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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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Formulating with humectants: One noted polyol combination mixes various polyols with glycerin in a ratio of glycerin/sorbitol/propylene glycol/water at 12/9/10/59 (40/20/40 glycerin/sorbitol/propylene glycol) to provide good skin hydration along with good skin aesthetics to aqueous-based hydrating moisturizers. Also, diglycerol and triglycerol are good additions to glycerin to slow the absorption of water from the skin and provide a better equilibrium of moisture externally and internally to skin, equating to better moisture modulation. They also tend to be recognized as contributing less tackiness to a cosmetic formula.
Finally, polysaccharides are also a class of humectants that can provide good skin hydrating properties, and in some cases, they impart the added benefit of desquamation of flaky skin. Hyaluronic acid, i.e. glycosaminoglycan, and chitosan, the deacetylation of chitin, are two popular ingredients. Hyaluronic acid is ubiquitous with the intercellular matrix of connective tissue.
Emollients: Emollient is a complex and misused term.7 It is most often associated with substances that have the ability to plasticize, soften and smooth the skin, usually by filling void spaces on the skin surface and replacing lost lipids in the SC. This term was originally used to describe skin plasticization based on NMF to slow transepidermal water loss (TEWL) based on the theory that moisture retention can keep the SC flexible; therefore, water loss prevention maintains softer and smoother skin. Emollients can also provide protection and lubrication on the skin surface to minimize chafing and enhance skin’s aesthetic smoothness and softness.
Emollient properties are described analytically by interfacial tension/spreading coefficients, coefficient of friction, solubility and polarity characteristics of the molecule. Interesting concepts such as those based on cascading emolliency,8 surface tension, polarity and solubility have been used to develop synergistic blends of emollient ingredients with differing skin aesthetics, spreading characteristics and a solubility similar to the SC.
Formulating with emollients: The chemistry of emollients is diverse and covers most fields of chemistry. The most popular emollients are based on hydrocarbons and their derivatives; mineral oil, fatty acid esters, vegetable oils, synthetic triglycerides and polymers (see Figure 1 and Commonly Used Emollients). Formulators should note the viscosity of a mineral oil, which is based on complex mixtures of varying carbon chain length-hydrocarbons purified to varying degrees, is a key determinant to the average molecular weight of the hydrocarbon mixture.