Characteristics and classification of surfactants

Surfactants refer to certain organic compounds that are not only soluble in water or other solvents, but also oriented at the phase interface and can change the properties of the interface I. They generally have the following characteristics:

(1) Amphiphilic structure From the chemical structure point of view, the surfactant molecule should have both lipophilic carbon-hydrogen bonds and hydrophilic functional groups;

(2) Solubility The surfactant is soluble in at least one phase of the liquid phase,

(3) When interfacial adsorption reaches equilibrium, the concentration of surfactant solute at the interface is greater than the concentration in the entire solution;

(4) Interface-oriented surfactant molecules will be oriented and arranged into molecular layers on the interface;

(5) Generation of micelles When the concentration of surfactant solute in the solvent reaches a certain value, its components will aggregate to form micelles. The limit value of this concentration is the critical micelle concentration (CMC for short);

(6) Multifunctional surfactant solutions usually have multiple complex functions, such as cleaning, foaming, wetting, emulsification, solubilization and dispersion, etc. –

The history of surfactant as a term is not long, but its application can be traced back to ancient times. Chinese people have used saponin and ancient Chinese saponin in ancient times. The Egyptians used saponaria to extract soap for washing clothes. This is actually a biological natural surfactant. Although this substance has a washing function, once it enters the human body’s biochemical circulation system, it will produce a certain degree of toxicity to the human body. Modern synthetic surfactants do not have this shortcoming.

The washing function of soap was discovered as far back as the Middle Ages. Since then, until the 19th century, soap has been the only synthetic surfactant. From a structural point of view, Soap is an alkali metal salt of a long carbon chain fatty acid, which certainly meets the definition we proposed earlier. Soap is also a type of surfactant. However, when classifying, they are usually listed as a separate category (i.e. soap) to show the difference from synthetic surfactants.

At the beginning of this century, the sensitivity of soap to water hardness and acidity attracted people’s attention. This shortcoming first produced a strong reaction in the textile industry. In 1917, the German chemist Gunshel successfully synthesized alkyl ligansulfonate. It has high foaming and wettability. Although this substance has not yet achieved the washing function of soap, it has laid the foundation for the future development of surfactants. The first synthetic surfactant used in the textile printing and dyeing industry was sulfonated oil (commonly known as Turkish red oil), which made it possible to use large quantities of additives that are not sensitive to water hardness and acidity. In the 1930s, German chemists conducted extensive research on surfactants and discovered hundreds of surfactants that were considered new at the time. This was the founding period of modern surfactant chemistry and the formation of synthetic surfactants and soaps. competition among them. After World War II, the rise of petrochemicals provided high-quality raw materials that were relatively cheap at the time. Changes in raw materials have prompted the surfactant industry to enter a period of rapid development, resulting in a confrontation between petrochemical resources and natural resources (such as animal and vegetable oils). In the past 20 years, petroleum resources have been strategically considered due to strategic considerations. , as well as improvements in oil crop production technology, are leading to further research into the possibility of using natural raw materials as basic raw materials for the surfactant industry. This tendency has been clearly seen in the oleochemical industry. However, it is currently impossible to change the proportion of existing raw material sources.

Theoretically, there are countless compounds that can be used as surfactants. The types of surfactants recorded in the literature include those that have been industrially produced. There are thousands of them, and the most commonly used classification method is to divide them into four categories: anionic, nonionic, cationic and amphoteric surfactants according to the charged characteristics of the ions in the molecular structure. Each category is then subdivided according to the characteristics of the functional groups.

(1) The hydrophilic group of the anionic surfactant has a negative charge, such as C”Hz, OSO”Na+, and its water-soluble group sulfate is negatively charged Sex.

Anionic surfactants. The most important among this type of surfactants is linear alkyl benzene sulfonate. In the future, it will also be the most important detergent. The most important surfactant in detergents and cleaning agents. Its output is second only to soap and ranks first among synthetic surfactants. Due to the advancement of production technology, in addition to alkyl sulfonates, the output of fatty alcohol aldehydes and fatty alcohol sulfates is also gradually increasing. Among surfactants based on petrochemical hydrocarbons, alkenesulfonic acid The proportion of salts and alkanesulfonate in the market has a tendency of increasing K. In addition, sulfonated fatty acid methyl esters also have a similar tendency.

(2) Nonionic surfactants do not have charged groups in the molecule, and their water solubility comes from the polyoxygen contained in the lower component. Vinyl quinone group and terminal hydroxyl group. 7 of them are arranged in a straight chainThe oxyethylene groups form a polyacyl structure, and their oxygen atoms and the hydroxyl group at the last end combine to provide water solubility.

Today, the most important nonionic surfactants are higher fatty alcohols (carbon number above 12) and the condensates of nonylphenol and ethylene oxide. Among them, fatty alcohol polyoxyethylene has surpassed nonylphenol polyoxyethylene in terms of output due to various reasons such as technical economy and application performance, and has a continued growth trend. The raw material fatty alcohol it advocates (G-6 alcohol) is provided by two sources: petrochemical products and natural oils.

(3) The hydrophilic group of the cationic surfactant has a positive charge, and the positive charge on the nitrogen atom provides the molecule with water solubility.

The most important cationic surfactant is the bis-octakyldyl bismethyl chloride with the above structure, which is mainly used as a softener and softener for textiles after washing. Antistatic agent.

(4) Amphoteric surfactants have both positive and negative groups that are soluble in water in the molecule. Such as c₁₂h_2jn^j (ch₉>₂chcoo, points The amphoteric structure of Yu enables it to have some special functions, such as being able to cooperate with any of the above types of surfactants. However, its output is small and it only has some special uses.

In 1982, the output of all surfactants in the world was 130,000 tons, of which soap accounted for 62% and synthetic surfactants accounted for 38%. Surfactants are widely used.