1 The Core Problem Addressed by Penetrating Agents: Enabling the Cleaning System to Reach the Soil First

When laundry detergent is dropped onto fabric, if the liquid remains only on the fiber surface, soils inside the fiber structure are difficult to contact sufficiently. When a kitchen cleaner is sprayed onto an oil film, if the droplets bead up, the alkali and surfactants can hardly act uniformly on the surface of the oily soil. In hard-surface cleaning, soils often remain in textures, micropores, and crevices. If the cleaning liquid cannot enter these areas, the subsequent soil-removal process will be limited. These are exactly the types of problems that penetrating agents are intended to solve.

Penetrating agents are not the main components responsible for detergency on their own. Their core function is to improve the contact state between the cleaning liquid and fibers, oily soils, solid surfaces, and pores. Here, “contact state” refers to whether the cleaning liquid can quickly and uniformly reach the soil surface, fiber gaps, or pore walls.

To understand penetrating agents, three key questions need to be addressed:

① Why is it difficult for water-based systems to enter the target areas?

② How do penetrating-agent molecules change the interfacial state?

③ Why do penetrating agents with different structures show different strengths and limitations?

2 Why Penetration Is Difficult: Interfacial Resistance Limits Liquid Entry

2.1 Water Has High Surface Tension and Tends to Contract into Droplets

Strong cohesive forces exist between water molecules. On many hydrophobic surfaces, pure water tends to contract into droplets rather than spread spontaneously. In the cleaning process, this means that a water-based cleaning liquid may remain only on the outer surface and may not quickly cover the edges of soils or enter the interior of fibers.

This phenomenon is common in household and personal care cleaning:

① When liquid is dropped onto fabric, the surface becomes wet, but internal wetting is slow.

② When a cleaner is sprayed onto an oil film, droplets partially aggregate and spread unevenly.

③ When micropores or textures are present on hard surfaces, the liquid has difficulty entering fine crevices.

Whether a liquid can spread is usually related to the contact angle. The smaller the contact angle, the more easily the liquid wets the surface; the larger the contact angle, the more the liquid tends to contract into droplets.

2.2 Oily Soils and Some Substrates Repel Water-Based Systems

Among the soils encountered in household and personal care cleaning, sebum, edible oils, mineral oils, and waxy soils are all hydrophobic. When a water-based cleaning system directly contacts such soils, it faces relatively high water–oil interfacial resistance. Some synthetic fibers, plastics, and coated surfaces are also not easily wetted by water. When a liquid cannot effectively wet the substrate, subsequent emulsification, dispersion, detachment, and rinsing are all affected.

2.3 Air in Pores Obstructs Liquid Entry

Air is usually present in fabrics, sponges, rough countertops, and crevices. For a liquid to enter these areas, it must first wet the pore walls and displace the air. If the liquid has poor wettability toward the pore walls, it can easily remain at the pore entrance and fail to penetrate further. The capillary-entry process is related not only to pore size, but also to the liquid’s surface tension, contact angle, and viscosity.

Therefore, so-called “penetration” is not forced penetration through the material structure. Rather, after interfacial resistance is reduced, the liquid enters fibers, pores, or soil crevices along wetted surfaces.

3 How Penetrating Agents Work: Amphiphilic Structures Change the Interfacial State

3.1 Penetrating-Agent Molecules Usually Have Amphiphilic Structures

Most penetrating agents are surfactants or co-surfactants. Their common structural feature is that the molecule contains both a hydrophilic part and a hydrophobic part. The hydrophilic part allows the molecule to disperse or dissolve in the aqueous phase, while the hydrophobic part makes the molecule tend to approach oily soils, air interfaces, or hydrophobic substrate surfaces. This amphiphilic structure is the basis of the function of penetrating agents.

A simplified structure can be represented as:

R—X

Here, R represents a hydrophobic hydrocarbon chain, and X represents a hydrophilic group. R enables the molecule to approach oily soils or hydrophobic surfaces, while X enables the molecule to remain in the aqueous phase. The function of a penetrating agent arises from the synergy between these two parts.

If the molecule is too hydrophilic, it tends to remain mainly in the aqueous phase and is difficult to adsorb effectively at the interface. If the molecule is too hydrophobic, its water solubility may be insufficient, affecting formulation stability. The key to a penetrating agent is an appropriate hydrophilic–hydrophobic balance.

3.2 Penetrating-Agent Molecules Preferentially Adsorb at Interfaces

Water–air, water–oil, and water–solid interfaces have relatively high interfacial energy. After a penetrating agent is added to a water-based system, its molecules migrate from the aqueous phase to these interfaces and become directionally arranged. The hydrophobic end tends to face the air, oily soil, or hydrophobic substrate, while the hydrophilic end faces the aqueous phase. This arrangement reduces the interfacial resistance between water and the target surface, making it easier for the liquid to spread.

3.3 Reduced Surface Tension and Interfacial Tension Enhance Liquid Spreading

After a penetrating agent adsorbs at the interface, it usually reduces the surface tension of the water-based system or changes the interfacial energy state of the water–oil and water–solid interfaces. Macroscopically, this often appears as a lower contact angle, a larger wetted area, or a faster wetting rate. Once the cleaning liquid changes from localized aggregation to uniform spreading, primary surfactants, alkalis, solvents, enzymes, and other components can more easily contact the soil.

3.4 After the Contact Angle Is Reduced, the Liquid More Easily Enters Fibers and Pores

For a liquid to enter fiber gaps and microporous structures, it must first wet the pore walls. When a penetrating agent reduces the contact angle, the water-based system can more easily advance inward along fiber surfaces, pore walls, or cracks in the soil.

The action process of a penetrating agent can be summarized as:

Interfacial adsorption → change in surface tension or interfacial energy state → improved wettability or faster dynamic spreading → enhanced capillary entry → fuller contact between cleaning components and soils.

3.5 Penetrating Agents Improve the Delivery Efficiency of the Cleaning System

Penetrating agents are mainly responsible for helping the cleaning system reach the location of the soil more quickly. The actual processes of oil emulsification, particle dispersion, saponification, enzymatic degradation, anti-redeposition, and related cleaning actions still require the combined effects of primary surfactants, builders, alkalis, solvents, enzymes, chelating agents, and other components.

4 How Structure Affects Penetration Performance

4.1 The Hydrophobic Chain Determines Interfacial Adsorption Ability

The hydrophobic chain in a penetrating-agent molecule is responsible for approaching oily soils, air interfaces, or hydrophobic substrates. If the hydrophobic chain is too weak, the molecule has insufficient adsorption ability at the interface. If the hydrophobic chain is too strong, water solubility may decrease, diffusion may slow down, and formulation stability may even be affected. Therefore, a longer hydrophobic chain is not necessarily better. An effective penetrating agent needs to strike a balance between interfacial adsorption ability and dispersibility in the aqueous phase.

4.2 The EO Chain Affects Water Solubility, Hydrophilicity, and Compatibility

Many nonionic penetrating agents contain EO chains. EO refers to the ethylene oxide structural unit. The EO chain provides hydrophilicity, enables the molecule to disperse in the aqueous phase, and affects the product’s cloud point, hydrophilic–lipophilic balance, foam behavior, wetting speed, and formulation compatibility.

Nonionic polyether surfactants containing EO chains can be represented by the general formula:

R—O—(CH₂CH₂O)n—H

Here, R is a hydrophobic alkyl or branched alkyl group, and n is the average number of EO units.

A higher EO number is not always better. If the EO chain is too short, water solubility may be insufficient. If the EO chain is too long, the molecule may become overly hydrophilic, and its tendency to adsorb at hydrophobic interfaces may decrease, so penetration efficiency will not necessarily be higher. The key in designing nonionic polyether-type penetrating agents is to select an appropriate ratio between the hydrophobic chain and the EO chain, so that the molecule can both remain stable in the aqueous phase and adsorb rapidly at the interface.

4.3 Branched or Double-Chain Structures Affect Dynamic Wetting Speed

Actual cleaning processes are not static. During spraying, wiping, soaking, and machine washing, the liquid surface and solid–liquid interface are constantly renewed. In such situations, a penetrating agent must not only reduce the final surface tension, but also migrate quickly and adsorb onto newly formed interfaces.

Branched structures, short-chain isomeric structures, and double-hydrophobic-chain structures are often used in rapid-wetting systems. Such structures usually help improve dynamic spreading efficiency. However, actual performance is also affected by concentration, temperature, electrolytes, solvents, substrate surface properties, and competitive adsorption with other surfactants in the formulation. Static surface tension alone is not sufficient for evaluation.

4.4 Ionic Hydrophilic Groups Determine Suitability in Specific Systems

Penetrating agents can be nonionic, anionic, or blended systems. Nonionic penetrating agents usually have good formulation compatibility. Anionic penetrating agents often have strong wetting and spreading ability, but their compatibility with cationic ingredients, salts, pH, and thickening systems must be considered. Structures such as phosphate esters and sulfonates are commonly used under more specific conditions. For example, high-alkali cleaning systems place greater emphasis on alkali resistance and stability, while rapid-wetting systems place greater emphasis on dynamic adsorption speed.

5 Structural Logic of Three Representative Product Types

5.1 Structural Types and Differences of Representative Products

Common JFC products are mostly nonionic penetrating agents based on fatty alcohol or branched alcohol polyoxyethylene ethers. Their product structures usually center on a hydrophobic alkyl chain and a polyoxyethylene hydrophilic chain. Different products may vary in average EO addition number, source of the hydrophobic alcohol, and active-matter content.

OEP-70 is an alkali-resistant penetrating agent, and its common structures are related to ethoxylated alcohol phosphate esters and their salts. Its structure contains a hydrophobic alkyl group and a polyoxyethylene chain, as well as a phosphate ester or salt-type hydrophilic group. Commercial products may differ in their monoester, diester, salt-form, and blended components.

DOSS / Penetrating Agent T usually corresponds to sodium dioctyl sulfosuccinate-type structures. Its structural characteristics are relatively clear: the core consists of double branched hydrophobic ester chains and a sulfonate anionic hydrophilic group. This structure helps explain its rapid dynamic wetting and spreading ability.

5.2 JFC: Nonionic Polyether-Type Penetrating Agent

JFC usually refers to nonionic penetrating agents based on fatty alcohol or branched alcohol polyoxyethylene ethers. JFC can be represented by the following general formula:

R—O—(CH₂CH₂O)n—H

Here:

① R represents the hydrophobic alkyl group derived from a fatty alcohol or branched alcohol;

② n represents the average number of EO units;

③ —O—(CH₂CH₂O)n—H is the polyoxyethylene hydrophilic segment.

The penetrating effect of JFC comes from the regulation of interfaces by this amphiphilic structure. After the molecules migrate to water–air, water–oil, and water–solid interfaces, they can reduce interfacial resistance, making it easier for the water-based system to wet fabrics, fiber gaps, and rough surfaces.

The advantage of JFC lies in its clear structural logic and obvious nonionic character. It usually has good formulation compatibility and is suitable for systems that need improved general wetting and fiber impregnation. The limitation of JFC is that its applicable conditions are not unlimited. In strongly alkaline, high-salt, high-temperature, or strictly low-foam systems, ordinary nonionic polyether-type penetrating agents may be limited by cloud point, foam behavior, alkali resistance, or formulation compatibility.

5.3 OEP-70: Alkali-Resistant Penetrating Agent

OEP-70 is commonly found in alkali-resistant penetrating-agent products. Its commonly associated structure is isooctyl alcohol polyoxyethylene ether phosphate ester or its salts, also known as a 2-ethylhexanol ethoxylated phosphate ester-type structure.

Alkali-resistant ethoxylated alcohol phosphate ester penetrating agents can be represented by the following typical structures.

① Monoester structure:

R—O—(CH₂CH₂O)n—P(=O)(OH)₂

② Salt forms can be represented as:

R—O—(CH₂CH₂O)n—P(=O)(ONa)(OH)

or further neutralized as:

R—O—(CH₂CH₂O)n—P(=O)(ONa)₂

③ Diester structure can be represented as:

[R—O—(CH₂CH₂O)n]₂—P(=O)—OH

Here:

① R is usually an isooctyl, 2-ethylhexyl, or similar branched hydrophobic alkyl group;

② n is the average number of EO units;

③ the phosphate ester group and its salt form provide anionic hydrophilic character;

④ commercial OEP-70 may be a mixture of monoesters, diesters, salt forms, and other components.

In systems such as commercial kitchen heavy-oil cleaning, industrial cleaning, strong-alkali degreasing, and textile pretreatment, the formulation may have a relatively high pH and high electrolyte concentration. Under these conditions, ordinary penetrating agents may show reduced stability, cloud-point changes, decreased wetting efficiency, or mismatched foam behavior.

The structural design goal of alkali-resistant penetrating agents such as OEP-70 is to maintain effective wetting and penetration ability under strongly alkaline conditions. Its action logic can be summarized as follows:

① The branched hydrophobic part helps the molecule approach oily soils or hydrophobic surfaces.

② The polyether chain provides dispersibility in the aqueous phase.

③ The phosphate ester or salt-type hydrophilic group improves adaptability under high-pH and electrolyte conditions.

④ Interfacial adsorption and wetting/penetration ability are maintained in strongly alkaline systems.

5.4 DOSS / Penetrating Agent T: Rapid Dynamic Wetting-Type Penetrating Agent

DOSS is the abbreviation for sodium dioctyl sulfosuccinate, also known as docusate sodium. Penetrating Agent T is also usually associated with dialkyl sulfosuccinate-type structures. DOSS is an anionic surfactant with the ability to reduce surface tension and promote wetting.

The representative structure of DOSS can be expressed as:

NaO₃S—CH(COOR)—CH₂—COOR

Here:

① R is 2-ethylhexyl;

② the two —COOR groups are ester groups;

③ —SO₃Na is the sulfonate anionic hydrophilic group.

The structural characteristics of DOSS are well suited to explaining rapid dynamic wetting. The two branched hydrophobic chains help the molecule adsorb and spread rapidly at the interface, while the sulfonate anionic hydrophilic group ensures dispersibility in the aqueous phase.

The key function of DOSS is not only how low the final surface tension can be reduced, but also whether it can rapidly adsorb onto newly formed interfaces within a short time. In actual cleaning processes such as spraying, soaking, and wiping, liquid interfaces are constantly changing. If the migration and adsorption speed of the penetrating agent is slow, it may fail to show a rapid penetration effect even if the final surface tension is low.

It should be noted that DOSS contains ester bonds, so its stability under strong acid, strong alkali, or high-temperature conditions requires careful evaluation. DOSS is more suitable for systems that emphasize rapid dynamic wetting and spreading. For more demanding systems such as strong-alkali degreasing, textile scouring, or high-temperature alkaline cleaning, preference should be given to alkali-resistant, verified, specialized wetting and penetrating agents, such as alkali-resistant phosphate esters or other surfactants suitable for high-pH conditions.

5.5 Structural Differences Among the Three Representative Product Types

6 Sources of Wetting and Penetration Functions in Household and Personal Care Formulations

Primary surfactants and co-surfactants in laundry detergents, dishwashing liquids, hard-surface cleaners, shampoos, and body washes may themselves also provide wetting, spreading, and auxiliary penetration functions.

For example, modern fatty alcohol ether surfactants such as alcohol ethoxylates (AE), alkyl polyglucoside surfactants (APG), and anionic–nonionic blended systems can all improve the wettability of liquids toward substrates and soils to varying degrees.

These raw materials also have amphiphilic structures and can participate in wetting and spreading processes. However, their design objective in household and personal care formulations is usually not simply to achieve the fastest possible penetration, but to balance detergency, foam, mildness, rinsability, odor, stability, and cost.

7 How to Determine Whether a Penetrating Agent Is Suitable: Returning to Structure, Interfaces, and Formulation Conditions

7.1 First Determine Whether the Problem Comes from Insufficient Wetting and Contact

If the formulation problem is slow liquid spreading, insufficient fabric wetting, or poor contact with the oily-soil surface, a penetrating agent may provide clear value. If the problem is insufficient oil emulsification, insufficient particle dispersion, insufficient alkalinity, or an incompatible enzyme system, simply increasing the amount of penetrating agent usually cannot solve the root cause.

7.2 Then Determine Whether the Structure Is Suitable for the Current System

Nonionic polyether-type structures are more suitable for basic wetting and formulation compatibility. Alkali-resistant structures are more suitable for high-pH, strong-cleaning systems. Double-chain anionic structures are more suitable for rapid dynamic wetting. Different structures correspond to different functional priorities and cannot simply be substituted for one another.

In high-alkali systems, priority should be given to alkali resistance and electrolyte compatibility. In household and personal care products that contact the skin, priority should be given to mildness, irritation potential, regulatory suitability, and formulation compatibility. In spraying, wiping, and short-contact-time systems, dynamic wetting speed needs to be emphasized.

7.3 Finally Determine Whether It Is Stable and Effective in the Complete Formulation

A penetrating agent that performs well in pure water does not necessarily perform well in a real formulation. Real formulations may contain primary surfactants, electrolytes, fragrances, preservatives, thickeners, solvents, alkalis, or acids at the same time. These components can all affect the solubility state, interfacial adsorption speed, foam behavior, and system stability of the penetrating agent.

The effectiveness of a penetrating agent should be evaluated based on its performance in the complete formulation. An excellent penetrating agent is one that can stably improve wetting and entry ability in the target system without disrupting the balance of the formulation.

8 Representative Chemical Classification Tables Related to Wetting and Penetration in Household and Personal Care Cleaning

Table 1 Wetting Agents, Penetrating Agents, and Dynamic Wetting-Related Products

Table 2 Anionic Surfactants for Household and Personal Care Cleaning Systems

Table 3 Mild Nonionic and Amphoteric Surfactants and Formulation Auxiliary Products

Note: The above are representative Aladdin products related to scientific research and formulation studies. They can be searched on the Aladdin website by product name, CAS number, or item number. These products are mainly intended for R&D and experimental research. Specific applications should be evaluated in combination with the SDS, regulatory requirements, and actual formulation testing. Among them, alkylphenol polyoxyethylene ether products such as nonylphenol polyoxyethylene ether and Triton X-100 can be used as classic nonionic surfactants and research references; however, in household and personal care, textile, and cleaning applications, environmental considerations, regulatory requirements, and substitution trends should be taken into account.

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