According to the table, surfactants suitable for use as oil-in-water emulsifiers have an HLB value of 3.5 to 6, while those for water-in-oil emulsifiers fall between 8 to 18.

② Determination of HLB Values (omitted).

07 Emulsification and Solubilization

An emulsion is a system formed when one immiscible liquid is dispersed in another in the form of fine particles (droplets or liquid crystals). The emulsifier, which is a type of surfactant, is essential for stabilizing this thermodynamically unstable system by decreasing the interfacial energy. The phase existing in droplet form in the emulsion is called the dispersed phase (or internal phase), while the phase forming a continuous layer is called the dispersion medium (or external phase).

① Emulsifiers and Emulsions

Common emulsions often consist of one phase as water or aqueous solution, and the other as an organic substance, such as oils or waxes. Depending on their dispersion, emulsions can be classified as water-in-oil (W/O) where oil is dispersed in water, or oil-in-water (O/W) where water is dispersed in oil. Moreover, complex emulsions like W/O/W or O/W/O can exist. Emulsifiers stabilize emulsions by lowering interfacial tension and forming monomolecular membranes. An emulsifier must adsorb or accumulate at the interface to lower interfacial tension and impart charges to droplets, generating electrostatic repulsion, or form a high-viscosity protective film around particles. Consequently, substances used as emulsifiers must possess amphiphilic groups, which surfactants can provide.

② Methods of Emulsion Preparation and Factors Influencing Stability

There are two main methods for preparing emulsions: mechanical methods disperse liquids into tiny particles in another liquid, while the second method involves dissolving liquids in molecular form in another and causing them to aggregate appropriately. The stability of an emulsion refers to its ability to resist particle aggregation that leads to phase separation. Emulsions are thermodynamically unstable systems with higher free energy, thus their stability reflects the time needed to reach equilibrium, i.e., the time it takes for a liquid to separate from the emulsion. When fatty alcohols, fatty acids, and fatty amines are present in the interfacial film, the membrane's strength significantly increases because polar organic molecules form complexes in the adsorbed layer, reinforcing the interfacial membrane.

Emulsifiers composed of two or more surfactants are called mixed emulsifiers. Mixed emulsifiers adsorb at the water-oil interface, and molecular interactions can form complexes that significantly lower interfacial tension, increasing the amount of adsorbate and forming denser, stronger interfacial membranes.

Electrically charged droplets notably influence the stability of emulsions. In stable emulsions, droplets typically carry an electric charge. When ionic emulsifiers are used, the hydrophobic end of the ionic surfactants is incorporated into the oil phase, while the hydrophilic end remains in the water phase, imparting charge to the droplets. Like charges between droplets cause repulsion and prevent coalescence, which enhances stability. Thus, the greater the concentration of emulsifier ions adsorbed on droplets, the greater their charge and the higher the stability of the emulsion.

The viscosity of the dispersion medium also affects emulsion stability. Generally, higher viscosity mediums improve stability because they stronger impede Brownian motion of droplets, slowing the likelihood of collisions. High-molecular-weight substances that dissolve in the emulsion can increase medium viscosity and stability. Additionally, high-molecular-weight substances can form robust interfacial membranes, further stabilizing the emulsion. In some cases, adding solid powders can similarly stabilize emulsions. If solid particles are fully wetted by water and can be wetted by oil, they will be retained at the water-oil interface. Solid powders stabilize the emulsion by enhancing the film as they cluster at the interface, much like adsorbed surfactants.

Surfactants can significantly enhance the solubility of organic compounds that are insoluble or slightly soluble in water after micelles have formed in the solution. At this time, the solution appears clear, and this capability is termed solubilization. Surfactants that can promote solubilization are called solubilizers, while the organic compounds being solubilized are referred to as solubilates.

08 Foam

Foam plays a crucial role in washing processes. Foam refers to a dispersive system of gas dispersed in liquid or solid, with gas as the dispersed phase and liquid or solid as the dispersion medium, known as liquid foam or solid foam, such as foam plastics, foam glass, and foam concrete.

(1) Foam Formation

The term foam refers to a collection of air bubbles separated by liquid films. Due to the considerable density difference between the gas (dispersed phase) and the liquid (dispersion medium), and the low viscosity of the liquid, gas bubbles quickly rise to the surface. Foam formation involves incorporating a large amount of gas into the liquid; the bubbles then rapidly return to the surface, creating an aggregate of air bubbles separated by a minimal liquid film. Foam has two distinctive morphological characteristics: first, the gas bubbles often assume a polyhedral shape because the thin liquid film at the intersection of bubbles tends to become thinner, ultimately leading to bubble rupture. Second, pure liquids cannot form stable foam; at least two components must be present to create a foam. A surfactant solution is a typical foam-forming system whose foaming capacity is linked to its other properties. Surfactants with good foaming ability are called foaming agents. Though foaming agents exhibit good foaming capabilities, the foam they generate may not last long, meaning their stability is not guaranteed. To improve foam stability, substances that enhance stability may be added; these are termed stabilizers, with common stabilizers including lauryl diethanolamine and oxides of dodecyl dimethyl amine.

(2) Foam Stability

Foam is a thermodynamically unstable system; its natural progression leads to rupture, thus reducing the overall liquid surface area and decreasing free energy. The defoaming process involves the gradual thinning of the liquid film separating the gas until rupture occurs. The degree of foam stability is primarily influenced by the rate of liquid drainage and the strength of the liquid film. Influential factors include:

① Surface Tension: From an energetic perspective, lower surface tension favors foam formation but does not guarantee foam stability. Low surface tension indicates a smaller pressure differential, leading to slower liquid drainage and thickening of the liquid film, both of which favor stability.

② Surface Viscosity: The key factor in foam stability is the strength of the liquid film, primarily determined by the robustness of the surface adsorption film, measured by the surface viscosity. Experimental results indicate that solutions with high surface viscosity produce longer-lasting foam due to enhanced molecular interactions in the adsorbed film that significantly increase membrane strength.

③ Solution Viscosity: Higher viscosity in the liquid itself slows the drainage of liquid from the membrane, thereby prolonging the liquid film's lifetime before rupture occurs, enhancing foam stability.

④ Surface Tension “Repair” Action: Surfactants adsorbed to the membrane can counteract the expansion or contraction of the film surface; this is called the repair action. When surfactants adsorb to the liquid film and expand its surface area, this reduces surfactant concentration at the surface and increases surface tension; conversely, contraction leads to an increased concentration of surfactant at the surface and subsequently reduces surface tension.

⑤ Gas Diffusion Through Liquid Film: Due to capillary pressure, smaller bubbles tend to have higher internal pressure compared to larger bubbles, leading to the diffusion of gas from small bubbles into larger ones, causing small bubbles to shrink and larger ones to grow, ultimately resulting in foam collapse. The consistent application of surfactants creates uniform, finely distributed bubbles and inhibits defoaming. With surfactants tightly packed at the liquid film, gas diffusion is hindered, thus enhancing foam stability.

⑥ Effect of Surface Charge: If the foam liquid film carries the same charge, the two surfaces will repel one another, preventing the film from thinning or being broken. Ionic surfactants can provide this stabilizing effect. In summary, the strength of the liquid film is the crucial factor determining foam stability. Surfactants acting as foaming agents and stabilizers must make closely packed surface absorbed molecules, as this significantly impacts interfacial molecular interaction, enhancing the strength of the surface film itself and thus preventing liquid from flowing away from the neighboring film, making foam stability more attainable.

(3) Destruction of Foam

The fundamental principle of foam destruction involves altering the conditions that produce foam or eliminating the stabilizing factors of the foam, leading to physical and chemical defoaming methods. Physical defoaming maintains the chemical composition of the foamy solution while altering conditions like external disturbances, temperature, or pressure changes, as well as ultrasonic treatment, all effective methods for eliminating foam. Chemical defoaming refers to the addition of certain substances that interact with the foaming agents to reduce the strength of the liquid film within the foam, reducing foam stability and achieving defoaming. Such substances are called defoamers, most of which are surfactants. Defoamers typically possess notable ability to reduce surface tension and can readily adsorb to the surfaces, with a weaker interaction among the constituent molecules, thus creating a loosely arranged molecular structure. Defoamer types are varied, but they are generally nonionic surfactants, with branched alcohols, fatty acids, fatty acid esters, polyamides, phosphates, and silicone oils commonly used as excellent defoamers.

(4) Foam and Cleaning

The amount of foam does not directly correlate with the efficacy of cleaning; more foam does not mean better cleaning. For instance, nonionic surfactants may produce less foam than soap, but they may have superior cleaning capabilities. However, in certain conditions, foam can aid the removal of dirt; for example, foam from washing dishes assists in carrying away grease, while cleaning carpets allows foam to remove dirt and solid contaminants. Moreover, foam can signal the effectiveness of the detergent; excessive fatty grease often inhibits bubble formation, causing either a lack of foam or diminishing existing foam, indicating low detergent efficacy. Additionally, foam can serve as an indicator for the cleanliness of rinsing, as foam levels in rinse water often decrease with lower detergent concentrations.

09 Washing Process

Broadly speaking, washing is the process of removing unwanted components from the object being cleaned to achieve a certain purpose. In common terms, washing refers to the removal of dirt from the surface of the carrier. During washing, certain chemical substances (like detergents) act to weaken or eliminate the interaction between the dirt and the carrier, transforming the bond between dirt and the carrier into a bond between dirt and detergent, allowing for their separation. Given that the objects to be cleaned and the dirt that needs removing can vary greatly, washing is a complicated process, which can be simplified into the following relationship:

Carrier • Dirt + Detergent = Carrier + Dirt • Detergent. The washing process can generally be divided into two stages:

1. The dirt is separated from the carrier under the detergent's action;

2. The separated dirt is dispersed and suspended in the medium. The washing process is reversible, meaning the dispersed or suspended dirt can potentially re-settle onto the cleaned item. Thus, effective detergents not only need an ability to detach dirt from the carrier but also to disperse and suspend the dirt, preventing it from resettling.

(1) Types of Dirt

Even a single item can accumulate different types, compositions, and amounts of dirt depending on its usage context. Oily dirt consists mainly of various animal and plant oils and mineral oils (like crude oil, fuel oil, coal tar, etc.); solid dirt includes particulate matter such as soot, dust, rust, and carbon black. Regarding clothing dirt, it can originate from human secretions like sweat, sebum, and blood; food-related stains like fruit or oil stains and seasonings; residues from cosmetics like lipstick and nail polish; atmospheric pollutants like smoke, dust, and soil; and additional stains like ink, tea, and paint. This variety of dirt can generally be categorized into solid, liquid, and special types.

① Solid Dirt: Common examples include soot, mud, and dust particles, most of which tend to have charges—often negatively charged—that adhere easily to fibrous materials. Solid dirt is generally less soluble in water but can be dispersed and suspended in detergents. Particles smaller than 0.1μm can be particularly challenging to remove.

② Liquid Dirt: These include oily substances that are oil-soluble, comprising animal oils, fatty acids, fatty alcohols, mineral oils, and their oxides. While animal and vegetable oils and fatty acids can react with alkalis to form soaps, fatty alcohols and mineral oils do not undergo saponification but can be dissolved by alcohols, ethers, and organic hydrocarbons, and can be emulsified and dispersed by detergent solutions. Liquid oily dirt is usually firmly adhered to fibrous materials due to strong interactions.

③ Special Dirt: This category consists of proteins, starches, blood, and human secretions like sweat and urine, as well as fruit and tea juices. These materials often firmly bind to fibers through chemical interactions, making them harder to wash out. Various types of dirt rarely exist independently, rather they mix together and adhere collectively to surfaces. Often, under external influences, dirt can oxidize, decompose, or decay, producing new forms of dirt.

(2) Adhesion of Dirt

Dirt clings to materials like clothing and skin due to certain interactions between the object and dirt. The adhesive force between dirt and the object can result from either physical or chemical adhesion.

① Physical Adhesion: Adhesion of dirt like soot, dust, and mud largely involves weak physical interactions. Generally, these types of dirt can be removed relatively easily due to their weaker adhesion, which mainly arises from mechanical or electrostatic forces.

A: Mechanical Adhesion**: This typically refers to solid dirt like dust or sand that adheres through mechanical means, which is relatively easy to remove, although smaller particles under 0.1μm are quite difficult to clean off.

B: Electrostatic Adhesion**: This involves charged dirt particles interacting with oppositely charged materials; commonly, fibrous materials carry negative charges, allowing them to attract positively charged adherents like certain salts. Some negatively charged particles can still accumulate on these fibers via ionic bridges formed by positive ions in the solution.

② Chemical Adhesion: This refers to dirt adhering to an object through chemical bonds. For example, polar solid dirt or materials like rust tends to adhere firmly due to the chemical bonds formed with functional groups such as carboxyl, hydroxyl, or amine groups present in fibrous materials. These bonds create stronger interactions, making it more difficult to remove such dirt; special treatments may be necessary to clean effectively. The degree of dirt adhesion depends on both the properties of the dirt itself and those of the surface it adheres to.

(3) Mechanisms of Dirt Removal

The objective of washing is to eliminate dirt. This involves utilizing the diverse physical and chemical actions of detergents to weaken or eliminate the adhesion between dirt and the washed items, aided by mechanical forces (like manual scrubbing, washing machine agitation, or water impact), ultimately leading to the separation of dirt.

① Mechanism of Liquid Dirt Removal

A: Wetness: Most liquid dirt is oily and tends to wet various fibrous items, forming an oily film over their surfaces. The first step in washing is the detergent's action that causes wetting of the surface.
B: Rollup Mechanism for Oil Removal: The second step of liquid dirt removal happens through a rollup process. The liquid dirt that spreads as a film on the surface progressively rolls into droplets due to the washing liquid's preferential wetting of the fibrous surface, ultimately being replaced by the washing liquid.

② Mechanism of Solid Dirt Removal

Unlike liquid dirt, the removal of solid dirt relies on the washing liquid's ability to wet both the dirt particles and the surface of the carrier material. The adsorption of surfactants on the surfaces of solid dirt and the carrier reduces their interaction forces, thereby lowering the adhesion strength of the dirt particles, making them easier to remove. Furthermore, surfactants, especially ionic surfactants, can increase the electric potential of solid dirt and the surface material, facilitating further removal.

Nonionic surfactants tend to adsorb on generally charged solid surfaces and can form a significant adsorbed layer, leading to reduced resettling of dirt. Cationic surfactants, however, may reduce the electric potential of dirt and the carrier surface, which leads to diminished repulsion and hampers dirt removal.

③ Removal of Special Dirt

Typical detergents may struggle with stubborn stains from proteins, starches, blood, and bodily secretions. Enzymes like protease can effectively remove protein stains by breaking down proteins into soluble amino acids or peptides. Similarly, starches can be decomposed to sugars by amylase. Lipases can help decompose triacylglycerol impurities which are often hard to remove through conventional means. Stains from fruit juices, tea, or ink sometimes require oxidizing agents or reductants, which react with the color-generating groups to degrade them into more water-soluble fragments.

(4) Mechanism of Dry Cleaning

The aforementioned points pertain primarily to washing with water. However, due to the diversity of fabrics, some materials may not respond well to water washing, leading to deformation, color fading, etc. Many natural fibers expand when wet and easily shrink, leading to undesirable structural changes. Thus, dry cleaning, typically using organic solvents, is often preferred for these textiles.

Dry cleaning is milder compared to wet washing, as it minimizes mechanical action that could damage clothes. For effective dirt removal in dry cleaning, dirt is categorized into three main types:

① Oil-soluble Dirt: This includes oils and fats, which dissolve readily in dry cleaning solvents.

② Water-soluble Dirt: This type can dissolve in water but not in dry cleaning solvents, comprising inorganic salts, starches, and proteins, which may crystallize once water evaporates.

③ Dirt that is Neither Oil- nor Water-soluble: This includes substances like carbon black and metallic silicates that do not dissolve in either medium.

Each dirt type requires different strategies for effective removal during dry cleaning. Oil-soluble dirt is methodologically removed using organic solvents due to their excellent solubility in nonpolar solvents. For water-soluble stains, adequate water must be present in the dry cleaning agent since water is crucial for effective dirt removal. Unfortunately, since water has minimal solubility in dry cleaning agents, surfactants are often added to help integrate water.

Surfactants enhance the cleaning agent's capacity for water and aid in ensuring the solubilization of water-soluble impurities within micelles. Additionally, surfactants can inhibit dirt from forming new deposits after washing, enhancing cleaning efficacy. A slight addition of water is essential for removing these impurities, but excessive amounts can lead to fabric distortion, thus necessitating a balanced water content in dry cleaning solutions.

(5) Factors Influencing the Washing Action

The adsorption of surfactants on interfaces and the resultant reduction of interfacial tension is crucial for removing liquid or solid dirt. However, washing is inherently complex, influenced by numerous factors across even similar detergent types. These factors include detergent concentration, temperature, dirt properties, fiber types, and fabric structure.

① Concentration of Surfactants: Micelles formed by surfactants play a pivotal role in washing. The washing efficiency dramatically increases once the concentration surpasses the critical micelle concentration (CMC), hence detergents should be used at concentrations higher than the CMC for effective washing. However, detergent concentrations above CMC yield diminishing returns, making excess concentration unnecessary.

② Effect of Temperature: The temperature has a profound influence on cleaning efficacy. Generally, higher temperatures facilitate dirt removal; however, excessive heat may have adverse effects. Raising the temperature tends to aid dirt dispersion and may also cause oily dirt to emulsify more readily. Yet, in tightly woven fabrics, increased temperature making fibers swell can inadvertently reduce removal efficiency.

Temperature fluctuations also affect surfactant solubility, CMC, and micelle counts, thus influencing cleaning efficiency. For many long-chain surfactants, lower temperatures reduce solubility, sometimes below their own CMC; thus, appropriate warming may be necessary for optimal function. Temperature impacts on CMC and micelles differ for ionic versus nonionic surfactants: increasing the temperature typically elevates the CMC of ionic surfactants, thus requiring concentration adjustments.

③ Foam: There is a common misconception linking foaming ability with washing effectiveness—more foam does not equal superior washing. Empirical evidence suggests that low-foaming detergents can be equally effective. However, foam may assist dirt removal in certain applications, such as in dishwashing, where foam helps displace grease or in carpet cleaning, where it lifts dirt. Moreover, foam presence can indicate whether detergents are functioning; excess grease can inhibit foam formation, while diminishing foam signifies reduced detergent concentration.

④ Fiber Type and Textile Properties: Beyond chemical structure, the appearance and organization of fibers influence dirt adhesion and removal difficulty. Fibers with rough or flat structures, like wool or cotton, tend to trap dirt more readily than smooth fibers. Closely woven fabrics may initially resist dirt accumulation but can hinder effective washing due to limited access to trapped dirt.

⑤ Hardness of Water: The concentrations of Ca²⁺, Mg²⁺, and other metallic ions significantly impact washing outcomes, particularly for anionic surfactants, which can form insoluble salts that diminish cleaning efficacy. In hard water even with adequate surfactant concentration, cleaning effectiveness falls short compared to distilled water. For optimal surfactant performance, the concentration of Ca²⁺ must be minimized to below 1×10⁻⁶ mol/L (CaCO₃ below 0.1 mg/L), often necessitating the inclusion of water-softening agents within detergent formulations.