Principles and Applications of Surfactants
Surfactants have a wide range of applications in the field of in vitro diagnostics. For example, in the field of blood cell analysis, surfactants are often used as an important component of hemolytic agents to realize the counting and classification detection of different types of blood cells; in the field of immunoassay, they are often used as wetting agents and detergents. In this paper, the basic knowledge of surfactants is outlined and summarized, which is of great significance in guiding the development of in vitro diagnostic reagents.
The concept and properties of surfactants
A surfactant is a substance that can significantly reduce the surface tension of a solution. The molecular structure of surfactants is amphiphilic, with one end consisting of a non-polar hydrocarbon chain (lipophilic group), with a hydrocarbon chain length generally above 8 carbon atoms, and the other end consisting of more than one polar group (hydrophilic group). Polar groups can be dissociated ions or non-dissociated hydrophilic groups, such as carboxylic acid, sulfonic acid, sulfuric acid, phosphoric acid, amino or amine groups and salts of these groups, or hydroxyl, amide, ether, carboxylic acid ester groups.
Schematic Structure of Surfactants
Surfactants are adsorbent and show positive adsorption in solution. When the concentration of surfactant in water is very low, the surfactant molecules are oriented at the water-air interface, with the hydrophilic groups facing towards water and the lipophilic groups facing towards the air, so that they aggregate to form a monomolecular layer on the surface of the solution, and the concentration of the surfactant on the surface layer of the solution is much higher than the concentration in the solution. Positive adsorption changes the nature of the solution surface, the outermost layer is hydrocarbon chain nature, reduces the surface tension of the solution, resulting in better wettability, emulsification, foaming and so on.
Directional arrangement of surfactant in water
Surface active solution in contact with the solid, but also in its surface adsorption can occur, so that the solid surface properties change. Nonpolar solid surface is monolayer adsorption, lipophilic group adsorption on the solid surface and hydrophilic group toward the air, and the adsorption amount does not increase with the increase of surfactant concentration, and even has a tendency to decrease. Polar solid surfaces are capable of both monolayer and multilayer adsorption. At low concentrations, the adsorption curve of surfactant molecules on polar solid surfaces is S-shaped, forming a monomolecular layer. When the concentration reaches the critical micelle concentration, the adsorption reaches saturation and shows bilayer adsorption, the arrangement of surfactant molecules is in the opposite direction of the first layer, the hydrophilic groups are directed toward the air, and the adsorption amount decreases with the increase of the solution temperature.
Classification of surfactants
According to the molecular structure and the dissociation nature of polar groups, surfactants are classified into anionic surfactants, cationic surfactants, amphoteric surfactants and nonionic surfactants.
Structure of different types of surfactants
Anionic surfactants play a role in the surface activity of the part of the anion, including advanced fatty acid salts of soaps (stearic acid, oleic acid, lauric acid, etc.), sulfated substances (sulfated castor oil, sodium dodecyl sulfate, etc.), sulfonates (sodium dioctylsuccinic acid sulfonate, sodium dodecylbenzene ring sulfonate, sodium hepatic bile acid, etc.).
Cationic surfactants play a role in surface activity is a cation, the main part of its molecular structure is a pentavalent nitrogen atom, so it is also known as quaternary ammonium compounds, such as dodecyltrimethylammonium chloride, dodecyltrimethylammonium bromide and so on.
Amphoteric surfactants have both positive and negatively charged groups in their structure, and can exhibit anionic or cationic surfactant properties in different pH media, including lecithin, amino acid-type and betaine-type amphoteric surfactants.
Nonionic surfactants do not dissociate in water, and the hydrophilic groups in the molecule are polyols such as glycerol, polyethylene glycol and sorbitol, and the lipophilic groups are long-chain fatty acids or fatty alcohols, alkyl groups, aryl groups, etc. The hydrophilic and lipophilic groups are combined through ester or ether bonds. The main types include alkyl glucosides (coco-glucosides, lauryl glucosides, etc.), fatty acid glycerides (glyceryl monostearate, etc.), polyols (sucrose fatty acid esters, dehydrated sorbitol fatty acid esters, polysorbates, etc.), polyoxymethylene types (Myrij, Brij, Perogol O, Emolphor, etc.), and polyoxymethylene-polyoxymethylene copolymers ( Poloxamer).
Physical and chemical properties of surfactants
(1) Critical micelle concentration
When the positive adsorption of surfactant saturation continue to add surfactant, its molecules are transferred to the solution, due to the presence of lipophilic groups, water molecules and surfactant molecules between the repulsive force is much greater than the attractive force, leading to the surfactant molecules rely on Van der Waals force aggregation of each other, the formation of lipophilic groups inward, hydrophilic groups outward in the water stable dispersion of the micelle. Within a certain temperature and concentration range, surfactant micelles have a certain number of molecular bonds, and different surfactant micelles have different numbers of molecular bonds. The lowest concentration at which surfactant molecules associate to form micelles is the following
(2) Hydrophilic Lipophilic Balance (HLB)
The selection of surfactants is usually based on the hydrophilic and lipophilic balance value, the combined affinity of hydrophilic and lipophilic groups for oil or water in surfactant molecules is called hydrophilic and lipophilic balance (HLB) value.HLB value is a relative value, it is stipulated that the HLB value of completely hydrophilic paraffin waxes is 0. Based on the experience, the HLB value of surfactants is in the range of 0~40, of which that of nonionic surfactants is 0~20. Hydrophilic surfactants have higher HLB values (HLB>9) and lipophilic surfactants have lower HLB values (HLB<9).
There is a close relationship between the HLB value of surfactant and its application; surfactants with HLB value of 3~6 are suitable for use as W/O emulsifiers, those with HLB value of 8~18 are suitable for use as O/W emulsifiers, those with HLB value of 13~18 are suitable for solubilizers and dispersing agents, and those with HLB value of 7~9 are suitable for use as wetting agents and penetrating agents.
Role of surfactants with different HLB values
HLB value of common surfactants
Application of surfactants
(1) Solubilizer
When the concentration of surfactant in aqueous solution reaches CMC, micelles will be formed, which can make the completely insoluble or slightly soluble substances in the solution enter into the micelles to be dissolved and become a thermodynamically stabilized solution, and play the role of solubilizing.
(2) Emulsifier
Surfactant can reduce the interfacial tension of two immiscible liquids (such as oil and water), so that one phase is dispersed in the other phase in the form of microdroplets to form a stable emulsion. The oil-water dispersion system formed by emulsification includes oil-in-water (O/W) and water-in-oil (W/O).
(3) Wetting and penetrating agent
Wetting is the process of replacing the gas on a solid surface with water. Surfactant can significantly reduce the surface tension of water, water on the solid surface can quickly diffuse, so that the wetting process occurs quickly, so it can be used as a wetting agent. Penetration and wetting effect is not essentially different, the former acts on the inside of the solid, the latter acts on the outside surface of the solid.
(4) Dispersant
Insoluble solids to tiny particles uniformly dispersed in the liquid system called dispersion or suspension, this role is called dispersion, can make the dispersion of surfactants called dispersant. Surfactant must have three roles to become a good dispersant. First, it must have good wetting properties, so that the liquid fully wet every solid particle, replace the air in the particles, and further make the solid particles broken into smaller crystals. Secondly, it must be able to significantly reduce the interfacial tension between the solid – liquid, increase the adsorption and compatibility between the solid – liquid, so that the energy present in the system is reduced. Finally, it must form an interfacial film of high mechanical strength around the solid particles in the form of a hydrated or charged layer to prevent aggregation between solid particles.
(5) Foaming agent
Bubbles is a thermodynamically unstable system, easy because the liquid film layer between the bubbles to produce liquid discharge phenomenon and small bubbles penetrate the merger of large bubbles, and the bubbles continue to rupture, the bubble disappears. If there is a surfactant in the liquid, the bubble surface can adsorb surfactant molecules and directional arrangement, to a certain extent, the bubble wall becomes a solid film, so that the bubble is not easy to merge. At the same time, the surfactant on the surface of the liquid directional arrangement, so that the surface tension of the liquid decreased significantly, resulting in a decrease in the pressure difference between the bubbles, thus slowing down the discharge rate. The above two aspects of the surfactant’s role in reducing the rupture capacity of the bubble, conducive to the formation and stable existence of bubbles.
(6) Detergent
The process of removing foreign matter or dirt from a solid surface submerged in some medium (mostly water) is called washing, and the chemicals that can play a role in washing are called detergents, and detergents are mostly made of surfactants as the main ingredient. The washing action is more complex, is the surfactant wetting, emulsification, dispersion, solubilization and other integrated action and stirring, kneading, water flow and other mechanical action of the joint results.
