SODIUM DODECYL SULFATE (SDS): STRUCTURE, PROPERTIES, USES - CHEMISTRY - 2025

The sulfate dodecyl sodium (SDS), is a synthetic anionic and amphiphilic organic salt. The apolar and hydrophobic part corresponds to the hydrocarbon chain; while the polar and hydrophilic part is due to sulfate, with a negative charge.

SDS is a surfactant: its molecules are placed on the surface of aqueous solutions, producing a decrease in surface tension. This feature allows it to be used in shampoo, toothpastes, soaps and other personal care and household cleaning items.

Structure of docile sodium sulfate. Source: Benjah-bmm27 via Wikipedia.

Sodium dodecyl sulfate binds to the protein and causes its unfolding, acquiring an elongated shape. Sulfate groups are exposed to water, acquiring negative charges proteins, the number of which is proportional to their molecular weight.

For this reason, the SDS allows the determination of the molecular weights of proteins by means of polyacrylamide electrophoresis.

Although a possible carcinogenic effect of SDS has been suspected, it has not been conclusively shown to be so. SDS can cause skin and eye irritation like other detergents.

Structure of the SDS

The first image showed the SDS molecule with a spheres and bars model. As can be seen, it has a hydrophobic and nonpolar carbonated tail (black and white spheres); and at the same time, a polar head, thanks to the presence of the group -SO ₄ ^- (yellow and red spheres).

Although not shown, there should be a purple sphere representing the Na ⁺ cation, right next to the -SO ₄ ^{- group}, interacting electrostatically.

That said, it is easy to see why this compound is amphiphilic; and also an anionic surfactant, since the charge of SDS is negative and it can attract other cations in addition to Na ⁺.

The molecule gives the false impression of being rigid, an immovable linear structure. However, it is just the opposite. The SDS could be compared to a "worm", whose carbonated tail exhibits multiple vibrations and rotations of its single bonds. This makes it capable of adopting different shapes or folds in the middle; in water, for example.

Crystals

Crystal structure of sodium dodecyl sulfate. Source: Benjah-bmm27

Although it is true that sodium dodecyl sulfate is a dynamic molecule, in the solid state it does not need to move too much, behaving like a "simple" rod. And thus, each molecule or bar is positioned in such a way as to increase the interactions between the carbonate tails and, at the same time, those of their ionic charges.

The result is the formation of a monoclinic crystal, where the SDS molecules are packed into several columns of bars (top image). The intermolecular forces that bind them are such that these crystals require a temperature of 206ºC to melt into the liquid phase.

Micelles

Outside the comfort of its crystals, the SDS molecule can no longer sit still; begins to twist its tail to increase or decrease interactions with the outside environment.

If this medium is water, a phenomenon called micelle formation occurs: the carbonated and hydrophobic tails join to arm themselves, while the polar heads, the -SO ₄ ^- groups, remain on the surface interacting with the H ₂ O molecules..

So the micelle takes on an oblate ellipsoid morphology (like Earth, but more squashed). In the glass they are as blocks of bars, and in an aqueous medium as ellipsoid micelles. What if the medium was oily? The micelle would be reversed: the polar heads SO ₄ ^- would go to the nucleus, while their carbonated tails would be exposed to the oil.

Properties

Names

- Sodium dodecyl sulfate (SDS).

- Sodium lauryl sulfate (SLS).

Molecular formula

C ₁₂ H ₂₅ O ₄ SNa.

Molar mass

288.378 g / mol.

Physical description

Various presentations: dry powder, liquid or wet solid. Its crystals have a white or creamy color.

Odor

Faint odor of fatty substances, odorless.

Melting point

206 ° C.

Water solubility

1 · 10 ⁵ mg / L. 1 g dissolved in 10 mL of an opalescent solution.

Density

1.01 g / cm ³.

Stability

It is stable under recommended storage conditions.

Decomposition

When heated to decomposition it emits a white smoke of sulfur oxide and sodium oxide.

Surface tension

39.5 dynes / cm at 25 ° C.

Critical molar concentration

It is 8.2 mM in pure water at 25 ° C.

Applications

Personal care and home cleaning

Dodecyl sulfate is a surfactant that is used in many products, such as hand soaps, foam baths, shaving creams, etc. It is also used to remove hard-to-remove stains from clothes, in cleaning floors and bathrooms, scrubbing kitchen items, etc.

Veterinary and human medicine

It is used as a repellent for fleas and ticks, present in animals. It is also used as a humectant in some antibiotics, for oral or topical use.

SDS is microbicidal against enveloped viruses such as HIV, types 1 and 2, and the herpes simplex virus (HSV-2). It also acts on non-enveloped viruses: papillomavirus, reovirus, rotavirus and poliovirus. However, this use is not approved.

The use of SDS has been suggested as a douche that provides protection against sexually transmitted viruses. Also, in the processing of breast milk to eliminate or reduce the possibility of HIV transmission via breastfeeding.

In addition to its antiviral action, SDS acts on pathogenic bacteria and fungi. SDS removes drug resistance and sex transfer factors from E. coli; and blocks the growth of numerous Gram positive bacteria.

At the laboratory

SDS binds to proteins causing their denaturation, supplying them with negative charges and a conformational change. This allows the determination of their molecular weights by polyacrylamide electrophoresis.

SDS is also used in the preparation of brain tissue samples for use in light microscopy. It is also used in the preparation of blood samples for counting the number of erythrocytes.

SDS is used in the purification of nucleic acids, due to its ability to dissolve membranes and its inhibitory action on the activity of RNase and DNase enzymes.

Additionally, SDS is used in the characterization of quaternary ammonium compounds.

Food additive

SDS is used as an emulsifier and thickener in foods. Improves the stability and texture of baked goods. Additionally, it is used as an auxiliary in beating dry egg products.

Industry

SDS is used in the electroplating industry, particularly nickel and zinc; as an emulsifier and penetrant in varnish and paint remover; in the formulation of injection molded explosives; and in solid rocket propellants as a foaming agent.

Risks

Constant exposure to SDS can cause skin hyperplasia in guinea pigs. Rabbits and humans are less susceptible to these injuries. Direct contact with SDS at a concentration ≤ 20% can cause moderate inflammation and irritation of the skin.

On the other hand, prolonged exposure to SDS can cause dermatitis, with signs of redness, swelling, and blistering.

SDS can cause eye irritation on contact. It can also be very reactive in some people, causing respiratory tract irritation and respiratory distress by inhalation.

In rare cases, ingestion of SDS can cause caustic injury. But, it has been reported in children who have ingested SDS, rapidly developing profuse vomiting, CNS depression, and respiratory distress.

No evidence of genotoxicity or teratogenic action has been found in SDS. In 242 patients with eczematous dermatitis, 6.4% of the patients were found to have an allergy, attributable to the use of the SDS.

References

1. Structural data from LA Smith, RB Hammond, KJ Roberts, D. Machin, G. McLeod (2000). Determination of the crystal structure of anhydrous sodium dodecyl sulphate using a combination of synchrotron radiation powder diffraction and molecular modeling techniques. Journal of Molecular Structure 554 (2-3): 173-182. DOI: 10.1016 / S0022-2860 (00) 00666-9.
2. Wikipedia. (2019). Sodium dodecyl sulfate. Recovered from: en.wikipedia.org
3. National Center for Biotechnology Information. (2019). Sodium dodecyl sulfate. PubChem Database. CID = 3423265. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Hammouda B. (2013). Temperature Effect on the Nanostructure of SDS Micelles in Water. Journal of research of the National Institute of Standards and Technology, 118, 151–167. doi: 10.6028 / jres.118.008
5. Elsevier. (2019). Sodium Dodecyl Sulfate. Science Direct. Recovered from: sciencedirect.com
6. Kumar, S., Thejasenuo, JK and Tsipila, T. (2014). Toxicological effects of sodium dodecyl sulfate. Journal of Chemical and Pharmaceutical Research 6 (5): 1488-1492.
7. Toxicology Data Network. (sf). Sodium lauryl sulfate. Toxnet. Recovered from: toxnet.nlm.nih.gov