Skin and Hair Cleansers 

Cleansing and conditioning the skin and hair are important aspects of maintaining the outward health and appearance of the body. An understanding of the mechanism of action and chemistry of products designed to improve the ability of the skin and hair to function is medically important.

Cleansing of the skin is a complex interaction between the stratum corneum barrier, environmental dirt, body secretions, and a surfactant. Washing of the skin is the single most common cause of dermatologic disease, yet it is necessary in terms of personal hygiene and health.^{[1, 2]}

Hair cleansing and conditioning are more complex interactions than skin cleansing because the surface to cleanse is greater, consisting of the scalp and all surfaces of each hair shaft. Products designed to cleanse the hair are known as shampoos. Products designed to beautify the cleansed hair are known as conditioners.

For additional information on personal grooming products, see Cosmeceuticals, Cosmetics, Moisturizers, and Nail Cosmetics.

Soap

Most cleansing is accomplished with soap, which is obtained through the chemical reaction between a fat and an alkali, resulting in a fatty acid salt with detergent properties. Modern refinements include adjustment of the alkaline pH to reduce skin irritation and to incorporate substances that prevent precipitation of calcium fatty acid salts in hard water, known as soap scum.^{[3, 4, 5, 6]}

Nevertheless, modern soap is basically a blend of tallow and nut oil, or the fatty acids derived from these products, in a 4:1 ratio. Increasing this ratio results in superfatted soaps designed to leave an oily film on the skin.

Bar and liquid cleansers can be divided into 3 basic types, as follows^{[7, 8]} :

• True soaps composed of long chain fatty acid alkali salts with a pH of 9-10
• Combars composed of alkaline soaps to which surface active agents have been added, also with a pH of 9-10
• Syndet (synthetic detergent) bars composed of synthetic detergents and fillers that contain less than 10% soap and that have an adjusted pH of 5.5-7

The goal in developing new synthetic detergents is to provide a product that is less irritating to the skin than traditional soaps are.

Common detergents in bar-type cleansers are sodium cocoate, sodium tallowate, sodium palm kernelate, sodium stearate, sodium palmitate, triethanolamine stearate, sodium cocoyl isethionate, sodium isethionate, sodium dodecyl benzene sulfonate, and sodium cocoglyceryl ether sulfonate. Detergents in liquid formulations are sodium laureth sulfate, cocoamido propyl betaine, lauric acid diethenolamine (lauramide DEA), sodium cocoyl isethionate, and disodium laureth sulfosuccinate.

The normal pH of the skin is acidic, between 4.5 and 6.5. Applying alkali soap theoretically raises the pH of the skin, making it feel dry and uncomfortable. However, healthy skin rapidly regains surfactant; induced irritation remains a controversial area under investigation.

Special additives to the basic soap types are responsible for the tremendous variety of soaps marketed today. Lanolin and paraffin may be added to a moisturizing syndet soap to create a superfatted soap, whereas sucrose and glycerin may be added to create a transparent bar. Adding olive oil instead of another form of fat distinguishes a castile soap.

Medicated soaps may contain benzoyl peroxide, sulfur, or resorcinol antibacterials (eg, triclocarban or triclosan). Triclocarban is excellent for eradicating gram-positive organisms, but triclosan eliminates both gram-positive and gram-negative bacteria. These soaps have a pH of 9-10 and may cause skin irritation.

Moisturizing syndet bar soaps contain sodium lauryl isethionate with a pH adjusted to 5-7 by using lactic or citric acid. These products are less irritating to the skin and are sometimes labeled beauty bars. Most bar soaps marketed by cosmetic companies are of this type.

Additives are also responsible for soap’s characteristic appearance, feel, and smell. For example, titanium dioxide is added in concentrations as high as 0.3% to opacify the bar and increase its optical whiteness. Pigments (eg, aluminum lakes), can color the bar without producing colored foam, which is considered an undesirable characteristic.

Foam builders (eg, sodium carboxymethylcellulose and other cellulose derivatives) can make the lather feel creamy. Perfume in concentrations of 2% or more also can be added to ensure that the soap bar smells pleasant until it is completely used up.

Lipid-free cleansers

Lipid-free cleansers are liquid products that clean without fats. They are applied to dry or moistened skin, rubbed to produce lather, and rinsed or wiped away. These products may contain water, glycerin, cetyl alcohol, stearyl alcohol, sodium laurel sulfate, and (occasionally) propylene glycol.

Lipid-free cleansers leave behind a thin, moisturizing film and can be used effectively to remove facial cosmetics and dirt in persons with sensitive or dermatitic skin. Lipid-free cleansers cause less cutaneous irritation in photoaged skin than other cleansers do. However, propylene glycol can cause stinging, and sodium laurel sulfate is a detergent.

Cleansing creams

Cleansing creams are applied to the face both to clean and to moisturize. They are composed of water, mineral oil, petrolatum, and waxes. The classic cream for facial cleansing is known as cold cream. Cold creams combine the effect of a lipid solvent (eg, beeswax or mineral oil) with the detergent action of borax, also known as decahydrate of sodium tetraborate. These products are popular for removing cosmetics and for cleansing dry skin.

Body washes

Body washes are a special subset of liquid synthetic detergents that combine mild skin cleansing with moisturizing and emollient qualities.^[9] They are applied with a puff that does not support bacterial growth to break the emulsion through the incorporation of generous amounts of air and water. High amounts of petrolatum can be incorporated in body wash emulsions to improve skin dryness and hydration.

Particulate abrasive scrubs

The recognition that exfoliation of desquamating corneocytes is desirable for producing smooth skin in maturing patients led to the introduction of abrasive scrubs. Abrasive scrubs incorporate polyethylene beads, aluminum oxide, ground fruit pits, or sodium tetraborate decahydrate granules to induce various degrees of exfoliation.

Aluminum oxide particles and ground fruit pits produce the most abrasive scrub. In general, products containing these rough-edged particulates are not appropriate for individuals with sensitive skin. Polyethylene beads (eg, those in Clinique 7th Day Scrub; Estée Lauder, New York, NY), which are smooth and round, produce more mild scrubbing. The least aggressive abrasion of the skin is achieved with products that contain sodium tetraborate decahydrate granules, which soften and dissolve during use.

The main problem with abrasive scrub products for epidermabrasion is related to the firm scrubbing granules, which do not deform when pressed hard against the skin.

Woven mesh sponges

Woven mesh products were introduced about the same time as abrasive scrubs. They also induce exfoliation, but this is achieved with an implement rather than with a particulate. The most popular product is composed of a nonwoven web sponge made of polyester fibers (Buf-Puf; 3M, St Paul, MN). Originally, this product was designed to remove open comedones, but later, the stiffness of the web was decreased, and the sponge was impregnated with a mild cleanser to yield products that were suitable for a variety of skin types.

Woven disposable face cloths

The desire for thorough but less abrasive cleansing led to the development of disposable cleansing cloths. These cloths are composed of a combination of polyester, rayon, cotton, and cellulose fibers held together by heat through a technique known as thermobonding. Additional strength is imparted to the wipe by hydroentangling the fibers, which is accomplished by entwining the individual rayon, polyester, and wood pulp fibers with high-pressure jets of water. This process eliminates the need for adhesive binders, thereby creating a soft, strong cloth.

The cloths are packaged dry and impregnated with a cleanser that foams modestly when a cloth is moistened. The type of cleanser in the cloth depends on whether strong sebum removal is required ( as with oily skin) or whether only modest sebum removal is required (as with dry skin). Humectants and emollients can also be added to the cloth to decrease barrier damage with cleansing or to smooth the skin scale present in xerosis.

In addition to the ingredients preapplied to the dry cloth, the weave of the cloth also determines its cutaneous effect. Two types of fiber weaves are used in facial products: open and closed. Open-weave cloths are so named because of the 2- to 3-mm windows in the cloth between the adjacent fiber bundles. This open weave softens the cloth and decreases the surface area in contact with the skin, yielding a milder exfoliating effect that is appropriate for people with dry or sensitive skin. Closed-weave cloths have a tighter weave and provide more aggressive exfoliation.

Ultimately, the degree of exfoliation achieved depends on the weave of the cloth, the pressure with which the cloth is stroked over the skin surface, and the length of time the cloth is applied.

Woven cleansing pouches

The cleansing pouch is a variation of fibered cloths; however, it can also be used as a metered delivery system for skin-cleansing and skin-conditioning agents. To create this pouch, a plastic membrane is placed between 2 fibered cloths containing holes of various diameters. The size of the holes determines how quickly the contents of the pouch are released onto the skin surface. Typically, the cleansing pouch produces less exfoliation than is achieved with a plain cleansing cloth.

Mechanized face brush

The newest mechanized technique for facial cleansing is the face brush (Clarisonic; Pacific Biosciences Laboratories, Bellevue, WA), developed by the same team of engineers and researchers that developed the Sonicare Toothbrush (Philips; Andover, MA). This toothbrush runs on a rechargeable battery attached to a miniaturized motor that creates an oscillatory motion of the brush head. This oscillatory sonic motion was developed to remove plaque from the teeth more thoroughly than manual brushes.

A revision of the enlarged brush head with soft, tufted bristles was developed for facial cleansing. The bristles of the face brush are designed to traverse facial dermatoglyphics, pores, and scars. The sonic motion of the brush also aids in dislodging facial debris, much like the sonic surgical-instrument cleansers that are used to clean liposuction cannulas and reusable injection needles.

Shampoos basically contain detergents, foaming agents, thickeners and opacifiers, conditioners, sequestering agents, pH adjusters, specialty additives, and other ingredients (eg, softeners, fragrances, preservatives).

Detergents are the primary components for sebum and dirt removal; however, excessive removal of sebum leaves the hair dull, susceptible to static electricity, and difficult to comb. Furthermore, consumers equate cleansing ability with abundant, long-lasting foam.

Excessive bubbles are not a technical requirement for good hair cleansing and bacteria removal, but shampoo manufacturers add increased amounts of detergents, in addition to foam boosters, to obtain the foam that consumers desire. This increased concentration of detergent creates the need for conditioners and other additives in shampoos to improve their cosmetic acceptability.

Detergents

Shampoos function by using detergents (also known as surfactants) that are both lipophilic (oil loving) and hydrophilic (water loving). The lipophilic component adheres to sebum, and the hydrophilic component allows water to rinse away the sebum.

Some of the most common synthetic detergents combined into shampoo formulations for various needs are as follows:

• Lauryl sulfates (sodium lauryl sulfate, triethanolamine lauryl sulfate, ammonium lauryl sulfate) – These are found in most shampoos as the main surfactant because they work well in both hard and soft water, produce rich foam, and are easy to remove; this group produces good cleansing but is hard on the hair
• Laureth sulfates (sodium laureth sulfate, triethanolamine laureth sulfate, ammonium laureth sulfate) – These produce rich foam, provide good cleansing, and leave hair in good condition; they also are a common main surfactant
• Sarcosines (lauryl sarcosine, sodium lauryl sarcosinate) – These are poor cleansers but are excellent conditioners; this group commonly is used as a secondary surfactant
• Sulfosuccinates (disodium oleamine sulfosuccinate, sodium dioctyl sulfosuccinate) – These are strong degreasers and commonly are used as a secondary surfactant in oily hair shampoos

These detergents are classified as anionic surfactants because of their negatively charged hydrophilic polar group.

Another group of detergents, the cationic detergents, is named for their positively charged polar group. They are relatively poor detergents and do not lather well, but their unpopularity is largely due to their incompatibility with other anionic surfactants. Some shampoos designed for use with dyed or bleached hair include cationic detergents because these are excellent at imparting softness and manageability.

The nonionic detergents, the second most popular group of detergents behind the anionic surfactants, possess no polar group. These are the mildest of all surfactants and are used in combination with anionic surfactants as a secondary cleanser. Examples are polyoxyethylene fatty alcohols, polyoxyethylene sorbitol esters, and alkanolamides.

The amphoteric detergents contain both an anionic and a cationic group, behaving as cationic agents at lower pH values and anionic agents at higher pH values. Detergents in this group are the betaines, sultaines, and imidazolinium derivatives. Ingredients such as cocamidopropyl betaine and sodium lauraminopropionate are used in baby shampoos because they do not irritate the eyes. These surfactants foam moderately well and leave the hair manageable, making them a good choice for chemically treated hair and fine hair.^[10]

Foaming agents

Foaming agents in shampoos introduce gas bubbles into the water. Many consumers believe that shampoos that generate copious foam are better cleansers than poorly foaming shampoos; this is not true. As the shampoo removes sebum from the hair, the amount of foam decreases because sebum inhibits bubble formation. This phenomenon accounts for the increased foam seen on the second shampooing, when most of the sebum has been removed.

Thickeners and opacifiers

Thickeners and opacifiers have no part in hair cleansing. They simply make the product more appealing to the consumer. Many people incorrectly believe that a thick shampoo is more concentrated than a thin shampoo; others want a shampoo that appears opaque or pearlescent.

Conditioners

Conditioners impart manageability, gloss, and antistatic properties to the hair. They are found in most shampoos for dry, damaged, or treated hair. They usually are fatty alcohols, fatty esters, vegetable oils, mineral oils, or humectants. Many different conditioners are used in dry hair shampoos, including hydrolyzed animal protein, glycerin, dimethicone, simethicone, polyvinylpyrrolidone (PVP), propylene glycol, and stearalkonium chloride.

Sequestering agents

Sequestering agents make shampoos function better than bar soaps in cleansing the hair. They chelate magnesium and calcium ions so that other salts or insoluble soaps, known as scum, are not formed. Without sequestering agents, shampoos would leave a film on the hair.

pH adjusters

Some shampoos contain ingredients designed to alter pH, which allowing them to be marketed as “pH balanced.” Most shampoos are alkaline, and this alkalinity can swell the hair shaft and render it more susceptible to damage. This is not a problem for patients with healthy, nonporous hair containing an intact cuticle. Patients who have damaged or chemically treated hair with a fragmented cuticle may wish to avoid hair swelling by selecting a shampoo that has an acid added to balance the pH.

Specialty additives

The key differences between similarly purposed shampoos manufactured by various personal care product companies are the fragrance and special care additives. Additives such as wheat germ oil (containing vitamin E) and panthenol (a form of vitamin B) are added mainly because they are believed to leave hair more silky and manageable. Other producers add fatty substances, such as plant extracts or mink oil. Proteins such as ribonucleic acid, collagen, and placenta may be added to act as conditioners. Some shampoos now include a chemical sunscreen.

Shampoos are formulated in liquids, gels, creams, aerosols, and powders. This article discusses only the liquid formulations, because these are the most popular. Several different types of liquid shampoos are available: basic shampoos (for normal, dry, oily, and chemically treated hair), baby shampoos, conditioning shampoos, medicated shampoos, and professional shampoos.

Basic shampoos

Basic shampoos may be selected from several formulations, depending on the amount of sebum production in the scalp and depending on the diameter and condition of the hair shaft. The label usually defines the intended purpose by stating “normal hair,” “oily hair,” “dry hair,” “damaged hair,” or “color-treated hair.” Some companies alter the concentrations of detergents and conditioners to make different formulations, but the ingredient lists may be identical for all formulations. Other product lines have different formulations for each type.

Normal-hair shampoos use lauryl sulfate detergents, which have good cleansing and minimal conditioning characteristics. These products work well for adults with moderate sebum production and coarse hair; however, they do not work well for persons with fine, unmanageable hair.

Oily-hair shampoos have excellent cleansing and minimal conditioning properties. They may use lauryl sulfate or sulfosuccinate detergents and are intended for adolescents with oily hair or persons who have extremely dirty hair. They can be drying to the hair shaft if used daily. Using a heavy conditioner after an oily-hair shampoo is self-defeating.

Dry-hair shampoos provide mild cleansing and good conditioning. Some companies recommend the same product for dry hair and damaged hair. These products are excellent for mature persons and for those who wish to shampoo daily. They reduce static electricity and increase manageability in fine hair; however, some products provide too much conditioning, which may result in limp hair.

Dry-hair shampoos may also cleanse so poorly that conditioner can build up on the hair shaft. This condition has been labeled “the greasies” in popular advertising and may account for the observation that hair sometimes has more body after a different shampoo is used.

Damaged-hair shampoos are intended for hair that has been chemically treated with permanent color, bleaching agents, permanent waving solutions, or straighteners. Hair can also be physically damaged by overcleansing, by excessive use of heated styling devices, and by vigorous brushing or combing. Long hair is more likely to be damaged than short hair because it undergoes a natural process known as weathering, whereby the scales of the cuticles are decreased in number from the proximal end to the distal end of the hair shaft.

As noted, damaged-hair shampoos may be identical to dry-hair shampoos, or they may contain mild detergents and more conditioners. Hydrolyzed animal protein is the superior conditioner for damaged hair because it can minimally penetrate the shaft and temporarily plug surface defects, resulting in hair with shine and a smooth feel. The protein should be hydrolyzed; large protein molecules cannot penetrate the hair shaft.

Baby shampoos

Baby shampoos are designed to be nonirritating to the eyes and to provide only mild cleansing because babies produce limited sebum. These shampoos contain detergents from the amphoteric group. Baby shampoos are also appropriate for use on mature hair and for individuals who wish to shampoo daily.

Conditioning shampoos

Conditioning shampoos may be labeled as such, or they may be labeled as shampoos for dry or damaged hair. Detergents used in conditioning shampoos generally are amphoterics and anionics of the sulfosuccinate type. These products sometimes are known as 1-step shampoos, because a conditioner need not be applied afterward.

Medicated shampoos

Medicated shampoos, also known as dandruff shampoos, contain additives, such as tar derivatives, salicylic acid, sulfur, selenium disulfide, polyvinylpyrrolidone-iodine complex, chlorinated phenols, or zinc pyrithione. Medicated shampoos have several functions: to remove sebum efficiently, to remove scalp scale, to decrease scalp scale production, and to act as an antibacterial or antifungal. The shampoo base removes sebum, and mechanical scrubbing removes scalp scale.^{[11, 12, 13]}

Tar derivatives commonly are used as anti-inflammatory agents. Sulfur and zinc pyrithione are used for their antibacterial or antifungal qualities. Menthol is added to some shampoos to produce a tingling sensation that some patients find esthetically pleasing.

Shampoos are not a common cause of cutaneous irritant or allergic contact dermatitis because they are in contact with the skin for only a relatively brief time before being rinsed off. However, eye irritation can be a problem. Some shampoos overcome eye irritation with the addition of imidazoline-type amphoteric surfactants, succinic ester sulfonates, silicone glycols, and fatty acid-peptide condensates.

Ingredients in shampoos that are possible sensitizers include formalin, parabens, hexachlorophene, fragrances, triclosan, and miranols.^[14]

Shampoos should be diluted to form a 1-2% aqueous solution for closed patch testing and a 5% aqueous solution for open patch testing. However, false-positive reactions due to irritation may still occur. A better assessment may be obtained by patch testing the individual ingredients separately.

The need for hair conditioners arose after technologic developments in detergents and shampoo formulation occurred. Originally, bar soaps were used to clean both the hair and the body. Most bar soaps possessed an alkaline pH, which caused the hair shaft to swell and left it unattractive and unmanageable. In addition, most homes used well water with a high mineral content. The combination of bar soap and hard water yielded soap scum that accumulated on the tub and on the hair. This soap scum left the hair harsh and dull, adding a source of scalp irritation.

The widespread use of municipal water sources and the development of liquid synthetic detergents formulated at a neutral pH with sequestering agents revolutionized hair shampooing. The newer shampoos left the hair soft and manageable and could be used more frequently without an adverse cosmetic result. These features led to the current practice of daily or every-other-day shampooing, which efficiently removes sebum from the hair shaft.

Sebum is, of course, the ideal hair conditioner. Excessive removal of sebum created the need for a synthetic sebumlike substance that could minimize static electricity, increase shine, improve manageability, and aid in maintaining a hairstyle. Accordingly, hair conditioners were developed in an attempt to supply the hair with the positive attributes of sebum while avoiding the greasy appearance indicative of excessive sebum and dirt.

Conditioners are liquids, creams, pastes, or gels that mimic sebum to make the hair manageable, glossy, and soft. Their role goes beyond maintaining the appearance of healthy hair: they are also designed to recondition hair that has been damaged by chemical or mechanical trauma. Common sources of trauma include excessive brushing, hot blow-drying, permanent hair waving, hair straightening, hair bleaching, and other mechanisms.

Damage to the hair shaft can also occur through environmental factors, such as exposure to sunlight, air pollution, wind, seawater, and chlorinated water in swimming pools; this type of hair damage is technically known as weathering. Obviously, because hair is nonliving tissue, any reconditioning that occurs is minimal and temporary until the next shampooing.

Ingredients

The first hair conditioners were developed in the early 1930s, when self-emulsifying waxes became available. These waxes were combined with protein hydrolysates, polyunsaturates, and silicones to give the hair improved feel and texture. Early sources of protein were gelatin, milk, and egg protein.

Currently, the most common ingredient in hair conditioners is silicone. Silicone is a lightweight oil that can leave a thin film on the hair shaft without creating the appearance of dirty hair. The amount of silicone left behind on the hair shaft determines whether the product is designed to add body to fine hair, for which minimal conditioning is desirable, or to straighten curly hair, for which maximal conditioning is desirable.

Mechanism of action

Healthy, undamaged hair is soft, resilient, and easy to disentangle. Unfortunately, the trauma caused by shampooing, drying, combing, brushing, styling, dyeing, and permanent waving damages the hair, making it harsh, brittle, and difficult to disentangle. Hair conditioners are designed to reverse this damage by decreasing static electricity, improving manageability, increasing shine, decreasing the number of split ends, and improving the flexibility of the hair.

Decreased static electricity

After combing or brushing, the hair shafts become negatively charged. These negatively charged shafts repel one another, preventing the hair from lying smoothly in a given style. Conditioners deposit positively charged ions on the hair shaft, neutralizing the electrical charge and minimizing frizzy hair. Frizzy hair due to static electricity is a greater problem in climates with low humidity, such as the southwestern United States, than elsewhere.

Improved manageability

In addition to decreasing static electricity, conditioners improve the manageability of hair (ie, the ease with which the hair can be combed and styled). By filling in the gaps around and between the cuticular scales, conditioners smooth the surface of the cuticle, improving manageability by decreasing the friction between hair shafts. A good-quality hair conditioner can reduce friction between hair shafts by as much as 50%. The reduction in friction also aids in disentangling of the hair after shampooing.

A subset of conditioners, known as cream rinses, is designed to make hair easier to comb after shampooing. A special subset of these products is designed to aid in combing children’s hair.

Increased shine

Most consumers equate shiny hair with healthy hair. Hair shine results from light reflected by individual hair shafts. The smoother the hair surface, the more light is reflected. Conditioners increase hair gloss primarily by increasing the adherence of the cuticular scale to the hair shaft and by placing a thin coating over the individual hairs.

Decreased number of split ends

Conditioners can also improve the health of the hair by temporarily repairing damage at the distal hair shaft, a condition known as split ends. Split ends occur when the cuticle is removed from the hair shaft and the soft keratin cortex and medulla are exposed to weathering and grooming trauma. Unable to withstand the damage, the protein of these structures splits or frays, much like a damaged textile fiber.

Conditioners temporarily reapproximate the frayed remnants of remaining medulla and cortex to strengthen the hair shaft and prevent breakage of the distal ends. However, the conditioner is removed with subsequent shampooing and must be reapplied after each contact with shampoo.

Improved flexibility

Conditioners can improve the flexibility of the hair (ie, the ability of the hair to withstand the forces of bending without fracturing). A hair coated with conditioner does not exhibit cuticle lifting, whereas a hair devoid of conditioner shows disruption of the cuticle. Flexibility is especially important in women with long hair to prevent hair breakage.

Although all types of hair benefit from the use of a conditioner, conditioners must be formulated for specific hair types—such as straight, fine, wavy, or kinky—to function optimally.

The geometry of the hair shaft determines the type of conditioner required to produce the best cosmetic result. For example, people with kinky hair may prefer a heavy conditioner that thickly coats the hair shaft to provide additional weight and straighten the unruly shafts. A kinky-hair conditioners allows the hair to lay in the desired style while appearing shiny and healthy, but this same conditioner applied to fine, thin hair makes it appear greasy and limp. Fine, straight hair is best conditioned minimally because additional straightening of the hair shafts is not a goal.

It may seem that the best way to avoid limp hair in persons with fine hair is to avoid use of a conditioner entirely, but this is not the case. Fine hair is particularly prone to weathering and damage from hair grooming because the increased number of hair fibers for a given weight increases the net surface area of the hair. This increased surface area is subject to static electricity and exposes proportionally more cuticular scales to damage.

Ingredients

Several active agents can be combined to achieve a hair conditioner designed for a given hair type. Categories of ingredients in hair conditioners include the following:

• Alkanolamides
• Glycols
• Lipids
• Quaternaries
• Polymers
• Protein derivatives
• Silicones

Of these, the quaternaries, polymers, and protein derivatives are most frequently used (see Table 1 below).

Table 1. Common Hair Conditioners (Open Table in a new window)

Quaternary conditioning agents (cationic detergents)

The quaternary conditioning agents—also known as quaternary ammonium compounds or quaternaries or quats—are cationic detergents.^{[15, 16]} The addition of the cationic quaternary conditioner neutralizes the anionic charge of the hair, decreasing static electricity and improving manageability. The attraction of the positively charged conditioner to the negatively charged hair shaft allows the hair care product to remain on the hair after it is rinsed with water.

Quaternary conditioning agents are used in conditioning shampoos (so-called 2-in-1 shampoos), which clean the hair but also leave a thin film of conditioner on the hair to improve its appearance. With repeated grooming and shampooing, the hair becomes weathered, and the cuticular scale it loosened.

The same effect is seen in woven textiles, such as cotton or wool sweaters, on which fuzz balls and pilling forms around areas of friction (eg, the elbows). These fuzz balls occur because the textile fibers are broken and rolled into a ball. Similarly, the cuticular scales also fracture, break, and clump, creating increased combing friction and a dull appearance.

Quaternary conditioners are excellent for increasing the adherence of the cuticular scales to the hair shaft, thereby enhancing the light-reflective abilities of the hair and adding shine and luster. These qualities make them an excellent choice for individuals with permanently dyed or permanently waved hair in which the cuticle is disrupted as part of the chemical process.

Film-forming conditioning agents

The second category of conditioners comprises the film-forming conditioning agents. Instead of functioning through electrical charge, as the quaternary agents do, these conditioners coat the hair shaft with a thin layer of polymer. They contain some of the new lightweight polymers used in hair sprays and styling products; the polymer most commonly selected is polyvinylpyrrolidone (PVP).

The polymer forms a coating over the hair shaft, filling in defects or missing areas of cuticular scale to create a smooth surface. This smooth surface reflects light, improving hair luster and shine. In addition, the polymer coating eliminates static electricity because of its cationic nature, improving the manageability of the hair.

Many makers of film-forming conditioners claim that they thicken hair. Although consumers may think that more hair is present on the scalp, this claim is based on the diameter of each hair shaft after the polymer film coats it. Indeed, the hair shafts are thickened, but not in the manner many consumers hope.

Film-formers are ideal in conditioners designed to straighten kinky or curly hair because a thick coating can be applied to help straighten the hair shafts. They are also found in products designed to add manageability to coarse hair; however, people with fine hair may find that the polymer coating makes the hair shaft limp and difficult to style.

Film-forming conditioners are most commonly used for hair that has been shampooed and towel dried. They are designed to remain on the hair shaft, whereas quaternary conditioners are applied after shampooing and rinsed before the hair is towel dried.

Protein-containing conditioning agents

Protein-containing conditioners are the most interesting agents and the most beneficial from a dermatologic standpoint.

As hair weathers, it loses its strength because cuticular scales are removed and the underlying cortex is damaged. This damage creates areas where the hair shaft contains holes. Protein from a conditioner can be deposited in these holes. Protein-containing conditioners can actually penetrate the damaged hair shaft and increase its fracture strength by 10%. Although a 10% improvement may seem insignificant, it may represent the difference between an intact hair shaft and one that is broken with the force of combing.

These proteins, derived from animal collagen, keratin, placenta, and other sources, are hydrolyzed to a particle size with a molecular weight of 1000-10,000 daltons, which is small enough to enter the hair shaft. The source of the protein is not as important as the size of the protein particle and its ability to enter and remain inside the hair shaft.

The ability of protein-containing conditioners to strengthen the hair shaft depends on contact time: the longer the protein conditioner is left in contact with the hair shaft, the greater the amount of protein that diffuses into the shaft. Therefore, proteins are used in short-contact instant conditioners applied after shampooing and rinsed for minimal protein penetration, as well as in leave-on conditioners applied before shampooing and left on the hair for 30 minutes before removal for greater penetration.

The amount of protein that penetrates the hair shaft determines the final result. However, protein diffusion is reversible—that is, any exogenous protein present in the hair shaft is removed during the next shampooing. Therefore, reapplication of the protein-containing conditioner is needed with each contact with water to maintain the effect.

Silicones

Silicones make up the newest major category of conditioning agents. These substances have virtually revolutionized hair conditioning, in terms of both conditioning shampoos and instant hair conditioners. Topical silicone is an amazingly safe material from a dermatologic perspective because it is hypoallergenic, noncomedogenic, and nonacnegenic.

Silicone originates from silica, which is found in sand, quartz, and granite. It derives its properties from the alternating silicon and oxygen bonds, known as siloxane bonds, which are exceedingly strong. These strong bonds account for the tremendous thermal and oxidizing stability of silicone. Silicone was first developed in the 1930s, when Franklin, Hyde, and McGragor discovered a method of extracting pure silica from raw quartzite and converting it to dimethyl silicone.

Silicone is resistant to decomposition from ultraviolet radiation, acids, alkalis, ozone, and electrical discharges. The silicone used in topical preparations is an odorless, colorless, nontoxic liquid. It is soluble in aromatic and halocarbon solvents but poorly soluble in polar and aliphatic solutes; it is immiscible and insoluble in water. To date, no cases of toxicity due to the use of topical silicone have been reported.

Because silicone is water resistant, some of it remains on the hair shaft after it is rinsed with water, and the residual silicone improves the manageability of the hair by reducing static electricity, minimizes tangles by decreasing friction, and imparts shine by smoothing roughened scales of the cuticles. In addition, silicone can form a thin, nongreasy film on the hair shaft; therefore, it does not create the limp appearance characteristic of other hair conditioning ingredients.