1. Surface tension
The contraction force per unit length on the surface of a liquid is called surface tension, measured in N • m-1.
2. Surface activity and surfactant
The property that can reduce the surface tension of solvents is called surface activity, and substances with surface activity are called surface active substances.
Surfactant refer to surface active substances that can form micelles and other aggregates in aqueous solutions, have high surface activity, and also have wetting, emulsifying, foaming, washing, and other functions.
3. Molecular structural characteristics of surfactant
Surfactant are organic compounds with special structures and properties that can significantly alter the interfacial tension between two phases or the surface tension of liquids (usually water), and have properties such as wetting, foaming, emulsification, and washing.
Structurally speaking, surfactants share a common characteristic of containing two different functional groups in their molecules. One end is a long-chain non-polar group that is soluble in oil but insoluble in water, known as a hydrophobic group or hydrophobic group. These hydrophobic groups are generally long-chain hydrocarbons, sometimes also organic fluorine, organosilicon, organophosphorus, organotin chains, etc. The other end is a water-soluble functional group, namely a hydrophilic group or hydrophilic group. The hydrophilic group must have sufficient hydrophilicity to ensure that the entire surfactant is soluble in water and has the necessary solubility. Due to the presence of hydrophilic and hydrophobic groups in surfactants, they can dissolve in at least one phase of the liquid phase. The hydrophilic and oleophilic properties of surfactants are called amphiphilicity.
4.Types of surfactants
Surfactants are amphiphilic molecules that have both hydrophobic and hydrophilic groups. The hydrophobic groups of surfactants are generally composed of long-chain hydrocarbons, such as straight chain alkyl C8-C20, branched chain alkyl C8-C20, alkylphenyl (with 8-16 alkyl carbon atoms), etc. The difference in hydrophobic groups mainly lies in the structural changes of carbon hydrogen chains, with relatively small differences, while there are more types of hydrophilic groups. Therefore, the properties of surfactants are mainly related to hydrophilic groups in addition to the size and shape of hydrophobic groups. The structural changes of hydrophilic groups are greater than those of hydrophobic groups, so the classification of surfactants is generally based on the structure of hydrophilic groups. This classification is mainly based on whether the hydrophilic groups are ionic, dividing them into anionic, cationic, nonionic, zwitterionic, and other special types of surfactants.
5. Characteristics of surfactant aqueous solution
① Adsorption of surfactants at interfaces
Surfactant molecules have lipophilic and hydrophilic groups, making them amphiphilic molecules. Water is a strongly polar liquid. When surfactants dissolve in water, according to the principle of polarity similarity and polarity difference repulsion, their hydrophilic groups are attracted to the water phase and dissolve in water, while their lipophilic groups repel water and leave the water. As a result, surfactant molecules (or ions) adsorb at the interface between the two phases, reducing the interfacial tension between the two phases. The more surfactant molecules (or ions) are adsorbed on the interface, the greater the decrease in interfacial tension.
② Some properties of adsorption membrane
Surface pressure of adsorption membrane: Surfactants adsorb at the gas-liquid interface to form an adsorption membrane. If a frictionless movable floating plate is placed on the interface and the floating plate pushes the adsorption membrane along the solution surface, the membrane exerts a pressure on the floating plate, 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, make its plane contact the water surface of the sink, rotate the platinum ring, the platinum ring is hindered by the viscosity of the water, and the amplitude gradually attenuates, according to which the surface viscosity can be measured. The method is: first conduct experiments on the pure water surface, measure the amplitude attenuation, then measure the attenuation after the formation of the surface facial mask, and calculate the viscosity of the surface facial mask from the difference between the two.
The surface viscosity is closely related to the firmness of the surface facial mask. Since the adsorption film has surface pressure and viscosity, it must be elastic. The higher the surface pressure and viscosity of the adsorption membrane, the greater its elastic modulus. The elastic modulus of surface adsorption film is of great significance in the process of foam stabilization.
③ Formation of micelles
The dilute solution of surfactants follows the laws of ideal solutions. The adsorption amount of surfactants on the surface of a solution increases with the concentration of the solution. When the concentration reaches or exceeds a certain value, the adsorption amount no longer increases. These excessive surfactant molecules in the solution are disordered or exist in a regular manner. Both practice and theory have shown that they form aggregates in solution, which are called micelles.
Critical micelle concentration: The minimum concentration at which surfactants form micelles in a solution is called the critical micelle concentration.
④ The CMC value of common surfactant.
6. Hydrophilic and oleophilic equilibrium value
HLB stands for hydrophilic lipophilic balance, which represents the hydrophilic and lipophilic equilibrium values of the hydrophilic and lipophilic groups of a surfactant, i.e. the HLB value of the surfactant. A high HLB value indicates strong hydrophilicity and weak lipophilicity of the molecule; On the contrary, it has strong lipophilicity and weak hydrophilicity.
① Regulations on 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 set to 0, while the HLB value of sodium dodecyl sulfate with strong water solubility is set to 40. Therefore, the HLB value of surfactants is generally within the range of 1-40. Generally speaking, emulsifiers with HLB values less than 10 are lipophilic, while emulsifiers with HLB values greater than 10 are hydrophilic. Therefore, the turning point from lipophilicity to hydrophilicity is approximately 10.
7. Emulsification and solubilization effects
Two immiscible liquids, one formed by dispersing particles (droplets or liquid crystals) in the other, are called emulsions. When forming an emulsion, the interfacial area between the two liquids increases, making the system thermodynamically unstable. To stabilize the emulsion, a third component - emulsifier - needs to be added to reduce the interfacial energy of the system. Emulsifiers belong to surfactants, and their main function is to act as emulsifiers. The phase in which droplets exist in an emulsion is called the dispersed phase (or internal phase, discontinuous phase), and the other phase connected together is called the dispersed medium (or external phase, continuous phase).
① Emulsifiers and emulsions
Common emulsions consist of one phase of water or aqueous solution, and the other phase of organic compounds that are immiscible with water, such as oils, waxes, etc. The emulsion formed by water and oil can be divided into two types based on their dispersion: oil dispersed in water forms a water in oil emulsion, represented by O/W (oil/water); water dispersed in oil forms a water in oil emulsion, represented by W/O (water/oil). In addition, complex water in oil in water W/O/W and oil in water in oil O/W/O emulsions may also form.
The emulsifier stabilizes the emulsion by reducing the interfacial tension and forming a monolayer facial mask.
Requirements for emulsifiers in emulsification: a: emulsifiers must be able to adsorb or enrich at the interface between the two phases, reducing interfacial tension; b: Emulsifiers must give particles an electric charge, causing electrostatic repulsion between particles or forming a stable, highly viscous protective film around the particles. So, substances used as emulsifiers must have amphiphilic groups to have emulsifying effects, and surfactants can meet this requirement.
② Preparation methods of emulsions and factors affecting emulsion stability
There are two methods for preparing emulsions: one is to use mechanical methods to disperse the liquid into small particles in another liquid, which is commonly used in industry to prepare emulsions; Another method is to dissolve a liquid in a molecular state in another liquid and then allow it to aggregate appropriately to form an emulsion.
The stability of emulsions refers to their ability to resist particle aggregation and cause phase separation. Emulsions are thermodynamically unstable systems with significant free energy. Therefore, the stability of an emulsion actually refers to the time required for the system to reach equilibrium, that is, the time required for a liquid in the system to separate.
When there are polar organic molecules such as fatty alcohol, fatty acid and fatty amine in the facial mask, the strength of the membrane increases significantly. This is because the emulsifier molecules in the interface adsorption layer interact with polar molecules such as alcohol, acid and amine to form a "complex", which increases the strength of the interface facial mask.
Emulsifiers composed of two or more surfactants are called mixed emulsifiers. Mixed emulsifiers adsorb on the water/oil interface, and intermolecular interactions can form complexes. Due to strong intermolecular interaction, the interfacial tension is significantly reduced, the amount of emulsifier adsorbed on the interface is significantly increased, and the density and strength of the formed interfacial facial mask are increased.
The charge of droplets has a significant impact on the stability of emulsions. Stable emulsions typically have droplets with electric charges. When using ionic emulsifiers, the emulsifier ions adsorbed on the interface insert their lipophilic groups into the oil phase, while the hydrophilic groups are in the water phase, thus making the droplets charged. Due to the fact that the droplets of the emulsion carry the same charge, they repel each other and are not easily agglomerated, resulting in increased stability. It can be seen that the more emulsifier ions adsorbed on the droplets, the greater their charge, and the greater their ability to prevent droplet coalescence, making the emulsion system more stable.
The viscosity of emulsion dispersion medium has a certain impact on the stability of emulsion. Generally, the higher the viscosity of the dispersing medium, the higher the stability of the emulsion. This is because the viscosity of the dispersing medium is high, which strongly hinders the Brownian motion of the liquid droplets, slows down the collision between the droplets, and keeps the system stable. Polymer substances that are usually soluble in emulsions can increase the viscosity of the system and enhance the stability of the emulsion. In addition, the polymer can also form a solid interface facial mask, making the emulsion system more stable.
In some cases, adding solid powder can also stabilize the emulsion. The solid powder is not in water, oil or at the interface, depending on the wetting ability of oil and water on the solid powder. If the solid powder is not completely wetted by water and can be wetted by oil, it will remain at the water oil interface.
The reason why the solid powder does not stabilize the emulsion is that the powder gathered at the interface does not strengthen the interface facial mask, which is similar to the interface adsorption emulsifier molecules. Therefore, the closer the solid powder particles are arranged at the interface, the more stable the emulsion will be.
Surfactants have the ability to significantly increase the solubility of organic compounds that are insoluble or slightly soluble in water after forming micelles in aqueous solution, and the solution is transparent at this time. This effect of micelles is called solubilization. Surfactants that can produce solubilizing effects are called solubilizers, and organic compounds that are solubilized are called solubilized compounds.
8. Foam
Foam plays an important role in the washing process. Foam refers to the dispersion system in which gas is dispersed in liquid or solid. Gas is the dispersion phase, and liquid or solid is the dispersion medium. The former is called liquid foam, while the latter is called solid foam, such as foam plastic, foam glass, foam cement, etc.
(1) Formation of foam
The foam here refers to the aggregation of bubbles separated by liquid film. Due to the large difference in density between the dispersed phase (gas) and the dispersed medium (liquid), and the low viscosity of the liquid, the foam can always rise to the liquid level quickly.
The process of forming foam is to bring a large amount of gas into the liquid, and the bubbles in the liquid return to the liquid surface quickly, forming a bubble aggregate separated by a small amount of liquid and gas
Foam has two remarkable characteristics in morphology: one is that bubbles as dispersed phase are often polyhedral, because at the intersection of bubbles, there is a tendency for the liquid film to become thinner, making the bubbles polyhedral. When the liquid film becomes thinner to a certain extent, the bubbles will break; Second, the pure liquid cannot form stable foam, but the liquid that can form foam is at least two or more components. The aqueous solution of surfactant is a typical system easy to generate foam, and its ability to generate foam is also related to other properties.
Surfactants with good foaming ability are called foaming agents. Although the foaming agent has good foam ability, the formed foam may not be able to maintain for a long time, that is, its stability may not be good. In order to maintain the stability of foam, a substance that can increase the stability of foam is often added to the foaming agent, which is called foam stabilizer. The commonly used foam stabilizers are lauroyl diethanolamine and dodecyl dimethyl amine oxide.
(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 bubble breaking. The defoaming process is the process in which the liquid film separating the gas changes thickness until it ruptures. Therefore, the stability of foam is mainly determined by the speed of liquid discharge and the strength of liquid film. There are several other influencing factors.
① Surface tension
From the energy point of view, low surface tension is more favorable for the formation of foam, but it cannot guarantee the stability of foam. Low surface tension, low pressure difference, slow liquid discharge speed, and slow liquid film thinning are conducive to the stability of foam.
② Surface viscosity
The key factor determining the stability of foam is the strength of the liquid film, which is mainly determined by the firmness of the surface adsorption film, measured by the surface viscosity. Experiments show that the foam produced by the solution with higher surface viscosity has a longer life. This is because the interaction between adsorbed molecules on the surface leads to the increase of membrane strength, thus improving the life of foam.
③ Solution viscosity
When the viscosity of the liquid itself increases, the liquid in the liquid film is not easy to be discharged, and the speed of the liquid film thickness thinning is slow, which delays the time of the liquid film rupture and increases the stability of the foam.
④ The 'repairing' effect of surface tension
Surfactants adsorbed on the surface of the liquid film have the ability to resist the expansion or contraction of the liquid film surface, which we refer to as repair effect. This is because there is a liquid film of surfactants adsorbed on the surface, and expanding its surface area will reduce the concentration of surface adsorbed molecules and increase surface tension. Further expanding the surface will require greater effort. Conversely, surface area shrinkage will increase the concentration of adsorbed molecules on the surface, reducing surface tension and hindering further shrinkage.
⑤ The diffusion of gas through a liquid film
Due to the existence of capillary pressure, the pressure of small bubbles in foam is higher than that of large bubbles, which will cause the gas in the small bubbles to diffuse into the low-pressure large bubbles through the liquid film, resulting in the phenomenon that the small bubbles become smaller, the large bubbles become larger, and finally the foam breaks. If surfactant is added, the foam will be uniform and dense when foaming, and it is not easy to defoamer. Since the surfactant is closely arranged on the liquid film, it is difficult to ventilate, which makes the foam more stable.
⑥ The influence of surface charge
If the foam liquid film is charged with the same symbol, the two surfaces of the liquid film will repel each other, preventing the liquid film from thinning or even destruction. Ionic surfactants can provide this stabilizing effect.
In conclusion, the strength of liquid film is the key factor to determine the stability of foam. As a surfactant for foaming agents and foam stabilizers, the tightness and firmness of the surface adsorbed molecules are the most important factors. When the interaction between the adsorbed molecules on the surface is strong, the adsorbed molecules are closely arranged, which not only makes the surface facial mask itself have high strength, but also makes the solution adjacent to the surface facial mask difficult to flow due to the high surface viscosity, so it is relatively difficult for the liquid film to drain, and the thickness of the liquid film is easy to maintain. In addition, closely arranged surface molecules can also reduce the permeability of gas molecules and thus increase the stability of foam.
(3) Destruction of foam
The basic principle of destroying foam is to change the conditions for producing foam or eliminate the stability factors of foam, so there are two defoaming methods, physical and chemical.
Physical defoaming is to change the conditions under which foam is generated while maintaining the chemical composition of foam solution unchanged. For example, external force disturbance, temperature or pressure change and ultrasonic treatment are all effective physical methods to eliminate foam.
The chemical defoaming method is to add some substances to interact with the foaming agent, reduce the strength of the liquid film in the foam, and then reduce the stability of the foam to achieve the purpose of defoaming. Such substances are called defoamers. Most defoamers are surfactants. Therefore, according to the mechanism of defoaming, defoamers should have a strong ability to reduce surface tension, be easily adsorbed on the surface, and have weak interactions between surface adsorbed molecules, resulting in a relatively loose arrangement structure of adsorbed molecules.
There are various types of defoamers, but they are mostly non-ionic surfactants. Non ionic surfactants have anti foaming properties near or above their cloud point and are commonly used as defoamers. Alcohols, especially those with branching structures, fatty acids and esters, polyamides, phosphates, silicone oils, etc., are also commonly used as excellent defoamers.
(4) Foam and washing
There is no direct relationship between foam and washing effect, and the amount of foam does not mean that the washing effect is good or bad. For example, the foaming performance of non-ionic surfactants is far inferior to soap, but their cleaning power is much better than soap.
In some cases, foam is helpful in removing dirt. For example, when washing tableware at home, the foam of the detergent can take away the oil drops washed down; When scrubbing carpet, foam helps to take away solid dirt such as dust and powder. In addition, foam can sometimes be used as a sign of whether the detergent is effective, because fatty oil stains can inhibit the foam of the detergent. When there is too much oil stains and too little detergent, there will be no foam or the original foam will disappear. Sometimes, foam can also be used as an indicator of whether the rinsing is clean. Because the amount of foam in the rinsing solution tends to decrease with the decrease of detergent content, the degree of rinsing can be evaluated by the amount of foam.
9. Washing process
In a broad sense, washing is the process of removing unwanted components 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 a carrier. During washing, the interaction between dirt and the carrier is weakened or eliminated through the action of some chemical substances (such as detergents), transforming the combination of dirt and carrier into the combination of dirt and detergent, ultimately causing the dirt and carrier to detach. 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 represented by the following simple relationship
Carrier • Dirt+Detergent=Carrier+Dirt • Detergent
The washing process can usually be divided into two stages: one is the separation of dirt and its carrier under the action of detergent; The second is that the detached dirt is dispersed and suspended in the medium. The washing process is a reversible process, and dirt that is dispersed or suspended in the medium may also re precipitate from the medium onto the laundry. Therefore, an excellent detergent should not only have the ability to detach dirt from the carrier, but also have good ability to disperse and suspend dirt, and prevent dirt from depositing again.
(1) Types of dirt
Even for the same item, the type, composition, and quantity of dirt will vary depending on the usage environment. Oil body dirt mainly includes animal and vegetable oils, as well as mineral oils (such as crude oil, fuel oil, coal tar, etc.), while solid dirt mainly includes smoke, dust, rust, carbon black, etc. In terms of clothing dirt, there are dirt from the human body, such as sweat, sebum, blood, etc; Dirt from food, such as fruit stains, edible oil stains, seasoning stains, starch, etc; Dirt brought by cosmetics, such as lipstick and nail polish; Dirt from the atmosphere, such as smoke, dust, soil, etc; Other materials such as ink, tea, paint, etc. It can be said that there are various and diverse types.
Various types of dirt can usually be divided into three categories: solid dirt, liquid dirt, and special dirt.
① Common solid dirt includes particles such as ash, mud, soil, rust, and carbon black. Most of these particles have a surface charge, mostly negative, and are easily adsorbed onto fibrous objects. Generally, solid dirt is difficult to dissolve in water, but can be dispersed and suspended by detergent solutions. Solid dirt with small particles is difficult to remove.
② Liquid dirt is mostly oil soluble, including animal and vegetable oils, fatty acids, fatty alcohols, mineral oils, and their oxides. Among them, animal and vegetable oils and fatty acids can undergo saponification with alkali, while fatty alcohols and mineral oils are not saponified by alkali, but can dissolve in alcohols, ethers, and hydrocarbon organic solvents, and be emulsified and dispersed by detergent aqueous solutions. Oil soluble liquid dirt generally has a strong interaction force with fibrous objects and adsorbs firmly on fibers.
③ Special dirt includes protein, starch, blood, human secretions such as sweat, sebum, urine, as well as fruit juice, tea juice, etc. Most of these types of dirt can strongly adsorb onto fibrous objects through chemical reactions. Therefore, washing it is quite difficult.
Various types of dirt rarely exist alone, often mixed together and adsorbed together on objects. Dirt can sometimes oxidize, decompose, or decay under external influences, resulting in the formation of new dirt.
(2) The adhesion effect of dirt
The reason why clothes, hands, etc. can get dirty is because there is some kind of interaction between objects and dirt. There are various adhesion effects of dirt on objects, but they are mainly physical adhesion and chemical adhesion.
① The physical adhesion of cigarette ash, dust, sediment, carbon black, and other substances to clothing. Generally speaking, the interaction between the adhered dirt and the contaminated object is relatively weak, and the removal of dirt is also relatively easy. According to different forces, the physical adhesion of dirt can be divided into mechanical adhesion and electrostatic adhesion.
A: Mechanical adhesion mainly refers to the adhesion of solid dirt such as dust and sediment. Mechanical adhesion is a weak adhesion method for dirt, which can almost be removed by simple mechanical methods. However, when the particle size of the dirt is small (<0.1um), it is more difficult to remove.
B: Electrostatic adhesion is mainly manifested by the action of charged dirt particles on objects with opposite charges. Most fibrous objects carry a negative charge in water and are easily adhered to by positively charged dirt such as lime. Some dirt, although negatively charged, such as carbon black particles in aqueous solutions, can adhere to fibers through ion bridges formed by positive ions (such as Ca2+, Mg2+, etc.) in water (ions act together between multiple opposite charges, acting like bridges).
Static electricity is stronger than simple mechanical action, making it relatively difficult to remove dirt.
③ Removal of special dirt
Protein, starch, human secretions, fruit juice, tea juice and other types of dirt are difficult to remove with general surfactants and require special treatment methods.
Protein stains such as cream, eggs, blood, milk, and skin excreta are prone to coagulation and denaturation on fibers, and adhere more firmly. For protein fouling, protease can be used to remove it. The protease can break down proteins in dirt into water-soluble amino acids or oligopeptides.
Starch stains mainly come from food, while others such as meat juices, paste, etc. Starch enzymes have a catalytic effect on the hydrolysis of starch stains, breaking down starch into sugars.
Lipase can catalyze the decomposition of some triglycerides that are difficult to remove by conventional methods, such as sebum secreted by the human body, edible oils, etc., to break down triglycerides into soluble glycerol and fatty acids.
Some colored stains from fruit juice, tea juice, ink, lipstick, etc. are often difficult to thoroughly clean even after repeated washing. This type of stain can be removed by oxidation-reduction reactions using oxidants or reducing agents such as bleach, which break down the structure of the chromophore or chromophore groups and degrade them into smaller water-soluble components.
From the perspective of dry cleaning, there are roughly three types of dirt.
① Oil soluble dirt includes various oils and fats, which are liquid or greasy and soluble in dry cleaning solvents.
② Water soluble dirt is soluble in aqueous solution, but insoluble in dry cleaning agents. It adsorbs onto clothing in the form of an aqueous solution, and after the water evaporates, granular solids such as inorganic salts, starch, proteins, etc. are precipitated.
③ Oil water insoluble dirt is insoluble in both water and dry cleaning solvents, such as carbon black, various metal silicates, and oxides.
Due to the different properties of various types of dirt, there are different ways of removing dirt during the dry cleaning process. Oil soluble dirt, such as animal and vegetable oils, mineral oils, and fats, are easily soluble in organic solvents and can be easily removed during dry cleaning. The excellent solubility of dry cleaning solvents for oil and grease is essentially due to van der Waals forces between molecules.
For the removal of water-soluble dirt such as inorganic salts, sugars, proteins, sweat, etc., it is also necessary to add an appropriate amount of water to the dry cleaning agent, otherwise water-soluble dirt is difficult to remove from clothing. But water is difficult to dissolve in dry cleaning agents, so to increase the amount of water, surfactants need to be added. The water present in dry cleaning agents can hydrate dirt and the surface of clothing, making it easy to interact with the polar groups of surfactants, which is beneficial for the adsorption of surfactants on the surface. In addition, when surfactants form micelles, water-soluble dirt and water can be solubilized into the micelles. Surfactants can not only increase the water content in dry cleaning solvents, but also prevent the re deposition of dirt to enhance the cleaning effect.
The presence of a small amount of water is necessary for removing water-soluble dirt, but excessive water can cause some clothes to deform, wrinkle, etc., so the water content in the dry detergent must be moderate.
Solid particles such as ash, mud, soil, and carbon black, which are neither water-soluble nor oil soluble, generally adhere to clothing by electrostatic adsorption or by combining with oil stains. In dry cleaning, the flow and impact of solvents can cause dirt adsorbed by electrostatic forces to fall off, while dry cleaning agents can dissolve oil stains, causing solid particles that combine with the oil stains and adhere to the clothes to fall off from the dry cleaning agent. The small amount of water and surfactants in the dry cleaning agent can stably suspend and disperse the solid dirt particles that fall off, preventing them from depositing on the clothes again.
(5) Factors affecting the washing effect
The directional adsorption of surfactants at the interface and the reduction of surface (interfacial) tension are the main factors for the removal of liquid or solid fouling. But the washing process is relatively complex, 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, type of fiber, and fabric structure.
① Concentration of surfactants
The micelles of surfactants in the 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 in order to achieve good washing effect. However, when the concentration of surfactants exceeds the CMC value, the increasing washing effect becomes less significant, and excessive increase in surfactant concentration is unnecessary.
When using solubilization to remove oil stains, even if the concentration is above the CMC value, the solubilization effect still increases with the increase of surfactant concentration. At this time, it is advisable to use detergent locally, such as on the cuffs and collars of clothes where there is a lot of dirt. When washing, a layer of detergent can be applied first to improve the solubilization effect of surfactants on oil stains.
② Temperature has a significant impact on the cleaning effect. Overall, increasing the temperature is beneficial for removing dirt, but sometimes excessive temperature can also cause adverse factors.
An increase in temperature is beneficial for the diffusion of dirt. Solid oil stains are easily emulsified when the temperature is above their melting point, and fibers also increase their degree of expansion due to the increase in temperature. These factors are all beneficial for the removal of dirt. However, for tight fabrics, the micro gaps between fibers are reduced after fiber expansion, which is not conducive to the removal of dirt.
Temperature changes also affect the solubility, CMC value, and micelle size of surfactants, thereby affecting the washing effect. Long carbon chain surfactants have lower solubility at low temperatures, and sometimes even lower solubility 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 leads to an increase in CMC value and a decrease in micelle size. This means that the concentration of surfactants should be increased in the washing solution. For non-ionic surfactants, increasing temperature leads to a decrease in their CMC value and a significant increase in their micelle size. It can be seen that appropriately increasing temperature can help non-ionic surfactants exert their surface activity. But the temperature should not exceed its cloud point.
In short, the most suitable washing temperature is related to the formula of the detergent and the object being washed. Some detergents have good cleaning effects at room temperature, while some detergents have significantly different cleaning effects for cold and hot washing.
③ Foam
People often confuse foaming ability with washing effect, believing that detergents with strong foaming ability have better washing effects. The results show that the washing effect is not directly related to the amount of foam. For example, using low foaming detergent for washing does not have a worse washing effect than high foaming detergent.
Although foam is not directly related to washing, foam is still helpful to remove dirt in some situations. For example, the foam of the washing liquid can carry away the oil drops when washing dishes by hand. When scrubbing the carpet, foam can also take away solid dirt particles such as dust. Dust accounts for a large proportion of carpet dirt, so carpet cleaner should have certain foaming ability.
Foaming power is also important for shampoo. The fine foam produced by the liquid when washing hair or bathing makes people feel comfortable.
④ Types of fibers and physical properties of textiles
In addition to the chemical structure of fibers affecting the adhesion and removal of dirt, the appearance of fibers and the organizational structure of yarns and fabrics also have an impact on the difficulty of dirt removal.
The scales of wool fibers and the flat strip like structure of cotton fibers are more prone to accumulate dirt than smooth fibers. For example, carbon black adhered to cellulose film (adhesive film) is easy to remove, while carbon black adhered to cotton fabric is difficult to wash off. For example, polyester short fiber fabrics are more prone to accumulating oil stains than long fiber fabrics, and the oil stains on short fiber fabrics are also more difficult to remove than those on long fiber fabrics.
Tightly twisted yarns and tight fabrics, due to the small micro gaps between fibers, can resist the invasion of dirt, but also prevent the cleaning solution from removing internal dirt. Therefore, tight fabrics have good resistance to dirt at the beginning, but it is also difficult to clean once contaminated.
⑤ The hardness of water
The concentration of metal ions such as Ca2+and Mg2+in water has a significant impact on the washing effect, especially when anionic surfactants encounter Ca2+and Mg2+ions to form calcium and magnesium salts with poor solubility, which can reduce their cleaning ability. Even if the concentration of surfactants is high in hard water, their cleaning effect is still much worse than in distillation. To achieve the best washing effect of surfactants, the concentration of Ca2+ions in water should be reduced to below 1 × 10-6mol/L (CaCO3 should be reduced to 0.1mg/L). This requires adding various softeners to the detergent.
Post time: Aug-16-2024