Surfactant Basics: Kraft Point, CMC, HLB, and Cleaning Applications

(1) Kraft Point, Cloud Point

Effect of temperature on solubilization:

• ★ Kraft Point: For ionic surfactants, as the temperature increases to a certain value, the solubility of the surfactant increases sharply. This temperature is the Kraft point.
• ★ Cloud Point: For nonionic surfactants, as the temperature increases to a certain value, the solubility of the surfactant decreases sharply and the solution becomes turbid. This temperature is the cloud point.
• Surfactant blending: The combination of surfactants with each other or with other compounds can enhance solubilization capacity and reduce usage.

(2) CMC (Critical Micelle Concentration)

★ Definition: As the concentration of a surfactant in water increases, the surfactant molecules at the surface form a tightly arranged monomolecular layer in an oriented manner. The excess molecules in the bulk phase also cluster together in small groups via hydrophobic groups to form micelles. The minimum concentration at which micelles begin to form is called the critical micelle concentration.

For surfactants in solution, the lowest concentration at which micelles start to form is the critical micelle concentration.

Micelle shapes: Spherical, rod-like, lamellar

★ Functions of micelles: Emulsification; foaming; dispersion; solubilization; catalysis

• Wetting: The phenomenon where a liquid contacts a solid surface, the original solid-gas interface disappears, and a new solid-liquid interface forms. It is a result of reduced solution surface tension and adsorption on the solution surface.
• Solubilization: Substances with strong lipophilicity can significantly increase solubility in micelles with similar properties, forming a transparent solution. This process is called solubilization. The solubilization system is a thermodynamically stable isotropic solution.
• Solubilization capacity: The saturated concentration of a solubilized substance dissolved in a surfactant solution of a certain concentration is called the solubilization capacity.
• Emulsification: The process in which one liquid phase is dispersed as droplets into another immiscible liquid phase, forming a heterogeneous liquid dispersion system (called an emulsion). This process is emulsification. The surfactant acts as an emulsifier, dispersing one liquid as very fine droplets evenly into the other.
• Foam: Air enters the solution, liquid films surround the gas to form bubbles. Due to buoyancy, the bubbles rise to the solution surface and eventually escape from the liquid surface to form a bimolecular film. It is a dispersion system of gas dispersed in liquid.

★ Factors affecting CMC:

1. Surfactant structure: Mainly includes the hydrocarbon chain length (C↑, CMC↓), number of hydrocarbon chain branches (more branches, weaker interaction between hydrocarbon chains, CMC↑), position of the polar group (polar group in the middle of the hydrocarbon chain, CMC↑), other substituents in the hydrocarbon chain (polar groups in the chain, CMC↑), hydrophilic group (CMC ionic > CMC nonionic).
2. External conditions: Temperature (T↑, CMC nonionic ↓).

(3) HLB Value (Hydrophilic-Lipophilic Balance)

★ Definition: Indicates whether the overall tendency of the molecule is hydrophilic or lipophilic after internal balancing. This measure of the combined hydrophilic-lipophilic effect strength is the HLB value of the surfactant.

The stronger the polarity of the polar group in the amphiphilic molecule, the higher the HLB value and the stronger the hydrophilicity; the longer the nonpolar group in the amphiphilic molecule, the lower the HLB value and the weaker the hydrophilicity.

Hydrophilicity = Hydrophilicity of hydrophilic group / Hydrophobicity of hydrophobic group

(4) EO Number (Number of Ethylene Oxide Units)

The EO number of nonionic polyoxyethylene surfactants:

• R (hydrophobic group) generally has the best wetting performance with C7~C11; wetting decreases above C12.
• When EO = 10~12, wetting is best;
• When EO > 12, wetting decreases sharply;
• When the EO number is low, wetting performance is also poor.

(5) Plateau Border

In foam, the intersection points of bubbles (usually three bubbles meet) form the so-called Plateau border, indicated at point A in the diagram.

As shown, B is the intersection of two bubbles, where the gas-liquid interface is relatively flat and can be approximated as a flat liquid surface. A is the intersection of three bubbles, where the liquid surface is concave. The internal pressure of the liquid at A is lower than that at the flat surface at B. Thus, the liquid pressure at B is greater than at A, causing liquid to automatically flow from B to A, thinning the liquid film at B. This is an automatic drainage process of foam.

When the liquid film becomes thin enough, it can lead to film rupture and foam collapse. Another drainage process is downward drainage due to gravity, which also thins the liquid film.

(6) Factors Affecting Foam Stability

Foam is a thermodynamically unstable system. After foam breaks, the total surface area of the system decreases, and energy is reduced. This is a spontaneous process, and foam will eventually collapse.

The foam collapse process mainly involves the thinning of the liquid film separating gas bubbles until it breaks. Therefore, foam stability mainly depends on the rate of liquid drainage and the strength of the liquid film. The main factors affecting foam stability are those that influence film thickness and surface film strength.

(7) Defoamers

Defoamer: A substance that can eliminate existing foam.

Antifoam (foam inhibitor): A substance that can prevent foam from forming.

★ Functions of defoamers:

1. Lower local surface tension
2. Destroy the elasticity of the interfacial film, preventing its self-repair
3. Reduce film viscosity
4. Solid particles

★ Defoaming mechanisms:

1. Defoamers reduce the local surface tension of the foam liquid film, causing foam collapse.
2. Defoamers destroy film elasticity, causing the liquid film to lose self-repair capability and collapse.
3. Defoamers reduce film viscosity, shortening foam lifetime and causing collapse.
4. Solid particle defoaming mechanism: The solid particle surface must be hydrophobic.

(8) Emulsions

★ I. Emulsion: A dispersion system formed by uniformly dispersing one or more liquids as droplets into another immiscible liquid.

★ II. Conditions for emulsion formation:

1. Two immiscible phases must exist;
2. An emulsifier must be present;
3. Appropriate stirring conditions must be provided.

III. Identifying emulsion type:

1. Dilution method: Oil-in-water (O/W) emulsions are miscible with water; water-in-oil (W/O) emulsions are miscible with oil.
2. Conductivity method: Based on the large difference in conductivity between water and oil. O/W emulsions have high conductivity and can cause a neon lamp in series to light up.
3. Filter paper wetting method: Water spreads well on paper. Drop the emulsion on filter paper. If the liquid spreads quickly, leaving a small oil droplet in the center, it is O/W type. If it does not spread, it is W/O type.
4. Viscosity method: Adding the dispersed phase generally increases the viscosity of the emulsion. Compare viscosity before and after adding water: if viscosity increases, it is W/O type; otherwise, O/W type.
5. Refractive index method: Differences in refractive indices of oil and water can also identify emulsion type.
6. Fluorescence method: Fluorescent dyes produce color under UV light; fluorescent dyes are usually oil-soluble.
7. Staining method: Add "Sudan III" (oil-soluble dye): In W/O emulsions, the whole system appears red; in O/W emulsions, the dye remains unchanged. Add methylene blue (water-soluble): In O/W emulsions, the whole system appears blue; in W/O emulsions, the dye remains unchanged.

★ IV. Characteristics of emulsions: Multi-phase system, large interfacial area, high surface free energy, thermodynamically unstable system.

★ V. Factors affecting emulsion stability:

1. Effect of surface tension: Low surface tension indicates that emulsions are easier to form, but it is not the only factor for stability.
2. Effect of interfacial film properties: ★ The strength and compactness of the interfacial film are important factors determining emulsion stability. Two aspects to note: (1) Use sufficient emulsifier; (2) Select emulsifiers with appropriate molecular structure.
3. Effect of interfacial charge: (1) Diffusion double layer; (2) Diffusion double layer in O/W emulsions; (3) Effect of electrolytes on emulsion stability. For W/O emulsions or emulsions formed by nonionic surfactants, electrolytes have little effect.
4. Effect of external phase viscosity: (1) Higher external phase viscosity increases resistance to droplet movement, making it harder for droplets to collide and coalesce; (2) Increased external phase viscosity strengthens the interfacial film, improving emulsion stability. The internal phase viscosity is generally considered to have little effect on stability.
5. Stabilizing effect of solid emulsifiers:

VII. Solid emulsifier: Solid particles that are both hydrophilic and lipophilic.

★ VIII. Factors affecting emulsion type: Mainly depends on the type of emulsifier.

1. HLB value: The HLB value of a surfactant can determine the type of emulsion formed: HLB 2~6: forms W/O emulsions; HLB 12~18: forms O/W emulsions. Emulsifier selection based on HLB: HLB↑, lipophilicity↓, < 8 lipophilic; HLB↑, hydrophilicity↑, > 8 hydrophilic.
2. Phase volume theory: According to this theory, when 0.2598 < φ < 0.7402, both O/W and W/O emulsions can form; when φ < 0.2498 or φ > 0.7402, only one type of emulsion can form.

(9) Solubility Rule (Bancroft Rule)

★ Rule: When a surfactant has greater solubility in one phase, that phase is usually the continuous phase (external phase). Conventionally, the distribution constant (k) is used to measure the solubility of emulsifier molecules in oil and water phases.

Distribution coefficient k = Concentration of emulsifier in water / Concentration of emulsifier in oil

When the distribution coefficient is relatively large, O/W emulsions are easily obtained; the larger the coefficient, the more stable the O/W emulsion. When the distribution coefficient is relatively small, W/O emulsions are obtained; the smaller the coefficient, the more stable the W/O emulsion.

(10) Special Surfactants

★ Unique properties of fluorosurfactants: "Three highs": high surface activity, high thermal stability, and high chemical stability. "Two repellencies": the fluorocarbon chain is both hydrophobic and oleophobic.

Fluorosilicone surfactant characteristics: "Three protections": water repellent, stain repellent, oil repellent.

Polymeric surfactants: Poor surface activity, good emulsification, foam stabilizer, good dispersibility and flocculation, thickening.

(11) Three Manifestations of Emulsion Instability

In emulsion theory, an important topic is creaming, inversion, and demulsification. These are three manifestations of emulsion instability.

• Creaming (stratification): An emulsion splits into two emulsions, one layer contains more dispersed phase than the other, and the opposite in the other layer. During creaming, the interfacial film is not destroyed, so creaming is not demulsification, but creaming eventually leads to demulsification.

(12) Dispersion Systems

Dispersion systems are thermodynamically unstable, and their stability is affected by time, pH, temperature, additives, etc.

Dispersion: Generally refers to the process of dispersing one substance into another to form a dispersion system. The dispersed phase is called the dispersed phase, and the other is the dispersion medium.

The stability of a sol under certain conditions depends on the potential energy of interaction between colloidal particles. The total potential energy equals the sum of van der Waals attraction potential energy and electrostatic repulsion potential energy from the double layer. That is, dispersion systems are subject to two forces: attraction + repulsion.

★ Dispersing action of surfactants:

1. Three basic processes of dispersing solid powder into liquid:
   1. Dispersion medium must completely wet the solid powder; liquid replaces air on the solid surface;
   2. Agglomerates of particles are de-agglomerated and dispersed;
   3. Prevent re-agglomeration of dispersed particles.
2. Role of surfactants in the above processes:
   1. For process a, based on the spreading coefficient.
   2. For process b (de-agglomeration and dispersion): Surfactants adsorb into "micro-cracks" of solids, reducing the mechanical work required for particle splitting. Ionic surfactants adsorb on particle surfaces, giving particles the same charge, causing repulsion and easy dispersion.
   3. For process c (preventing re-agglomeration): In water as dispersion medium, surfactants adsorb with hydrophilic groups oriented toward the water phase, thus reducing the interfacial tension between particles and water, improving dispersion system stability.

(13) Solubilization

★ Definition: In an aqueous solution, the presence of surfactants can significantly increase the solubility of organic substances that are originally insoluble or only slightly soluble in water, compared to pure water. This effect is called solubilization.

★ Characteristics of solubilization:

1. Solubilization only becomes apparent above the CMC, when a large number of micelles are formed.
2. Solubilization differs from hydrotropy. Hydrotropy refers to increasing solubility by using a mixed solvent. For example, a substance insoluble in water can be dissolved by adding another water-miscible solvent. Hydrotropy requires large amounts of water-miscible organic solvents.
3. Solubilization differs from emulsification. Solubilization is a spontaneous process; the resulting solution is homogeneous and transparent, a thermodynamically stable system. Emulsification is not spontaneous; it produces a thermodynamically unstable polydisperse system.
4. Solubilization also differs from general dissolution. Dissolution significantly affects colligative properties such as freezing point depression and osmotic pressure.

★ Modes of solubilization:

1. Solubilization into the core of micelles.
2. Solubilizate molecules intercalate with surfactant molecules in the palisade layer.
3. Solubilization onto the surface of micelles (adsorbed on the micelle surface).
4. Solubilization between the polar polyoxyethylene layers outside the micelle.

The order of solubilization capacity for the above four modes: d > b > a > c

Factors affecting solubilization capacity:

1. Surfactant structure
2. Solubilizate structure
3. Electrolytes
4. Organic additives
5. Temperature

a. Effect of surfactant structure on solubilization:

★ What are the structural characteristics of surfactants? Amphiphilic molecules: hydrophobic group and hydrophilic group.

Hydrophobic group: carbon chain length, branched or linear, presence of unsaturated bonds or other polar groups, containing fluorine or silicon, etc.

Hydrophilic group: type of ion, type of ionic group, counterion of hydrophilic group, length of polyoxyethylene chain, etc.

Two basic principles:

1. The larger the micelle or the greater its aggregation number, the stronger the solubilization capacity.
2. The lower the CMC, the stronger the solubilization capacity.

Micelle aggregation number: The number of surfactant molecules or ions that associate to form one micelle. The larger the aggregation number, the larger the micelle.

(14) Detergency (Washing Action)

★ Factors affecting detergency:

1. Properties of the fabric and soil, and the binding state between soil and fabric.
2. Composition of the wash liquor:
   1. Properties and concentration of surfactants; (★ Factors affecting surfactant detergency?)
   2. Properties and concentration of builders;
   3. Properties and concentration of suspended matter (e.g., dirt) brought in by the fabric;
   4. pH of the wash liquor;
   5. Changes in composition and properties of the wash liquor during washing.
3. Physical and mechanical conditions of the washing process.
4. Relative quantities of soil, fabric, and wash liquor, etc.
5. Washing temperature.
6. Soil particle size.

Solubilization during washing: Micellar solubilization is not the main factor in detergency; surface activity is the primary factor affecting washing performance.

★ Role of surfactants in the washing process?

The length of the hydrophobic chain of surfactants affects washing performance. Generally, longer hydrocarbon chains give better washing performance. However, if the chain is too long, solubility becomes poor, and washing performance decreases.

Washing process:

1. Adsorption of detergent onto oil and fiber surfaces.
2. Wetting and penetration of soil.
3. Detachment of soil.
4. Emulsification and dispersion of soil.

(15) Adsorption on Solid Surfaces

Reasons for adsorption on solid surfaces: Molecules inside a solid experience symmetric intermolecular forces, while molecules on the surface experience asymmetric forces. The inward side is subject to greater force from internal molecules, while the outward side is subject to less force. Therefore, when gas molecules or solute molecules in solution collide with the solid surface, they are attracted and remain on the surface.

Surface adsorption: The phenomenon where the concentration of gas or liquid on a solid surface is higher than its bulk concentration is called solid surface adsorption.

Adsorption: The transfer of a substance from one phase to another phase is called adsorption.

Factors affecting adsorption:

1. Adsorbent structure: (a) Specific surface area; (b) Pore structure; (c) Surface chemical properties.
2. Adsorbate properties: For a given adsorbent, the adsorption effect varies due to differences in adsorbate properties.
3. Operating conditions: Adsorption is an exothermic process; low temperature favors adsorption, while increasing temperature favors desorption.

(16) Nonionic Surfactants

Nonionic surfactants do not ionize in aqueous solution. Their hydrophilic groups mainly consist of a certain number of oxygen-containing groups (ether or hydroxyl) that form hydrogen bonds with water to achieve solubility.

Characteristics of nonionic surfactants:

1. They are the second largest class of surfactants, with production volume only lower than anionic surfactants.
2. Since nonionic surfactants do not dissociate into ions in aqueous solution, they have high stability, are not affected by acids, bases, or salts, and have strong hard water resistance.
3. Good compatibility with other surfactants and additives; can be used in combination with anionic, cationic, and amphoteric surfactants.
4. Because they do not ionize in solution, they do not readily adsorb strongly on general solid surfaces.
5. The physicochemical properties of polyoxyethylene-type nonionic surfactants are strongly dependent on temperature; as temperature increases, they become insoluble in water (cloud point phenomenon). However, sugar-based nonionic surfactants have normal temperature dependence, with solubility increasing as temperature rises.
6. Nonionic surfactants have high surface activity: low surface tension in aqueous solution, low CMC, high micelle aggregation number, strong solubilization, good emulsifying and detergency power.
7. Compared to ionic surfactants, nonionic surfactants generally have poorer foaming properties, making them suitable for low-foam detergents and other low-foam formulations.
8. Nonionic surfactants carry no charge in solution and do not bind to proteins, thus they have low toxicity and less skin irritation.
9. Most nonionic surfactant products are in liquid or paste form, unlike ionic surfactants.

Classification by hydrophilic group structure:

1. Polyethylene glycol type: Prepared by the addition reaction of hydrophobic raw materials containing active hydrogen with ethylene oxide.
2. Polyol type: Partial esters formed by fatty acids with polyols such as glycerol, sorbitan, sucrose, etc.

★ Factors affecting cloud point of nonionic surfactants:

The cloud point increases with an increase in the number of ethylene oxide adducts. However, above 100°C, the rate of increase becomes very slow. Influencing factors include: (1) Type of hydrophobic group; (2) Length of hydrophobic carbon chain; (3) Effect of hydrophilic group; (4) Effect of additives.

★ CMC of nonionic surfactants is generally 1-2 orders of magnitude lower than that of anionic surfactants. Reasons:

1. Nonionic surfactants do not ionize, carry no charge, have no electrostatic repulsion, and easily form micelles.
2. The hydrophilic portion of the molecule is relatively large in volume, relying only on polar atoms to form hydrogen bonds with water. Compared to ionic surfactants, the interaction with the solvent is weaker, making micelle formation easier.

Trends: (1) As the length of the hydrophobic carbon chain increases, hydrophilicity decreases and CMC decreases. (2) As the degree of polyoxyethylene polymerization increases, hydrophilicity increases and CMC increases.

★ Water number (for nonionic surfactants): 1.0 g of nonionic surfactant is dissolved in 30 mL of dioxane. Water is added dropwise to the solution until it becomes turbid. The volume of water consumed is the water number.

Factors affecting surface tension:

1. Effect of hydrophobic group functional groups: Different hydrophobic groups yield different surface tensions.
2. Effect of hydrophilic group: As the polyoxyethylene chain length increases (i.e., EO number increases), surface tension increases.
3. Temperature: As temperature increases, surface tension decreases.

(17) Amphoteric Surfactants

Characteristics of amphoteric surfactants:

1. Have an isoelectric point.
2. Can be blended with all other types of surfactants.
3. Low toxicity, mild to skin and eyes.
4. Hard water resistance and high electrolyte tolerance.
5. Good biodegradability.

2. Relationship between CMC and pH: Generally, the CMC of amphoteric surfactants increases with increasing pH.

3. Effect of pH on solubility and foaming: (1) At around pH 4 (isoelectric point), solubility and foam volume are both at a minimum. (2) pH > 4: fast foaming, abundant and large foam, solubility increases rapidly. (3) pH < 4: foam volume and solubility are also relatively high.

4. Adsorption on substrates and bactericidal activity vs. pH: At pH below the isoelectric point, they exhibit cationic surfactant characteristics: high adsorption on wool and hair, strong affinity, and relatively strong bactericidal power. At pH above the isoelectric point, they exist as anions, and the above properties are poor.

5. Relationship between CMC and carbon chain length for betaine surfactants: lgcmc = A - Bn, where n is the number of carbon atoms in the alkyl chain, constants A = 1.5~2, B = 29. Additionally, the types of cationic and anionic groups also affect CMC. Quaternary ammonium salts have higher CMC than quaternary phosphonium salts. Anionic group order: -COO- > -SO3- > -OSO3-.

6. Solubility and Krafft point of amphoteric surfactants: For betaine type, increasing the number of carbon atoms between the carboxyl group and nitrogen from 1 to 3 has little effect on solubility and Krafft point. Typically, carboxybetaine amphoteric surfactants have Krafft points below 4~18°C, while most sulfobetaines have Krafft points between 20~89°C. Sulfate betaines are all above 90°C.

★ Lime soap dispersing rate (LSDR): The amount (in grams) of dispersant required to keep 100 g of sodium oleate dispersed in water with a hardness of 333 mg/L as CaCO3, such that no lime soap precipitation occurs.

(18) Cationic Surfactants

In aqueous solution, they exhibit positive charge, forming positively charged surface-active ions.

Main uses: Bactericides, fabric softeners, mineral flotation agents, phase transfer catalysts, and antistatic agents.

Water solubility: Surfactants with alkyl chains of C15 or less are easily soluble in water; those with C15 or more have lower water solubility and are difficult to dissolve. Single long-chain alkyl quaternary ammonium salts are soluble in polar solvents but insoluble in nonpolar solvents. Double long-chain alkyl quaternary ammonium salts are almost insoluble in water but soluble in nonpolar solvents. Unsaturated groups in the alkyl chain of quaternary ammonium salts increase their water solubility.

Krafft temperature point: When the surfactant solution is supersaturated, the Krafft point is the triple point where ionic surfactant monomers, micelles, and undissolved solid surfactant coexist. A higher Krafft point indicates the surfactant is more difficult to dissolve and has lower solubility; conversely, a lower Krafft point indicates easier dissolution and better solubility.

The Krafft point generally has a linear relationship with the length of the surfactant hydrophobic carbon chain: Krafft point = a + bn (a and b are constants, n is the number of carbon atoms in the chain). According to this relationship, the longer the carbon chain (larger n), the higher the Krafft point.

(19) Anionic Surfactants

Characteristics of anionic surfactants:

1. Krafft point: The solubility change with temperature shows a distinct break point. At relatively low temperatures, solubility increases very slowly with temperature. When the temperature reaches a certain value, solubility increases rapidly with temperature. This temperature is the Krafft point. Generally, ionic surfactants have a Krafft point.
2. Generally, they have poor compatibility with cationic surfactants, easily forming precipitates or causing turbidity. However, under specific conditions, blending with cationic surfactants can greatly enhance surface activity.
3. Poor hard water resistance. Sensitivity to hard water follows the order: carboxylates > phosphates > sulfates > sulfonates.
4. Introducing a short polyoxyethylene chain between the hydrophobic chain and the anionic head group can greatly improve salt tolerance.
5. Introducing a short polyoxypropylene chain between the hydrophobic chain and the anionic head group can improve solubility in organic solvents but also reduces biodegradability.
6. Carboxylates easily precipitate free carboxylic acids in acid; sulfates can undergo autocatalytic decomposition rapidly in acid; other types of anionic surfactants are generally stable under normal conditions.
7. Anionic surfactants are important ingredients in household detergents, industrial cleaners, dry cleaning agents, and wetting agents.