Application of surfactants in highway engineering

Surfactants have the functions of emulsification, demulsification, foaming, dispersion, flocculation, wetting, etc., and are widely used in petroleum, metallurgy, mining, machinery, construction, road, aviation and other fields. In this paper, the application of surfactants as asphalt emulsifiers in highway engineering is introduced in depth, which is conducive to its popularization and application.

0 Introduction

surfactant (surfactant, also known as surfactant) is an indispensable chemical assistant in the development of industry and agriculture, which has the advantage of small dosage and large harvest. Especially after the Second World War, with the development of petrochemical, the rapid rise of synthetic surfactant industry, further promote the application of surfactants in various aspects, such as petroleum, textile, pesticide, medical, metallurgy, mining, machinery, construction, road, aviation, food, environmental protection, washing and dyeing in various fields have been widely used. This paper focuses on the application of surfactant as asphalt emulsifier in highway engineering.

1 Definition of surfactant

It has been found in the long-term production practice that the solution of some substances can greatly change the surface properties of the solvent even when the concentration is very small, and make it suitable for certain requirements in production, such as reducing the surface tension or interfacial tension of the solvent, increasing wetting, washing, emulsification and foamability. Soap, which is often used in daily life, is one of these substances. A remarkable feature of such substances as soap is that the addition of a small amount to water can reduce the surface tension of water by a great deal.

With the progress of science and technology and the development of production, people have conducted in-depth studies on the properties and effects of such substances, and have given a more precise definition of surfactant, that is, surfactant is a very low concentration that can greatly reduce the surface tension (or liquid-liquid interfacial tension) of solvents (generally water). Change the surface state of the system to produce wetting and anti-wetting, emulsification and demulsification, dispersion and condensation, foaming and defoaming and solubilization and a series of chemical substances.

2. Structural characteristics of surfactants

Surfactant molecules are composed of two distinct parts (as shown in Figure 1) : one is an oil-friendly group (also known as a hydrophobic group), and the other is a water-friendly group (also known as an oleophobic group), which has an affinity with water. This structural feature of surfactants makes them soluble in water, the hydrophilic group is attracted by water molecules, and the hydrophilic group is repelled by water molecules. In order to overcome this unstable state, it is only possible to occupy the surface of the liquid surface, extend the oil-philic group into the weather, and extend the water-philic group into the water.

Although the molecular structure of surfactants is characterized by amphiphilic molecules, not all amphiphilic molecules are surfactants, and only amphiphilic substances with sufficient length of the lipophilic part are surfactants.

For example, in the fatty acid sodium salt series, the compounds with a small number of carbon atoms (sodium formate, sodium acetate, sodium propionate, sodium butyrate, etc.) have oleophilic and hydrophilic groups, and have surface activity, but they do not have soap effects, so they cannot be called surfactants. Only when the number of carbon atoms is increased to a certain extent, the fatty acid sodium shows obvious surface activity and has general soapy properties. Most of the natural animal and vegetable oils are fatty acid esters containing C10~C18, and if these acids are combined with a hydrophilic group, they will become a certain lipophilic and hydrophilic surfactant, and have good solubility.

3 Application of surfactants in highway engineering

3.1 Surfactants and asphalt emulsifiers

Asphalt emulsifier is a kind of surfactant, emulsifier and detergent have the same adsorption, orientation, colloidal ion formation, reduce the interfacial tension, but as an emulsifier also need to have film production, especially for asphalt emulsifier, but also need to have the appropriate carbon chain alkane, in order to better emulsify with asphalt.

3.2 Classification of asphalt emulsifiers

According to whether the hydrophilic group of the emulsifier is charged or not, the emulsifier is divided into ionic type and non-ionic type. Ionic emulsifiers can be divided into cationic, anionic and zwitterionic types due to the different charges of hydrophilic groups after ionization in water.

3.2.1 Emulsification

Anionic asphalt emulsifier raw materials are cheap and easy to obtain, the process is simple, so the earliest production of emulsified asphalt is anionic emulsified asphalt, generally for medium crack type, but also some slow crack type, can be used for slurry sealing, penetration, surface treatment and so on. Although the anionic emulsifier has the advantage of price, it has a great impact on the original properties of asphalt, and there are many problems in the construction process, so the comprehensive effect of cost, construction effect and construction quality must be considered when applying.

3.2.2 Cationic emulsifier

This kind of emulsifier developed late, but the practice has proved that it has better adhesion with various minerals, and the forming speed is fast, the early strength is high and the amount is small, which not only plays the advantages of anionic emulsifier, but also makes up for its shortcomings, and has attracted much attention since its development. Cationic asphalt emulsifiers are diverse and classified in different ways. They are usually divided according to their chemical structure. Common ones are alkylamines, quaternary ferric salts, woody amines, imidazolines and so on.

3.2.3 Zwitterionic emulsification

There are both acidic and basic groups in the zwitterionic emulsifier molecule, and it is easy to form "inner salt". The characteristic of zwitterionic emulsifier aqueous solution is that its electrification changes with the change of pH value. Its resistance to hard water calcium dispersion is strong, and other types of emulsifiers have good compatibility, but the price is high.

3.2.4 Non-ionic emulsifier

Non-ionic emulsifiers are mostly obtained by the reaction of ethylene oxide with active hydrogen compounds (such as phenols, alcohols, carboxylic acids, amines, etc.), and its activity is not only related to hydrophobic alkyl, but also related to the length of the polyoxyethylene chain. It has high surface activity, stability and good emulsification ability, good compatibility with other emulsifiers and auxiliaries, and has a certain integration effect on metal ions. Its activity is independent of the pH of the solution, and the emulsion is most stable when formed at the Phase Inversion Temperature (PIT).

Working principle of asphalt emulsifier 3.3

3.3 Low surface tension

When the emulsifier concentration is very low, there are very few emulsifier molecules, and it is impossible to accumulate more emulsifier molecules at the interface between air and water. On the surface, there is almost still direct contact between air and water, and the surface tension is almost unchanged, still close to the surface tension in the state of pure water, as shown in Figure 3a.

When the emulsifier concentration is properly increased, the emulsifier molecules quickly gather on the water surface, reducing the contact surface between air and water, and thus rapidly reducing the surface tension, as shown in Figure 3b.

When the emulsifier concentration further increases to a certain value, a large number of emulsifier molecules gather on the surface of the aqueous solution and form a single molecular film covering the surface of the solution, so that the aqueous solution is completely isolated from the air and the surface tension tends to be stable. If the concentration of emulsifier is increased slightly, the emulsifier molecules can no longer aggregate on the surface of water, but form micelles or micelles with oil - philic groups in and water - philic groups out. The lowest Concentration at which micelles or micelles begin to form is often referred to as the Critical Micelle Concentration (CMC).

3.3.2 Micellation

After reaching the critical micelle concentration, if the emulsifier concentration continues to increase, the surface tension will no longer decrease. Because the surface has formed a single molecular film, the emulsifier molecules tend to merge and converge into micelles, and the number of micelles in the emulsion will continue to increase.

3.3.3 Chemical action

Emulsification of asphalt is an important aspect of emulsification. After adding an emulsifier to the oil-water solution, the two groups of the emulsifier produce a directional arrangement, connecting the two interfaces of the oil and water, so as to prevent them from repelling each other. After mixing and dispersing, the asphalt can be stably dispersed in the water in the form of particles.

peroration

Taking asphalt emulsifier as an example, this paper gives a comprehensive introduction and analysis of the structure, working principle and application of surfactants. Surfactants can effectively reduce the surface tension of water, strongly adsorb surface active molecules on various other interfaces, and often have a fixed directional adsorption. It is this directional adsorption that makes the surfactant have many functions such as emulsification, demulsification, foaming, dispersion, coagulation and wetting. Asphalt emulsifiers work by the emulsification of surfactants. Whether from the economic performance, or from the perspective of environmental protection, cold construction will be an important trend in the development of highway engineering in the 21st century, and emulsifier is the core of this technology