Basic theoretical knowledge of surfactants

1.Surface tension

The contraction force of any unit length of the liquid surface is called surface tension, and the unit is N•m-1.
2.Surfactants and surfactants

The properties that can reduce the surface tension of solvents are called surfactants, and substances with surfactants are called surfactants. Surfactants that can associate molecules in aqueous solutions and form micelles and other associatives, and have high surface activity, as well as wetting, emulsifying, foaming, washing and other effects, are called surfactants.

3.Molecular structure characteristics of surfactants

Surfactants are organic compounds with special structure and properties, which can significantly change the interfacial tension between the two phases or the surface tension of liquids (generally water), and have wetting, foaming, emulsifying, washing and other properties. In terms of structure, surfactants have a common feature, that is, their molecules contain two groups with different properties, one end is a long chain non-polar group, which can be soluble in oil but not in water, that is, the so-called hydrophobic group or hydrophobic group, this hydrophobic group is generally a long chain of hydrocarbons, sometimes also organic fluorine, silicone, organic phosphorus, organic tin chain, etc. At the other end is a water-soluble group, that is, a hydrophilic group or a hydrophilic group. The hydrophilic group must be hydrophilic enough to ensure that the entire surfactant is soluble in water and has the necessary solubility. Since surfactants contain hydrophilic and hydrophobic groups, they are soluble in at least one phase in the liquid phase. This hydrophilic and lipophilic property of surfactants is called amphiphilicity.

4.Types of surfactants

A surfactant is an amphiphilic molecule with both hydrophobic and hydrophilic groups. The hydrophobic groups of surfactants are generally composed of long chains of hydrocarbons, such as linear alkyl C8~C20, branched-chain alkyl C8~C20, alkyl phenyl (alkyl carbon atom number is 8~16), etc. The difference between hydrophobic groups is mainly in the structural changes of hydrocarbon chains, which are smaller, while there are more types of hydrophilic groups, so the properties of surfactants are not only related to the size and shape of hydrophobic groups, but also mainly related to hydrophilic groups. The structure of hydrophilic groups changes more than that of hydrophobic groups, so the classification of surfactants is generally based on the structure of hydrophilic groups. This classification is based on whether the hydrophilic group is ionic or not, and it is divided into anionic type, cationic type, nonionic type, zwitterionic type and other special types of surfactants.

5.Characteristics of aqueous surfactant solution

(1) Adsorption of surfactant at the interface

Surfactant molecules have lipophilic and hydrophilic groups, which are amphiphilic molecules. Water is a highly polar liquid, when the surfactant is dissolved in water, according to the principle of polar similarity attraction and polar phase repulsion, its hydrophilic group and water are attracted to each other and dissolved in water, and its lipophilic group and water repel each other and leave the water, resulting in surfactant molecules (or ions) adsorbed on the interface between the two phases, so that the interfacial tension between the two phases is reduced. The more surfactant molecules (or ions) are adsorbed at the interface, the greater the interfacial tension decreases.
(2) Some properties of the adsorption film
Surface pressure of the adsorption film: The surfactant is adsorbed at the gas-liquid interface to form an adsorption film, such as placing a frictionless movable float on the interface, and pushing the adsorption film along the solution surface with the float, the membrane generates a pressure on the float, which is called surface pressure.
Surface viscosity: Like surface pressure, surface viscosity is a property exhibited by insoluble molecular films. Suspend a platinum ring with a thin metal wire, so that its plane touches the water surface of the sink, rotate the platinum ring, the platinum ring is hindered by the viscosity of the water, the amplitude gradually attenuates, according to which the surface viscosity can be measured, the method is: first conduct an experiment on the surface of pure water, measure the amplitude attenuation, and then measure the attenuation after the formation of the surface film, and find the viscosity of the surface film from the difference between the two.
Surface viscosity is closely related to the firmness of the surface film, because the adsorption film has surface pressure and viscosity, it must be elastic. The greater the surface pressure and viscosity of the adsorption film, the greater its elastic modulus. The elastic modulus of the surface adsorption film is of great significance in the process of foam stabilization.
(3) micelle formation
The dilute solution of the surfactant obeys the laws followed by the ideal solution. The adsorption capacity of surfactant on the surface of the solution increases with the increase of solution concentration, and when the concentration reaches or exceeds a certain value, the adsorption capacity does not increase, and these excessive surfactant molecules are disordered in the solution, or exist in a regular way. Both practice and theory show that they form associations within a solution, which are called micelles.
Critical micelle concentration: The lowest concentration at which surfactants form micelles in solution is called critical micelle concentration.
(4) CMC value of common surfactants (omitted).

6.Hydrophilic and lipophilic balance value

HLB is the abbreviation of hydrophile lipophile balance, which represents the hydrophilic lipophile balance value of the surfactant, that is, the surfactant HLB value. The large HLB value indicates that the molecule has strong hydrophilicity and weak lipophilicity; On the contrary, lipophilicity is strong and hydrophilic is weak.
(1) Provisions of HLB value
The HLB value is a relative value, so when formulating the HLB value, as a standard, the HLB value of paraffin without hydrophilic properties is 0, and the HLB value of sodium lauryl sulfate with strong water solubility is 40. Therefore, the HLB value of surfactants is generally within the range of 1~40. Generally speaking, emulsifiers with an HLB value of less than 10 are lipophilic, while emulsifiers greater than 10 are hydrophilic. Therefore, the turning point from lipophilic to hydrophilic is about 10.
Based on the HLB value of surfactants, their possible uses can be roughly understood, as shown in Table 1-3.
Table 1-3 HLB range and its application performance

HLB value	Use	HLB value	Use
1.5-3	W/O type defoamer	8-18	O/W type emulsifier
3.5-6	W/0 type emulsifier	13-15	Detergent
7-9	Wetting agent	15-18	Solubilizer

From the above table, it can be seen that the HLB value of surfactant suitable for water-in-oil emulsifier is 3.5~6, while the HLB value of oil-in-water emulsifier is 8~18.

(2) Determination of HLB value (omitted).

7.Emulsification and solubilization

Two insoluble liquids, one formed by particles (droplets or liquid crystals) dispersed in the other, is called emulsion. When forming the emulsion, due to the increase of the boundary area between the two liquids, this system is thermodynamically unstable, and in order to stabilize the emulsion, a third component – emulsifier needs to be added to reduce the interface energy of the system. Emulsifiers belong to surfactants, and their main function is to emulsify. The phase in the emulsion where droplets exist is called the dispersed phase (or inner phase, discontinuous phase), and the other phase connected into a piece is called the dispersion medium (or outer phase, continuous phase).
(1) Emulsifiers and emulsions
Common emulsions, one phase is water or aqueous solution, and the other phase is immiscible organic matter with water, such as grease, wax, etc. The emulsion formed by water and oil can be divided into two types according to its dispersion: oil dispersed in water to form oil-in-water emulsion, expressed as O/W (oil/water): water-in-oil emulsion formed in water dispersed oil, expressed as W/O (water/oil). In addition, complex water-in-oil-in-water W/O/W types and oil-in-oil O/W/O types may also be formed.
Emulsifiers stabilize emulsions by reducing interfacial tension and forming monomolecular interface films.
Requirements for emulsifiers in emulsification: a: Emulsifiers must be able to adsorb or enrich at the interface of the two phases, so that the interfacial tension is reduced; b: The emulsifier must impart an electric charge to the particles, causing electrostatic repulsion between the particles, or forming a stable, particularly viscous protective film around the particles. Therefore, the substance used as an emulsifier must have amphiphiles to play an emulsifying role, and surfactants can meet this requirement.
(2) Preparation method of emulsion and factors affecting the stability of emulsion
There are two ways to prepare emulsion: one is to use mechanical method to disperse the liquid in tiny particles in another liquid, which is mostly used in industry to prepare emulsion; The other is to dissolve a liquid in another liquid in a molecular state, and then let it aggregate properly to form an emulsion.
The stability of an emulsion refers to the ability to resist the aggregation of particles that lead to phase separation. Emulsions are thermodynamically unstable systems with large free energy. Therefore, the stability of the emulsion actually refers to the time it takes for the system to reach equilibrium, that is, the time it takes for a liquid in the system to separate.
When there are polar organic molecules such as fatty alcohols, fatty acids and fatty amines in the interface membrane, the membrane strength increases significantly. This is because the emulsifier molecules interact with polar molecules such as alcohols, acids and amines in the interfacial adsorption layer to form “complexes”, which increases the strength of the interfacial film.
Emulsifiers composed of more than two surfactants are called mixed emulsifiers. The mixed emulsifier is adsorbed at the water/oil interface, and intermolecular interaction can form a complex. Due to the strong intermolecular action, the interfacial tension is significantly reduced, the adsorption capacity of the emulsifier at the interface is significantly increased, and the density and strength of the interfacial film formed are increased.
The charge of the liquid beads has a significant impact on the stability of the emulsion. Stable emulsions, the beads of which are generally charged. When using ionic emulsifiers, the lipophilic group of the emulsifier ions adsorbed at the interface is inserted into the oil phase, and the hydrophilic group is in the aqueous phase, so that the liquid beads are charged. Because the beads of the emulsion have the same charge, they repel each other and are not easy to coalesce, so that the stability is increased. It can be seen that the more emulsifier ions adsorbed on the beads, the greater its charge, the greater the ability to prevent the coalescence of the beads, and the more stable the emulsion liquid system.
The viscosity of the emulsion dispersion medium has a certain impact on the stability of the emulsion. Generally, the greater the viscosity of the dispersion medium, the higher the stability of the emulsion. This is because the viscosity of the dispersion medium is large, which has a strong hindrance effect on the Brownian motion of the liquid beads, which slows down the collision between the liquid beads and keeps the system stable. Polymer substances that can usually be soluble in emulsions can increase the viscosity of the system and increase the stability of the emulsion. In addition, polymers can also form a strong interface film, making the emulsion liquid system more stable.
In some cases, adding solid powder does not stabilize the emulsion. Solid powder is not in water, oil or at the interface, depending on the oil, the water has not wetted the solid powder, if the solid powder is not completely wetted by water, and can be wetted by oil, it will remain on the water-oil interface.
The reason why solid powder does not stabilize the emulsion is that the powder aggregated at the interface does not strengthen the interface film, which is similar to the interface adsorption emulsifier molecules, so the tighter the solid powder material is arranged on the interface, the more stable the emulsion.
After the formation of micelles in aqueous solution, surfactants have the ability to significantly increase the solubility of insoluble or slightly soluble organic matter in water, and the solution is transparent at this time, and this effect of micelles is called solubilization. Surfactants that can produce solubilization are called solubilizers, and solubilized organic matter is called solubilizers.

8.Foam
Foam plays an important role in the washing process. Foam refers to the dispersion system in which the gas is dispersed in a liquid or solid, the gas is the dispersed phase, and the liquid or solid is the dispersion medium, the former is called liquid foam, and the latter is called solid foam, such as foam plastic, foam glass, foam cement, etc.

(1) Formation of bubbles
By foam we mean an aggregate of bubbles separated by a liquid film. Due to the large difference in density between the dispersed phase (gas) and the dispersion medium (liquid), and the low viscosity of the liquid, the bubbles always rise to the liquid surface quickly.
The process of foam formation is to bring a large amount of gas into the liquid, and the bubbles in the liquid quickly return to the liquid surface, forming an aggregate of bubbles separated by a small amount of liquid gas
Foam has two significant characteristics in morphology: one is that the bubble as a dispersed phase is often polyhedral shape, because at the intersection of bubbles, there is a tendency of the liquid film to thin the bubble to become a polyhedron, when the liquid film becomes thinner, it will cause the bubble to burst; Second, the pure liquid cannot form a stable foam, and the liquid that can form foam is at least two or more components. The aqueous solution of surfactant is a typical system that is prone to foaming, and its ability to form foam is also related to other properties.
Surfactants with good foaming power are called foaming agents. Although the foaming agent has good foaming ability, the foam formed may not necessarily last for a long time, that is, its stability is not necessarily good. In order to maintain the stability of the foam, substances that can increase the stability of the foam are often added to the foaming agent, which is called a foam stabilizer, and the commonly used foam stabilizers are lauroyl diethanolamine and lauryl dimethylamine oxides.

(2) Stability of foam
Foam is a thermodynamically unstable system, and the final trend is that the total surface area of the liquid in the system decreases and the free energy decreases after the bubble bursts. The defoaming process is the process of the liquid film separating the gas from thick to thin until it breaks. Therefore, the stability of the foam is mainly determined by the speed of discharge and the strength of the liquid film. There are also the following influencing factors.
(1) Surface tension
From an energy point of view, low surface tension is more beneficial for foam formation, but it does not guarantee foam stability. The surface tension is low, the pressure difference is small, the liquid discharge speed is slow, and the liquid film is thinned slowly, which is conducive to the stability of the foam.
(2) Surface viscosity
The key factor that determines the stability of the foam is the strength of the liquid film, which is mainly determined by the firmness of the surface adsorption film, which is measured by the surface viscosity. Experiments have proved that the foam life generated by solutions with higher surface viscosity is longer. This is because the interaction between surface adsorption molecules leads to an increase in film strength, which increases the life of the foam.
(3) Solution viscosity
When the viscosity of the liquid itself increases, the liquid in the liquid film is not easy to discharge, and the thickness of the liquid film becomes thinner more slowly, which delays the rupture time of the liquid film and increases the stability of the foam.
(4) The “repair” effect of surface tension
Surfactants adsorb on the surface of the liquid film and have the ability to resist the expansion or contraction of the liquid film surface, which we call repair. This is because the liquid film adsorbed by surfactant on the surface expands its surface area, which will reduce the concentration of adsorbed molecules on the surface and increase the surface tension. Further enlarging the surface will require more work. On the contrary, surface area shrinkage will increase the concentration of surface adsorption molecules, that is, reduce the surface tension, which is not conducive to further shrinkage.
(5) Diffusion of gas through liquid film
Due to the existence of capillary pressure, the pressure of the small bubble in the foam is higher than that of the large bubble, which will cause the gas in the vesicle to diffuse into the low-pressure bubble through the liquid film, causing the bubble to become smaller, the large bubble to become larger, and finally the bubble to burst. If surfactants are added, the foam can be made uniform and fine during foaming, and it is not easy to defoam. Because the surfactant is tightly arranged on the liquid film, it is difficult to breathe, making the foam more stable.
(6) The influence of surface charge
If the liquid foam film has the same symbol of charge, the two surfaces of the liquid film will repel each other, preventing the liquid film from becoming thinner or even destroyed. Ionic surfactants can play this stabilizing role.
In conclusion, liquid film strength is a key factor in determining foam stability. As a foaming agent and foam stabilizer, the tightness and firmness of the surface adsorption molecule arrangement are the most important factors. When the surface adsorption molecules interact strongly, the adsorption molecules are arranged tightly, which not only makes the surface film itself have high strength, but also makes the solution adjacent to the surface film difficult to flow due to the high surface viscosity, and the liquid film is relatively difficult to drain, and the thickness of the liquid film is easy to maintain. In addition, tightly packed surface molecules reduce the permeability of gas molecules and thus increase the stability of the foam.

(3) Bubble destruction
The basic principle of destroying foam is to change the conditions for foam formation or eliminate the stabilizing factors of foam, so there are two methods of defoaming: physical and chemical.
Physical defoaming is to change the conditions of foam production while maintaining the chemical composition of the foam solution, such as external force disturbance, temperature or pressure change, and sonication are all effective physical methods to eliminate foam. The chemical defoaming method is to add certain substances to interact with the foaming agent to reduce the strength of the liquid film in the foam and then reduce the stability of the foam to achieve the purpose of defoaming. Most of the defoamers are surfactants. Therefore, according to the mechanism of defoaming, the defoamer should have a strong ability to reduce surface tension, and it is easy to adsorb on the surface, and the interaction between surface adsorption molecules is weak, and the arrangement structure of adsorption molecules is relatively loose.
There are various types of defoamers, but they are basically nonionic surfactants. Nonionic surfactants have anti-foam properties near or above their turbidity point and are often used as defoamers. Alcohols, especially alcohols with branched structures, fatty acids and fatty acid esters, polyamides, phosphates, silicone oils, etc., are also commonly used excellent defoamers.
(4) Foam and washing
There is no direct connection between foam and washing, and the amount of foam does not indicate the quality of the washing effect. For example, nonionic surfactants have much less foaming properties than soap, but their stain removal power is much better than soap.
In some cases, foam can still be helpful in removing dirt. For example, when washing tableware at home, the foam of the washing liquid can take away the oil droplets that have been washed down; When scrubbing carpets, foam helps to remove dirt and powder solid dirt. In addition, foam can sometimes be used as a sign of whether the detergent is effective, because fatty oil stains have an inhibitory effect on the foam of the detergent liquid, when there is too much oil and less washing dose, there will be no foam formation, or the original foam will disappear. Foam can sometimes be used as an indicator of whether the rinse is clean, because the amount of foam in the rinse solution tends to decrease with the decrease of detergent content, so the degree of rinsing can be evaluated by the amount of foam.

9.Washing process

Broadly speaking, washing is the process of removing unwanted ingredients from the object being washed and achieving a certain purpose. Washing in the usual sense refers to the process of removing dirt from the surface of the carrier. In washing, through the action of some chemicals (such as detergents, etc.) to weaken or eliminate the interaction between dirt and carrier, the combination of dirt and carrier is transformed into the combination of dirt and detergent, and finally the dirt and carrier are separated, because the object to be washed and the dirt to be removed are diverse, so washing is a very complex process, and the basic process of washing can be expressed by the following simple relationship
Carrier • Dirt + Detergent = Carrier + Dirt • Detergent
The washing process can usually be divided into two stages: one is that the dirt is separated from its carrier under the action of detergent; Second, the detached dirt is dispersed and suspended in the medium. The washing process is a reversible process, dispersed, and dirt suspended in the medium may also resettle from the medium onto the washed object. Therefore, in addition to the ability to free dirt from the carrier, an excellent detergent should also have a good ability to disperse and suspend dirt to prevent dirt from being re-deposited.
（1）. Types of dirt
Even if it is the same item, if the environment in which it is used, the type, composition and amount of dirt will be different. Oil fouling is mainly some animal, vegetable oil and mineral oil (such as crude oil, fuel oil, coal tar, etc.), and solid dirt is mainly soot, gray soil, rust, carbon black, etc. As far as dirt on clothes is concerned, there are dirt from the human body, such as sweat, sebum, blood, etc.; Dirt from food, such as fruit stains, edible oil stains, condiment stains, starch, etc.; There is dirt brought by cosmetics, such as lipstick, nail polish, etc.; dirt from the atmosphere, such as smoke, dust, dirt, etc.; Others such as ink, tea, paint, etc. It can be said that there are many kinds and varieties.
All kinds of dirt can usually be divided into three categories: solid dirt, liquid dirt and special dirt.
①Solid dirt 
Common solid fouling includes particles such as ash, mud, earth, rust, and carbon black. Most of these particles are charged on the surface, most of which are negatively charged and easy to adsorb on fiber items. Generally, solid dirt is difficult to dissolve in water, but it can be dispersed and suspended by detergent solutions. Solid dirt with small particles is more difficult to remove.
② Liquid dirt 
Most of the liquid dirt is oil-soluble, including animal and vegetable oils, fatty acids, fatty alcohols, mineral oils and their oxides. Among them, animal and vegetable oils, fatty acids can be saponified with alkalis, while fatty alcohols and mineral oils are not saponified by alkalis, but can be soluble in alcohols, ethers and hydrocarbon organic solvents, and are emulsified and dispersed by detergent aqueous solutions. Oil-soluble liquid dirt generally has a strong force on fiber items, and is more firmly adsorbed on fibers.
③ Special dirt 
Special dirt includes protein, starch, blood, human secretions such as sweat, sebum, urine, juice, tea, etc. Most of this kind of dirt can be strongly adsorbed on fiber items through chemical action. Therefore, it is more difficult to wash.
Dirt rarely exists alone, often mixes together, and adheres to items together. Dirt can also sometimes oxidize, decompose or decay under external influences, resulting in new dirt.

（2）.Adhesion of dirt

The reason why clothes, hands, etc. can get dirt is because there is some kind of interaction between objects and dirt. The adhesion of dirt to objects is diverse, but it is nothing more than physical adhesion and chemical adhesion.
① Adhesion of soot, dust, sediment, carbon black and other clothes is physical adhesion
Generally speaking, the effect between the dirt through this adherence and the contaminated object is relatively weak, and the dirt is easier to remove. According to the different forces, the physical adhesion of dirt can be divided into mechanical adhesion and electrostatic force adhesion.
A: Mechanical adhesion This type of adhesion mainly refers to the adhesion of some solid dirt (such as dust and sediment). Mechanical adhesion is a relatively weak adhesion method of dirt, which can almost be removed by simple mechanical methods, but when the texture of dirt is relatively small (<0.1um), it is more difficult to remove.
B: Electrostatic force adhesion Electrostatic force adhesion is mainly manifested in the action of charged dirt particles on the opposite electric body. Most fibrous items are negatively charged in water and can easily be adhered to by certain positively charged dirt, such as lime. Although some dirt has a negative charge, such as carbon black particles in aqueous solution, it can be attached to the fibers through ion bridges formed by positive ions (such as Ca2+, Mg2+, etc.) in water (such as Ca2+, Mg2+, etc.).
The static effect is stronger than the simple mechanical effect, so it is relatively difficult to remove dirt.
②Chemical adhesion
Chemical adhesion refers to the phenomenon in which dirt acts on objects through chemical or hydrogen bonds. For example, polar solid dirt, proteins, rust, etc. adhere to fiber items, fibers contain carboxyl groups, hydroxyl groups, amide and other groups, these groups and fatty acids of oily dirt, fatty alcohols are easy to form hydrogen bonds. The chemical force is generally strong, so the dirt is more firmly bound to the object. This type of dirt is difficult to remove by ordinary methods and requires special methods to deal with it.
The firmness of dirt adhesion is related to the nature of the dirt itself and the nature of the adherent. Generally, particles are easy to adhere to fibrous items. The smaller the solid dirt particle, the stronger the adhesion. Polar dirt on surfaces such as cotton and glass adheres more firmly than non-polar dirt. The adhesion strength of non-polar dirt is greater than that of polar dirt such as polar fat, dust, clay, etc., and it is less easy to remove and clean.

(3) Dirt removal mechanism
The purpose of washing is to remove dirt. In a medium with a certain temperature (mainly water). Use various physical and chemical reactions generated by detergents to weaken or eliminate the effect of dirt and washed items, and under a certain mechanical force (such as hand rubbing, stirring of washing machines, water impact), separating dirt from washed items to achieve the purpose of decontamination.
① Removal mechanism of liquid dirt
A: Most of the wetting liquid dirt is oil-based. Oil stains can wet most of the fiber items and spread into an oil film on the surface of the fiber material. The first step in the washing action is to moisten the surface with the washing solution. For ease of illustration, the surface of the fiber can be regarded as a smooth solid surface.
B: Detachment of oil stains – shrinkage mechanism The second step of the washing action is the removal of oil stains, and the removal of liquid dirt is achieved by a curling method. 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 rolled up into oil beads step by step, which was replaced by the washing liquid and finally left the surface under a certain external force.
② Removal mechanism of solid dirt
The removal of liquid dirt is mainly through the preferential wetting of the dirt carrier by the washing solution, while the removal mechanism of solid dirt is different. Due to the adsorption of surfactants on the surface of solid dirt and its carriers, the interaction between dirt and the surface is reduced, and the adhesion strength of dirt textures on the surface is reduced, so the dirt textures are easily removed from the carrier surface.
Not only that, the adsorption of surfactants, especially ionic surfactants, on the surface of solid dirt and its carriers may increase the surface potential of solid dirt and its carrier surfaces, which is more conducive to the removal of dirt. Solid or general fiber surfaces are usually negatively charged in aqueous media, so diffuse double layers can be formed on dirt particles or solid surfaces. Due to the mutual repulsion of the same charge, the adhesion strength of the dirt particles in the water on the solid surface will be weakened. When anionic surfactant is added, because anionic surfactant can simultaneously increase the negative surface potential of the dirt particle and the solid surface, so that the repulsion force between them is more enhanced, so the adhesion strength of the particles is further reduced, and the dirt is easier to remove.
Although the interfacial potential cannot be significantly changed, the adsorbed nonionic surfactant often forms a certain thickness of adsorption layer on the surface, which helps to prevent the re-deposition of dirt.
For cationic surfactants, because their adsorption will reduce or eliminate the negative surface potential of the dirt particles and their carrier surfaces, which reduces the repulsion between the dirt and the surface, which is not conducive to the removal of dirt. In addition, after the adsorption of cationic surfactants on the solid surface, they often turn the solid surface into hydrophobic, which is not conducive to surface wetting and washing.
③ Removal of special dirt Proteins, starches, human secretions, juices, tea juices and other dirt are difficult to remove with ordinary surfactants and require special treatment methods.
Protein dirt such as cream, eggs, blood, milk, skin excrement and other proteins are easy to coagulate and denature on the fibers, and the adhesion is relatively strong. For protein fouling, it can be removed using proteases. The proteases in it break down the proteins in the dirt into water-soluble amino acids or oligopeptides.
Starch stains mainly come from food, others such as gravy, paste, etc., amylase has a catalytic effect on the hydrolysis of starch dirt, so that starch is decomposed into sugars.
Lipase can catalyze the decomposition of some trifatty acid glycerides that are difficult to remove by ordinary methods, such as sebum secreted by the human body, edible oil, etc., so that trifatty acid glycerides can be decomposed into soluble glycerol and fatty acids.
Some colored stains from juice, tea, ink, lipstick, etc., are often difficult to wash thoroughly even after repeated washing. Such stains can be removed by some oxidizing agents or reducing agents such as bleaching powder, which destroy the structure of the chromogenic or color-aiding groups and degrade them into smaller water-soluble components.

(4) Dirt removal mechanism of dry cleaning

The above is actually for the washing effect of water as a medium. In fact, due to the different types and structures of clothes, some clothes are inconvenient or not easy to wash clean, and some clothes are even deformed and faded after washing, for example: most natural fibers absorb water and are easy to swell, and after drying, they are easy to shrink, so they will be deformed after washing; Washed wool products also often shrink, and some woolen products are easy to pill and lose color after washing. Some silks feel worse after washing with water, lose their luster, etc. These clothes are often cleaned and stained. The so-called dry cleaning generally refers to the washing method in organic solvents, especially non-polar solvents.
Compared with water washing, dry cleaning is a gentler washing method. Because dry cleaning does not require much mechanical action, it will not cause damage, wrinkle and deformation to the clothes, and dry cleaning agents rarely cause expansion and contraction like water. As long as the technology is properly handled, the clothes can achieve excellent effects such as no deformation, no fading and longer service life after dry cleaning.
From the perspective of dry cleaning, there are roughly three types of dirt.
①Oil-soluble dirt
Oil-soluble dirt includes various oils and greases, which are liquid or greasy and soluble in dry cleaning solvents.
②Water-soluble dirt
Water-soluble dirt is soluble in aqueous solution, but insoluble in dry cleaning agents. It is adsorbed on clothes in the state of aqueous solution. After the water evaporates, granular solids, such as inorganic salts, starch, and proteins, are precipitated.
③Oil-water-insoluble dirt
Oil-water-insoluble dirt is neither soluble in water nor dry cleaning solvents, such as carbon black, silicates and oxides of various metals, etc.

Due to the different properties of various dirt, there are different ways of removing dirt during dry cleaning. Oil-soluble dirt, such as animal and vegetable oils, mineral oils and greases, is easily soluble in organic solvents and is easier to remove in dry cleaning. The excellent solubility of dry cleaning solvents on oils and greases essentially comes from the van der Waals forces between molecules. For the removal of water-soluble dirt such as inorganic salts, sugars, proteins, sweat, etc., an appropriate amount of water must also be added to the dry cleaning agent, otherwise the water-soluble dirt is difficult to remove from the clothes. However, water is difficult to dissolve in dry cleaning agents, so surfactants need to be added to increase the amount of water. The water present in the dry cleaning agent can hydrate the surface of dirt and clothing, so that it is easy to interact with the polar groups of the surfactant, which is conducive to the adsorption of the surfactant on the surface. In addition, water-soluble dirt and water can be dissolved in micelles when surfactants form micelles. In addition to increasing the content of water in 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 excess water will cause some clothes to deform, wrinkle, etc., so the amount of water in dry cleaning agents must be moderate. Solid particles such as ash, mud, soil and carbon black that are neither water-soluble nor oil-soluble are generally adsorbed by electrostatic forces or combined with oil stains and attach to clothes. In dry cleaning, the flow and impact of solvent can make the dirt adsorbed by electrostatic force fall off, and the dry cleaning agent can dissolve the oil stains, so that the solid particles combined with the oil stains and attached to the clothes fall off in the dry cleaning agent, and the small amount of water and surfactant in the dry cleaning agent can make those solid dirt particles that fall off can be stably suspended and dispersed to prevent them from being deposited on the clothes again.

(5) Factors affecting washing effect
The directional adsorption of surfactants on the interface and the reduction of surface (interfacial) tension are the main factors in the removal of liquid or solid fouling. But the washing process is more complicated, and even the washing effect of the same type of detergent is affected by many other factors. These factors include the concentration of detergent, temperature, nature of dirt, fiber type, fabric structure, etc.
①Concentration of surfactant
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 achieve good washing effect. However, when the concentration of surfactant is higher than the cmc value, the incremental cleaning effect is not obvious. It is not necessary to increase the concentration of surfactant too much.
When removing oil stains by means of solubilization, even if the concentration is above the cmc value, the solubilization still increases with the increase of surfactant concentration. At this time, it is advisable to use detergent in local areas. For example, there is a lot of dirt on the cuffs and collars of clothes. When washing, you can first apply a layer of detergent to improve the solubilizing effect of surfactant on oil stains.
②Temperature has a very important impact on decontamination
Generally speaking, increasing the temperature is beneficial to the removal of dirt, but sometimes too high a temperature can also cause adverse factors. The increase in temperature is conducive to the diffusion of dirt. Solid oil dirt is easily emulsified when the temperature is higher than its melting point. The fiber also increases the degree of puffing due to the increase in temperature. These factors are conducive to the removal of dirt. But for tight fabrics, the micro-gaps between fibers are reduced after the fibers are puffed, which is detrimental to the removal of dirt. Temperature changes also affect the solubility, cmc value, micelle size, etc. of surfactants, thus affecting the washing effect. The solubility of surfactants with long carbon chains is small when the temperature is low, and sometimes the solubility is even lower than the cmc value. In this case, the washing temperature should be appropriately increased. 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 decreases the micelle size, which means that the concentration of surfactant in the washing solution must be increased. For non-ionic surfactants, the increase in temperature causes the cmc value to decrease, while the micelle amount increases significantly. It can be seen that appropriately increasing the temperature will help the non-ionic surfactants to exert their surface active effects. But the temperature should not exceed its cloud point.
In short, the most suitable washing temperature is related to the detergent formula and the objects being washed. Some detergents have good cleaning effects at room temperature, while some detergents have very different decontamination effects between cold washing and hot washing.
③Bubble
People tend to confuse foaming ability with washing effect, thinking that detergents with strong foaming power have better washing effect. Research results show that the washing effect is not directly related to the amount of foam. For example, using low-foaming detergent for washing has no worse washing effect than high-foaming detergent.
Although foam is not directly related to washing, in some cases, foam can still help remove dirt. For example, when washing dishes by hand, the foam of the washing liquid can carry away the washed oil droplets. When scrubbing carpets, foam can also take away solid dirt particles such as dust. Dust accounts for a large proportion of carpet dirt, so carpet cleaners should have a certain foaming ability.
Foaming power is also important for shampoos. The fine foam produced by the liquid during shampooing or bathing makes people feel lubricated and comfortable.
④ Types of fibers and physical properties of textiles
In addition to the chemical structure of the fiber affecting the adhesion and removal of dirt, the appearance of the fiber and the organizational structure of the yarn and fabric also have an impact on the ease of dirt removal.
The scales of wool fibers and the curved, flat ribbon-like structure of cotton fibers accumulate dirt more easily than smooth fibers. For example, carbon black stained on cellulose film (viscose film) is easy to remove, but carbon black stained on cotton fabric is difficult to remove. Another example is that polyester short fiber fabrics are easier to accumulate oil stains than long fiber fabrics, and oil stains on short fiber fabrics are also more difficult to remove than long fiber fabrics.
Tightly twisted yarns and compact fabrics can resist the intrusion of dirt due to the small micro-gaps between fibers, but they can also prevent the washing liquid from expelling internal dirt. Therefore, compact fabrics have good stain resistance at the beginning, but once stained, it is more difficult to wash.
⑤Water hardness
The concentration of metal ions such as Ca2 and Mg2 in water has a great influence on the cleaning effect. In particular, the calcium and magnesium salts formed by anionic surfactants when encountering Ca2 and Mg2 ions have poor solubility, which will reduce its decontamination ability. Even with higher concentrations of surfactants in hard water, the decontamination effect is still much worse than in distillation. In order for the surfactant to exert the best cleaning effect, the Ca2 ion concentration in the water must be reduced to less than 1×10-6mol/L (CaCO3 must be reduced to 0.1mg/L). This requires adding various water softeners to the detergent.