The basic role of Surfactants

     Surfactants are amphiphilic molecules that enable them to have two interfacial (surface) adsorption functions in aqueous solutions.

      First, the surface tension of water can be rapidly reduced through "positive adsorption", reflecting the wetting and penetrating effects of surfactants;

      Second, through "micellarization", a large number of micelles can be formed in water and the interfacial tension between the two phases can be effectively reduced, so that liquids, solids, and gases can exist stably in water, which reflects the emulsification, dispersion, foaming, and emulsification of surfactants. Solubilization, etc. The washing action is a comprehensive process in which the surfactant exerts various functions such as wetting, emulsifying, dispersing, foaming, and solubilizing. Below, we briefly discuss the wetting, emulsifying, dispersing, foaming, solubilizing, washing and other action principles of surfactants and their reaction principles.

(1) Wetting and penetration

      Cut a piece of grey cloth and put it lightly on the water surface, the piece of grey cloth will stay on the water surface for a period of time and then slowly sink to the bottom. If a small amount of surfactant JFC is added to the water, we find that the fabric placed on the water surface will sink to the bottom immediately, which is a common method to test the wetting ability of surfactants.

      Generally speaking, wetting is the process by which one fluid is replaced by another on a solid surface. Therefore, wetting involves at least three phases, two of which are fluid and one is solid. In the dyeing and finishing process, it is mostly a process in which the gas (a fluid) on the surface of the fiber (solid) is replaced by water (another fluid).

      The wetting speed of grey fabrics in pure water is slow, because the surface tension of water is large, so it cannot spread quickly on the fiber surface, and cannot quickly replace the air in the grey fabrics; after adding surfactants to water, the surface tension of water drops significantly , so that the water can quickly spread on the fiber surface and replace the air quickly, thus speeding up the wetting process. Therefore, the surfactant that can make the wetting process happen quickly is called wetting agent or penetrant, and the role played by surfactant in this process is called wetting or penetrating effect.

      There is no essential difference between wetting and penetrating. The former acts on the surface of the solid and the latter acts on the interior of the solid. The same surfactant can be used for both, so the wetting agent can also be called a penetrating agent.

     The wetting and penetrating effects of surfactants are due to their ability to significantly reduce the surface tension of water.

     The fabric is different from the general solid plane. It is a porous system. There are countless capillaries of different sizes and interconnections between the yarns, between the fibers and between the microstructures inside the fibers. Therefore, in the dyeing and finishing process , the wetting ability of the fabric is often measured by the capillary effect.

      After adding a small amount of wetting and penetrating agent to the dyeing and finishing working solution, the capillary effect of the fabric can be significantly improved, ensuring the smooth progress of the dyeing and finishing process.

      As a surfactant used as a wetting and penetrating agent, its molecular chain length should be moderate, and its HLB value should be appropriate. The branched chain in the hydrophobic group will significantly improve its wetting ability. The hydrophilic group of ionic surfactants is located in the molecular The one in the center of the chain has the best wettability, and the wettability decreases when the surfactant molecule is introduced into the second hydrophilic group. When polyoxyethylene type nonionic surfactant is used as penetrating agent at high temperature, it should be used together with anionic surfactant to improve its thermal stability. The wetting and penetrating agent used under strong acid and alkali conditions should Fully consider its chemical stability to avoid decomposition failure.

      Products that are often used as wetting and penetrating agents in dyeing and finishing include penetrating agent JFC, penetrating agent T, pulling powder BX, penetrating agent 5881, mercerizing penetrating agent MP and other brands.

(2) Emulsification

      Two immiscible liquids, one of which is dispersed in the other in the form of droplets, is called emulsification.

      Emulsification is often not automatic or permanent. For example, vigorously stirring oil and water together can form a temporary emulsification state, but once the stirring is stopped, the oil and water are immediately divided into upper and lower layers. This is because there is a large interface between oil and water. After the oil turns into droplets under the action of stirring, the contact area between oil and water will increase greatly, the surface energy will increase rapidly, and it will become an unstable system, so that once the stirring is stopped, it will be divided into two layers and recover. It becomes the stable state with the smallest contact area between the two phases. If a certain amount of appropriate surfactant is added to the oil and water, and then stirred, the hydrophilic group extends to the water and the hydrophobic group extends to the oil due to the ability of the surfactant to adsorb directionally on the oil-water interface, thereby reducing the amount of oil. - The interfacial tension between water reduces the interfacial energy of the system.

      While reducing the interfacial tension, the surfactant molecules are closely adsorbed around the oil droplets to form an adsorption film with a certain mechanical strength. When the oil droplets contact and collide with each other, the adsorption film can prevent the oil droplets from agglomerating, thus making the emulsion stable exist. Such surfactants that enable smooth emulsification are called emulsifiers.

      If an ionic surfactant is selected as an emulsifier, an electric double layer and a hydration layer will also be formed on the oil-water interface, which can further prevent the aggregation of oil droplets. If a non-ionic surfactant is used as an emulsifier, a relatively firm hydration layer will be formed around the oil droplets to prevent aggregation.

      The oil-water dispersion system formed by emulsification is called emulsion, and there are two types of emulsion.

      One is the oil-in-water type (oil/water type), which is expressed as O/W. The oil-in-water type is that oil-in-water liquid is dispersed in water in particulate form, in which oil is the inner phase (discontinuous phase) and water is the outer phase ( continuous phase);

      The other is the water-in-oil type (water/oil type), which is expressed by W/O. The water-in-oil type is that water is dispersed in the oil in the form of particles, wherein the water is the inner phase (discontinuous phase), and the oil is the outer phase ( continuous phase).

      Generally speaking, emulsifiers with strong hydrophilicity are easy to form oil/water emulsions, while emulsifiers with strong hydrophobicity are easy to form water/oil emulsions.

     Emulsifiers are all surfactants, but not all surfactants can be good emulsifiers. Only surfactants that can form stable micelles in water have good emulsifying and dispersing ability. Emulsifiers should have appropriate HLB values, such as nonionic surfactants, whose HLB values are between 8 and 18 to form oil/water emulsions, and between 3 and 6, they can form water/oil emulsions; emulsifiers It should have a similar molecular structure with the emulsified material, and should be able to significantly reduce the interfacial tension between the emulsified material and water; the emulsifier should have a strong hydration effect to form a hydration layer around the emulsified particles or make the emulsified particles with Higher charge to prevent the aggregation of emulsified particles.

     In the process of dyeing and finishing, some emulsified working fluids are often used, most of which are oil/water emulsions. Frequently used emulsifiers such as Pingping O series, Span-Tween series, EL and so on.

(3) Dispersion

     The system formed by uniformly dispersing insoluble solid substances in liquid with tiny particles is called dispersion or suspension. The dispersed solid particles are called the dispersed phase (internal phase), and the dispersed liquid is called the dispersion medium (external phase). The two functions of emulsification and dispersion are very similar, the main difference is that the internal phase of the emulsion is a liquid, while the internal phase of the dispersion is a solid.

    A surfactant must have three roles to be a good dispersant. First, it must have good wetting properties, so that the liquid can fully wet each solid particle, replace the air in the particle, and further break the solid particle into smaller crystals. Secondly, it must be able to significantly reduce the interfacial tension between solid and liquid, increase the adsorption and compatibility between solid and liquid, and reduce the energy existing in the system. Finally, it must form a mechanically strong interfacial film around the solid particles in the form of a hydration layer or a charged layer to prevent the aggregation between solid particles.

    For the dispersed solid, the particle volume must be reduced as much as possible. The smaller the particle volume, the more favorable it is for the wetting, differentiation and adsorption of the surfactant, and the formation of an interfacial film around it. For example, disperse dyes must be pre-processed and ground into tiny particles below 2 μm in order to form a relatively stable suspension dyeing working solution under the action of dispersants. Despite this, dispersion is still a thermodynamically unstable system. Compared with emulsion, it has more unstable factors and greater instability, and is more prone to coagulation and stratification, which affects normal use. Therefore, the dispersion working solution should not be stored for too long, and it is best to use it now. Dispersions are widely used in dyeing and finishing, such as the preparation of disperse dyes and vat dyes. Commonly used dispersants include diffusing agent NNO, dispersing agent WA, etc. Among them, there are more anionic surfactants.

(4) Foaming effect

     The state in which the gas is dispersed in the liquid is called bubbles, and the dispersed system formed by a large number of bubbles is called foam, and the ability to promote the formation of foam is called foaming. Foams are similar to emulsions and suspensions, except that the internal phase is gas instead of liquid and solid.

      Foam is easier to generate and stably exist under the action of surfactants. Surfactants that can promote the formation of foam are called foaming agents or foaming agents, and surfactants that can promote the stable existence of foams are called foam stabilizers.

     The formed foam is also a thermodynamically unstable system, and it is easy to cause the bubbles to burst continuously and the foam to disappear because of the liquid film layer between the bubbles and the merger of the small bubbles penetrating the large bubbles. If there are surfactants in the liquid, because the surface of the bubbles can adsorb the surfactant molecules, when these directional molecules reach a certain level on the surface of the bubbles, the bubble wall becomes a solid film, so that the bubbles are not easily merged; Due to the directional arrangement of the surfactant on the liquid surface, the surface tension of the liquid is significantly reduced, and the internal pressure difference between the bubbles is reduced, so the liquid discharge speed is slowed down. The above two functions of surfactants reduce the bursting ability of bubbles, which is beneficial to the formation and stable existence of foams.

    Foam has a certain effect on the removal and suspension of dirt. There are also some new processes that rely on foaming agents in dyeing and finishing, such as foam dyeing, foam printing and other new processes. However, in the process of dyeing and finishing, more occasions require low foam or no foam, so how to suppress and defoam more people's attention.

(5) Solubilization

      Substances that are completely insoluble or slightly soluble in the solvent enter the micelle formed by the surfactant to be dissolved and become a thermodynamically stable solution. This phenomenon is called solubilization, and the formed transparent solution is called a solubilizing solution or micelle. Solution, the solubilized substance is called solubilizer, and the surfactant that solubilizes is called solubilizer. Solubilization is both distinct and related to emulsification and dispersion.

The difference is:

(1) Emulsification is limited to the dispersion system formed between liquid and liquid, dispersion is limited to the dispersion system formed between solid and liquid, and the dissolved substance by solubilization can be either liquid or solid.

(2) Emulsification and dispersion formed a thermodynamically unstable multiphase dispersion system, while solubilization formed a thermodynamically stable homogeneous system.

(3) The appearance is obviously different, the emulsion and dispersion are mostly milky white and suspended, while the solubilizing solution is transparent.

Its connections are:

      Solubilization can be regarded as the limit stage and ideal state of emulsification or dispersion, and there is a way of mutual transformation between them. For example, an emulsion can also become a microemulsion—the appearance changes from milky white to transparent, which is close to the solubilizing solution; when the solubilizing solute is added to the solubilizing solution to a certain amount, the solubilizing solution changes It is an emulsion—the appearance changes from transparent to milky white.

     The solubilizing solution is also fundamentally different from the true solution. The true solution is that organic or inorganic substances are dissolved in the solvent in the form of molecules or ions, while the solubilized solubilizer "dissolved" in the solubilizing solution is a "molecule" that is much larger than the molecule. Group" form enclosed in micelles.

     The surfactant used as a solubilizer must reach a sufficient concentration in the solution and form a sufficient number of micelles in the solution to ensure the smooth generation of the solubilization effect, and the number of rod-shaped, layered and other advanced micelles formed. The more, the more obvious the solubilization effect will be.

     Solubilization also has many special effects on dyeing and finishing. For example: disperse dyes are solubilized by appropriate solubilizers, and their solubility in water will be significantly improved, which is conducive to the stability of the working solution and improves the dyeing effect; many high-grade silicon-containing soft products The finishing agent needs to be prepared into a very stable microemulsion and a solubilizing solution to produce an excellent finishing effect; in the process of removing dirt on the fabric, solubilization also plays an important role in decontamination.

(6) washing action

     The process of removing foreign matter or dirt from a solid surface immersed in a certain medium (mostly water) is called washing, and the chemicals that can play a washing role are called detergents.

     The washing action is more complicated, which is the result of the comprehensive action of surfactants such as wetting, emulsifying, dispersing, and solubilizing, as well as mechanical action such as stirring, rubbing, and water flow. Taking the fabric as an example, the washing process can be represented by the following formula:

Fabric · Dirt + Detergent

Fabric Detergent + Dirt Detergent

Fabric + Detergent

During the washing process, dirt is removed by the following specific ways:

(1) The detergent does directional interface adsorption on the fiber surface and the dirt surface, and further wets and penetrates between the fiber and the dirt (where they are in contact with each other).

(2) Under the action of detergent separation and substitution, the binding force between dirt and fibers is weakened, and the fibers are separated from the fibers under the action of machinery or water flow.

(3) The detached dirt is emulsified, dispersed or solubilized by the detergent in the aqueous solution, so that it will not be deposited back to the surface of the fabric.

(4) The dirt and detergent are washed away with the aqueous solution, and the residual detergent adsorbed on the surface of the fabric is also washed away together.

    Dirt is generally divided into two categories: oily dirt and solid dirt. Oily dirt is mostly composed of animal, vegetable oil, mineral oil, etc. The solid dirt is mainly dust, rust, carbon black, etc. The oily dirt and the fabric are bound together by intermolecular attraction in the form of surface bonding, which is not easy to remove, while the solid dirt and the fabric are connected in the form of point bonding, which is easier to remove.

    In fact, oily dirt and solid dirt are mostly mixed together to form mixed dirt, so it is only necessary to take measures to remove the oily dirt. When the oily dirt is effectively removed, the solid dirt is washed away together. The removal process of oily soils follows a "curling" mechanism.

The HLB value of the surfactant used as a detergent should be appropriate. Usually, the molecular structure is linear, and the surfactant with the hydrophilic group at the end has a stronger cleaning effect.

Since fibers are negatively charged in water, anionic surfactants have excellent washing effect on fabrics. This is because the interface layer formed by anionic detergents around dirt is also negatively charged, which makes the difference between fabric and dirt. A certain repulsive force is generated, which is conducive to the separation of dirt from the surface of the fabric and stably suspended in the aqueous solution.

   The cationic surfactant will make the dirt surface positively charged, but will strengthen the adsorption between the dirt and the fabric, so the cationic surfactant should not be used as a detergent. The cmc value of non-ionic surfactants is very low, and they have strong decontamination ability at low concentrations, especially for hydrophobic synthetic fibers, such as polyester fabrics, which have better washing effect and are often used as detergents.

   The use effect of detergent is not only related to the molecular structure and type of detergent, but also closely related to the concentration, temperature, pH value of washing liquid, fabric type, mechanical action and many other factors.

   There are many kinds of detergents suitable for dyeing and finishing, such as detergent AS, detergent AES, detergent LS, detergent 105, detergent LAS and other products. The washing process using organic solvent as the cleaning medium is called dry cleaning.

Excerpted from: "Dyeing and Finishing Technology"