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The Science of Rendering Beauty12th Installment

The Science of Rendering Beauty12th Installment

This column is a follow-up to our previous column on cosmetic products and the role rheology plays in their development. Regarding this subject, many male readers may perhaps inquire, what are cosmetics anyway? Probably most may agree that cosmetics are beauty aids. However underscoring this purpose are specific functions and benefits that the product provides. It is generally said that cosmetics add moisture to the skin and help in moisture retention (skincare and haircare). Enhancing the beauty (make-up) of the user is, of course, another goal. Other attributes such as fragrance and body cleansing are also considered to fall within this product category.
More recently, there have been increasing demands for cosmetic products which provide additional functions and benefits such as skin lightening, treatment for rough skin, and sunscreen protection as well enhancements in comfort-in-use (smoothness, non-stickiness). Rheology plays an integral role in the development of these products.

The foundation of cosmetics - emulsions

From the rheological standpoint, emulsions - creams and milky lotions - represent the basic form of cosmetics. Emulsions are a combination of two substance phases - a solvent and a solute. Emulsions serve as the foundation of cosmetic products to which are added thickening agents that control suppleness as well as various active ingredients such as salts, collagen and other types of polymers, functional enzymes, pigments, fragrances, and other substances.
Emulsions can largely be categorized into two types. Many cosmetic products are comprised of small droplets of oil suspended in water and are referred to as O/W (oil-in-water) emulsions. Milk is an O/W emulsion. The other type is the opposite, that is, where tiny droplets of water are suspended in oil. This is referred to as W/O (water-in-oil) emulsions. A common W/O emulsion is mayonnaise.
O/W emulsions are frequently used in cosmetic products because both their water and oil phases are applied at the same time. With water-based O/W emulsions, the product elicits a refreshing feeling upon application that is followed by anointment of moisture-retaining oil which provides an overall, much-desired sensation of comfort.
To put it another way, the technology of emulsions is the key in manipulating these solutions to achieve the variety of attributes users demand in such products.
Generally, the smaller the size of the particles dissolved in the solute, the more stable the overall emulsion. This type of emulsion provides a feeling of freshness when applied. Conversely, larger-sized oil particles enhance suppleness which impart a stronger sensation of moistness.
As we have indicated in previous columns however, these emulsions have drawbacks. As the size of dispersed particles increases, emulsion stability is lost and it becomes more difficult to sustain uniformity in particle dispersion. To deal with this problem, a certain cosmetics manufacturer has embraced two approaches.
One method achieves emulsion stability by encasing dispersed oil particles in oil which are solid shells at room temperature. The hardness of this shell can be manipulated by changing the composition of the solidified oil.
The other method involves the creation of what is referred to as water-in-oil-in-water (W/O/W) emulsions. W/O/W emulsions are comprised of an O/W emulsion base where small water droplets are entrapped within larger oil droplets that in turn are dispersed in a continuous water phase. These oil particles are thus somewhat akin to blisters which greatly increase in size. Instead of simply becoming large-sized particles however, these blisters are stable. When such emulsions are applied, shear causes the particles to collapse imparting, it is said, suppleness and an enhanced feeling of freshness to the product. With such promising advantages, cosmetics manufacturers are investing a great deal of proprietary knowhow into W/O/W emulsions.

Application of nano-size particles

Let us explore the development of even smaller-sized emulsion particles, using as example cosmetic products which protect the skin from the effects of the sun.
Sunscreen products function to protect the skin by blocking out harmful ultraviolet rays. The degree of protection provided by such products is indicated by an SPF (sun protection factor) value. Sun blocking preparations are, at the minimum, composed of a mixture of UV-absorbing substances and substances which have diffusion properties. White titanium oxide is frequently employed as a diffusing agent but when applied, the whitish color is a noticeable detraction and an undesirable trait. One method employed to resolve this problem is to create nano-sized titanium oxide particles.
Nano-sized titanium oxide particles diffuse UV-containing sunlight reducing the amount of skin exposure to harmful rays and tones down the substances chalky appearance. The result is a product which provides enhanced SPF protection with greater transparency - product attributes which respond to the tastes and needs of the market.
In this manner, rheological achievements such as these will increasingly be embraced by industry as a critical aspect in the process of development of new cosmetic products which can respond to future expectations and diversity in needs.

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