Application of Surfactants in Highway Engineering

Surfactants (also known as surface-active substances) are indispensable chemical auxiliaries in the development of industry and agriculture, with the advantage of achieving significant results with small dosage. Especially after World War II, with the development of petrochemical industry, the rapidly emerging synthetic surfactant industry has further promoted the application of surfactants in various fields, such as petroleum, textiles, pesticides, medical treatment, metallurgy, mining, machinery, construction, roads, aviation, food, environmental protection, washing and dyeing, etc. This article will focus on introducing the application of surfactants as asphalt emulsifiers in highway engineering.

1.Definition of Surfactants

People have found in long-term production practice that solutions of some substances can significantly alter the surface properties of solvents even at very low concentrations, making them suitable for certain production requirements, such as reducing the surface tension or interfacial tension of the solvent, increasing wettability, detergency, emulsifying and foaming properties, etc. Soap, which is often used in daily life, is one such substance. A notable characteristic of substances like soap is that adding a small amount to water can greatly reduce the surface tension of water.

With the advancement of science and technology and the development of production, people have conducted in-depth research on the properties and functions of such substances and have given a relatively precise definition of surfactants. That is, a surfactant is a chemical substance that can significantly reduce the surface tension (or liquid-liquid interfacial tension) of a solvent (usually water) at very low concentrations, change the surface state of the system, thereby producing a series of effects such as wetting and anti-wetting, emulsification and demulsification, dispersion and coagulation, foaming and defoaming, and solubilization.

2.Structural characteristics of surfactants

Surfactant molecules consist of two parts with completely different properties: one part is the lipophilic group (also known as the hydrophobic group) which has an affinity for oil, and the other part is the hydrophilic group (also known as the oleophobic group) which has an affinity for water. This structural characteristic of surfactants causes, when they dissolve in water, the hydrophilic groups to be attracted by water molecules, while the lipophilic groups are repelled by water molecules. To overcome this unstable state, they have to occupy the surface of the liquid, with the lipophilic groups extending into the atmosphere and the hydrophilic groups extending into the water.

Although the structural characteristic of surfactant molecules is that they are amphiphilic molecules, not all amphiphilic molecules are surfactants. Only amphiphilic substances with a sufficiently long lipophilic part are surfactants.

For example, in the series of fatty acid sodium salts, compounds with a small number of carbon atoms (such as sodium formate, sodium acetate, sodium propionate, sodium butyrate, etc.) all have lipophilic and hydrophilic groups and possess surface activity, but they do not function as soap and therefore cannot be called surfactants. Only when the number of carbon atoms increases to a certain extent do sodium fatty acids exhibit obvious surface activity and possess the general properties of soap. Most natural animal and plant oils and fats are fatty acid esters containing 10 to 18 carbon atoms. If these acids are combined with a hydrophilic group, they will become surfactants with a certain degree of lipophilicity and hydrophilicity, and have good solubility.

3.Application of Surfactants in Highway Engineering

3.1. Surfactants and asphalt emulsifiers

Asphalt emulsifier is a type of surfactant. Emulsifiers and detergents share properties such as adsorptivity, orientation, the ability to form colloidal ions, and the ability to reduce interfacial tension. However, as an emulsifier, it also needs to have film-forming properties. Especially for asphalt emulsifiers, they need to have alkanes with an appropriate carbon chain to better emulsify with asphalt.

3.2. Classification of asphalt emulsifiers

Emulsifiers are classified into ionic and non-ionic types based on whether the hydrophilic groups of the emulsifier molecules carry charges when the emulsifiers are dissolved in water. Ionic emulsifiers are further divided into cationic, anionic, and amphoteric ionic types due to the differences in the charges carried by their hydrophilic groups after ionization in water.

Anionic asphalt emulsifier raw materials are cheap and easily available, and the production process is simple. Therefore, the earliest produced emulsified asphalt was anionic emulsified asphalt, which is generally of the medium-set type, and there are also some slow-set types. It can be used for slurry sealing, penetration, surface treatment, etc. Although anionic emulsifiers have price advantages, they have a great impact on the original properties of asphalt, and many problems occur during the construction process. Therefore, when applying them, it is necessary to consider the comprehensive effects of cost, construction effect and construction quality.

Cationic emulsifier although it developed relatively late, practice has shown that it has better adhesion to various mineral materials, with fast forming speed, high early strength, and low dosage. It not only gives play to the advantages of anionic emulsifiers but also makes up for their shortcomings, thus attracting much attention since its development. Cationic asphalt emulsifiers have a wide variety of types and different classification methods. They are usually classified according to their chemical structures, and the common ones mainly include alkyl amines, quaternary iron salts, lignin amines, imidazolines, etc.

Zwitterionic emulsifier molecules contain both acidic and basic groups, and they easily form “inner salts”. A characteristic of aqueous solutions of zwitterionic emulsifiers is that their electrical charge changes with variations in pH value. They have strong calcium dispersion ability in hard water and good compatibility with other types of emulsifiers, but their price is relatively high.

Most non-ionic emulsifiers are obtained by the reaction of ethylene oxide with compounds containing active hydrogen (such as phenols, alcohols, carboxylic acids, amines, etc.). Their activity is related not only to the hydrophobic alkyl groups but also to the length of the polyoxyethylene chains. They possess high surface activity, stability, and good emulsifying ability, exhibit good compatibility with other emulsifiers and their additives, and have a certain chelating effect on metal ions. Their activity is independent of the pH value of the solution, and the emulsion formed at the Phase Inversion Temperature (PIT) is the most stable.

3.3. The working principle of asphalt emulsifiers

When the concentration of the emulsifier is extremely low, there are very few emulsifier molecules. At the interface between air and water, it is impossible for a large number of emulsifier molecules to accumulate. On the surface, it is almost still in direct contact with air and water, and the surface tension remains almost unchanged, still close to the surface tension of pure water.

When the concentration of the emulsifier increases appropriately, the emulsifier molecules quickly gather on the water surface, reducing the contact area between air and water, thereby causing the surface tension to drop rapidly.

When the concentration of the emulsifier further increases and reaches a certain value, a large number of emulsifier molecules accumulate on the surface of the aqueous solution, forming a monomolecular film covering the surface of the solution, which completely isolates the aqueous solution from the air and stabilizes the surface tension. If the concentration of the emulsifier is slightly increased further, the emulsifier molecules can no longer accumulate on the water surface, but instead self-assemble into micelles or micellar aggregates with the lipophilic groups pointing inward and the hydrophilic groups pointing outward. The minimum concentration at which micelles or micellar aggregates are about to start forming is usually called the Critical Micelle Concentration (CMC).

After reaching the critical micelle concentration, if the concentration of the emulsifier continues to increase, the surface tension will no longer decrease. Since a monomolecular film has already formed on the surface, the emulsifier molecules tend to merge and move closer together, continuing to aggregate into micelles, thereby causing the number of micelles in the emulsion to increase continuously.

The emulsification of asphalt is an important aspect of emulsifying action. After adding an emulsifier to an oil-water solution, the two groups of the emulsifier arrange themselves in a directional manner, connecting the two interfaces of oil and water, thereby preventing them from repelling each other. After stirring and dispersing, asphalt can stably disperse in water in the form of fine particles.

Conclusion

Taking asphalt emulsifiers as an example, this article provides a comprehensive introduction and analysis of the structural characteristics, working principles, and application status of surfactants. Surfactants can effectively reduce the surface tension of water, strongly adsorb surfactant molecules on various other interfaces, and often have a certain degree of directional adsorption. It is this directional adsorption that enables surfactants to have multiple functions such as emulsification, demulsification, foaming, dispersion, coagulation, and wetting. Asphalt emulsifiers work by utilizing the emulsifying effect of surfactants. Whether from the perspective of economic performance or environmental protection, cold construction is bound to be an important trend in the development of highway engineering in the 21st century, and emulsifiers are the core of this technology. The research and improvement of emulsifier performance will surely have a profound impact on cold construction.