The contraction force per unit length of a liquid's surface is referred to as surface tension, measured in units of N•m⁻¹.
The property that reduces the surface tension of a solvent is termed surface activity, and substances exhibiting this property are known as surfactant substances.
Surfactants are substances that can form molecular aggregates, such as micelles, in aqueous solutions, exhibiting high surface activity. They also possess wetting, emulsifying, foaming, and detergent properties.
Surfactants are organic compounds with unique structures and properties. They significantly alter the interfacial tension between two phases or the surface tension of a liquid (typically water). They exhibit properties such as wetting, foaming, emulsification, and detergent action.
Structurally, surfactants share a common feature: their molecules have two different types of groups. One end comprises a long-chain nonpolar group that is soluble in oil but insoluble in water—referred to as a hydrophobic or lipophilic group. This hydrophobic group is typically a long-chain hydrocarbon but can also include organic fluorine, organic silicon, organic phosphorus, or organic tin chains. The other end is a water-soluble group, known as a hydrophilic group. The hydrophilic group must exhibit sufficient water affinity to ensure the surfactant's solubility in water, with the necessary degree of dissolution. Due to the presence of both hydrophilic and hydrophobic groups, surfactants can dissolve in at least one phase of the liquid. This amphiphilic nature, both water-attracting and oil-attracting, is a characteristic property of surfactants.
Surfactants are amphiphilic molecules with both hydrophobic and hydrophilic groups. The hydrophobic group of surfactants is typically composed of long-chain hydrocarbons, such as straight-chain alkyl groups (C8 to C20), branched alkyl groups (C8 to C20), alkyl benzene groups (alkyl carbon atoms ranging from 8 to 16), and others. The differences in hydrophobic groups are primarily in the structural variations of the hydrocarbon chain, with relatively small variations. On the other hand, there are many types of hydrophilic groups. Therefore, the properties of surfactants are not only related to the size and shape of the hydrophobic group but also to the hydrophilic group.
The structural variations in the hydrophilic group are larger than those in the hydrophobic group. Consequently, the classification of surfactants is generally based on the structure of the hydrophilic group. This classification primarily considers whether the hydrophilic group is ionic, dividing surfactants into anionic, cationic, nonionic, amphoteric, and other special types of surfactants.
① Adsorption at the Interface
Surfactant molecules possess both hydrophilic and hydrophobic groups, making them amphiphilic. Water is a strongly polar liquid, and when surfactants dissolve in water, based on the principle of like poles attracting and unlike poles repelling, the hydrophilic group is attracted to water, causing the surfactant to dissolve in water, while the hydrophobic group is repelled by water. As a result, surfactant molecules (or ions) adsorb at the interface between the two phases, reducing the interfacial tension. The more surfactant molecules (or ions) adsorb at the interface, the greater the reduction in interfacial tension.
② Properties of Adsorption Films
Surface Pressure of Adsorption Films: Surfactants adsorb at the gas-liquid interface, forming adsorption films. If a frictionless, movable float is placed at the interface and pushed along the solution surface by the adsorbed film, the film exerts pressure on the float. This pressure is known as surface pressure.
Surface Viscosity: Similar to surface pressure, surface viscosity is a property exhibited by an insoluble molecular film. Using a fine metal wire suspending a platinum ring, with the plane of the ring in contact with the water surface in a water trough, rotating the platinum ring encounters resistance due to the viscosity of the adsorbed film. The amplitude gradually decreases, allowing the measurement of surface viscosity. The difference between the amplitude decay in pure water and that with the formed surface film yields the viscosity of the surface film.
Surface viscosity is closely related to the firmness of the adsorption film. Since the adsorption film has surface pressure and viscosity, it must possess elasticity. The greater the surface pressure and viscosity of the adsorption film, the higher its elastic modulus. The elastic modulus of the surface adsorption film is significant during the stable foam process.
③ Formation of Micelles
Dilute solutions of surfactants in water follow the laws of ideal solutions. The adsorption at the solution surface increases with increasing solution concentration. When the concentration reaches or exceeds a certain value, the adsorption does not increase further. The excess surfactant molecules in the solution are either randomly dispersed or organized in a certain pattern. Both practical and theoretical considerations indicate that they form aggregates in the solution, known as micelles.
Critical Micelle Concentration (CMC): The lowest concentration at which surfactants in a solution form micelles is called the critical micelle concentration.
④ Common CMC Values of Surfactants.
HLB is an abbreviation for Hydrophile Lipophile Balance, representing the balance between the hydrophilic and lipophilic groups in a surfactant molecule, indicated by the surfactant's HLB value. A higher HLB value indicates stronger hydrophilicity and weaker lipophilicity, while a lower HLB value indicates stronger lipophilicity and weaker hydrophilicity.
① Definition of HLB Values
HLB values are relative values. Therefore, when establishing HLB values as standards, the HLB value for hydrophobic paraffin wax is set at 0, and the HLB value for the water-soluble sodium dodecyl sulfate is set at 40. Thus, the HLB values for surfactants generally fall within the range of 1 to 40. Generally, emulsifiers with HLB values less than 10 are lipophilic, while those with HLB values greater than 10 are hydrophilic. The transition point from lipophilic to hydrophilic is typically around 10.
A system where two immiscible liquids, one dispersed as particles (droplets or liquid crystals) in the other, is termed an emulsion. The formation of an emulsion increases the interfacial area of the two liquid phases, making the system thermodynamically unstable. To stabilize the emulsion, a third component, an emulsifier, is added to reduce the interfacial energy of the system. Emulsifiers belong to surfactants and play a crucial role in emulsification. In an emulsion, the phase existing as droplets is called the dispersed phase (or internal phase, discontinuous phase), and the continuous, uniform phase is known as the dispersing medium (or external phase, continuous phase).
① Emulsifiers and Emulsions
Common emulsions involve water or aqueous solutions as one phase and an organic substance immiscible with water, such as oil or wax, as the other phase. Emulsions formed by water and oil can be classified based on the dispersion: water dispersed in oil forms water-in-oil (W/O) emulsions, and oil dispersed in water forms oil-in-water (O/W) emulsions. Additionally, complex multiple emulsions such as water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O) may also occur.
Emulsifiers function by lowering interfacial tension and forming a monomolecular interface film to stabilize emulsions.
Requirements for emulsifiers in the emulsification process: a. Emulsifiers must adsorb or accumulate at the interface of the two phases, reducing interfacial tension. b. Emulsifiers must impart charges to particles, inducing electrostatic repulsion between particles or forming a stable, high-viscosity protective film around the particles. Therefore, substances used as emulsifiers must have amphiphilic groups, and surfactants meet this requirement.
② Preparation Methods for Emulsions and Factors Affecting Emulsion Stability
There are two methods for preparing emulsions:
Mechanical method: This involves dispersing one liquid into another using mechanical means. This method is commonly used in industrial emulsion preparation.
Molecular state method: In this method, a liquid is dissolved in molecular form in another liquid, and then it is appropriately aggregated to form an emulsion.
Emulsion stability refers to the ability to resist particle aggregation leading to phase separation. Emulsions are thermodynamically unstable systems with high free energy. Therefore, emulsion stability refers to the time required for the system to reach equilibrium, i.e., the time needed for one liquid to separate in the system.
The presence of polar organic molecules such as fatty alcohols, fatty acids, and fatty amines in the interface membrane significantly increases the membrane strength. This is because in the adsorption layer at the interface, emulsifier molecules interact with polar molecules like alcohols, acids, and amines to form "complexes," leading to increased membrane strength.
Emulsifiers composed of two or more surfactants are termed mixed emulsifiers. Mixed emulsifiers adsorb at the water/oil interface, and intermolecular interactions can form complexes. Due to strong intermolecular interactions, interfacial tension significantly decreases, and the adsorption of emulsifiers at the interface increases, resulting in increased membrane density and strength.
The charge on liquid droplets has a significant impact on the stability of emulsions. Stable emulsions generally have charged liquid droplets. When using ionic emulsifiers, the adsorbed emulsifier ions insert their hydrophobic bases into the oil phase, leaving the hydrophilic bases in the water phase, resulting in charged liquid droplets. As emulsion droplets with the same charge repel each other, they are less likely to coalesce, increasing stability. Therefore, the more emulsifier ions adsorbed on the droplets, the greater the charge, and the greater the ability to prevent droplet coalescence, leading to increased stability.
The viscosity of the dispersing medium in the emulsion has a certain influence on emulsion stability. Generally, the higher the viscosity of the dispersing medium, the higher the stability of the emulsion. This is because the higher viscosity of the dispersing medium imposes a stronger hindrance on the Brownian motion of the droplets, slowing down their collisions and maintaining system stability. High-molecular-weight substances that can dissolve in emulsions usually increase the system's viscosity, enhancing emulsion stability. Additionally, high-molecular-weight substances can form a robust interface membrane, further stabilizing the emulsion system.
In some cases, adding solid powders can also contribute to emulsion stability. Solid powders may be present in water, oil, or at the interface, depending on the wettability of the solid powder by water or oil. If the solid powder is wetted by both water and oil, it will remain at the water-oil interface.
The reason solid powders contribute to emulsion stability is that when aggregated at the interface, they enhance the interface membrane, similar to the adsorption of emulsifier molecules at the interface. The tighter the arrangement of solid powder particles at the interface, the more stable the emulsion.
Surfactants, after forming micelles in aqueous solutions, have the ability to significantly increase the solubility of organic substances that are insoluble or poorly soluble in water. At this stage, the solution appears transparent. This action of micelles is known as solubilization. Surfactants capable of producing solubilization are called solubilizers, and the organic substances solubilized are referred to as solubilizates.
Foam plays a crucial role in the washing process. Foam refers to a dispersed system where gas is dispersed in a liquid or solid, with gas as the dispersed phase and liquid or solid as the dispersing medium. The former is called liquid foam, while the latter is called solid foam, such as foam plastics, foam glass, foam concrete, etc.
(1) Formation of Foam
The foam referred to here is the aggregation of bubbles separated by a thin liquid film. Due to the significant difference in density between the dispersed phase (gas) and the dispersing medium (liquid), coupled with the low viscosity of the liquid, bubbles can quickly rise to the liquid surface.
The process of forming foam involves introducing a large amount of gas into the liquid, and the gas bubbles in the liquid quickly return to the liquid surface, forming an aggregation of bubbles separated by a small amount of liquid.
Foam has two significant characteristics in terms of its structure: first, the bubbles, as the dispersed phase, often have a polyhedral shape. This is because at the intersection of bubbles, there is a tendency for the liquid film to thin, resulting in polyhedral bubbles. When the liquid film thins to a certain extent, it leads to bubble rupture. Second, pure liquid cannot form stable foam; a liquid that can form foam must have at least two components. A typical system with good foam-forming ability is a solution of surfactants in water, and its foam-forming ability is related to other properties.
Surfactants with good foaming ability are called foaming agents. Although foaming agents have good foaming ability, the foam they generate may not necessarily maintain stability for an extended period. To maintain foam stability, substances that increase foam stability are often added to foaming agents. These substances are called foam stabilizers, and common foam stabilizers include lauryl diethanolamide and the oxides of dodecyl dimethylamine.
(2) Stability of Foam
Foam is a thermodynamically unstable system, and its ultimate tendency is for the system's total surface area to decrease and free energy to decrease after foam rupture. The process of foam breakdown involves the thinning of the liquid film separating the gas, leading to rupture. Therefore, the stability of foam is mainly determined by the speed of liquid drainage and the strength of the liquid film. Other influencing factors include:
① Surface Tension: Lower surface tension is favorable for foam formation from an energy perspective. However, low surface tension does not guarantee foam stability. Lower surface tension results in a smaller pressure difference, slowing down liquid drainage and contributing to foam stability.
② Surface Viscosity: The key factor determining foam stability is the strength of the liquid film, which is measured by surface viscosity. Experiments show that solutions with higher surface viscosity form foam with a longer lifespan. This is because higher surface viscosity enhances the strength of the interface membrane, thus increasing foam stability.
③ Solution Viscosity: An increase in the viscosity of the liquid itself slows down the drainage of liquid from the liquid film, delaying the thinning of the liquid film and thus increasing foam stability.
④ "Repair" Effect of Surface Tension: Surfactants adsorbed on the liquid film have the ability to resist the expansion or contraction of the liquid film, known as the repair effect. Adsorbed surfactant molecules reduce the concentration of adsorbed molecules when the surface area expands, increasing surface tension. Conversely, when the surface area contracts, the concentration of adsorbed molecules increases, reducing surface tension, hindering further contraction.
⑤ Diffusion of Gas Through the Liquid Film: Due to capillary pressure, the pressure inside small bubbles in foam is higher than that in large bubbles, leading to the diffusion of gas from small bubbles through the liquid film to larger bubbles. This phenomenon causes small bubbles to shrink and large bubbles to expand, eventually leading to foam rupture. Adding surfactants during foaming can result in uniform and fine foam, which is less prone to defoaming. This is because surfactants tightly arrange themselves on the liquid film, making gas diffusion difficult and increasing foam stability.
⑥ Influence of Surface Charge: If the liquid film of foam has the same charge, there will be mutual repulsion between the two surfaces, preventing the thinning and rupture of the liquid film. Ionic surfactants can play this stabilizing role.
In summary, the strength of the liquid film is the key factor determining foam stability. Surfactants, as foaming agents and foam stabilizers, depend on the closeness and firmness of the arrangement of adsorbed molecules on the surface. When intermolecular interactions are strong, the arrangement of adsorbed molecules is tight, not only giving the surface membrane itself higher strength but also making the solution near the surface membrane less flowable, impeding liquid drainage, and maintaining the thickness of the liquid film. Additionally, tightly arranged surface molecules can reduce the permeability of gas molecules, further increasing foam stability.
(3) Foam Breakdown
The basic principle of breaking foam is to change the conditions that produce foam or eliminate the stabilizing factors of foam. Therefore, there are physical and chemical methods of defoaming.
Physical defoaming involves changing the conditions that produce foam while keeping the chemical composition of the foam solution unchanged. Methods include external disturbances, changes in temperature or pressure, and ultrasonic treatment, all of which are effective physical methods for eliminating foam.
Chemical defoaming involves adding certain substances that interact with foaming agents to reduce the strength of the liquid film in foam, thereby reducing foam stability and achieving defoaming. Such substances are called defoamers, most of which are nonionic surfactants. Therefore, according to the mechanism of defoaming, defoamers should have a strong ability to reduce surface tension, easy adsorption on the surface, and weaker intermolecular interactions between adsorbed molecules, resulting in a loose arrangement of adsorbed molecules.
Defoamers come in various types, but they are generally nonionic surfactants. Nonionic surfactants have antifoaming properties near or above their cloud point, making them suitable as defoamers. Alcohols, especially those with branched structures, fatty acids and esters, polyamides, phosphate esters, and silicone oil are commonly used excellent defoamers.
(4) Foam and Washing
Foam and washing do not have a direct connection; the amount of foam does not necessarily indicate the effectiveness of washing. For example, nonionic surfactants have much less foaming ability than soap, but their cleaning power is much superior to soap.
In some cases, foam is helpful in removing dirt. For example, when washing dishes at home, the foam of the washing liquid can carry away the washed-off oil droplets. When scrubbing carpets, foam helps to remove solid dirt such as dust and powder. Additionally, foam can sometimes serve as an indicator of the effectiveness of the detergent. Greasy oil stains inhibit foam generation in the detergent solution. When there is an insufficient amount of detergent, or excessive oil stains, foam may not be generated, or the existing foam may disappear. Foam can also serve as an indicator of the cleanliness of rinsing. The amount of foam in the rinsing solution often decreases as the detergent content decreases, allowing the evaluation of the rinsing level based on the amount of foam.
In a broad sense, washing is the process of removing unwanted components from the object to achieve a certain purpose. In a typical sense, washing refers to the process of removing dirt from a surface. During washing, the interaction between dirt and the substrate is weakened or eliminated by the action of certain chemicals (such as detergents), transforming the bond between dirt and the substrate into a bond between dirt and the detergent. Ultimately, this results in the separation of dirt from the substrate. As the objects to be washed and the types of dirt vary widely, washing is a highly complex process. The basic process of washing can be represented by the following simple relationship:
Substrate••Dirt + Detergent = Substrate + Dirt•Detergent
The washing process can generally be divided into two stages: first, under the action of the detergent, the dirt is separated from its substrate, and second, the detached dirt is dispersed or suspended in the medium. The washing process is a reversible process, and dirt dispersed or suspended in the medium may also re-precipitate onto the washed items. Therefore, an excellent detergent should not only have the ability to remove dirt from the substrate but also possess good capabilities for dispersing and suspending dirt, preventing the re-deposition of dirt.
(1) Types of Dirt
Even for the same item, the types, components, and quantities of dirt can vary depending on the usage environment. Oil-based dirt mainly includes animal and plant oils as well as mineral oils (such as crude oil, fuel oil, coal tar, etc.). Solid dirt includes particles such as soot, ash, rust, and carbon black. For clothing, dirt can come from body secretions like sweat, sebum, blood, food stains such as fruit, cooking oil, condiments, starch, cosmetics such as lipstick, nail polish, atmospheric pollutants like dust, ash, mud, and others like ink, tea, paint, etc. There is a wide variety of dirt types.
Various types of dirt can generally be categorized into three major classes: solid dirt, liquid dirt, and special dirt.
① Solid Dirt: Common solid dirt includes particles like ash, mud, soil, rust, and carbon black. These particles usually carry a charge, mostly negative, making them easily adsorb onto fibrous items. Solid dirt is generally less soluble in water but can be dispersed or suspended in detergent solutions.
② Liquid Dirt: Liquid dirt is mostly oil-soluble, including animal and vegetable oils, fatty acids, fatty alcohols, mineral oils, and their derivatives. Animal and vegetable oils and fatty acids can undergo saponification with alkali, while fatty alcohols and mineral oils, although not saponifiable by alkali, can dissolve in alcohols, ethers, and hydrocarbon-based organic solvents, being emulsified and dispersed in detergent water solutions. Oil-soluble liquid dirt generally exerts a strong affinity for fibers and adheres firmly to them.
③ Special Dirt: Special dirt includes proteins, starch, blood, human secretions such as sweat, sebum, urine, as well as fruit juice, tea, etc. This type of dirt often adsorbs strongly onto fibrous items through chemical interactions, making it more challenging to wash.
Various types of dirt rarely exist alone and are often mixed together, co-adsorbing onto items. Moreover, dirt can oxidize, decompose, or putrefy under external influences, leading to the generation of new dirt.
(2) Adhesion Mechanisms of Dirt
Objects such as clothes or hands acquire dirt due to some form of interaction between the object and the dirt. The adhesion of dirt to the object can be broadly categorized into two types: physical adhesion and chemical adhesion.
① Physical Adhesion: This includes the adhesion of particles such as soot, dust, mud, and carbon black to clothing, which is primarily a form of mechanical adhesion. In general, dirt adhered through physical means has relatively weak interactions with the contaminated object, making it easier to remove. Depending on the nature of the forces involved, physical adhesion can be further classified into mechanical force adhesion and electrostatic force adhesion.
A: Mechanical Force Adhesion: This refers to the adhesion of solid dirt particles (such as dust and mud) and is relatively weak. Dirt adhered through mechanical force adhesion is easily removed through mechanical methods, although it becomes more challenging when the dirt particles are very small (≤0.1 μm).
B: Electrostatic Force Adhesion: This mainly occurs when charged dirt particles adhere to objects with opposite charges. Most fibrous materials are negatively charged in water, making them easily attract positively charged dirt particles, such as lime. Even negatively charged dirt particles, like carbon black particles in aqueous solutions, can be attached to fibers through ion bridges (similar to bridges formed by ions between multiple opposite charges) formed by positive ions (e.g., Ca2+, Mg2+).
② Mechanism of Solid Dirt Removal
The removal of liquid dirt is mainly through the preferential wetting of the dirt carrier by the washing liquid. However, the mechanism for the removal of solid dirt is different. During the washing process, it mainly involves the wetting of the dirt particles and their carrier's surface by the washing liquid. Due to the adsorption of surfactants on the surface of solid dirt and its carrier, the interaction between dirt and the surface is reduced, lowering the adhesion strength of dirt particles to the surface, making it easier to remove them from the carrier's surface.
Moreover, surfactants, especially ionic surfactants, adsorbed on the surface of solid dirt and its carrier may increase the surface potential of the carrier's surface, making it more favorable for dirt removal. Solid or general fiber surfaces usually carry a negative charge in water medium, so dirt particles in water have reduced adhesion strength to the solid surface. When anionic surfactants are added, they simultaneously increase the negative surface potential of both dirt particles and the solid surface, enhancing the repulsive force between them. Therefore, the adhesion strength of the particles is further reduced, making dirt easier to remove.
Non-ionic surfactants can adsorb on generally charged solid surfaces. Although they may not significantly change the interface potential, the adsorbed non-ionic surfactants often form a certain thickness of adsorption layer on the surface, helping prevent further deposition of dirt.
As for cationic surfactants, their adsorption tends to reduce or eliminate the negative surface potential of dirt particles and their carrier's surface. This reduces the repulsion between dirt and the surface, making it unfavorable for dirt removal. Additionally, after adsorption of cationic surfactants on the solid surface, it often turns the solid surface into hydrophobic, hindering surface wetting and thus impeding the washing process.
(3) Removal of Special Stains
Proteins, starch, human secretions, fruit juice, tea, and similar stains are difficult to remove with regular surfactants and require special treatment.
Protein stains like cream, eggs, blood, milk, and skin secretions tend to coagulate and denature on fibers, making them strongly adhesive. Proteinases can be used to remove protein stains, breaking down proteins into water-soluble amino acids or oligopeptides.
Starch stains mainly come from food, including meat juices and pastes. Amylases catalyze the hydrolysis of starch stains, converting them into sugars.
Lipases catalyze the decomposition of some triglyceride stains that are hard to remove with conventional methods, such as sebum and edible oils, breaking them down into soluble glycerol and fatty acids.
Stains with color, like those from fruit juice, tea, ink, lipstick, etc., are often challenging to completely clean even with repeated washing. These stains can be treated with oxidizing or reducing agents, like bleach, to undergo redox reactions. This process disrupts the structure of chromophores or auxochromes, degrading them into smaller water-soluble components for removal.
(4) Dry Cleaning Mechanism
The discussions above primarily address washing with water as the medium. However, due to differences in garment types and structures, some fabrics are not convenient or easy to clean with water. Certain fabrics may deform or fade after washing; for example, most natural fibers swell when absorbing water and shrink upon drying, leading to deformation. Wool products may shrink after water washing, and some woolen items are prone to pilling and color fading. Silk may lose its luster and feel worse after water washing. Dry cleaning is often used for these garments, typically involving washing with organic solvents, especially non-polar solvents.
Compared to water washing, dry cleaning is a gentler method. Since dry cleaning does not require significant mechanical action, it minimizes the risk of damage, wrinkles, and deformation to clothing. Dry cleaning agents, unlike water, seldom cause expansion and contraction. With proper technical handling, dry cleaning can result in garments that maintain their shape, color, and extended lifespan.
From a dry cleaning perspective, various stains can be broadly categorized into three types.
① Oil-Soluble Stains: These include various oils and fats, existing in liquid or greasy form and soluble in dry cleaning solvents.
② Water-Soluble Stains: These stains dissolve in aqueous solutions but are insoluble in dry cleaning agents. They are initially adsorbed onto clothing in a water-soluble state, forming solid particles upon water evaporation, such as inorganic salts, starch, proteins, etc.
③ Oil-and-Water-Insoluble Stains: These stains do not dissolve in water or dry cleaning solvents. Examples include carbon black, silicates, and oxides of various metals.
Due to the different natures of these stains, various methods are employed in the dry cleaning process for their removal. Oil-soluble stains, like vegetable and mineral oils, are easily soluble in organic solvents, making them readily removable in dry cleaning. The excellent solubilizing ability of dry cleaning solvents for oils and fats essentially stems from intermolecular van der Waals forces.
For the removal of water-soluble stains such as inorganic salts, sugars, proteins, sweat, etc., an appropriate amount of water must be added to the dry cleaning agent; otherwise, water-soluble stains are challenging to remove from clothing. Since water is less soluble in dry cleaning agents, surfactants need to be added to increase the water content. The water present in dry cleaning agents hydrates stains and clothing surfaces, facilitating interaction with the polar groups of surfactants. Additionally, when surfactants form micelles, water-soluble stains and water can be solubilized into the micelles. Besides increasing the water content in dry cleaning solvents, surfactants also prevent redeposition of stains, enhancing the stain removal effect.
A small amount of water is necessary for the removal of water-soluble stains, but excessive water can lead to deformation and wrinkling of clothing. Therefore, the water content in dry cleaning agents must be moderate.
Non-water-soluble and non-oil-soluble stains, such as ash, mud, soil, and carbon black particles, are typically adsorbed or attached to clothing by electrostatic forces or combined with oil stains. In dry cleaning, the flow and impact of the solvent can dislodge statically adsorbed stains, while the solvent can dissolve oil stains, causing solid particles attached to clothing to fall into the dry cleaning agent. The small amount of water and surfactants in the dry cleaning agent stabilizes and disperses the detached solid stain particles, preventing them from redepositing onto clothing.
(5) Factors Affecting Washing Effects
The directed adsorption of surfactants at interfaces and the reduction of surface (interface) tension are the primary factors for removing liquid or solid stains. However, the washing process is complex, and even similar detergents' washing effects are influenced by many other factors. These factors include the concentration of the detergent, temperature, nature of stains, types of fibers, and the structure of fabrics.
①Surfactant Concentration
The micelles formed by surfactants in the detergent solution play a crucial role in the washing process. The washing effect sharply increases when the concentration reaches the critical micelle concentration (cmc). Therefore, the concentration of the detergent in the solvent should be higher than the cmc value for optimal washing. However, once the surfactant concentration exceeds the cmc value, the increase in washing effect becomes less significant, and excessive surfactant concentration is unnecessary.
When using the solubilizing effect to remove oil stains, even if the concentration is above the cmc value, the solubilizing effect increases with the higher concentration of surfactant. In such cases, it is advisable to use detergent more concentrated in specific areas with more dirt, such as applying a layer of detergent foam to the cuffs and collars of clothing to enhance the solubilizing effect on oil stains.
② Temperature
Temperature has a significant impact on the stain removal process. In general, raising the temperature is advantageous for stain removal, but excessively high temperatures may lead to adverse effects.
Increased temperature facilitates the diffusion of stains, and solid grease stains are easily emulsified at temperatures higher than their melting point. The swelling of fibers also increases with temperature, promoting stain removal. However, for closely woven fabrics, the reduction in microgaps between fibers after fiber swelling is unfavorable for stain removal.
Temperature changes also affect the solubility of surfactants, cmc values, micelle sizes, etc., influencing washing effectiveness. For surfactants with long carbon chains, their solubility is lower at lower temperatures, and sometimes it may even be lower than the cmc value. In such cases, it is necessary to raise the washing temperature appropriately. The effect of temperature on cmc values and micelle sizes varies for ionic and non-ionic surfactants. For ionic surfactants, an increase in temperature generally raises the cmc value and reduces micelle size, implying that the concentration of surfactant in the washing solution needs to be increased. For non-ionic surfactants, an increase in temperature leads to a decrease in the cmc value and a significant increase in micelle size. Therefore, a moderate increase in temperature helps non-ionic surfactants exert their surfactant properties. However, the temperature should not exceed their cloud point.
In summary, the most suitable washing temperature depends on the detergent's formulation and the object being washed. Some detergents exhibit good washing effects at room temperature, while for others, there can be a significant difference in the stain removal efficacy between cold and hot washing.
③Foam
People often mistakenly associate foaming ability with washing effectiveness, believing that detergents with strong foaming power have better washing results. Research results indicate that there is no direct relationship between washing effectiveness and the amount of foam. For example, low-foam detergents can provide good washing results, and their effectiveness is not inferior to high-foam detergents.
Although foam is not directly related to washing, it can still assist in stain removal in certain situations. For example, when handwashing dishes, the foam from the detergent can carry away oil droplets washed off. When scrubbing carpets, foam can also help remove solid stain particles like dust and dirt. Since dust accounts for a significant proportion of carpet stains, carpet cleaning agents should have a certain foaming ability.
Foaming ability is essential for shampoos as well. The fine and dense foam produced during hair washing or bathing provides a smooth and comfortable sensation.
④Types of Fibers and Physical Properties of Textiles
In addition to the chemical structure of fibers affecting the adhesion and removal of stains, the visual morphology of fibers and the organizational structure of yarn and fabric also influence the difficulty of stain removal.
The scales on wool fibers and the curved, flat, and band-like structure of cotton fibers make them more prone to accumulating stains. For example, carbon black adheres more easily to cellulose films (sticky gel films) and is easy to remove, while carbon black on cotton fabric is challenging to wash off. Additionally, short polyester fiber fabrics are more likely to accumulate oil stains than long fiber fabrics, and oil stains on short fiber fabrics are more difficult to remove than on long fiber fabrics.
Tightly twisted yarn and tightly woven fabrics, due to smaller microgaps between fibers, resist the invasion of stains but also hinder the washing solution from expelling internal stains. Therefore, tightly woven fabrics initially exhibit good stain resistance but may face difficulties in stain removal once soiled.
(5) Water Hardness
The concentration of metal ions such as Ca2+ and Mg2+ in water has a significant impact on washing effectiveness. Particularly, anionic surfactants encounter poor solubility when forming calcium and magnesium salts with Ca2+ and Mg2+ ions, reducing their stain-removing ability. Even if the concentration of surfactants is relatively high in hard water, the washing effect is much inferior compared to distilled water. To achieve the optimal washing effect with surfactants, the concentration of Ca2+ ions in water should be reduced to below 1 × 10-6 mol/L (CaCO3 should be reduced to 0.1 mg/L) or lower. This requires the addition of various water softeners to the detergent.
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