Sodium dodecyl sulfate

Structure

SDS is in the family of organosulfate compounds, and has the formula, CH₃(CH₂)₁₁SO₄Na; that is, it is the sodium salt of a 12-carbon alcohol that has been esterified to sulfuric acid. An alternative description is that it is an alkyl group with a pendant, terminal sulfate group attached. As a result of its hydrocarbon tail, and its anionic "head group", it has amphiphilic properties that allow it to form micelles, and so act as a detergent.

Physicochemical properties

The critical micelle concentration (CMC) in pure water at 25 °C is 8.2 mM, and the aggregation number at this concentration is usually considered to be about 62. The micelle ionization fraction (α) is around 0.3 (or 30%).

Production

SDS is synthesized by treating lauryl alcohol with sulfur trioxide gas, oleum, or chlorosulfuric acid to produce hydrogen lauryl sulfate. The resulting product is then neutralized through the addition of sodium hydroxide or sodium carbonate. Lauryl alcohol can be used in pure form or may be derived from either coconut or palm kernel oil by hydrolysis (which liberates their fatty acids), followed by hydrogenation. When produced from these sources, commercial samples of these "SDS" products are actually not pure SDS, rather a mixture of various sodium alkyl sulfates with SDS being the main component. For instance, SDS is a component, along with other chain-length amphiphiles, when produced from coconut oil, and is known as sodium coco sulfate (SCS). SDS is available commercially in powder, pellet, and other forms (each differing in rates of dissolution), as well as in aqueous solutions of varying concentrations.

Cleaning and hygiene

SDS is mainly used in detergents for laundry with many cleaning applications. It is a highly effective surfactant and is used in any task requiring the removal of oily stains and residues; for example, it is found in higher concentrations with industrial products including engine degreasers, floor cleaners, and car wash soaps.

In lower concentrations, it is found in toothpastes, shampoos, shaving creams, and bubble bath formulations, for its ability to create a foam (lather), for its surfactant properties, and in part for its thickening effect.

Food additive

Sodium dodecyl sulfate, appearing as its synonym sodium laurel sulfate (SLS), is considered as a generally recognized as safe (GRAS) ingredient, for food use according to the guidelines published in 21 CFR 172.822. It is used as an emulsifying agent and whipping aid. SLS is reported to temporarily diminish perception of sweetness.

Principal applications

Sodium lauryl sulfate, in science referred to as sodium dodecyl sulfate (SDS), is used in cleaning procedures, and is commonly used as a component for lysing cells during DNA extraction, and for denaturing proteins in preparation for electrophoresis in the SDS-PAGE technique.

In the case of the SDS-PAGE application, the compound works by disrupting non-covalent bonds in the proteins, and so denaturing them, i.e., causing the protein molecules to lose their native conformations and shapes. The association of SDS molecules with protein molecules imparts an associated negative charge to the molecular aggregate formed; this negative charge is significantly greater than the original charge of that protein. The electrostatic repulsion that is created by SDS binding forces proteins into a rod-like shape, thereby eliminating differences in shape as a factor for electrophoretic separation in gels.

Miscellaneous applications

SDS is used in an improved technique for preparing brain tissues for study by optical microscopy. The technique, which has been branded as CLARITY, was the work of Karl Deisseroth and coworkers at Stanford University, and involves infusion of the organ with an acrylamide solution to bind the macromolecules of the organ (proteins, nucleic acids, etc.), followed by thermal polymerization to form a brain–hydrogel" (a mesh interspersed throughout the tissue to fix the macromolecules and other structures in space), and then by lipid removal using SDS to eliminate light scattering with minimal protein loss, rendering the tissue quasi-transparent.

Along with sodium dodecylbenzene sulfonate and Triton X-100, aqueous solutions of SDS are popular for dispersing or suspending nanotubes, such as carbon nanotubes (CNTs).

Niche uses

SDS has been proposed as a potentially effective topical microbicide, for intravaginal use, to inhibit and possibly prevent infection by various enveloped and non-enveloped viruses such as the herpes simplex viruses, HIV, and the Semliki Forest virus.

Carcinogenicity

SDS is not carcinogenic when consumed or applied directly, even to amounts and concentrations that exceed amounts used in standard commercial products. The earlier review of the Cosmetic Ingredient Review (CIR) program Expert Panel in 1983 reported that SDS (there, abbreviated SLS, for sodium lauryl sulfate) in concentrations up to 2%, in a year-long oral dietary studies in dogs, gave no evidence of tumorigenicity or carcinogenicity, and that no excess chromosomal aberrations or clastogenic effects were observed in rats fed up to 1.13% sodium lauryl sulfate in their diets for 90 days, over those on a control diet. The 2005 review by the same group indicated that further available data lacked any available suggestion that SDS or the related ammonium salt of the same amphiphile could be carcinogenic, stating that "Despite assertions to the contrary on the Internet, the carcinogenicity of these ingredients is only a rumor;" both studies conclude that SDS appears "to be safe in formulations designed for discontinuous, brief use followed by thorough rinsing from the surface of the skin. In products intended for prolonged contact with skin, concentrations should not exceed 1%."

Sensitivity

Like all detergent surfactants, sodium lauryl sulfate removes oils from the skin, and can cause skin and eye irritation. It has been shown to irritate the skin of the face, with prolonged and constant exposure (more than an hour) in young adults. SDS may worsen skin problems in individuals with chronic skin hypersensitivity, with some people being affected more than others. In animal studies SDS appears to cause skin and eye irritation.

Oral concerns

The low cost of SDS, its lack of impact on taste, its potential impact on volatile sulfur compounds (VSCs, which contribute to malodorous breath), and its desirable action as a foaming agent have led to the use of SDS in the formulations of toothpastes. A series of small crossover studies (25-34 patients) have supported the efficacy of SLS in the reduction of VSCs, and its related positive impact on breath malodor, although these studies have been generally noted to reflect technical challenges in the control of study design variables. While primary sources from the group of Irma Rantanen at University of Turku, Finland conclude an impact on dry mouth (xerostomia) from SLS-containing pastes, a 2011 Cochrane review of these studies, and of the more general area, concludes that there "is no strong evidence… that any topical therapy is effective for relieving the symptom of dry mouth." A safety concern has been raised on the basis of several studies regarding the effect of toothpaste SDS on aphthous ulcers, commonly referred to as canker or white sores. A consensus regarding practice (or change in practice) has not appeared as a result of the studies. As Lippert notes, of 2013, "very few… marketed toothpastes contain a surfactant other than SLS [SDS]," and leading manufacturers continue to formulate their produce with SDS.

Interaction with fluoride

Some studies have suggested that SLS in toothpaste may decrease the effectiveness of fluoride at preventing dental caries (cavities). This may be due to SLS interacting with the deposition of fluoride on tooth enamel.