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.

ANALYSIS OF LAUNDRY DETERGENTS

ANALYSIS OF LAUNDRY DETERGENTS- Powders, Bars/ Cakes and Liquids: 

Detergents are graded on the basis of their active matter, carbonate and poly phosphate content.

POWDERS:

Active matter/ Active detergent (% LAS)

Total Alkalinity (% Na₂CO₃)

Total salt (% NaCl)

Total phosphates

Bulk density & Tapped density for powders

Reflectance (%)

Moisture (%)

Sieve Analysis

pH

CAKES

Active matter/ Active detergent (% LAS)

Total Alkalinity

Total salt (% NaCl)

Total phosphates

Total fatty matter (TFM) for detergent soap

Moisture (%)

pH

Gritt/ Smoothness

LIQUIDS

Viscosity

Specific gravity

pH

Nearly all detergents are alkaline because nearly all soils and most staining agents are acidic. As a result, an alkaline cleaner performs best at removing these soils because they bring these acids into solution.

LAUNDRY DETERGENTS FORMULATION

LAUNDRY DETERGENTS: Powders/ Bars or Cakes/ Liquids

General purpose detergents are suitable for all washable fabrics. Powders are especially effective in lifting out clay and ground-in dirt. Liquids work best on oily soils and for pretreating soils and stains. Light duty detergents are used for hand or machine washing lightly soiled items and delicate fabrics. Cake or detergent bars are used.

LAUNDRY DETERGENT FORMULATION

In addition to the surfactant modern detergent formulations contain a number of other substances to improve the detergency, to bleach, to lessen redeposition of dirt, to brighten, or simply to reduce the cost of the formulation. A substance added to a surfactant to increase its detergency is called a builder. The commonly used ingredients are as follows-

1.      SURFACTANTS

Linear alkylbenzene sulphonic acid (LABSA), Linear alkene benzene sulphonate (LABS), Sodium laureth sulphate, or sodium lauryl ether sulphate (SLES), Sodium lauryl sulphate (SLS), Alpha olefin sulphonate (AOS), Methyl ester sulphonates (MES), Alkyl polyglyucosides (APG), Lauryl alcohol ethoxylates (LAE). Detergent surfactants are made from a variety of petrochemicals (derived from petroleum) and/or oleo chemicals (derived from fats and oils). 

Role: Multiple roles including- Cleaning (soil removal), Foaming, Wetting agent, Antiredeposition agent, Dispersing agent.

Mechanism: 
A) Surfactant converts greasy and oily dirt into micelle that become dispersed in water. Solubilisation of oil, achieved by micellar action When the detergent dissolves in water, detergent molecules group themselves around an oil droplet (a). The water-repelling part of the molecules (light area) projects into the droplet, while the water-soluble part (dark area) remains in the water (b). The oil is held in suspension by the emulsifying action of the detergent and is carried away with the dirty water (c).
B) Surfactants decreases the surface tension of water, making it better wetting agent .
C)Removal of particulate soil from the fabric, achieved by electrostatic repulsion. Surfactant keep the greasy micelle in suspension and prevents them redepositing until they can be washed away.
D) Suspension of soiling matter in the wash liquor.

 

2.     BUILDERS

Soda ash or Sodium carbonate, Sodium tripolyphosphates, Sodium silicate, Zeolite (special form of clay - hydrated sodium aluminium silicate), Sodium Citrate.

Role: Water softeners, added to inactivate hard water minerals. This is done either by

·         Sequestration or chelation (holding hardness minerals in solution),

·         Precipitation (forming an insoluble substance), or

·         Ion exchange (trading electrically charged particles).

Complex phosphates and sodium citrate are common sequestering builders. Sodium carbonate and sodium silicate are precipitating builders. Sodium aluminosilicate (zeolite) is an ion exchange builder. Since detergents are more effective in soft water, these products increase cleaning power. Builders can also supply and maintain alkalinity, which assists cleaning, especially of acid soils; help keep removed soil from redepositing during washing; and emulsify oily and greasy soils.

Alkalis: Ex. Sodium carbonate, Sodium silicate and Sodium hydroxide. The potassium analogue (potassium carbonate, potassium silicate and potassium hydroxide) is used for the liquid detergents due to their higher solubility.
Role:
1. Neutralization of acidic soils
2. Saponification of fatty soil
3. Enhancement of soil removal, for example, with anionics
4. Improving soil suspension and preventing re-deposition
5. Optimising bleach conditions
6. Optimising enzyme conditions

3.     FILLERS

Sodium chloride, Sodium sulphate, China clay, Dolomite, Calcite and Water

Role: To lower the cost of formulation and maintain the physical properties like to make detergents fluid or to turn fluidized detergents into powders. They're used to change the consistency of the detergent, to make it more pourable, more soluble and to help it disperse evenly.

Sodium sulfate is added to make the detergent powder flow freely. Without it, the detergent would stick together and become one big block.

4.     ENZYMES

Amylase, Protease, Cellulase, Proteinase and Savinase

Role: To break down soils to simpler forms for removal by detergent.

·         Proteases: break down protein stains (such as blood, dairy products, eggs, meat, mud, and grass), into small units called “peptides”),

·         Amylases: break down carbohydrate (such as starch, potato, pasta and rice) into smaller molecules called “oligosaccharides” or “monosaccharides”.

·         Lipases (break down fats (or “lipids”) such as butter and oil)

Mechanism: Enzymes break down large molecules such as proteins, carbohydrates and fats into smaller segments. These smaller segments are either water-soluble, or are of size and polarity compatible with surfactants meaning that they can be suspended in solution. Whatever the type of stain, after its enzymatic breakdown, surfactants suspend the resulting fragments in solution.   Most enzymes are destroyed by high temperatures, i.e. above 60 °C. They are usually most effective at warm water temperatures (e.g. 40 °C), however some enzymes are best for use in cold water.

Colour and fabric care: Some enzymes act to smooth cotton fabrics by cleaving fibres that protrude from the surface. A smoother cotton surface means that soils are less readily taken up by fibres, and are more easily liberated.

Enzymes can also help remove fuzz and pills, and can assist colour protection of fabrics.

5.     ANTIREDEPOSITION AGENTS

Sodium carboxymethyl cellulose, Polyethylene glycol (PEG), Polyvinyl alcohol, Polyvinyl pyrrolidone.

Role: To prevent soils from resettling after removal during washing.

Mechanism: Anti-redeposition agents increase the negative charge on the fabric surface, so that the surface repels Mechanism: soil particles because these are also negatively charged.

6.     BLEACHING AGENTS

Sodium perborate, Hydrogen Peroxide, Chlorine

Role: Bleaches (chlorine and oxygen) whiten and brighten fabrics and help remove stubborn stains.

Mechanism: Active oxygen bleaches work by oxidation of the stain. This means that the active component of the bleach accepts electrons from the stain, resulting in either:

·         Cleavage of chemical bonds in the stain and its breakdown, following which fragments can be suspended in solution by surfactant action.

·         A change in the oxidation state of the stain, rendering it colourless

Liquid chlorine bleach (usually in a sodium hypochlorite solution) can also disinfect and deodorize fabrics. Oxygen (color-safe) bleach is gentler and works safely on almost all washable fabrics. The active ingredient in oxygen bleach is hydrogen peroxide.

7.     OPTICAL BRIGHTENERS (Fluorescent Whitening Agent)

CBSX (Disodium distearyl biphenyl sulphonate), Tinolux, Tinopal, coumarins, and stilbenes etc.

Role: To create a whitening effect. Optical brighteners enhance the light reflected from the fabric surface and can make fabrics appear whiter and brighter, helping to keep them looking newer for longer.

Mechanism: Bluings/ Optical Brighteners contain a blue dye or pigment taken up by fabrics in the wash or rinse. Bluing absorbs the yellow part of the light spectrum, counteracting the natural yellowing of many fabrics.

Different colours exist because light can have many different wavelengths, depending on the nature of the substance that the light is being emitted or reflected from. The colour that is observed comes from a combination of all the wavelengths of light that reach the eye. Humans can only see a small fraction of all the wavelengths of light that exist, known as the visible spectrum; white is a combination of all the wavelengths in the visible spectrum. Just beyond the visible spectrum lies the ultraviolet region.

Optical brighteners attach to fabrics, absorb invisible ultraviolet light and convert it to visible blue-violet light. The blue light that is emitted interacts with the yellow light emitted by the fabric, giving an overall appearance of whiteness.

8.     Fabric softeners

Cationic surfactants such as long-chain amines and long-chain quaternary ammonium compounds.

Role: Fabric softeners, added to the final rinse or dryer, make fabrics softer and fluffier; decrease static cling, wrinkling and drying time; impart a pleasing fragrance and make ironing easier.

Mechanism: Softness, smoothness and reduced crinkling: Fabric softeners are cationic surfactants, meaning that their polar head-groups bear a positive charge. These are attracted to the negatively charged fabric surface and associate with the fibres. With the positively charged headgroup associated with the fabric, the fatty tail protrudes from the surface and imparts a feeling of softness or smoothness to the fabric. The layer of molecules on the surface may also endow the fabric with some water-proofing properties.

Reduced static: Electrical charge, or static, builds up on fabrics as the fibres rub together, especially in the clothes drier. When fabric softeners coat the fibres (as described above), they provide a lubricating coat that enables fibres to rub together without the same static build up. The coat of fabric softener molecules also conducts electricity, and so allows discharge of any static that has built up.

9.     HYDROTROPES

Short-chain aromatic sulfonates such as xylenesulfonate, cumenesulfonate, some glycol ether sulphates, and urea.

Role: Hydrotropes, or solubilisers, assist in maintaining the pouring characteristics of liquid detergents by preventing gel formation or separation into layers in the bottle.

Mechanism: Hydrotropes are similar to surfactants in that they have two domains: a polar (hydrophilic) domain and a non-polar (hydrophobic) domain. Despite these similarities they are typically smaller and less linear than surfactant molecules. They interrupt the formation of surfactant micelles in the bottle that can lead to high viscosity gel structures and insoluble phases. In this way, inclusion of hydrotropes in liquid detergent formulations help maintain a uniform composition throughout the liquid detergent and also maintain the pouring properties required for ease of use.

10.PRESERVATIVES

EDTA

Role: All organic detergent ingredients, such as the surfactants and enzymes, are biodegradable. This means that they can be broken down by bacteria in the environment. Preservatives prevent this spoilage by killing bacteria.

11.FRAGRANCES

Role: Pleasant smell. They can also enhance the mood and help create pleasant associations with ‘doing the laundry’.

12.PROCESSING AIDS

Role: To maintain the physical characteristics of laundry detergents during processing, storage and consumer use.

·         DESICCANTS:

Sodium sulphate

Role: Desiccants are able to bind multiple molecules of water, forming “hydrates”. This effectively locks up any moisture that enters the detergent, maintaining a dry, free-flowing powdered detergent. Sodium sulphate is a common example.

·         SOLVENTS:

Ethanol and propanol

Role: Solvents help dissolve the detergent ingredients in liquid laundry detergents. Some solvents that are miscible (can mix) with water are able to dissolve ingredients that water alone does not dissolve. This maintains a uniform composition throughout the liquid detergent. Alcohols are one example of solvents that have an additional effect of lowering the freezing point of liquid laundry detergents. This prevents disruption to the physical properties of the detergent that would be caused by crystal formation during cold storage. Examples are ethanol and propanol.

13.FOAM REGULATORS

Lauryl alcohol ethoxylates, soaps, siloxanes and paraffins

Role: Foam regulators either prevent the formation of foam by disrupting the surfactants at the air-water interface of the forming bubble, or cause foam bubbles to collapse by forming hydrophobic bridges across multiple bubbles.

Foam regulators inhibit the formation of suds during the washing cycle. Particularly in front-loading washing machines, mechanical energy comes from the wash items colliding with the sides of the rotating tub. High amounts of a foam cushion the collision impact and decrease the effectiveness of the wash.

 TYPICAL COMPOSITION DIFFERENT TYPES A DETERGENT POWDERS AND DETERGENTS BARS.

POWDER