Formulation and Use of Surfactants In Toothpastes

Transcription

1 Formulation and Use of Surfactants In Toothpastes Adam C Shanebrook CE527: Colloid and Surface Phenomenon Dr. M. Tsianou April 8, 2004

2 The primary goals of toothpastes are to remove all debris on teeth and gums with out damaging tooth enamel or gum tissue. Through the combination of brushing, to provide scrapping and agitation, and all the agents in the toothpaste, which will be discussed, efficient cleaning of the teeth and gums is achieved. The exact composition of a toothpaste varies from brand to brand and any special goals involved like whitening or fluorination properties. A rough outline of the components and their concentrations is; 10-40% abrasive agent, 20-70% humectant, 5-30% water, 1-2% binder, 1-3% of detergent/surfactant, and small concentrations of flavoring, colorings, preservatives and therapeutic agents such as calcium fluoride (CaF 2 ). Tooth enamel is composted mostly (96 wt. %) by the mineral hydroxyapatite (Ca 10 (PO 4 ) 6 2(OH)) in hexagonal packing order. It is important that the abrasives in the paste are not capable of damaging this enamel along with making sure that no reactions between the toothpaste and the enamel occur. By far the common detergent in toothpastes is Sodium Dodecyl Sulfate (SDS), also known as Sodium Lauryl Sulfate. SDS is the most used detergent in all consumer cosmetics, from shampoo to mouthwashes. It is composed of hydrophobic hydrocarbon chain which contains twelve carbons, a sulfate group (SO - 4 ) as the polar head group and a sodium, which dissociates from the molecule when dissolved in water. The twelve carbon chain provides enough of the hydrophobic effect that micelles and bubble films can form while still being able to be dispersed in an aqueous solution. Other surfactant

3 types are used in toothpastes, but are not very chemically different. Examples of these other types are sodium N-lauryl sarcosinate and sodium lauryl sulfoacetate. In the actual use of the toothpaste, the SDS serves two purposes. The first purpose is to generate a foam to coat the teeth. This provides that there is always a layer of the toothpaste solution between the teeth/gums and the bristles of the toothbrush. The toothpaste foam forms by the agitation of brush and the addition of water. Toothpaste foam is different than typical foam in that liquid domain is actually a slurry of the solid and fluid components of the paste. The foam is partially stabilized by the arrangement of the surfactant molecules. At each of the two sides of the bubble film the hydrophobic chains of the SDS point into the air domains. Between these layers, the thin paste/water domain exists. The presence of the foam reduces the surface interactions (tensions) and enables the cleaning/treatment agents to penetrate into tiny spaces between and on teeth. The amount of surfactant used in toothpaste is strongly determined by having the proper amount of foam generation. Too little foam being generated would lead to ineffective tooth coating, too much foam would lead to user discomfort and inconvenience (technically speaking: V foam >> V mouth ). The second purpose for including a surfactant is for the stabilization of food debris. After the debris has been scraped off the tooth enamel/gum tissue by the brush bristles and paste abrasives, it goes in to the bulk brushing solution. This debris interacts with the surfactant to form a small number of micelles. This process effectively prevents the plaque and tartar from redepositing onto the teeth as brushing continues. Due to some allergies/irritations caused by the SDS, some SDS-free toothpastes, which contain no substitution for the loss of a surfactants, have been developed. It can be

4 perceived that these products are somewhat inferior, due to the lack of a significant cleaning component of the paste. To remedy these problems while keeping SDS in the formula, new additives have been developed. He most prominent of these is Betaine. Betaine (also known as trimethylglycine) has been proven to reduce the instances of Xerostomia in toothpaste users. Xerostomia is the medical term for chronic dry mouth, often associated with the use of toothpaste containing SDS. The Betaine adsorbs itself to skin tissue where it is then heavily hydrated. This forms a protective coating of water between mouth tissue and the SDS. Turning to the functionalities of the other toothpaste ingredients we see that the most abundant ingredient is humectant. Humectant is a material that makes the paste retain it water, even when exposed to the air. Humectants are often composed of chemicals like sorbitol and glycerol. The binders in the paste are actually a natural colloid, which stabilizes the multiple phase present in the paste. It is a large design concern that the paste does not separate in to solid and liquid phases as the age of the paste increases. The abrasive in toothpaste is probably the most important element. The abrasive scrapes any and all debris off the teeth and gums. There are a variety of different abrasives, each with different properties, most notably hardness. If the abrasive is too soft, it does not have the capability to scrape off any plaque, if it is too hard, the abrasive can cause cervical tooth abrasions and create notches in the enamel that can lead to cavities. Abrasive types include hydrated silica, sodium bicarbonate, hydrated alumina and dicalcium phosphate dehydrate. The most important marketing/consumer satisfaction elements lie in the toothpaste s flavor and flow properties. Most of the cleaning agents in toothpaste have a

5 rather offensive flavor. SDS is known for it s strong disagreeable taste. To counter these bad flavors a number of ingredients are added to adjust the flavor. Oils of various mints, cinnamon, menthol and citrus are often used to appeal to the consumer. When a toothpaste is squeezed onto the brush, it is desired that the paste retain it shape and not show any type of flow until brushing is commenced. Although this criteria may seen remedial, much formulation and testing goes into its optimization. Natural gums and synthetic polymers are often used to perfect the mechanical and flow properties of the toothpaste. Toothpaste formulations change often due to consumer trends. Through all variations with whitening agents, tartar control ingredients and fluorination components, surfactants have always proved to be an effective component. Although SDS and other surfactants have come under scrutiny for their irritation of tissue, preventative mechanisms and other, less harsh surfactants will continue to be developed for future, better toothpastes.

6 References Evans, D. Fennell and Håkan Wennerström, The Colloidal Domain, Wiley, New York, Prencipe, M, James Masters, K.P. Thomas and James Norfleet, Squeezing a Better Toothpaste, Chemtech of the American Chemical Society, December Rantanan, Irma, Jorma Tenovuo, Effects of a Betaine-containing Toothpaste on Subjective Symptoms of Dry Mouth: A Randomized Clinical Trial, Journal of Contemporary Dental Practices, Vol. 4, No. 2, May 15, Reynolds, Eric C., Contents of Toothpaste Safety Implications, Aust. Prescr. 1994;17: