August 4, 2022 Longchang Chemical

Basic theory of surfactants (3)

Washing in the usual sense refers to the process of removing dirt from the surface of a carrier. In washing, the interaction between dirt and carrier is weakened or eliminated by the action of some chemical substances (such as surfactant substances like detergents), so that the combination of dirt and carrier is changed into the combination of dirt and detergent, and finally the dirt is separated from carrier. The basic process of washing can be expressed by a simple relationship: carrier – dirt + detergent = carrier + dirt – detergent.

The washing process can usually be divided into two stages: first, under the action of detergent, the dirt is separated from its carrier; second, the detached dirt is dispersed and suspended in the medium. The washing process is a reversible process, dispersed, suspended in the medium of dirt may also be re-precipitated from the medium to the washed object. Therefore, a good detergent in addition to the ability to make the dirt from the carrier, but also should have a better ability to disperse and suspend dirt, to prevent the redeposition of dirt.

A. Adhesion of dirt. Clothes, hands, etc. can get dirt because there is some kind of interaction between the object and the dirt. Dirt in the object of the adhesion of a variety of roles, but no more than physical adhesion and chemical adhesion of two. Soot, dust, mud, sand, carbon black and other adhesion on the clothing is a physical adhesion. Generally speaking, through this adhesion of dirt, and the role of the stained object is relatively weak, the removal of dirt is also relatively easy. According to the different forces, the physical adhesion of dirt can be divided into mechanical adhesion and electrostatic force adhesion.

1, mechanical adhesion refers to the adhesion of some solid dirt (such as dust, sand). Mechanical adhesion is a relatively weak adhesion of dirt, almost can be removed by purely mechanical methods, but when the dirt is relatively small (<0.1um), it is more difficult to remove; electrostatic force adhesion is mainly in the role of charged dirt particles on oppositely charged objects. Most fibrous items are negatively charged in water and are easily adhered by some positively charged dirt, such as lime type. Some dirt, although negatively charged, such as carbon black particles in aqueous solutions, can be attached to fibers by ionic bridges (ions between multiple anisotropic charges, acting together with them in a bridge-like manner) formed by positive ions in water (such as Ca2+ ﹑ Mg2+, etc.). The electrostatic effect is stronger than the simple mechanical action, so the dirt removal is relatively difficult.

2、Chemical adhesion. Chemical adhesion refers to the phenomenon of dirt acting on the object through chemical or hydrogen bonding. Such as polar solid dirt, protein, rust and other adhesion on fiber items, fibers contain carboxyl, hydroxyl, amide and other groups, these groups and oily dirt fatty acids, fatty alcohols are easy to form hydrogen bonds. The chemical force is generally stronger, and thus the dirt is more firmly bonded on the object. This kind of dirt is difficult to remove by normal methods, and special methods are needed to deal with it. The degree of adhesion of dirt is related to the nature of the dirt itself and the nature of the object to which it is adhered. Generally, particles are easy to adhere on fibrous items. The smaller the solid dirt, the stronger the adhesion. Hydrophilic objects such as cotton, glass and other polar dirt on the surface to adhere more firmly than non-polar dirt. The adhesion strength of non-polar dirt is greater than polar dirt such as polar fat, dust, clay, etc., which is not easy to remove and clean.

Second, the mechanism of dirt removal. The purpose of washing is to remove dirt. In a certain temperature medium (mainly water as the medium), the use of detergents produced by a variety of physical and chemical effects, weaken or eliminate the role of dirt and washed items, under the action of certain mechanical forces (such as hand-rubbing, washing machine agitation, water impact), so that dirt and washed items from the purpose of decontamination.

1、The mechanism of liquid dirt removal. Most of the liquid dirt is oily dirt, oil can wet most of the fiber items, more or less spread into a layer of oil film on the surface of the fiber material. The first step of the washing action is the wetting of the surface by the washing solution, which can be seen as a smooth solid surface of the fiber. The removal of liquid dirt is achieved by a kind of convolution. The liquid dirt originally exists on the surface in the form of a spread oil film, and under the preferential wetting action of the washing liquid on the solid surface, i.e. the surface of the fiber (the action of the wetting agent), it is curled up into oil beads step by step, replaced by the washing liquid, and finally leaves the surface under the action of certain external force.

2、The mechanism of solid dirt removal. It is mainly the wetting of the dirt mass and its carrier surface by the washing liquid. Due to the adsorption of surfactant on the surface of solid dirt and its carrier, the interaction between the dirt and the surface is reduced, and the adhesion strength of the dirt mass on the surface is reduced, so the dirt mass is easily removed from the surface of the carrier. Not only that, the adsorption of surfactants, especially ionic surfactants, on the surface of solid dirt and its carrier is likely to increase the surface potential of solid dirt and its carrier surface, which is more favorable to the removal of dirt. Solid or general fiber surfaces are usually negatively charged in aqueous media, and therefore, a diffuse double electric layer can be formed on the dirt mass or solid surface. Since same-sex charges repel each other, the adhesion strength of dirt plasmas on solid surfaces in water is weakened. When anionic surfactant is added, because the anionic surfactant can simultaneously increase the negative surface potential of dirt plasmas and solid surfaces, the repulsive force between them is enhanced, thus, the adhesion strength of plasmas is reduced even more and dirt can be removed more easily.

Non-ionic surfactants can produce adsorption on the general charged solid surface, and although they cannot significantly change the interfacial potential, the adsorbed non-ionic surfactants tend to form a certain thickness of adsorption layer on the surface, which helps prevent the redeposition of dirt. For cationic surfactants, as their adsorption will reduce or eliminate the negative surface potential of the dirt mass and its carrier surface, which makes the repulsion between the dirt and the surface lower, thus not conducive to the removal of dirt; furthermore, after the adsorption of cationic surfactants on the solid surface, the solid surface often becomes hydrophobic, thus not conducive to the wetting of the surface, and not conducive to washing.

3. Removal of special dirt. Protein, starch, human secretions, juice, tea juice and other such dirt are difficult to be removed by general surfactants, and special treatment methods are needed.

Third, the decontamination mechanism of dry cleaning. The above introduction is for the washing effect of water as the medium, the so-called dry cleaning generally refers to the washing method in organic solvents, especially in non-polar solvents. Compared to water washing, dry cleaning is a more gentle way of washing. Because dry cleaning does not require much mechanical action, the clothing does not cause damage ﹑ wrinkles and deformation, while dry cleaning agent is not like water, rarely produce expansion and contraction effect. As long as the technology is properly handled, you can dry clean clothes to achieve no deformation, no fading and extend the life of the excellent results.

Due to the different nature of various kinds of dirt, there are different ways of action for the removal of dirt in the dry cleaning process. Oil-soluble dirt, such as animal and plant oils, mineral oils and grease, etc., are easily soluble in organic solvents, and are easier to remove in dry cleaning. The excellent solubility of dry cleaning solvents for oil and grease essentially comes from the van der Waals forces between molecules. For water-soluble dirt such as inorganic salts, sugars, proteins, sweat and other removal, but also must be added to the dry cleaning agent in the right amount of water, otherwise water-soluble dirt is difficult to remove from the clothing. But water is more difficult to dissolve in the dry cleaning agent, so to increase the amount of water, but also need to add surfactants. The presence of water in the dry cleaning agent can make the surface of dirt and clothing hydrated, so that it is easy to interact with the polar groups of surfactants, which is conducive to the adsorption of surfactants on the surface. In addition, when surfactants form micelles, water-soluble dirt and water can be solubilized into the micelles. In addition to increasing the water content in the dry cleaning solvent, surfactants can also play a role in preventing the redeposition of dirt to enhance the decontamination effect. The presence of a small amount of water is necessary to remove water-soluble dirt, but excessive water will lead to some clothing deformation, wrinkling, etc., so the content of water in dry cleaning agents must be moderate.

Dirt that is neither water-soluble nor oil-soluble, such as ash, mud, soil and carbon black and other solid particles are generally adsorbed by static electricity or combined with oil and dirt attached to the clothing. In dry cleaning, solvent flow ﹑ impact can make the electrostatic force adsorption of dirt off, and dry cleaning agent can dissolve the oil, so that the combination of oil and dirt and attached to the clothing of solid particles off in the dry cleaning agent, dry cleaning agent in a small amount of water and surfactants, so that those off the solid dirt particles can be stable suspension ﹑ dispersion, to prevent its re-deposition to the clothing.

Fourth, the factors affecting the role of washing.

1. The concentration of surfactant. The micelles of surfactants in the solution play an important role in the washing process. When the concentration reaches the critical micelle concentration (cmc), the washing effect increases sharply. Therefore the concentration of detergent in the solvent should be higher than the cmc value to have a good washing effect. However, when the concentration of surfactant is higher than the cmc value, the increment of washing effect is not obvious, and it is unnecessary to increase the concentration of surfactant too much. When removing oil stains by solubilization, the solubilization effect increases with the concentration of surfactant even if the concentration is above cmc value. For example, if there is more dirt at the cuffs and collars of clothes, a layer of detergent can be applied when washing to improve the solubilization effect of surfactant on oil.

2、Temperature has a very important influence on the decontamination effect. In general, raising the temperature is good for dirt removal, but sometimes too high temperature can also cause unfavorable factors. The increase of temperature is beneficial to the diffusion of dirt, solid oil scale is easily emulsified when the temperature is higher than its melting point, and the fiber also increases the degree of expansion due to the increase of temperature, all these factors are beneficial to the removal of dirt. However, for compact fabrics, the microgap between fibers decreases after fiber expansion, which is unfavorable to dirt removal.

Temperature change also affects the solubility of surfactants, cmc value, micelle size, etc., which affects the washing effect. The solubility of surfactants with long carbon chains is smaller when the temperature is low, and sometimes the solubility is even lower than the cmc value, so the washing temperature should be raised appropriately. The effect of temperature on cmc value and micelle size is different for ionic and nonionic surfactants. For ionic surfactants, an increase in temperature generally increases the cmc value and decreases the micelle size, which means that the concentration of surfactant in the washing solution should be increased. For nonionic surfactants, an increase in temperature leads to a decrease in their cmc value and a significant increase in micelle volume, which shows that an appropriate increase in temperature can help nonionic surfactants to exert their surface active effect. However, the temperature should not exceed its cloud point. The most suitable washing temperature is related to the detergent formulation and the object to be washed. Some detergents have good washing effect at room temperature, while some detergents have much different decontamination effect between cold and hot washing.

3, foam. People are often used to foam capacity and washing effect, that the foaming power of the detergent washing effect is good. In fact, the washing effect and the amount of foam is not directly related, with low foam detergent for washing, the washing effect is not worse than high foam detergent.

Although foam is not directly related to washing, in some occasions, foam can help to remove dirt, for example, when washing dishes by hand, the foam of the detergent can carry away the oil drops. When scrubbing carpets, foam can also take away dust and other solid dirt particles, carpet dirt accounted for a large proportion of dust, so carpet cleaning agents should have a certain foaming ability. Foaming power is also important for shampoo, shampoo or bath when the liquid produces a fine foam to make people feel lubricated and comfortable.

4, the variety of fibers and the physical properties of textiles. In addition to the chemical structure of the fiber affects the adhesion and removal of dirt, the appearance of the fiber form and the organization of the yarn and fabric have an impact on the ease of dirt removal.

5, the water hardness. The concentration of Ca2+, Mg2+ and other metal ions in the water has a great influence on the washing effect, especially when the anionic surfactant encounters Ca2+ and Mg2+ ions forming calcium and magnesium salts, which are less soluble and will reduce its decontamination ability. In hard water, even if the concentration of surfactant is higher, the decontamination effect is still much worse than in distillation. To make the surfactant play the best washing effect, the concentration of Ca2+ ion in water should be reduced to 1×10-6mol/L (CaCO3 should be reduced to 0.1mg/L) or less. This requires the addition of various water softeners to the detergent.

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