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Formula Anatomy Deciphered—Feminine Hygiene Products
By: Luigi Rigano, PhD, Studio Rigano Industrial Consulting Laboratories
Posted: December 4, 2012, from the December 2012 issue of Cosmetics & Toiletries.
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The formulation strategy is to select the most delicate among the available cleansing systems. The action required by personal hygiene surfactants is a quick wetting of the skin and the residual conditioning of the intimate body parts without excessive simultaneous degreasing of the skin. Moreover, accurate rinse-off is requested, as residues of surfactant should not be left on the skin before the next cleansing/rinse. Rinse-off actions are frequently incomplete, for the anatomic complication of the area and the limited time dedicated to washing that area. Moreover, hair in the region impedes complete wash. Personal hygiene surfactants do not often produce high foam, but rather respect the skin equilibrium in the treated zone. The most used strategy to assemble the cleansing complex is blending. A high foaming surfactant such as sodium, magnesium or ammonium lauryl ether sulfate at 1–6% w/w is modified (i.e., made less aggressive for the skin) by blending with one or more mild surfactants at 1–4% w/w. Absolute concentrations depend on the foreseen dilution ratio of the final product with water. Lauryl sulfates and lauryl ether sulfates are almost never or should never be used alone due to their substantivity for the skin, excess foaming power and poor resistance to long-term hydrolysis at acidic pH.13
The most delicate surfactants for feminine cleansing are the anionic surfactants alkyl sulfosuccinates, sodium lauryl sulfoacetate and hydrolyzed proteins condensed with vegetal fatty acids, with some amphoterics like alkyl amidopropyl betaine and cocoamphoglycinates also being gentle. Among the nonionics, alkyl polyglucosides, ethoxylated glycerides or alcohols and their blends are frequently used to minimize skin degreasing. A recent alkyl glucoside line was launched by the Dow Chemical Company. The concentration of active surfactants is generally low for the above explained considerations (i.e., easy rinse-off, no residue left on the skin). Sometimes, surfactants’ concentration is just enough to reach the critical micelle concentration, as in delicate makeup removers. Recently Quillaja tree saponins have been used for their mildness, controlled foam properties and skin compatibility.14, 15 A quillaja foaming agent is offered by BioOrganic Concepts.
In respect to the skin area and mildness, lather boosters are almost never used in these systems for the same reason, as foam must be transient and decay rapidly, making rinse-off more efficient. (Read more on designing mild products.) Nevertheless, amine oxides are used in some formulations because they have skin conditioning properties at an acidic pH. An additional strategy to ensure mildness is to add some super-fatting agents like fatty acid glycerides or hydrolized proteins in the cleansing formula to reduce the aggressiveness of surfactants toward skin keratin. An interesting innovative way of eliminating soil from the skin surface is the use of a nonaggressive, non skin-dehydrating emulsifier. For example, sucrose esters, particularly sucrose laurate, do not irritate skin and do not interfere with the skin’s ability to produce and retain water. In addition, they perform some antimicrobial action toward several bacteria, including those producing bad odors, and against fungi of the Candida species without posing any harm to the resident bacterial flora.16 Recently, zinc coceth sulfate has been proposed for its antimicrobial and deodorant action, in combination with cocoyl glutamate-derived surfactants.13 Formula 1 is an example of a typical feminine cleansing formula with a surfactant system. The last ingredient is just for opacifying the product.
Acidity buffering: The second main characteristic of intimate cleansers dedicated to women is the pH value; according to many gynecologists, it should be as close as possible to 3.5–4.5, especially when the product is diluted with water. Following the blending with water, the pH of a diluted personal cleansing formula decreases to about 7. In order to prevent this phenomenon, it is a common practice to create a system buffered at an acidic pH that could maintain its value even after dilution. The ideal buffer system is partially neutralized lactic acid, as it is a well-known and safe physiological substance. Its concentration depends on the foreseen dilution ratio in the formula. It is generally buffered with an alkaline material, including the amino acid arginine. Citric acid is sometimes used, even if it is more prone to bacterial attack. Carefully buffered acids concentration can reach 5%.
Thickening: The fluidity of the intimate cleansing product must allow for proper dosage due to the area being intricately configured and allow for the use of a cleansing aid such as a sponge. Thickeners used are usually pH insensitive like hydroxyethyl cellulose, mineral clays and aluminum silicates, derived from smectic clays. These last two possess good soil-absorbing power and skin adhesion. In addition, they are efficient in removing dead cell residue and soil from the skin surface. Consequently, they work as nontraditional cleansers for the skin at 0.2–1.5%, without any foam production.