Cleaning Your Clothes with Chemistry

In this section, we will see the laundry formulation and how the formulation can clean your clothes.
 
The soap recipe is basically an ancient technology. A soap-like material was found in clay cylinders dating to 2800 BC during excavation of Babylon. The inscription confirmed that fats were boiled with ashes, but the material was used as a hair styling aid. Then, in the Ebers Papyrus from ancient Egypt (ca 1500 BC) is the oldest known medical textbook and one of the content is a soap-like recipe. It describes combining animal and vegetable oils with alkaline salts to form a soap-like material for was washing, as well as for treating skin diseases. Moreover, in ancient Rome at Mount. Sapos which was the place for regular animal sacrifices and the fats ran off from the hill into the Tiber river with the ashes and it was found as the best place to wash clothes at the river bank.

The Ebers Papyrus

Nowadays, soaps are generally manufactured from alkaline hydrolysis of saturated (saponification) with the by-product of glycerol which is also useful.

Saponification reaction

If the alkaline is NaOH, hard soaps are formed; but if KOH is used, soft soaps are formed. Furthermore, both soaps forms 'scum' in hard water due to insoluble calcium salts. In these days soap is marketed as chips, flakes, beads, and also in powdered form.

Before we see the laundry formulation, it might be better to see the nature of the soil, dirt, or stain on our clothes. Firstly is the visible stains, which the successful of laundry can be judged on their removal. The example of visible stains is food stains (starch, egg, milk, gravy, meat), grass, blood, mud, ink, paint, etc. Secondly, is the soil that is generated by our bodies and microflora (up to 1.5 x 10⁶ bacteria / cm²). In fact, our body shed more than 1 billion skin cells, excrete around a litre of sweat, and generate tens of grams of sebum (triglycerides, fatty acids, waxy esters, cholesterol) each day. Thirdly is the residues from personal care products such as lotions, creams, deodorants, shampoos, make-up, and hair sprays. Lastly is micro-particulates such as soot, smoke, clay, sand, and dust. Moreover, the micro-particulates is formed from air pollution.

As the main objective of laundry products is to remove soil, dirt, stains from fabric while depositing dyes, fragrances, brigteners, etc. This condition must be achieved in mildly alkaline (pH 9 in USA, pH 10.5 in Europe) dilute aqueous solution within short times using cheap and safe ingredients and these are non-trivial constraints.

To remove the stains, there are six primary steps in the laundry mechanism:

1. Soil rehydration. Dirt has often dried into the fabric and its removal requires rehydration.
2. Soil removal from fabric by chemical and mechanical action
3. Soil dispersal into smaller, water-soluble fragments
4. Prevention of redeposition of removed soil or fugitive dye
5. Bleaching of residual soils
6. Substrate modification to improve consumer satisfication such as fragrances, soil release agents, optical brighteners, and fabric conditioners.

The Laundry Formulation

Surfactants

Surfactants are the most important component in formulation and it has function to lower the interfacial tension which is the important for foam formation. Moreover, consumers like seeing a lot of foam which provides confidence in the product, but in modern washing machines it clean better in absence of foam. There are three main types of synthetic surfactants:

• Anionic surfactants which generate foam by lowering the surface tension.

Common anionic surfactants: sodium dodecylsulphate (above) and sodium dodecylbenzenesulphonate (below).

 Moreover, sodium dodecylbenzenesulphonate can be synthesised as shown below.

The synthesis of anionic surfactant (sodium alkylbenzenesulfonate)

• Non-ionic surfactants maintain foam stability and it is important for hand-washing

Synperonic A7

• Cationic surfactants

Cetylpyridinium chloride

Surfactant mixtures are often found in cleaning formulations, for example 'Fairy Liquid' (P&G) contains at least six different surfactants. Moreover, anionic and non-ionic surfactants mixtures are used in most laundry formulations, and anionics dissolve faster, but non-ionics produce lower surface tensions.

The properties of surfactant solution in laundry formulation especially the anionic surfactants. The anionic surfactants exhibit a Krafft point or Krafft temperature (T_K). If the temperature is above this critical temperature (T_K), surfactant solubility rapidly increases and leading to the formation of surfactant aggregates known as micelles. In the other sides, if the temperature is below T_K, surfactant solubility is low and no micelles are present, which has an important consequence.

The phase diagram of surfactant

If the surfactants solubility is low, it means no interfacial activity, so no solubilisation of oily stains. Moreover, Krafft temperature means different surfactants are required for laundry formulations in USA and Europe since different wash temperatures. Besides that, the structure of surfactants affect the Krafft temperature, as the hydrophobic tail longer, T_K the increases. In this diagram below, it is shown the mechanism of detergency.

The cleaning mechanism of surfactant

Firstly, the hydrophobic tail of the surfactants binds with the fat or oil stain to reduce the interfacial tension between the stain and the fabric. Then, by mechanical agitation the stain will be detached from the fabric substrate to form a globular structure, but this the polar head of the surfactants can bind again on the substrate. Hence, to solve this problem, there are some adsorbed surfactants to prevent re-adsorption of fat globule.

Builders

Apart from surfactants and anti-caking agent (to ensure the detergent in powder form), builders constitute the bulk of most detergent powder formulations. There are important roles of builders in laundry such as:

• water softeners,
• a source alkalinity/buffer effect,
• dispersants,
• anti-corrosion agents,
• bleach stabilisers,
• processing aids.

Generally, builders are water softeners which are based on Si, C, and P compounds and builders present as a sodium salts of:

• Silicates (Ca²⁺ selectivity)

• Aluminosilicates

• Microporous zeolites. It softens the water by ion exchange.

Na-zeolites structure. Na⁺ is represented by purple colour in zeolites cage.

• Carbonates or carboxylates. It softens the water by precipitating Ca²⁺ and Mg²⁺ ions.

• Tripolyphospate (TPP) which does not have Ca²⁺/Mg²⁺ selectivity, but TPP is banned in EC formulation due to aquatic catastrophe.

All the builders above remove Ca²⁺ and Mg²⁺, and release Na⁺.

Bleaches

Bleaches in laundry formulations has a function to oxidise stains and occasionally dyes which involves of removal of electrons from conjugated C=C bonds, such as in carrot juice, tomato juice, and orange juice. Typically, bleaches are based on either H₂O₂ and/or peracids, and H₂O₂ is a common bleaching agent in liquid formulations. In the other sides, in detergent powder, as solid source of H₂O₂ is sodium perborate hydrate [(NaBO₂H₂O₂)₂.H₂O] and H₂O₂ is released by hydrolysis reaction.

However, H₂O₂ is not very effective at lower wash temperature (below 60°C). Hence, for lower temperature washes (e.g. in USA), the more effective peracids are used and the synthesis of peracids is shown below.

Enzymes

Enzymes are bio-catalysts, so only small amounts are required (typically levels in wash liquor are below 1 ppm). Enzymes either act on stains directly or act on the proteins, starch, or fats that cause stains to adhere to fabric and it allow certain reactions to occur at lower temperature (e.g. hydrolysis of triglycerides). Besides that, enzyme activity can be optimised at room temperature 37°C and pH 7.4 and it also depends on other factors such as water hardness, chlorine content in water supply, ionic strength, bleach and presence of surfactants.

Typically, there are 4 types of enzymes are used in laundry formulations:

• Protease. It cleaves amide bonds non selectively into smaller units such as oligopeptides and it works for blood, egg, grass, spinach, milk, meat, etc. However it has problem as it can degrade itself (also amylase, lipase, and cellulase) and attacks wool or silk.
• Amylase works to change insoluble starch (e.g. potato, pasta, rice, or chocolate) into soluble starch. It acts synergistically with protease, but it can sensitive to Ca²⁺ and high was temperature.
• Lipase catalyses hydrolysis of fatty acid esters, so it remove oily stains (e.g. butter, gravy, oil, lipstick) at lower temperature.
• Cellulase degrades cellulose (cotton), so it aids removal of adsorbed soot, clay by degrading cellulose fibrils that adhere to these particulates. Moreover, it also reduces microfibril formation (minimises optical dullness).

Fluorencent Whitening Agents (FWA) or Optical Brighteners

An FWA enhances the white appearance of fabrics by absorbing UV light and re-emitting this in blue region of visible spectrum and this compensate for the yellow tinge often acquired by aged fabrics.

The mechanism of FWA (above) and common FWA compound (below)

Anti-Redeposition (ARD) polymer

An ARD polymers adsorb onto the surface of the desorbed particulates to prevent their redeposition onto the fabric and the mechanism are shown below.

The redeposition mechanism ARD polymer

There are two examples of ARD polymers that is commonly used in laundry formulation, sodium carboxymethyl cellulose and polyethoxylated poly(ethylene imine). Polyethoxylated poly(ethylene imine) can work well for polyester substrates via strong adsorption onto soil particles (e.g clay) in solution. In the other sides, sodium carboxymethyl cellulose works well for cotton, but by different mechanism that adsorbs onto the substrate.

However, it is very difficult to find an ARD polymer that works well for all substrate (in the worst case scenario is the deposition is promoted). For example, polyethoxylated poly(ethylene imine) adsorbs onto both clay and cotton. Moreover, 10-20 ppm or ARD polymer are required in the wash.

Anti-Dye Transfer (ADT) Polymers

ADT polymers has a function to minimise the 'one rogue red sock = pink washing' effect. This effect is basically when the red dye from the sock is released during the washing and adsorbs onto the other washing to give the pink colour. Generally, fabric dyes have very diverse structures, but it can be classified into 2 main groups:

Reactive red 198

• Reactive dyes. Covalently bonded; excess dye is only released during first wash.
• Direct dyes. More easily released.

There are 3 common ADT polymers that is used in laundry formulation:

Common ADT polymers

• Poly(N-vinyl pyrrolidone) or PNVP. It is cheap and widely used for more than 40 years
• Poly(N-vinyl pyrrolidone-stat-N-vinylimidazole) or PNVP-stat-NVIMZ
• Poly(4-vinylpyridine-N-oxide) or P4VP-N-Ox.

PNVP-stat-NVIMZ and P4VP-N-Ox are improved anti-dye transfer properties (stronger dye binding) but more expensive.

ADT agents form physical complexes with a wide range of dyes in aqueous solution and it prevents released dyes from re-depositing onto white fabrics. Moreover, the efficiency of ADT depends on:

• dye chemistry,
• concentration of dye and polymer,
• polymer-dye binding stength,
• soiling levels (it makes ADT less effective in heavily soiled washes).

Repel-O-Tex PF

At the washing process, there is also a problem when soil release requires deposition of a suitable polymer into fabric but laundry formulations are optimised to prevent adsorption from the wash. Moreover, 'polyester' fabrics are actually comprise polye(ethylene terephthalate) or PET.

Poly(ethylene terephthalate or PET

Then, to address this problem a polymer called Repel-O-Tex PF which is a bifunctional multi-block copolymer that contains both adsorbing blocks and soil release blocks and it mimicks the structure of PET.

Repel-O-Tex Polyester Fiber

Hence, the soil release mechanism for polyester fabrics is:

However, the mechanism of Repel-O-Tex PF adsorbs onto the polyester fabrics is unknown.

Fabric Conditioners

The consumer perceptions about conditioner is to confer pleasant smell and get softer, less clingy clothes. Then, typical formulation comprises lamellat mesophases of cationic surfactants. The structure of the conditioners is a quaternary amine with long n-alkyl chain and two-tail nature of surfactant dictates mesophase structure.

The general structure of fabric conditioner

Cationic surfactant adsorbs as a (or partial) bilayer onto anionic fabric substrate as shown in figure below to form a lubricious, anti-static cationic surfactant bilayer.

The mesophase structure

Fabric conditioner formulations typically contain fragrance and also sometimes PDMS (as the lubricant) in the form of emulsion droplets. Moreover, fabric conditioners are formulated at low pH to avoid Hofmann elimination.

To ensure fabric conditioners works effectively, all anionic surfactants must be removed from the wash by a rinse cycle before all cationic fabric conditioner is added, other wise it would form insoluble surfactant complexes.

Most of the consumers prefers the fragrance are deposited onto the clothes which confers the confidence in product. However, there is technical problem which is difficult to deposit fragrance in presence of lots of surfactants and typically more than 90% of (expensive) fragrance is lost during wash. This matter can be solved by delivering the fragrance within microcapsules as a part of fabric conditioner formulation.

Microcapsule fragrance synthesis

The microcapsule can be made from the fragrance as oil droplets into melamine and formaldehyde in interfacial copolymerisation. Therefore, the fragrance-loaded microcapsule can be made with dimension of 30 μm in diameter with 1 μm thick of copolymer wall. During the washing, microcapsules are caught by garment 'net', and then during the consumer acitivity, it is activated the fragrance release due to the copolymer walls are easily to be ruptured.

Fragrance release mechanism

Future Challenges for Laundry Science 

As shown in table above, the conditions of laundry habits in different region is different with another. Besides that, the broad array of fabrics used by consumers as well. Hence, optimising laundry formulation for global markets is complicated.

To sum up, there are future challenges for laundry science excluding different conditions as we have seen above. The challenges are:

• Substrate. New textile finishes, novel fibres, etc.
• Value-added benefits: anti-wrinkle agents, improved soil released polymers. increased perfume retention, antibacterial agents, etc.
• Lower wash temperatures due to energy conservation
• Stressed water conditions. In developing nations, many people still use dirty water to wash the clothes.