The basic role of surfactants

The surfactant is an amphiphilic molecule that allows it to have two interfacial (surface) adsorption functions in aqueous solution. Firstly, “positive adsorption” can rapidly reduce the surface tension of water, reflecting the wetting and penetration of surfactants. Second, “micelleization” can form large amounts of micelles in water and effectively reduce the interphase between the two phases. Interfacial tension makes liquids, solids, and gases stable in water, embodying the emulsifying, dispersing, foaming, and solubilizing effects of surfactants. The washing action is a comprehensive process in which the surfactant exerts various functions such as wetting, emulsification, dispersion, foaming, and solubilization.

(A) Wetting and Osmosis In general, wetting is the process of replacing one fluid on a solid surface with another fluid. Therefore, wetting affects at least three phases, where two phases are fluids and one phase is solid. In the dyeing and finishing process, the gas (a fluid) on the surface of the fiber (solid) is mostly replaced by water (another fluid). The slow wetting of grey fabrics in pure water is due to the fact that the surface tension of water is too large to rapidly spread on the surface of the fibers and the air in the grey fabrics cannot be quickly replaced; after the surfactants are added to the water, the surface tension of the water drops significantly. This allows the water to spread rapidly on the fiber surface and quickly replace the air, thus speeding up the wetting process. Therefore, the surfactant that can make the wetting process happen quickly is called wetting agent or penetrant. The role of surfactant in this process is called wetting or infiltration.

There is no essential difference between wetting and osmosis. The former acts on a solid surface and the latter on solids. Both can use the same surfactant, so the wetting agent can also be called a penetrant.

Surfactants have wetting and penetration because they can significantly reduce the surface tension of water.

Analyze the wetting and penetrating action of the surfactant with the droplets equilibrated in the solid plane: or cos θ = (rs-rsL)/rL

The θ angle can be used to measure the degree of wetting of a liquid by a solid:

θ = 0°, the droplets flatten on the solid surface, indicating complete wetting;

0°<θ<90°, the droplets are in the form of a convex lens, indicating partial wetting;

θ>90°, droplets are difficult to spread, indicating no wetting;

θ = 180°, the droplets are spherical on the surface of the solid, indicating complete non-wetting.

Thus, the smaller the angle θ, the better the wetting performance.

The angle θ is dominated by the three forces rs, rL, and rsL, where rs is determined by the solid species as a constant. Only by trying to reduce the rL and rsL forces as much as possible can the cosθ become larger, the θ angle decrease, and wetting The performance becomes better. The addition of surfactant to the liquid not only can rapidly reduce the surface tension rL of the liquid, but also can reduce the interfacial tension rsL of the solid-liquid interfacial tension so that the angle θ can be reduced and the wetting performance of the liquid can be improved.

The fabric is different from the general solid plane. It is a porous system. Numerous mutually intersecting capillary tubes of different sizes are distributed between the yarns, between the fibers, and between the fine structures inside the fibers. Therefore, during the dyeing and finishing process, The wetting ability of fabrics is often measured by the capillary effect. After adding a small amount of wetting and penetrating agent to the dyeing and finishing working fluid, the capillary effect of the fabric can be significantly improved, ensuring the smooth dyeing and finishing process.

(B) Emulsification Two mutually incompatible liquids, one of which is dispersed in droplets in the other, this effect is called emulsification. Emulsification often does not occur automatically or persists. For example, when the oil and water are stirred together vigorously, although it can also form a temporary emulsified state, once the stirring is stopped, the oil and water are immediately divided into two layers. This is because there is a large interface between oil and water. Tension, after the oil becomes a droplet under stirring, the contact area between oil and water will increase greatly and the surface energy will increase rapidly, becoming an unstable system with high internal energy, so that once the stirring is stopped, it will be divided. For two floors, recovery becomes the minimum steady state with two-phase contact area. If a suitable amount of surfactant is added to the oil and water and agitated, the surfactant has the capability of directional adsorption at the oil-water interface, the hydrophilic group extends to the water, and the hydrophobic group extends to the oil, thereby reducing the oil - The interfacial tension between waters reduces the interface energy of the system. While reducing the interfacial tension, the surfactant molecules are closely adsorbed around the oil droplets to form an adsorption membrane with a certain mechanical strength. When the oil droplets contact each other and collide, the adsorption membrane can prevent the accumulation of oil droplets, thereby stabilizing the emulsion. exist. This kind of surfactant that can make emulsification work smoothly is called emulsifier.

If an ionic surfactant is selected as an emulsifier, an electric double layer and a hydration layer will also form on the oil-water interface, both of which will further prevent the accumulation of oil droplets. If non-ionic surfactants are used as emulsifiers, a relatively strong hydration layer will form around the oil droplets to prevent condensation.

The oil-water dispersion formed by emulsification is called an emulsion. There are two types of emulsions. One is an oil-in-water type (oil/water type), expressed in O/W. The oil-in-water type is an oily liquid that is dispersed in water in particulate form, where the oil is the internal phase (discontinuous phase) and the water is the external phase ( Continuous phase); the other type is water-in-oil (water/oil type) expressed in W/O. The water-in-oil type is water in fine particles dispersed in oil, in which water is the internal phase (discontinuous phase), The oil is the external phase (continuous phase). In general, hydrophilic emulsifiers tend to form oil/water emulsions, while highly hydrophobic emulsifiers readily 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 capabilities. Emulsifiers should have appropriate HLB values, such as non-ionic surfactants, with an HLB value between 8 and 18 to form an oil/water emulsion, and between 3 and 6 to form a water/oil emulsion; The emulsion should have a similar molecular structure and should significantly reduce the interfacial tension between the emulsified product and water. The emulsifier should have a strong hydration effect to form a hydration layer around the emulsified particles or to emulsify the particles. Higher charge to prevent aggregation of emulsified particles.

(C) Dispersion The system in which insoluble solid substances are dispersed uniformly in liquids in the form of fine particles is called a dispersion or suspension. This effect is called dispersion, and the surfactant that can make the dispersion smoothly occur is called For dispersants. The dispersed solid particles are called dispersed phase (internal phase), and the dispersed liquid is called dispersed media (external phase). The two effects of emulsification and dispersion are very similar. The main difference is that the internal phase of the emulsion is a liquid and the internal phase of the dispersion is a solid.

Surfactants must have three actions in order to be a good dispersant. First of all, it must have good wetting properties, so that the liquid fully wets each solid particle, replaces the air in the particle, and further breaks the solid particle into smaller crystals. Second, it must be able to significantly reduce the interfacial tension between solids and liquids, increase the ability of solids-liquids to adsorb and mix, and reduce the energy present in the system. Finally, it must form an interface film with high mechanical strength around the solid particles in the form of a hydration layer or a charged layer to prevent aggregation between the solid particles.

For the dispersed solids, the particle volume must be reduced as much as possible. The smaller the particle size, the more favorable the surfactant is to wet, differentiate, and adsorb, forming an interfacial film around it. For example, disperse dyes must be pre-processed and ground into fine particles of 2 μm or less in order to form a relatively stable suspension dyeing working fluid under the action of a dispersant. In spite of this, the dispersion is still a thermodynamically unstable system. Compared with the emulsion, the instability factor is greater, the instability is greater, and the phenomenon of aggregation and delamination is more likely to occur, which affects the normal use. Therefore, the dispersion working fluid should not be stored for too long.

(D) Foaming Gas The state of the gas dispersed in the liquid is called a bubble, and a large number of bubbles together to form a dispersed system is called a foam. The ability to promote foam formation is called foaming. The dispersed gas is called dispersed phase (internal phase), and the dispersed liquid is called dispersion medium (external phase). Foams are similar to emulsions and suspensions, except that the internal phase is a gas, not a liquid or a solid. Foams are more easily generated and stably present under the action of surfactants. Surfactants that promote foam formation are called foaming agents or foaming agents. Surfactants that promote the stable presence of foams are called foam stabilizers.

The resulting foam is also a thermodynamically unstable system, which is prone to cracking and foam disappearing due to the combined effect of the liquid film layer between the bubbles and the small bubbles that penetrate the large bubbles. If there is a surfactant in the liquid, since the surface of the bubble can adsorb surfactant molecules, when these aligned molecules reach a certain degree on the surface of the bubble, the bubble wall becomes a firm film, so that the bubbles do not easily merge; Due to the alignment of the surfactant on the surface of the liquid, the surface tension of the liquid is significantly reduced, and the internal pressure difference between the air bubbles is reduced, so that the drainage speed is slowed down. The above two functions of the surfactant reduce the bursting ability of the air bubbles, and are favorable for the formation and stable existence of the foam.

The foam has a certain effect on the removal and suspension of the dirt. There are also some processes that are completed by the foaming agent in the dyeing and finishing process, such as foam dyeing, foam printing and other new processes.