In a broad sense, washing is the process of removing unwanted components from the object to be washed and achieving some purpose. Washing in the usual sense refers to the process of removing dirt from the surface of the carrier. In washing, the interaction between the dirt and the carrier is weakened or eliminated by the action of some chemical substances (e.g., detergent, etc.), so that the combination of dirt and carrier is changed into the combination of dirt and detergent, and finally the dirt is separated from the carrier. As the objects to be washed and the dirt to be removed are diverse, washing is a very complex process and the basic process of washing can be expressed in the following simple relationships.

Carrie··Dirt + Detergent= Carrier + Dirt·Detergent

The washing process can usually be divided into two stages: firstly, under the action of the detergent, the dirt is separated from its carrier; secondly, the detached dirt is dispersed and suspended in the medium. The washing process is a reversible process and the dirt dispersed and suspended in the medium may also be re-precipitated from the medium to the object being washed. Therefore, a good detergent should have the ability to disperse and suspend dirt and prevent redeposition of dirt, in addition to the ability to remove dirt from the carrier.

(1) Types of dirt

Even for the same item, the type, composition and amount of dirt can vary depending on the environment in which it is used. Oil body dirt is mainly some animal and vegetable oils and mineral oils (such as crude oil, fuel oil, coal tar, etc.), solid dirt is mainly soot, ash, rust, carbon black, etc. In terms of clothing dirt, there is dirt from the human body, such as sweat, sebum, blood, etc.; dirt from food, such as fruit stains, cooking oil stains, condiment stains, starch, etc.; dirt from cosmetics, such as lipstick, nail polish, etc.; dirt from the atmosphere, such as soot, dust, mud, etc.; others, such as ink, tea, coating, etc. It comes in various types.

The various types of dirt can usually be divided into three main categories: solid dirt, liquid dirt and special dirt.

 

① Solid dirt

Common solid dirt includes particles of ash, mud, earth, rust and carbon black. Most of these particles have an electrical charge on their surface, most of them are negatively charged and can be easily adsorbed on fiber items. Solid dirt is generally difficult to dissolve in water, but can be dispersed and suspended by detergent solutions. Solid dirt with smaller mass point is more difficult to remove.

② Liquid dirt

Liquid dirt is mostly oil-soluble, including plant and animal oils, fatty acids, fatty alcohols, mineral oils and their oxides. Among them, plant and animal oils, fatty acids and alkali saponification can occur, while fatty alcohols, mineral oils are not saponified by alkali, but can be soluble in alcohols, ethers and hydrocarbon organic solvents, and detergent water solution emulsification and dispersion. Oil-soluble liquid dirt generally has a strong force with fiber items, and is more firmly adsorbed on fibers.

③ Special dirt

Special dirt includes proteins, starch, blood, human secretions such as sweat, sebum, urine and fruit juice and tea juice. Most of this type of dirt can be chemically and strongly adsorbed on fiber items. Therefore, it is difficult to wash.

The various types of dirt are rarely found alone, but are often mixed together and adsorbed onto the object. Dirt can sometimes be oxidized, decomposed or decayed under external influences, thus creating new dirt.

（2）Adhesion of dirt

Clothes, hands etc. can be stained because there is some kind of interaction between the object and the dirt. Dirt adheres to objects in a variety of ways, but there are no more than physical and chemical adhesions.

①The adhesion of soot, dust, mud, sand and charcoal to clothing is a physical adhesion. Generally speaking, through this adhesion of dirt, and the role between the stained object is relatively weak, the removal of dirt is also relatively easy. According to the different forces, the physical adhesion of dirt can be divided into mechanical adhesion and electrostatic adhesion.

A: Mechanical adhesion

This type of adhesion mainly refers to the adhesion of some solid dirt (e.g., dust, mud and sand). Mechanical adhesion is one of the weaker forms of adhesion of dirt and can be removed almost by purely mechanical means, but when the dirt is small (<0.1um), it is more difficult to remove.

B：Electrostatic adhesion

Electrostatic adhesion is mainly manifested in the action of charged dirt particles on oppositely charged objects. Most fibrous objects are negatively charged in water and can easily be adhered to by certain positively charged dirt, such as lime types. Some dirt, although negatively charged, such as carbon black particles in aqueous solutions, can adhere to fibers through ionic bridges (ions between multiple oppositely charged objects, acting together with them in a bridge-like manner) formed by positive ions in water (e.g., Ca2+， Mg2+ etc.).

Electrostatic action is stronger than simple mechanical action, making dirt removal relatively difficult.

② Chemical adhesion

Chemical adhesion refers to the phenomenon of dirt acting on an object through chemical or hydrogen bonds. For example, polar solid dirt, protein, rust and other adhesion on fiber items, fibers contain carboxyl, hydroxyl, amide and other groups, these groups and oily dirt fatty acids, fatty alcohols are easy to form hydrogen bonds. The chemical forces are generally strong and the dirt is therefore more firmly bonded to the object. This type of dirt is difficult to remove by the usual methods and requires special methods to deal with it.

The degree of adhesion of dirt is related to the nature of the dirt itself and the nature of the object to which it is adhered. Generally, particles adhere easily to fibrous items. The smaller the texture of the solid dirt, the stronger the adhesion. Polar dirt on hydrophilic objects such as cotton and glass adhere more strongly than non-polar dirt. Non-polar dirt adheres more strongly than polar dirt, such as polar fats, dust and clay, and is less easy to remove and clean.

(3) Dirt removal mechanism

The purpose of washing is to remove dirt. In a medium of a certain temperature (mainly water). Using the various physical and chemical effects of the detergent to weaken or eliminate the effect of dirt and washed objects, under the action of certain mechanical forces (such as hand rubbing, washing machine agitation, water impact), so that the dirt and washed objects from the purpose of decontamination.

① Mechanism of liquid dirt removal

A：Wetting

Liquid soiling is mostly oil-based. Oil stains wet most fibrous items and spread more or less as an oil film on the surface of the fibrous material. The first step in the washing action is the wetting of the surface by the washing liquid. For the sake of illustration, the surface of a fiber can be thought of as a smooth solid surface.

B: Oil detachment - curling mechanism

The second step in the washing action is the removal of oil and grease, the removal of liquid dirt is achieved by a kind of coiling. The liquid dirt originally existed on the surface in the form of a spread oil film, and under the preferential wetting effect of the washing liquid on the solid surface (i.e., the fiber surface), it curled up into oil beads step by step, which were replaced by the washing liquid and eventually left the surface under certain external forces.

② Mechanism of solid dirt removal

The removal of liquid dirt is mainly through the preferential wetting of the dirt carrier by the washing solution, while the removal mechanism for solid dirt is different, where the washing process is mainly about the wetting of the dirt mass and its carrier surface by the washing solution. Due to the adsorption of surfactants on the solid dirt and its carrier surface, the interaction between the dirt and the surface is reduced and the adhesion strength of the dirt mass on the surface is reduced, thus the dirt mass is easily removed from the surface of the carrier.

In addition, the adsorption of surfactants, especially ionic surfactants, on the surface of the solid dirt and its carrier has the potential to increase the surface potential on the surface of the solid dirt and its carrier, which is more conducive to the removal of the dirt. Solid or generally fibrous surfaces are usually negatively charged in aqueous media and can therefore form diffuse double electronic layers on dirt masses or solid surfaces. Due to the repulsion of homogeneous charges, the adhesion of dirt particles in the water to the solid surface is weakened. When an anionic surfactant is added, because it can simultaneously increase the negative surface potential of the dirt particle and the solid surface, the repulsion between them is more enhanced, the adhesion strength of the particle is more reduced, and the dirt is easier to remove.

Non-ionic surfactants are adsorbed on generally charged solid surfaces and although they do not significantly change the interfacial potential, the adsorbed non-ionic surfactants tend to form a certain thickness of adsorbed layer on the surface which helps to prevent redeposition of dirt.

In the case of cationic surfactants, their adsorption reduces or eliminates the negative surface potential of the dirt mass and its carrier surface, which reduces the repulsion between the dirt and the surface and is therefore not conducive to dirt removal; furthermore, after adsorption on the solid surface, cationic surfactants tend to turn the solid surface hydrophobic and are therefore not conducive to surface wetting and therefore washing.

③ Removal of special soils

Protein, starch, human secretions, fruit juice, tea juice and other such dirt are difficult to remove with normal surfactants and require special treatment.

Protein stains such as cream, eggs, blood, milk and skin excreta tend to coagulate on the fibers and degeneration and get stronger adhesion. Protein soiling can be removed by using proteases. The enzyme protease breaks down the proteins in the dirt into water-soluble amino acids or oligopeptides.

Starch stains mainly come from foodstuffs, others such as gravy, glue etc. Amylase has a catalytic effect on the hydrolysis of starch stains, causing starch to break down into sugars.

Lipase catalyzes the decomposition of triglycerides, which are difficult to remove by normal methods, such as sebum and edible oils, and breaks them down into soluble glycerol and fatty acids.

Some colored stains from fruit juices, tea juices, inks, lipstick etc. are often difficult to clean thoroughly even after repeated washing. These stains can be removed by a redox reaction with an oxidizing or reducing agent such as bleach, which destroys the structure of the color-generating or color-auxiliary groups and degrades them into smaller water-soluble components.

（4）Stain removal mechanism of dry cleaning

The above is actually for water as the medium of washing. In fact, due to the different types of clothing and structure, some clothing using water washing is not convenient or not easy to wash clean, some clothing after washing and even deformation, fading, etc., for example: most natural fibers absorb water and easy to swell, and dry and easy to shrink, so after washing will be deformed; by washing wool products also often appear shrinkage phenomenon, some woolen products with water washing is also easy to pilling, color change; Some silks hand feeling turns worse after washing and lose their luster. For these clothes often use the dry-cleaning method to decontaminate. The so-called dry cleaning generally refers to the washing method in organic solvents, especially in non-polar solvents.

Dry cleaning is a gentler form of washing than water washing. Because dry cleaning does not require much mechanical action, it does not cause damage, wrinkling and deformation to clothing, while dry cleaning agents, unlike water, rarely produce expansion and contraction. As long as the technology is properly handled, the clothes can be dry cleaned without distortion, color fading and extended service life.

In terms of dry cleaning, there are three broad types of dirt.

①Oil-soluble dirt Oil-soluble dirt includes all kinds of oil and grease, which is liquid or greasy and can be dissolved in dry cleaning solvents.

②Water-soluble dirt Water-soluble dirt is soluble in aqueous solutions, but not in dry cleaning agents, is adsorbed on clothing in an aqueous state, water evaporates after the precipitation of granular solids, such as inorganic salts, starch, protein, etc.

③Oil and water insoluble dirt Oil and water insoluble dirt is neither soluble in water nor soluble in dry cleaning solvents, such as carbon black, silicates of various metals and oxides , etc.

Due to the different nature of various types of dirt, there are different ways of removing dirt in the dry-cleaning process. Oil-soluble soils, such as animal and vegetable oils, mineral oils and greases, are easily soluble in organic solvents and can be removed more easily in dry cleaning. The excellent solubility of dry-cleaning solvents for oils and greases essentially comes from the van der Walls forces between molecules.

For the removal of water-soluble dirt such as inorganic salts, sugars, proteins and sweat, the right amount of water must also be added to the dry-cleaning agent, otherwise water-soluble dirt is difficult to remove from the clothing. However, water is difficult to dissolve in the dry-cleaning agent, so to increase the amount of water, you also need to add surfactants. The presence of water in the dry-cleaning agent can make the surface of the dirt and clothing hydrated, so that it is easy to interact with the polar groups of surfactants, which is conducive to the adsorption of surfactants on the surface. In addition, when surfactants form micelles, water-soluble dirt and water can be solubilized into the micelles. In addition to increasing the water content of the dry-cleaning solvent, surfactants can also play a role in preventing the re-deposition of dirt to enhance the decontamination effect.

The presence of a small amount of water is necessary to remove water-soluble dirt, but too much water can cause distortion and wrinkling in some clothes, so the amount of water in the dry-cleaning agent must be moderate.

Dirt that is neither water-soluble nor oil-soluble, solid particles like ash, mud, earth and carbon black, is generally attached to the garment by electrostatic forces or in combination with oil. In dry cleaning, the flow of solvent, impact can make the electrostatic force adsorption of dirt off, and dry-cleaning agent can dissolve the oil, so that the combination of oil and dirt and attached to the clothing of solid particles off in the dry-cleaning agent, dry cleaning agent in a small amount of water and surfactants, so that those off the solid dirt particles can be stable suspension, dispersion, to prevent its re-deposition to the clothing.

（5）Factors affecting washing action

The directional adsorption of surfactants at the interface and the reduction of surface (interfacial) tension are the main factors in the removal of liquid or solid dirt. However, the washing process is complex and the washing effect, even with the same detergent type, is influenced by many other factors. These factors include the concentration of the detergent, the temperature, the nature of the soiling, the type of fiber and the structure of the fabric.

① Surfactant concentration

The micelles of surfactants in solution play an important role in the washing process. When the concentration reaches the critical micelle concentration (CMC), the washing effect increases sharply. Therefore, the concentration of detergent in the solvent should be higher than the CMC value to have a good washing effect. However, when the concentration of surfactant is higher than the CMC value, the incremental increase in washing effect is not obvious and it is not necessary to increase the concentration of surfactant too much.

When removing oil by solubilization, the solubilization effect increases with increasing surfactant concentration, even when the concentration is above CMC. At this time, it is advisable to use detergent in a local centralized manner. For example, if there is a lot of dirt on the cuffs and collar of a garment, a layer of detergent can be applied during washing to increase the solubilizing effect of the surfactant on the oil.

②Temperature has a very important influence on the decontamination action. In general, increasing the temperature facilitates the removal of dirt, but sometimes too high a temperature can also cause disadvantages.

The increase in temperature facilitates the diffusion of dirt, solid grease is easily emulsified at temperatures above its melting point and the fibers increase in swelling due to the increase in temperature, all of which facilitate the removal of dirt. However, for compact fabrics, the microgaps between the fibers are reduced as the fibers expand, which is detrimental to the removal of dirt.

Temperature changes also affect the solubility, CMC value and micelle size of surfactants, thus affecting the washing effect. The solubility of surfactants with long carbon chains is low at low temperatures and sometimes the solubility is even lower than the CMC value, so the washing temperature should be raised appropriately. The effect of temperature on the CMC value and micelle size is different for ionic and non-ionic surfactants. For ionic surfactants, an increase in temperature generally increases the CMC value and reduces the micelle size, which means that the concentration of surfactant in the washing solution should be increased. For non-ionic surfactants, an increase in temperature leads to a decrease in the CMC value and a significant increase in micelle volume, so it is clear that an appropriate increase in temperature will help the non-ionic surfactant to exert its surface-active effect. However, the temperature should not exceed its cloud point.

In short, the optimum washing temperature depends on the detergent formulation and the object being washed. Some detergents have a good detergent effect at room temperature, while others have a much different detergency between cold and hot washing.

③ Foam

It is customary to confuse foaming power with washing effect, believing that detergents with high foaming power have a good washing effect. Research has shown that there is no direct relationship between the washing effect and the amount of foam. For example, washing with low foaming detergents is no less effective than washing with high foaming detergents.

Although foam is not directly related to washing, there are occasions when it helps to remove dirt, for example, when washing dishes by hand. When scrubbing carpets, foam can also take away dust and other solid dirt particles, carpet dirt accounts for a large proportion of dust, so carpet cleaning agents should have a certain foaming ability.

Foaming power is also important for shampoos, where the fine foam produced by the liquid during shampooing or bathing leaves the hair feeling lubricated and comfortable.

④ Varieties of fibers and physical properties of textiles

In addition to the chemical structure of the fibers, which affects the adhesion and removal of dirt, the appearance of the fibers and the organization of the yarn and fabric have an influence on the ease of dirt removal.

The scales of wool fibers and the curved flat ribbons of cotton fibers are more likely to accumulate dirt than smooth fibers. For example, carbon black stained on cellulose films (viscose films) is easy to remove, while carbon black stained on cotton fabrics is difficult to wash off. Another example is that short-fiber fabrics made of polyester are more prone to accumulate oil stains than long-fiber fabrics, and oil stains on short-fiber fabrics are also more difficult to remove than oil stains on long-fiber fabrics.

Tightly twisted yarns and tight fabrics, due to the small gap between the fibers, can resist the invasion of dirt, but the same can also prevent the washing liquid to exclude the internal dirt, so tight fabrics start to resist dirt good, but once stained washing is also more difficult.

⑤ Hardness of water

The concentration of Ca2+, Mg2+ and other metal ions in the water has a great influence on the washing effect, especially when the anionic surfactants encounter Ca2+ and Mg2+ ions forming calcium and magnesium salts which are less soluble and will reduce its detergency. In hard water, even if the concentration of surfactant is high, the detergency is still much worse than in distillation. For the surfactant to have the best washing effect, the concentration of Ca2+ ions in the water should be reduced to 1 x 10-6 mol/L (CaCO3 to 0.1 mg/L) or less. This requires the addition of various softeners to the detergent.