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Skin Care Moisturizers
By: Eric S. Abrutyn, TPC2 Advisors Ltd., Inc.
Posted: November 30, 2010, from the December 2010 issue of Cosmetics & Toiletries.
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Other measurement tools that provide insight into the performance of a moisturizer include the following.
Irritation/redness: This measurement utilizes skin patch methodology to determine the ability of an ingredient to cause skin irritation and/or countenance of redness. Skin flexibility/elasticity: This approach determines the elastic and plastic characteristics of skin. Most measurements are made with a cutometer that vacuum sucks a defined area of the skin surface, recording visco-elastic properties.
Permittivity: Permittivity measures polarity; emollients with high permittivity are highly polar, exhibiting greater affinity for water.22 By matching the formula’s permittivity closely to the permittivity of healthy skin, formulators can stabilize all the ingredients in the formula to achieve optimum performance on the skin. This is particularly important for multicomponent color cosmetics. Some high permittivity ingredients include lauryl lactate, diisopropyl sebecate and isostearyl hydroxystearate, and low permittivity ingredients include isoparaffin, isododecane and dimethicone.
Contact angle: Contact angle (CA) measures the angle formed by a liquid droplet on a solid surface at the air/solid/liquid contact point, which determines the degree to which a liquid wets or spreads on a solid; CA = 0 degrees indicates complete wetting, and CA = 180 degrees indicates complete non-wetting. Low spreadability emollients work well for an active requiring enhanced substantivity and low wash-off characteristics, e.g., eye treatments, sunscreens and lip products. Low contact angle ingredients include cyclosiloxanes, isoparaffin, isododecane and mineral oil. High contact angle ingredients include diisostearyl maleate, hydrogenated castor oil, dimethicone and dimethiconol. The addition of hydroxyl groups allows for hydrogen bonding interactions between the emollient and other more polar materials, as well as between emollient molecules.
Interfacial tension: Interfacial tension (IFT) is an indicator of the ease with which emulsions are formed. Knowing the density of the two phases, researchers can calculate the IFT by measuring the curvature of the droplet. High IFT ingredients include mineral oil, isododecane and hydrogenated polyisobutene; low IFT ingredients include lauryl lactate, neopentyl glycol dicaprate and diisostearyl maleate.