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Foams in Cosmetics: Functionality and Physical Structure
The description of foams by the layman has also become the descriptive terminology employed by technologists, despite the vague and personalized characteristics thus included. Advertising slogans may include identical descriptors. It appears proper, therefore, to identify at least some of the adjectives used to characterize the term "foam."
Copious foam probably refers to the amount of foam and describes an (unidentified) quantity of foam. The term also implies that the foam is rich. The term rich foam may be identical with the term copious foam but may also describe a dense or tight foam. When used descriptively in this vein, a rich foam identifies primarily a foam consisting of small bubbles.
The description of a foam as rich, dense or tight implies that the foam is strong enough to resist deformation. A more precise descriptor for this last feature may be a firm or cushioning foam, i.e., one that exhibits a yield point or at least some sort of viscoelasticity.
A billowing foam could be viewed as the opposite of a cushioning foam. The volume of such a foam is large or copious; the bubbles in such a foam are also large; and the foam is likely to collapse when subjected to physical stress. A lacy foam is in principle similar to a billowing foam and most commonly describes a foam which lacks the attributes of richness or tightness.
A lacy foam also describes a dry foam, i.e., a foam in which the bubbles are separated from each other by relatively thin aqueous membranes which possess little or no mechanical strength unless the aqueous films include substances other than surfactants. A typical example is the lacy or billowing foam found on rivers and lakes into which household or industrial effluent is discharged. Proteins, saponins, and other detritus are required to generate these unsightly, remarkably stable, floating islands of dry foams.
It is important to remember that cosmetic foams need not be stable for prolonged periods of time. From the moment of foam generation the foam ages, undergoing changes in structure and physical behavior. As a rule, an originally copious, dense, and wet foam changes into a lacy and dry foam with age. Acceleration of this type of pattern, i.e., rapid aging with early collapse, is required in products based on quick-breaking foams. Such foams are normally delivered from aerosol or (less frequently) pump containers. The freshly delivered foam does not run off the application site but is formulated to collapse on the skin within a few minutes to provide a film of (viscous) aqueous liquied containing active constituents.
Functional Attributes of Cosmetic Foams
Foam in cosmetic products is an important consumer attribute, but foam or its quality contributes only marginally to the functional performance of cosmetic preparations on body tissues. This rather harsh indictment is based on the absence in the technical literature--as examined by this review--of any evidence that foam is required for the performance of cosmetic products. Aerosol foams are included, since no evidence is available that the unfoamed (slightly modified) concentrate is not as effective as the foaming product.
It is important for formulators and environmentalists to realize that foam in a cosmetic product contributes little to performance. Some typical citations on the desirable attributes of foams in cosmetics follow (underlining added for emphasis):
* Foam is one of the dominant factors that determines the commercial value of such cosmetic products as
soap, shampoo, shaving foam, cleansing foam, tooth
paste, etc. * The acceptability of many consumer products is
closely linked to the quality and texture of the foam
they produce (e.g., shaving foams and creams,
shampoos, etc.). * The ability of a shampoo to create a desirable lather in
use is one of the major factors in the selection of
surfactants. This important psychological stimulus ... may
be a primary factor in the selection of a shampoo. * No dentifrice would receive wide consumer
acceptance today if it did not create foam in the mouth
"Convenience" or, better, speed and ease of use are commonly cited benefits for aerosols, especially shaving foams. There are no data documenting performance benefits of an aerosol shave product over a properly formulated soap (gel) with or without a conventional shaving brush. As a matter of fact, the latter is less costly and ecologically more correct.
Cosmetic foams are water based, i.e., the individual foam bubbles are surrounded by …