Sodium lauryl sulphate (SLS)

INDEX

·         SYNONYM

·         STRUCTURAL FORMULA

·         APPEARANCE

·         GENERAL

·         SYNTHESIS

·         REACTION

·         FOAM

·         APPLICATIONS

·         EFFECTS

SYNONYM:

·         Sodium lauryl sulphate (SLS)

·         sodium dodecyl sulphate (SDS/ NaDS)

·         lauryl sodium sulphates

·         lauryl sulfate sodium salt

·         sodium n-dodecyl sulfate

·         Sulfuric acid, monododecyl ester, sodium salt

·         Sodium dodecanesulfate

·         Dodecyl alcohol, hydrogen sulfate, sodium salt

·         Akyposal SDS

·         Duponol

·         Coconut surfactant/ Sodium coco /Cocoyl sulfate (a combination of sodium lauryl sulfate- usually around 50% and sodium myristyl and palmityl sulfate (longer chain hydrocarbon tails))

STRUCTURAL FORMULA: CH3-(CH2)11-O-SO3-Na+

APPEARANCE: White powder

GENERAL: Sodium lauryl sulphate (SLS) is a synthetic detergent (cleaning agent) and surfactant (which means it makes bubbles). It has a high pH as it is an alkali substance and has the appearance of a white powder.Sodium lauryl sulfate is sometimes referred to as the coconut surfactant becauseit can be manufactured from coconut oil. Being derived from inexpensive coconut and palm oils, Sodium coco-sulfate is essentially the same compound, but made from less purified coconut oil. Sodium lauryl sulfate (SLS) is a cheap, very effective cleansing and foaming agent (foams quickly). It is probably the most commonly used anionic surfactant in the personal-care business. It's an ingredient in a wide range of personal care products such as soap, shampoo and conditioners, bubble bath, moisturisers, cleansers, facial scrubs and shaving cream and toothpastebut in lower concentrations. Sodium lauryl sulfate is used to remove oily stains because it has a thickening effect that helps form lather. It is used in dishwashing liquids and laundry detergent.It's also used, in much higher concentrations, in industrial products such as car wash soap, engine degreasers, and floor (carpet) cleaners.

SLS is rarely added to baby products. Instead, baby soaps and shampoos often use a closely related but mildercleanser called sodium laureth sulfate (SLES). The SLS products tend to clean better, foam better and are usually less expensive.

SLS is added to soaps, bubble baths and toothpastes for its thickening effect and its ability to create lather. In cleaning products SLS allows oil and grease to be washed off by water and SLS can alsoassist lathering, while in other products it maintains the product's stability. It is used in so many products because it is a cheap, highly effective cleansing and foaming agent. The foaming effect of soaps does not improve their ability to clean, but is added for visual reasons (Morelli and Weston 1987). In many toothpastes, it helps thicken the toothpaste and make it easier to spread across the teeth.

Sodium lauryl sulfate (SLS), has an amphiphilic properties due to C12 chain (lipophilic) attached to a sulfate group (hydrophilic). This bifunctionality in one molecule provides the basic properties useful in cleaners and detergents. SLS is used as a wetting agent in textiles, foaming and cleaning agent in detergent, cosmetic emulsifier, and sometimes in toothpastes. 

SYNTHESIS: Sodium lauryl sulfate has the chemical formula C12H25SO4Na. Sodium lauryl sulfate (SLS) is synthesized by reacting lauryl alcohol (dodecanol) with sulfuric acid (sulfation reaction). Sulfation reaction produces hydrogen lauryl sulfate that is neutralized by addition of sodium carbonate. The industrially practiced method typically uses sulfur trioxide gas for sulfation. Lauryl alcohol is in turn usually derived from either coconut or palm kernel oil by hydrolysis, which liberates their fatty acids, followed by hydrogenation. Due to this synthesis method, commercial samples of SDS are often a mixture of other alkyl sulfates, dodecyl sulfate being the main component. SDS is available commercially in powder and pellet forms. It seems the pellet form dissolves faster than the powder form in water. Purification is accomplished through repeated extraction. It is available commercially in both broad-cut and purified forms.

Also it is formed by combining sulfonic acid and dodecanol in a process known as esterification. This product is then neutralised with sodium carbonate to give sodium lauryl sulfate. 

REACTION: Synthesis Equation of Sodium Lauryl Sulfate

C12H25OH + H2SO4 ------> C12H25HSO4

Lauryl alcohol + Sulfuric acid ---> Hydrogen lauryl sulfate

C12H25HSO4 + Na2CO3 ------> NaC12H25SO4

Hydrogen lauryl sulfate + Sodium carbonate----> Sodium lauryl sulfate

ACTION: Both SLS and SLES are very effective ingredients used in cleansing products and as creams and lotions. In this function, surfactants wet body surfaces, emulsify or solubilize oils, and suspend soil. These ingredients contribute foaming and lathering properties to cleansing products and bubble baths.The part which has Na+ and (SO4) 2- is the polar part due to charges on these ions.Polar part issoluble in water and is hydrophilic. The alkyl C12H25---- is the part without charge separation and is nonpolar. It is insoluble in water and hydrophobic.Sodium lauryl sulfate is a material that decompose to release a gas under certain conditions (typically high temperature), which can be used to turn a liquid into a foam. This molecule has 12 carbon atoms in its hydrophobic tail group and has a low critical micelle concentration, which means it has relatively good cleansing capabilities. SLS is used so commonly because it is a very good surfactant (agent that helps water be more effective). Products without SLS simply will not foam as well and many people don't like that. Many people want lots of foam from their shampoos and body washes. SLS is a great way to get that foam.

Illustration of a micelle

Above a critical concentration SLS will form micelles in water. The concentration at which micelles start to form is the critical micelle concentration, CMC. At this point, surface tension becomes independent of concentration. An SLS micelle is spherical and will have a diameter of roughly twice the length of SLS. It will contain 20-50 molecules, the sulfate heads will face outwards forming the face of the sphere pointing towards the water. The long hydrocarbon chains with then form the interior of the spherical micelle. The CMC of SLS is roughly 8.1 mol m-3 at 25°C.

Basically, a surfactant such as sodium lauryl sulfate affects your skin by partially dissolving the cell membranes of your skin cells. This anionic surfactant is also able to penetrate right into living cells. Cell membranes are made up primarily of lipid (another name for fat) molecules. The molecules of the lipids which compose the membrane of cells are very similar in nature to anionic surfactant molecules. And, it is therefore not surprising that some surfactants are very readily absorbed into the skin cell membranes.Sodium lauryl sulfate is one of the most readily absorbed surfactants, in fact, and it has been chosen for experimental purpose to enhance the penetration of different elements in living systems.

The damage taking place to your skin when you use this surfactant is not noticeable to you at first. Only at sufficiently high concentrations of sodium lauryl sulfate on your skin will you detect the damage that it is causing. At lower concentrations, you will still suffer cellular damage; you just will not be able to tell it is happening.

FOAM:

SLS is an excellent foaming agent, and this is one of the reasons it's included in many personal care products, such as toothpaste. Sodium lauryl sulphate gives thick, rich foam and cleanses the hair. However, its ability to foam has a negligible effect on the functional performance of the product, it allows the hands to work the shampoo through the hair. This helps mechanical removal of dirt. Foaming properties are actually added to meet a consumer demand. The amount and quality of foam produced is associated by consumers as an indicator as to whether the product is working. This myth is propagated by advertising companies as it is a visual, tangible feature of SLS, and it would be hard to show the cleaning process otherwise.

For foam to exist there must be a substituent with the bulk of the liquid to lower surface tension. If one were to shake a bottle of water, air bubbles would be trapped briefly but they would be short-lived due to the high surface tension and instability of the bubble. Hence, on the addition of the surfactant SLS the surface tension of the bubble is lowered and thus has more stability and a longer life-span.

SLS's foaming properties do have a use in dentifrice besides consumer satisfaction although its performance does rely upon it heavily. The foaming action allows the polishing agent to be suspended and detergency properties to reach otherwise inaccessible areas and cavities in the mouth. SLS also shows antimicrobial effects on bacterial flora or the mouth and hence is the most commonly chosen surfactant for toothpastes.

APPLICATION:

·         Personal care products: It is probably the most commonly used anionic surfactant in the personal-care business.

o   Shampoo, soap, bubble bath&detergents: Cleansing agent, surface-active agent, foaming agent.

o   Creams, lotion and medical preparations: Emulsifying, foaming, wetting, dispersing agent.

o   Toothpaste: foaming, wetting, and dispersing agent.

·         Industrial applications: It's also used, in much higher concentrations, in industrial products such as car wash soap, engine degreasers, and floor cleaners.

·         Insecticides: Emulsifier, wetting agent and adjuvant.

·         Antibacterial and antimicrobial properties: Sodium lauryl sulfate has antibacterial and antimicrobial properties, making it effective in inhibiting the growth of harmful, disease-causing pathogens. It is used in mouth rinses, hand soaps, and various other oral care products to eliminate microbial agents (protozoans, fungi, bacteria, and viruses). Sodium laurl sulfate is commonly available and an ingredient in high-quality cleaning agents used in various capacities.

·         Varnish and paint remover: Emulsifier and penetrant

·         Solid rocket propellants: anti-foaming agent

·         Foods: Emulsifier, whipping agent and surfactant. In food it is used as an emulsifier and whipping agent (e.g. dried egg products) as well. It also appears in insecticides as a wetting agent, that is, it helps the insecticide disperse evenly over the plants' surfaces.

·         Laboratory applications: It can be used to aid in lysing cells during DNA extraction and for unraveling proteins in SDS-PAGE. SLS-PAGE stands for sodium lauryl sulfate polyacrylamide gel electrophoresis. Sodium lauryl sulfate, in science referred to as sodium dodecyl sulfate (SDS) or Duponol, is commonly used in preparing proteins for electrophoresis in the SDS-PAGE technique. It's a widely used biochemical technique that separates proteins based on their molecular weight. SLS binds to the proteins in solution, much like it would bind to dirt in detergency. It then unfolds the proteins and gives them a uniform negative charge across the protein. This enables them to be easily identified when passing through the gel by measuring their mobility (see photo, right), and this value will be proportional to the logarithm of their molecular weight.

·         Biocide: Another use of SLS was documented in a study recently. PG-300995 is an anti-HIV agent, but is a poorly soluble drug in solution. The addition of SLS at different pH levels was investigated, and found to be an efficient surfactant, aiding solvation at high enough concentrations.

·         Medicinal applications: In medicine, sodium lauryl sulfate is used rectally as a laxative in enemas, and as an excipient on some dissolvable aspirins and other fiber therapy caplets.

·         Shark repellant: Evidence suggests that surfactants such as sodium lauryl sulfate can act as a shark repellent at concentrations on the order of 100 parts per million. However, this does not meet the desired "cloud" deterrence level of 0.1 parts per million

EFFECTS:

SLS doesn't cause cancer or harm the eyes, the claims are unfounded. Like most detergents, however, SLS can irritate skin, causing eczema and dry, coarse, or reddened skin, particularly in high concentrations or when used often.According to the International Programme on Chemical Safety, sodium lauryl sulfate acts as a skin and eye irritant that may impact short and long-term health. Babies have a thinner epidermis (the outer layer of skin) than adults do. This means babies are more sensitive to soaps and detergents. A product containing sodium lauryl sulfate won't irritate the skin of most babies and children, provided it's rinsed away promptly.

SLS in Shampoo: It cleans hair so thoroughly, it strips (floorings) it clean, leaving it so stripped that you have to pour on a chemical conditioner to be able to manage it. But more important than its stripping effect on hair, SLS has highly adverse effects on the body. Sodium Lauryl Sulfate (SLS) - a synthetic surfactant frequently found in shampoo. Admittedly, its cleaning power is strong - too strong. If you have dandruff, dermatitis, canker sores, or other irritated tissues or skin, it could be due to SLS.It is a type of caustic cleanser that corrodes the hair follicles, causing problems with hair growth.

Sodium lauryl sulfate is a regular member of the list of ingredients in a toothpaste. It causes microscopic damage to the oral tissue. This is said to have caused canker (cancer/ plague) sores (wounds) in many people. SLS containing toothpaste may experience skin irritation around the mouth or on the face.  In order to avoid canker sores, bad breath and dry mouth, you should be using sodium lauryl sulfate free toothpaste. Over time, prolonged exposure may lead to dermatitis in sensitive individuals. SLS consumption is also linked to gastrointestinal distress in the form of diarrhea and vomiting.

IRRITATION INTENSITY OF SULFATES

From the table you can see that olefin sulfonate which some call a 'mild' surfactant is actually comparable to the most irritating surfactant SLS. Meanwhile SLES which is regarded as a 'harsh sulfate' is much gentler generally producing a moderate reaction if left on skin 5 days after washing.