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Structure and Performance of Non-Ionic Surfactants

Jan. 04, 2021

Surfactants are a class of amphiphilic molecules, that is, one end of the surfactant molecule is a lipophilic group, and the other end is a hydrophilic group. The hydrophobic groups of nonionic surfactants generally have the following types: linear alkyl, branched alkyl, alkyl phenyl, rosin derivatives, polyoxypropylene, perfluoropolyoxypropylene, fluoroalkyl and poly Siloxane groups, etc.; the most common hydrophilic groups of nonionic surfactants are polyoxyethylene, sugar groups and polyols. In addition, N, S, P, O atoms and combinations thereof can also constitute nonionic surface active The hydrophilic group of the agent.

1.The influence of non-ionic surface active structure on the critical micelle concentration

As the concentration of surfactant in water increases, its adsorption on the surface will not continue to increase after increasing to a certain extent, but will form micelles in water. The minimum concentration of surfactant that can form micelles in water is called the critical micelle concentration of the surfactant, or cmc for short. CMC is one of the most important physical quantities of surfactants. After reaching cmc, various properties of the surfactant solution will undergo mutations.

The cmc of surfactant is closely related to its lipophilic and hydrophilic structure. Generally speaking, the smaller the hydrophilic group of Nonionic Surfactants, the lower the cmc value. Taking fatty alcohol polyoxyethylene ether as an example, the same fatty alcohol, the longer the polyoxyethylene chain, the higher the cmc value; For the same hydrophilic group, the larger the lipophilic group of the nonionic surfactant, the lower the cmc value. According to the general rule, for nonionic surfactants with the same hydrophilic group, the lipophilic group increases by 2 carbon atoms, and the cmc value is reduced to 1/10 of the original.

Nonionic Surfactants

Nonionic Surfactants

2. The effect of non-ionic surface active structure on surface tension

①The influence of hydrophilic group structure

For nonionic surfactants, if they have the same hydrophobic group, the larger the hydrophilic group, the higher the surface tension. This is because when the surfactant adsorbs on the water and air interface, the hydrophilic group has an adverse effect on the limit adsorption capacity. The larger the hydrophilic group, the lower the limit adsorption capacity of the surfactant, the less dense the adsorption monolayer on the surface, and the higher the surface tension.

②The influence of hydrophobic base structure

For surfactants with the same hydrophilic group, different hydrophobic groups also have a greater impact on surface tension. This is determined by the interaction between the molecules of the hydrophobic substance itself and the density of the arrangement of the monolayer adsorbed on the surface. For example: the gravitational force between carbon-fluorocarbon molecules> the gravitational force between carbon-hydrocarbon molecules. Therefore, the surface tension of fluorosurfactants with carbon-fluorocarbon as the hydrophobic base is much lower than that of ordinary carbon- Hydrocarbon surfactants.

In the surfactants with polyoxypropylene as the hydrophobic base, the oxygen atom repels the forces between the hydrophobic groups. The surface tension of this type of surfactant is significantly higher than that of the hydrocarbon-hydrophobic base. Surfactant, for example, polyoxypropylene is a hydrophobic base polyether product, the surface tension can reach 40mN/m or more.

In addition, for surfactants with the same molecular formula but different configurations of hydrophobic groups, for example, polyoxyethylene ethers prepared from linear fatty alcohols and branched alcohols, even if they have the same EO number, those of branched fatty alcohol polyoxyethylene ethers The surface tension will also be lower, because the branched alcohols cover a larger area on the surface, making the surface performance closer to hydrocarbons.

Influence of non-ionic surface active structure on wetting and permeability

①Surfactants have suitable molecular weight and HLB. The fatty alcohols used as nonionic surfactants generally used as penetrants are C8-11 alcohols. If the hydrophobic base is too large, the surfactant will diffuse slowly and the penetration rate will be affected. The hydrophobic base is too small, the cmc of the surfactant is high, the ability to reduce the surface tension is poor, and the penetration effect is not good

②The penetration of branched alcohol is stronger than that of polyoxyethylene ether prepared from linear alcohol. This may have two reasons; one is that the surface tension of branched alcohol polyoxyethylene ether is lower, which is beneficial to wetting; The product of branched chain alcohol diffuses fast in water

③The position of the hydrophilic group in the molecule also affects the diffusion rate of the surfactant. Generally, the hydrophilic group has a strong penetration force in the middle of the molecule.

The penetration rate of non-ionic surfactants is closely related to the cloud point. When the use temperature is far below the cloud point, its hydrophilicity is too strong, and the surface activity is reduced. At this time, the penetration rate is not fast; when the use temperature exceeds the cloud point, the Ionic surfactants will also precipitate out of the water, and the penetration ability will decrease rapidly; only when the temperature is slightly lower than the cloud point, the penetration rate will reach the fastest.

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