Introduction to Chemistry in a Tube of Toothpaste

Throughout history, people have used many methods to clean their teeth. Some ancient people chewed on frayed twigs. Others rubbed their teeth with cloth or swabbed them in vinegar. The Egyptians may have been the first people to use toothpaste when they mixed powdered pumice (a porous rock formed in volcanic eruptions) with vinegar and applied it to their teeth in the 100's B.C. Some toothpastes still contain pumice, but science has since supplied other toothpaste ingredients that the ancient Egyptians never dreamed existed.

The Work of Toothpaste

Toothpaste has two functions—cosmetic and hygienic. It improves one's appearance by removing stains from teeth. The hygienic function of toothpaste helps prevent tooth decay by removing food particles and plaque from teeth and delivering other decay preventing substances such as fluoride. Plaque is a sticky substance that forms on teeth. It is made up primarily of common oral bacteria and products of those bacteria. Plaque also contains some saliva and components that dissolve from food onto the teeth. If plaque is not removed, the bacteria digest the sugar in foods and beverages to form a family of acids that slowly erode a tooth's enamel (hard outer coating).

A major purpose of brushing is to prevent plaque from turning into tartar. Tartar, also called calculus, is a hard mineral substance that is chemically similar to tooth enamel. It forms along the gum line when calcium salts in saliva collect on the teeth along with dead bacteria in plaque. Tartar above the gum line does not hurt the teeth, but because it quickly absorbs stains and turns brown, it is a cosmetic problem. Tartar below the gum line can cause gingivitis (inflamed gums), which may lead to periodontitis (loss of bone that supports teeth) and eventually loss of teeth.

All toothpastes are mixtures of an abrasive, a detergent, a thickener, a humectant (moisturizer), water, and other ingredients such as flavoring agents, coloring, and sweeteners. Most toothpastes also contain a fluoride compound that helps prevent tooth decay. Some toothpastes contain compounds that fight tartar formation.

Other toothpastes contain desensitizing agents to reduce the sensitivity of tooth surfaces below the gum line. This sensitivity can develop if the gums recede or are worn away by brushing too hard with an extra firm toothbrush. Some toothpastes also contain a bleaching agent that may make teeth look whiter.

The Benefits of Abrasives

Abrasives provide the cleaning power in toothpastes. They give toothpaste a slightly gritty texture designed to polish teeth and remove plaque, food remnants, and stains.

The most commonly used abrasives are hydrated silica, calcium carbonate, aluminum oxides, and various phosphates of calcium, or aluminum. Calcium carbonate is the compound of which chalk is made, but the form used in toothpaste is not simply ground up chalk. The compound is chemically reformed into finer particles than those found in chalk. Likewise, sand contains pure silica, but the form used in toothpaste is gentler than that in sand. Harsher abrasives include anhydrous dicalcium phosphate, zirconium silicate, crystalline silica, and pumice. Those abrasives are commonly used by the dentist to remove tartar and stains. Some European toothpastes even contain abrasives made of tiny granules of hard plastic.

Most abrasives used in toothpaste pose no harm to teeth because tooth enamel is made almost entirely of hydroxyapatite, a form of calcium phosphate that is about as hard as the mineral topaz or the steel in most knives.

But toothpastes designed to remove tough stains, such as those from tobacco and coffee, may contain harsher abrasives than do regular brands. Some experts say that the frequent use of highly abrasive toothpastes over many years has the potential to damage tooth enamel.

Harsher abrasives may also contribute to the loss of cementum at the base of the teeth. Cementum is a thin layer of calcium, magnesium, and phosphorus that covers the dentin (the main material of which teeth are composed) below the gum line. If the cementum is worn away—from overly vigorous brushing or the aging process—the sensitive dentin is exposed. Repeated brushing with harsh abrasives may also wear away exposed dentin, increasing sensitivity.

Cleaning Ingredients

Detergents, like abrasives, also provide cleaning power. They loosen food remnants and plaque so that the toothbrush can scrub them away. Detergents contain cleaning agents called surfactants. Surfactants are complex molecules (groups of linked atoms) that attach to stains at one end and to water at the other. The water pulls the surfactant and the stain away from the surface to which they are attached. In toothpaste, surfactants help pull food particles and stains away from teeth. Surfactants alone are not as effective as abrasives in removing plaque and stains, however.

Chemicals in toothpaste detergents can also help prevent tooth decay. The most frequently used toothpaste detergent, sodium lauryl sulfate, can slow the growth of some plaque forming bacteria. Another detergent, sodium lauryl sarcosinate, may inhibit the chemicals that plaque forming bacteria use to digest sugar.

Baking soda, one of the oldest forms of tooth cleaners, acts both as a gentle abrasive and as a detergent. In some toothpastes, it is also used to fight decay causing microbes. Baking soda has the disadvantage of dissolving quickly in saliva and water, however. Once dissolved, baking soda is no longer abrasive, though it continues to function as a detergent.

Enamel Strengtheners

Fluoride—most frequently in the form of sodium fluoride, sodium monofluorophosphate, or stannous fluoride—combats tooth decay by strengthening enamel and, according to some researchers, by inhibiting bacteria's formation of acids that attack enamel. Although enamel is the hardest substance in the body, constant attack by the acids in saliva and plaque can dissolve it.

Fluoride's action is not completely understood, but chemists believe that the ingredient works through a series of chemical reactions called remineralization, in which fluoride is transformed into fluoride rich compounds called apatites that chemically bond with enamel to strengthen it and help protect it from dissolving in acid.

Calcium can help strengthen enamel, but excess calcium can contribute to tartar build-up. Calcium that is naturally present in saliva can be a source of excess calcium. Tartar control agents in toothpaste work by slightly changing the chemistry of saliva and teeth to inhibit the build-up of calcium on teeth. These changes inhibit calcium from crystallizing into tartar once the calcium binds to plaque. In this way, less tartar forms, and the tartar that does form is often easier to remove. Tartar control agents in toothpaste do not work below the gum line and thus have no clear effect on gingivitis.

Easing the Ouch

Toothpastes with ingredients designed to desensitize teeth are intended for people whose teeth are extremely sensitive to heat and cold and to brushing. As many as 1 in 7 adults develop sensitive teeth, usually after age 45. This typically happens when the gums recede, exposing the sensitive cementum and dentin below the gum line.

Desensitizing toothpastes work differently depending on the desensitizing compounds they contain. Potassium nitrate, strontium chloride, and sodium fluoride are common desensitizing compounds. Fluorides help rebuild a thin layer of cementum on the tooth's exposed root, a process similar to the way in which fluoride remineralizes enamel. Strontium chloride blocks tiny tubules (channels) that lead from root surfaces to the nerves. In doing so, the compound disrupts pathways that help produce pain signals. Although chemists are not completely certain how potassium nitrate works, they believe it also alters chemical signals in the tubules. It and the other desensitizing compounds must be used regularly to be effective.

Compounds For A Whiter Smile

Bleaching agents for whitening teeth date to at least the early 1900's. Today, the most common bleaching agent in toothpaste is a compound called carbamide peroxide, which in the mouth forms hydrogen peroxide. The hydrogen peroxide in turn breaks down into water and oxygen. The free oxygen is a powerful bleaching agent because it combines with materials in the stains and lightens their color. A few brands of toothpaste contain a different type of bleaching agent, citric acid, which is found in lemons and other citrus fruits and acts as a mild bleach.

Toothpastes do not contain enough bleaching agents to lighten heavy stains beyond the normal effect of abrasives, however. Products called home tooth bleaching kits claim to offer stronger whitening power. But according to the American Dental Association (ADA), these products may be dangerous due to their high peroxide content. In 1989, the ADA warned that the regular use of home bleaching kits may temporarily damage the soft tissues of the mouth, delay healing of already damaged tissue, or damage tooth pulp by traveling down tubules in the enamel to the pulp. None of these kits had received approval from the ADA as of mid-1993.

Binders, Moisturizers, Preservatives, and Colors

The remaining ingredients in toothpaste perform functions unrelated to cleaning the teeth. Toothpastes need something to keep the solid and the liquid ingredients together, and a binding agent performs this function. Most such agents are some form of cellulose, a gel, or an extract of seaweed. Without the binder, the toothpaste would separate into a liquid portion and a sort of mush and would have to be stirred, like paint, before each use.

In addition, moisturizers keep the toothpaste from drying out. The most commonly used moisturizers are glycerin and sorbitol.

Whenever the cap is left off the toothpaste tube, molds or bacteria can get inside. Most manufacturers add a preservative such as methylparaben or propylparaben to the toothpaste to prevent the microbes' growth.

To make toothpaste visually appealing, manufacturers add coloring agents, which usually are food colorings. Some coloring agents are natural, and others are artificial. White, pink, green, and blue are the favored colors of toothpaste. White toothpastes often contain titanium dioxide, a brilliant white pigment used in some paints, to make the toothpaste appear whiter and brighter.

Choosing A Toothpaste

Puzzled over which type of toothpaste to buy? The ADA considers most major brands of toothpaste to be about equally effective for people with healthy teeth and gums. One indication of effectiveness is the ADA seal. To receive the seal, toothpaste manufacturers must submit results of long-term scientific studies that prove their product's safety and effectiveness. Despite the ordinary appearance of your favorite toothpaste, a lot of science went into that dab of goo.