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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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Lubricity: Lubricity impacts skin feel and is a measure of the coefficient of friction (CofF) between two surfaces with the emollient of interest between them. High CofF ingredients include triisostearyl citrate and diisostearyl maleate, and low CofF ingredients include isostearyl neopentanoate, dimethicone and C12-15 alkyl benzoate.
Humectants exist in various different chemistries. They provide topical moisture to the skin surface that can equilibrate the moisture in the SC, providing a reservoir of moisture on an as-needed basis. When combined with occlusive agents and other emollients, humectants can minimize skin moisture from desorbing to the hygroscopic film on the skin surface.23 As noted, among the most popular ingredients is glycerin, which is still the most effective.24, 25 It has a very safe and long history of use, high hygroscopicity, is effective at low levels (4%), can provide some desquamation of the skin, and is easy to formulate, with consumer-acceptable aesthetics. Other humectants are listed in the sidebar on Page 18, Commonly Used Humectants.
Occlusive agents: Occlusive agents are ingredients that can reduce the porosity of the skin’s surface by providing an envelope over the skin, thus reducing or preventing evaporation of moisture. Common occlusive agents include petrolatum and alkyldimethicones.
Emollients: Emollients used in cosmetic and personal care products are typically in the family of fats and oils (lipids). Optimum-performing synthetic emollients are capable of mimicking natural ingredients. Some natural emollients are: water, aloe vera, seed and nut oils, fruit butters, fruit and vegetable juices. These materials prevent water loss from the dermis and epidermis and have been reported to repair wounds due to their enhancement of epidermal proliferation. Other humectants are listed in the sidebar on Page 20, Commonly Used Emollients.
Key properties of emollients include good spreading properties, low toxicity/skin irritation, and good oxidative stability. Additional important formulating considerations are as follows.
- It is important for formulators to understand that unsaturated oils such as olive oil have double bonds that can react with oxygen, especially when heated. This oxidation process can produce off colors and odors in lipids, causing them to go rancid and render them unusable.
- Silicones such as cyclopentasiloxane and dimethicone can be added to increase slip and emolliency.
- Oils that contain high levels of essential fatty acids (EFAs) can be used to supplement loss of the lipids (oils) found naturally within the skin; linoleic acid is an example of an EFA.
- Long-chain alcohols, also called fatty alcohols, are useful as emollients and emulsion stabilizers. Their polar hydroxyl groups orient to the water phase and their fatty chains orient to the oil phase. Balancing the length of the carbon chain—i.e., C16 vs. C18 vs. C22—with the alkoxylated derivative can dramatically change the play time of a lotion during rubout and the perception of moisturization on the skin post dry-down.
- Lanolin, derived from sheep’s wool and often called wool grease, has been used for centuries for its unique composition of complex sterols, fatty alcohols and fatty acids. Cholesterol, a cyclic molecule called a sterol, is a major component of lanolin. The polar hydroxyl groups of sterols and alcohols enable the lanolin to absorb and hold water.
- Waxes are composed primarily of long-chain esters that are solid at room temperature. Common waxes used in cosmetics are beeswax, candelilla, carnauba, polyethylene and paraffin. The melting point of waxes varies widely depending on their unique composition and chain length. By combining waxes having different properties, such as high shine, flexibility and brittleness, optimal cosmetic performance can be achieved. Oftentimes waxes are combined with compatible oils to achieve the desired softness.
- Combining esters and hydrocarbons have several effects on the bulk properties; isododecane/diisopropyl adipate combo increases polarity of hydrocarbon, which is reflected in a higher permittivity and lower IFT. Intermolecular interactions due to the ester groups also cause an increase in viscosity, which leads to a decrease in spreading (indicated by an increase in the CA). Another factor affecting bulk behavior of esters is the degree of branching on the hydrocarbon moieties. The length of the hydrocarbon chains attached to the ester groups can affect the properties of an ester.
- Esters having good spreading properties are often useful for dispersing pigments and improving the incorporation of micro-fine oxides into sun products. They can also improve the smoothness and lightness of oils and reduce tackiness without affecting emollient characteristics.