Comprehensive Guide to Sulfate Surfactants: From Structure to Application

1. Representative Sulfate Surfactants

In personal care formulations, the most commonly used sulfate surfactants are alkyl sulfates and alkyl ether sulfates derived from fatty alcohols.
Typical representatives include Sodium Lauryl Sulfate (SLS), also known as Sodium Lauryl Sulfate (SDS), and Sodium Laureth Sulfate (SLES).
The common feature of these surfactants is a hydrophilic head group with a sulfate ester group (–OSO₃⁻) connected to a hydrophobic hydrocarbon chain, but they differ in the number of polyoxyethylene (ethoxy) units and the cations** (such as Na⁺ vs. NH₄⁺).

1.2 Key Physical Properties

• HLB Value:Sulfate surfactants are highly hydrophilic.

  
  

  For example, SLS (sodium lauryl sulfate) has an HLB value as high as approximately 40, far exceeding the conventional 0–20 HLB scale used for nonionic emulsifiers. This reflects its extremely high water solubility and excellent compatibility with oil-in-water (O/W) systems. SLES (sodium lauryl ether sulfate) also has a similarly high HLB value (approximately 40), which stems from its structure combining ionic sulfate groups and polyether segments, giving it good stabilizing properties in aqueous formulations.
  In contrast, many milder nonionic surfactants typically have HLB values ​​in the teens or even lower. Such a high HLB indicates that these sulfates are strongly biased towards the aqueous phase in formulations and contribute almost nothing to lipophilicity.

• CMC Value:refers to the lowest concentration at which surfactant monomers begin to self-assemble into micelles.

  
  

  The CMC of SLS is approximately 8.2 mM at 25°C (equivalent to 0.2–0.3% by weight), which is low for an anionic surfactant containing a C₁₂ alkyl chain, indicating good surface activity even at low concentrations.
  Introducing an ethoxylated (EO) unit further reduces the CMC: the CMC of SLES is typically around 0.8–1 mM, about an order of magnitude lower than that of SLS. Studies show that further ethoxylation has limited effect on CMC beyond the first EO unit. This low CMC value means that only small amounts of SLS/SLES are needed in formulations to achieve effective micelle formation and surface tension reduction.
  In water, SLS micelles aggregate at approximately 60 and have a high charge density (ionization degree of approximately 30%); SLES micelles have a slightly lower aggregate number due to their larger ethoxylated structure (e.g., laureth-3 sulfate reported aggregates around 40).

• Solubility & Krafft Point:

  As sodium salts of sulfated alcohols, both SLS and SLES can form clear solutions in water, but their solubility varies with temperature.
  

  • The Kraff point of the SLS is approximately 16°C.
    Below this temperature, its solubility decreases sharply, and hydrated crystals may form. For example, a 0.1–0.5% SLS solution may not dissolve completely at about 21°C and needs to be heated to above 25°C to dissolve completely.

  • The ethoxy structure of SLES significantly lowers the Kraff point.
    It maintains good solubility even at temperatures close to 0°C, making it more suitable for stable formulations in personal care products at low temperatures.

  Both exhibit good solubility in both soft and hard water, but high electrolyte concentrations can affect their solubility (see next section for details). They are almost insoluble in nonpolar solvents and have limited solubility in short-chain alcohols such as ethanol.

• Salt effect & pH stability:

  
  

  As ionic surfactants, alkyl sulfates exhibit a common “salting-out” effect at high salt concentrations:

  • Adding electrolytes (especially polyvalent cations) can lower their CMC and may trigger phase separation or precipitation.
    For example, in neutral solutions, SLS (SDS) will precipitate from aqueous solutions when the Na⁺ concentration exceeds approximately 0.4 M. The water solubility of its sodium salt is limited (approximately 0.5 M at room temperature), and beyond this range, cation depletion will lead to the formation of solid or liquid crystal precipitates.

  • Divalent cations (such as Ca²⁺ and Mg²⁺ in hard water) are more likely to cause sulfates to form insoluble salt precipitates.
    Although SLS is more resistant to hard water and less likely to form “soap scum” than soap (fatty acid salts), it can still precipitate calcium lauryl sulfate or magnesium lauryl sulfate under high concentration Ca²⁺/Mg²⁺ conditions.
    The ethoxy structure of SLES gives it higher electrolyte tolerance. When the degree of ethoxylation is ≥2, it can prevent precipitation in moderately hard water and reduce adsorption to salt or soil surfaces.

  • Therefore, SLES is more favored in formulations that require resistance to ionic interference, such as shampoos.

  In terms of pH stability, sulfate surfactants are very stable in the neutral to alkaline range and can be safely used in alkaline detergents without hydrolysis.

  • However, in strongly acidic environments (pH ＜ ~4), especially at high temperatures, its sulfate half-ester structure may slowly hydrolyze, releasing sulfuric acid and the corresponding alcohols.

  • SLS and SLES are relatively stable at room temperature, even in slightly acidic formulations (such as personal care products with pH 5–7).

  • In contrast, sulfonates (such as alkylbenzene sulfonates) are stable even under strong acid conditions, so sulfate esters are generally not used in strongly acidic formulations, or their stability and irritation need to be protected by co-surfactants (such as amphoteric surfactants).

• Summary of physicochemical properties:

  • Lauryl sulfate (LS) is a highly hydrophilic surfactant with extremely high HLB values;

  • It can form micelles even at low concentrations, effectively reducing surface tension;

  • It is soluble in water, but SLS requires a temperature above the Krafft point, while SLES is also soluble at low temperatures;

  • It performs well in the presence of appropriate electrolytes, but may precipitate in high-salt or hard water;

  • It is stable under neutral to slightly acidic conditions, but may hydrolyze under strong acid/high temperature conditions;

2. Performance in Personal Care

2.1 Foaming:

Sulfate surfactants are favored for their excellent foaming ability:

• SLS (sodium lauryl sulfate) can quickly generate a large amount of foam, with large bubbles rich in air;

• SLES (sodium lauryl ether sulfate), on the other hand, forms a finer, creamier foam with greater stability, and is therefore often used as a primary foaming agent in shampoos and shower gels.

During use, the abundant foam produced by SLS gives users the impression of “powerful cleaning,” which is one of the reasons for its widespread use. In standard foam tests (such as Ross-Miles foam height), SLS and SLES perform far better than most other types of surfactants.

While foam does not directly participate in the cleaning process, it helps the product distribute on the skin and hair and enhances the user experience.

In addition, sulfate surfactants and amphoteric surfactants (such as betaines) have a foam synergistic effect, which can maintain foam stability during use.

For example, a shampoo containing about 10% SLES can produce a rich lather even in the presence of oil or conditioning ingredients, thanks to its strong foaming properties.

2.2 Cleaning ability

Stain removal and cleaning power

Lauryl sulfate is a highly effective cleaning agent, excelling at removing grease, dust, and particulate dirt. Its superior cleaning power stems from two main factors:

1. 1. Strong wetting effect: Reduces the surface tension of water, allowing it to penetrate deep into the oil layer;
   2. Micelle formation:Encapsulates and dissolves oily dirt.

SLS is particularly effective at removing grease and is often used in experiments to break down and dissolve proteins and lipids (such as protein denaturation in SDS-PAGE).

In personal care, sulfate-containing facial cleansers are effective at removing sebum, makeup residue, and other impurities.

Dirt emulsification ability

When the concentration exceeds the CMC, the micelles formed by SLS/SLES can effectively emulsify oily dirt and carry it into the wash water. However, they themselves have a weak ability to suspend particulate heavy dirt, so high molecular weight polymers or auxiliary surfactants are often added to the formulation to enhance the anti-redeposition ability.

Sulfate surfactants are perfectly adequate for handling minor oil stains (such as sebum and cosmetics) that come into contact with daily.

These types of products often produce a “squeaking” cleaning sensation after use, which is a sign that they thoroughly remove grease. However, this strong cleaning power can also cause irritation.

2.3 Skin and eye irritation

One known drawback of sulfates is their relatively strong irritant properties:

• SLS;It is considered a relatively irritating surfactant that can penetrate and denature skin proteins, damaging the skin barrier and causing discomfort such as erythema and itching. It has even been used in positive-control irritation tests in dermatology.

• Even at concentrations of just a few percent, SLS can cause discomfort with prolonged exposure.

In contrast, SLES is milder. Its ethoxylated structure increases molecular size, reducing its ability to penetrate the skin and thus causing less irritation.

Research by the CIR (Cosmetic Ingredient Review) expert panel indicates that while SLES can be somewhat irritating at high concentrations, it does not trigger allergic reactions. In actual consumer products, SLES, after being formulated with optimized ingredients, exhibits very low levels of irritation.

The concentration of SLES used in regular shampoos and facial cleansers is 10–15%, and they are products that require short rinsing times, so they are relatively safe for the skin.

To further alleviate dryness, conditioning agents, plant oils, or polymers are often added to the formula.

• ALS (Ammonium Lauryl Sulfate);Slightly milder than SLS (possibly because NH₄⁺ has a gentler effect on the skin than Na⁺), but still not as mild as SLES.

In summary, SLES is generally superior to SLS in hair care products due to its gentler nature. Baby shampoos and products for sensitive skin often completely avoid SLS, and sometimes even SLES, opting instead for gentler alternatives such as sulfosuccinate, APG, and amino acid surfactants.

2.4 Compatibility techniques with other ingredients

Compatibility and synergistic effects with surfactants

As anionic surfactants, SLS and SLES are compatible with most anionic and nonionic surfactants, but not with cationic surfactants.

1. When coexisting with cationic conditioning agents such as quaternary ammonium salts, they will form insoluble complexes or neutralize and fail to work. Therefore, they should not coexist in the same formulation (for example, it is okay to use conditioner after shampoo, but they should not be mixed together).

2. It exhibits synergistic effects with amphoteric surfactants (such as betaines) and nonionic surfactants (such as alkyl glycosides). These synergistic effects include:

   • Enhances foam richness and stability;

   • Reduces irritation: Amphoteric surfactants also inhibit protein denaturation, making the formula gentler on the skin;

   • Improves salt thickening response, making the product easier to formulate into an ideal gel texture.

   For example, mixing SLES with betaine (such as CAPB) in a 3:1 weight ratio can significantly improve foam quality and reduce irritation.

3. Sulfate surfactants can also be combined with fatty alcohol amides (such as cocamide DEA) to further enhance the foam emulsion feel and viscosity.

4. Adding a small amount of amphoacetate or sulfosuccinate (such as sulfosuccinate) can further improve mildness and foam stability.

5. In addition, it can be combined with nonionic compounds (such as polysorbates and alkyl polyglucosides) to regulate foam or dissolve fragrances.

Viscosity and foam control

Viscosity Control:

• • Sulfate surfactants respond to salt by initially increasing viscosity upon salt addition (due to salt shielding the charge repulsion between micelles, forming rod-shaped micelles), then decreasing viscosity with excess salt.

  • SLES exhibits a significant viscosity peak upon salt addition, which is beneficial for preparing thick shampoos;

  • Combining it with CAPB can optimize this salt thickening effect.

SLS itself is usually a high-concentration paste (30% active), already possessing a certain viscosity. Adding salt may actually decrease viscosity due to the salting-out effect; therefore, SLES is usually the preferred choice for products requiring high viscosity.

Regarding Foaming:

• Sulfate surfactants foam rapidly;

• Adding a small amount of foam stabilizer (such as betaine or amine oxides) can extend foam life, making it suitable for applications requiring long-lasting foam, such as shaving foam and bubble baths.

2.5 Summarize

Sulfate surfactants offer high-performance advantages in personal care:

• Rich foam;

• Excellent cleansing power;

• Synergistic optimization of foam, viscosity, and gentleness with co-surfactants.

Their main limitation lies in their inherent irritation, especially SLS. However, by using gentler derivatives (such as SLES and ALS) and optimizing formulations (co-surfactants, conditioners), a good balance between cleansing power and skin-friendliness can be achieved. When properly designed, SLES/ALS-based products can provide the comfort consumers expect while maintaining cleansing power.

3. Synthetic Route

3.1 Production Overview

Sulfate surfactants such as SLS (sodium lauryl sulfate) and SLES (sodium lauryl ether sulfate) are typically prepared through a two-step industrial process:

1. Sulfation:

   The fatty alcohol is reacted with a sulfating agent to generate a sulfate ester intermediate;

2. Neutralization:

   The acidic intermediate is reacted with a base to generate the final surfactant salt.

The fatty alcohols used are primarily of natural origin (such as lauryl alcohol extracted from coconut oil or palm kernel oil) or synthetic origin (such as those obtained through the Ziegler or Oxo processes). The carbon chain length is generally C₁₂–C₁₄ to achieve optimal performance.

Step 1: Sulfation

Taking lauryl alcohol (CH₃–(CH₂)₁₁–CH₂OH) as an example, it can be sulfated in the following way:

• Traditional Method:

  Uses concentrated sulfuric acid (H₂SO₄), but this method is an equilibrium reaction requiring excess acid, produces numerous byproducts, and easily corrodes equipment; therefore, it is rarely used now.

• Modern Method:

  Chlorosulfonic acid (ClSO₃H): Reacts with lauryl alcohol to produce lauryl sulfuric acid and hydrogen chloride gas (HCl), which needs to be recovered using a water or alkaline scrubbing system;

  Sulfur trioxide (SO₃) gas: The preferred industrial method. SO₃ is usually diluted in an inert gas (such as air or nitrogen) for use in continuous reactors (such as thin-film reactors) with alcohols.

• Key Reaction Control Points:

  Use a molar ratio close to 1:1 to control side reactions;

  The reaction is highly exothermic, requiring cooling to maintain the temperature below 50°C;

  Prevent byproducts such as coking, discoloration, or sulfonone formation;

  The product is an acidic form of sulfate ester (such as lauryl sulfuric acid).

This step is typically carried out in a continuous reaction system, with water added to adjust to the standard concentration.

Step 2: Neutralization reaction

The acidic sulfate intermediate is immediately neutralized by a base to produce the desired salt product:

1. SLS:

   Neutralizes with sodium hydroxide (NaOH) or sodium carbonate to form sodium lauryl sulfate;

2. ALS(ammonium lauryl sulfate):

   Neutralizes with ammonia (NH₃ or NH₄OH);

3. SLES:

   Neutralizes with NaOH after sulfation to form sodium laureth sulfate.

This step is typically carried out in a continuous reaction system, with water added to adjust to the standard concentration.

Byproducts may contain trace amounts of sodium sulfate (Na₂SO₄), which may originate from residual sulfuric acid or excess alkali. Commercial-grade SLS/SLES often contain 1–3% Na₂SO₄ impurities.

To improve color, the final product is often lightly bleached with hydrogen peroxide to remove residual unsaturated impurities.

3.2 Pretreatment steps for SLES: Ethoxylation

In the preparation of SLES, lauryl alcohol is first ethoxylated by adding 2–3 moles of EO (ethylene oxide) under high pressure to generate lauryl ether alcohol.

This mixture (called lauryl alcohol ethoxylate) is then sulfated and neutralized as described above.

Therefore, the production process of SLES is:

Ethoxylation → Sulphation → Neutralization

Ethoxylation is carried out using a catalyst (such as KOH), and the product is a mixture with an EO unit distribution. Common products are laureth-2 sulfate and laureth-3 sulfate. While a higher degree of ethoxylation improves mildness, it also increases cost, resulting in diminishing returns.

3.3 Raw Material Sources (Feedstocks)

The main sources of fatty alcohol raw materials for the production of SLS/SLES include:

1. 1. Natural sources:

      Coconut oil, palm kernel oil: → Hydrolyze into fatty acids → Hydrogenate into alcohols (such as lauryl alcohol C₁₂, myristyl alcohol C₁₄);

      The resulting mixture of “coconut alcohols” (containing approximately 70% C₁₂ and 30% C₁₄) can be sulfated to sodium coco-sulfate.

This is why SLS is often labeled as a “natural ingredient derived from coconut oil”.

1. 1. Synthetic Sources:

      Aliphatic alcohols prepared using processes such as Ziegler or Oxo have a wide chain length distribution. Some commercial products (such as pareth sulfate) are ethoxylated sulfates of petroleum-derived alcohols.

   2. Other raw materials include:

      Sulfur (for SO₃).

      Ethylene (for EO).

The resulting mixture of “coconut alcohols” (containing approximately 70% C₁₂ and 30% C₁₄) can be sulfated to sodium coco-sulfate.

The source of these raw materials determines the economic viability and environmental impact of the product.

3.4 Process Technology and Efficiency

Modern production utilizes continuous reaction equipment, with precise control of reaction conditions:

• Thin-film reactors: enable efficient contact between alcohols and SO₃, completing the conversion within seconds;

• High conversion rate: main product yield is typically >98%, with minimal byproducts;

• High atom economy: theoretically, all SO₃ enters the target product;

• Heat recovery systems: commonly used to absorb exothermic reactions; the process is usually carried out under vacuum or an inert atmosphere to remove HCl or excess heat.

Ethoxylation processes are energy-intensive, requiring heating and pressurization, and may also produce trace byproducts (such as 1,4-dioxane).

Neutralization processes are also continuously controlled, effectively utilizing alkali to ensure uniform product quality.

Therefore, SLS/SLES production is a mature, high-yield, and cost-effective industrial process, explaining its low cost and widespread use.

3.5 Safety and Environmental Considerations

The production of sulfate surfactants involves hazardous chemicals:

• SO₃ and ClSO₃H are highly corrosive and toxic substances, requiring closed reactors and gas absorption systems (e.g., for HCl absorption);

• Factories must strictly adhere to safety operating procedures and engineering control measures to ensure personnel health;

• Environmentally friendly: minimal organic waste, with byproducts primarily being inorganic salts (e.g., NaCl, Na₂SO₄), which can be treated or recycled;

• Energy consumption is moderate, and many manufacturers have implemented thermal integration optimization.

One noteworthy issue is:

Trace amounts of 1,4-dioxane (a potential carcinogen) can be formed during ethoxylation. Reputable manufacturers employ vacuum degassing or other purification steps to control its concentration in the final product to below a few ppm and are subject to regulatory monitoring.

Overall, the production of SLS/SLES is a mature, safe, and efficient chemical process.

4. Summarize

SLS/SLES Overall Performance Advantages:

Highly efficient foaming + powerful cleaning + low cost;
Produces the dense foam consumers expect;
Compatible with common dyes, fragrances, and conditioning agents;
High water solubility, can be formulated into clear or pearlescent products;
Cleans thoroughly, easy to rinse, with minimal residue (especially noticeable with SLS); Fast foaming, suitable for various formulation systems.