoate 10.0

Formula 2. Adjusted anhydrous sunscreen formulations (SPF 25)

Ingredient a (% w/w) b (%w/w) c (% w/w)
Ethanol 45.0 42.0 50.0
VA/Butyl Maleate/Isobornyl Acrylate Copolymer 2.0 2.0 2.0
Avobenzone 3.0 3.0 3.0
Oxybenzone 6.0 6.0 6.0
Homosalate 15.0 15.0 15.0
Octisalate 5.0 5.0 5.0
Octocrylene 8.0 8.0 8.0
Diisopropyl Adipate 7.0 8.5 4.5
Isodecyl Neopentanoate 9.0 10.5 6.5

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