Washing and decontamination of surfactants

1 Washing process

The decontamination process involves complex surface phenomena, mechanical fluid dynamics, and chemical or biological effects. The decontamination process is roughly the removal of liquid oil pollution, which is through wetting, rolling, emulsification, solubilization and other functions, supplemented by mechanical force, which is suspended in the medium and removed; The removal of solid dirt is made by wetting and interfacial charge dispersion.

In the washing process, detergent is indispensable. Detergent has the following functions in the washing process: First, reduce the surface tension of water to improve the wettability of water on the surface of the wash. The wetting of the washing liquid on the washing items is a prerequisite for the completion of the washing process, and the washing liquid must have good wettability on the washing items, otherwise the washing effect of the washing liquid is not easy to play. For artificial fibers (such as polypropylene, polyester, polyacrylonitrile, etc.), undegreased natural fibers, etc., the critical surface tension is lower than the surface tension of water, and therefore the wettability of water on it cannot reach a satisfactory degree. After the addition of detergent, the surface tension of water can generally be reduced to 30mN/m. Therefore, in addition to teflon, the aqueous solution of detergent will have good wettability on the surface of its articles, which will promote the dirt from the surface of its articles and produce a washing effect. Another effect is that the dirt that has been separated from the solid surface can be well dispersed and suspended in the washing medium, so that it is no longer deposited on the solid surface. The washing process can be expressed as: solid surface dirt + detergent + medium = solid surface · detergent · medium + dirt · detergent · medium.

In the washing process, the factors that affect the washing efficiency are the adhesion strength between solid and dirt, the adhesion strength between solid surface and detergent, and the adhesion strength between detergent and dirt. The adhesion between the solid surface and the detergent is strong, which is conducive to the removal of dirt from the solid surface, and the adhesion between the detergent and the dirt is strong, which is conducive to preventing the redeposition of dirt. In addition, there are different binding forces between surfaces of different properties and dirt of different properties. Therefore, they have different adhesion strengths. In aqueous medium, non-polar dirt is not easy to be washed because of its hydrophobicity. The non-polar dirt on the non-polar surface is more difficult to remove than the hydrophilic surface because it can be adsorbed on the non-polar surface by van der Waals force and has higher adhesion strength between the three. Polar dirt is easier to remove on hydrophobic non-polar surfaces than on more polar hydrophilic surfaces.

2. The relationship between the structure of the surfactant and the washing effect

Due to the complex interaction between dirt and surfactants, the relationship between the purification power of surfactants and their chemical structure is very complicated. For the decontamination of liquid oily dirt, because the removal process of oily dirt is mainly subject to the solubilization mechanism, it can be said that any surfactant that is conducive to improving the spatial structure of solubilization can dissolve the oil well and remove it. Similarly, if the dirt removal process is mainly subject to the emulsification mechanism, surfactants with appropriate HLB value for emulsification have stronger emulsification deconfouling ability than other surfactants. At low concentration, the ability of non-ionic surfactants to remove oil and prevent oil redeposition is higher than that of anionic surfactants with similar structure, because the cmc of non-ionic surfactants is very low.

As mentioned earlier, the directivity of surfactant molecules adsorbed at the solid-liquid interface plays an important role in washing. During the washing process, the surfactants are oriented so that their hydrophilic groups are oriented towards the aqueous phase, otherwise they cannot remove dirt and prevent redeposition. Therefore, the washing behavior of the surfactant in the washing solution is closely related to the polarity of the solid surface and the ionic properties of the surfactant. For example, both anionic and non-ionic surfactants have good performance on non-polar solid surfaces (such as polyester or nylon). On objects with high hydrophilicity such as cotton or cellulose, anionic surfactants have better performance than non-ionic surfactants. This is because the solid surface has high hydrophilicity and produces polar attraction and hydrogen bonding with the polyoxyethylene unit of the surfactant, thus forcing it to Orient and expose more hydrophobic groups to the water phase. Or arrange the surfactant molecules parallel to the surface of the solid. This directional arrangement can increase or at least not decrease the free energy at the dirt-water and solid-water interfaces, thus preventing the removal of dirt. Generally, cationic surfactants are rarely used as detergents, because they can be arranged in reverse on the solid surface to form a water-repellent surface, and when the solid surface is negatively charged, it is particularly easy to form a water-repellent film.

Obviously, the degree of adsorption of surfactant molecules on the solid surface and the orientation arrangement have a great influence on the behavior of surfactant in the washing process. Therefore, the washing power can be improved by changing the structure of the surfactant. As mentioned earlier, increasing the length of the hydrocarbon chain will increase the decontamination capacity of the surfactant. Surfactants with branch chains and hydrophilic groups in the middle of carbon chains have low washing capacity. For surfactants with a given number of carbon atoms and end groups, when the carbon chain has a straight chain structure and the hydrophilic group is at the end group position, they have the greatest washing capacity. Generally, with the increase of the length of the hydrophilic group and the movement from the middle chain to the end group, the washing ability of the surfactant increases. However, if the chain length is too large, the solubility of the surfactant is reduced, and the washing effect is decreased.

Although the straight-chain surfactants with hydrophilic group structure at the end group show the best washing ability under ideal conditions, when there are electrolytes and high-value cations in the washing solution, the solubility of the surfactants is reduced, which affects the washing ability and can not achieve the best washing effect. In this case, surfactants with hydrophilic groups located within the chain have a high washing capacity.

The properties of hydrophilic groups of surfactants also have a great influence on the washing capacity. If the saturated carbon chain is surrounded, the directional arrangement of adsorption is affected, thus affecting the washing capacity. For polyoxyethylene non-ionic surfactants, the polyoxyethylene chain increases, and the adsorption effect on the solid surface decreases, resulting in the decrease of washing capacity or even disappearance. When the polyoxyethylene chain is inserted between the hydrophobic group and the anionic group (such as fatty alcohol polyoxyethylene ether sulfate), the washing properties of this surfactant are superior to sulfates without the polyoxyethylene chain.

To sum up, it can be summarized as follows.

① The washing capacity of the surfactant increases with the increase of the hydrophobic chain within the allowable limit of solubility.

② Given the number of carbon atoms in the hydrophobic chain, the straight-chain surfactant has greater washing capacity than the branchable surfactant.

③ The surfactant with hydrophilic group on the end group has better washing effect than the hydrophilic group in the chain.

④ For non-ionic surfactants, when the turbidity point of the surfactant is slightly higher than the use temperature of the solution, the best washing effect can be achieved.

For polyoxyethylene non-ionic surfactants, an increase in the length of the polyoxyethylene chain (as long as sufficient solubility is achieved) often leads to a decrease in washing capacity.