Removal of liquid dirt
General man-made fibers such as polypropylene, polyester, polyacrylonitrile, etc. as well as non-degreased natural fibers (raw cotton, raw wool, etc.) belong to low-energy surfaces, which are hydrophobic, i.e., not easily wetted by water. Therefore, oil can easily adhere to the surface of these substances and form a spreading oil film.
The contact angle of the fouling is greater than 90° and it can be removed completely;
The first step in the decontamination process is the wetting of the fiber surfaces with water. Although the critical surface tension for wetting of these surfaces is generally not lower than 30 mN – m-1 , i.e. they are not easily wetted by water, they are easily wetted by surfactant solutions. After degreasing treatment of cotton, wool and other natural fibers itself has good water wettability.
The second step in the decontamination process is the detachment of oil from the fiber surface. This is achieved by the so-called “shrinkage” mechanism. Since the aqueous detergent solution preferentially wets the fiber surface, the contact angle of the oil contamination on the surface will accordingly increase, and the oil contamination will curl up into oil beads from the spreading thin film, and then be washed away from the surface by the liquid stream, as shown in Fig. (b).
According to the definition of adhesion work, the adhesion work Wso and Wsw of oil and water on the solid surface are:
Where γs, γw, γ0 are the surface tension of the solid, aqueous solution and oil, respectively; γsw and γso are the interfacial tension of the solid and aqueous solution and oil, respectively. Subtracting the two equations and combining them with Young’s equation yields:
Asw-Aso = γso -γsw = γwoCOSθw
where γwo is the oil/water interfacial tension; Asw and Aso are the adhesion tensions of the aqueous solution and oil on the solid surface, respectively; θw is the contact angle of water on the solid surface.
When θw→0, the oil is swept away. Therefore, the above equation shows that the important parameter that determines whether the oil can be swept away is the adhesion tension of the two liquids, rather than the simple work of adhesion. When θw is zero, oil and surface contact angle of 180 °, oil can be spontaneously detached from the solid surface; when θw <90 °, oil can also be detached from the solid surface through the impact of the liquid flow, as shown in Figure (a); but when θw> 90 ° (oil contact angle θ0 <90 °), even if there is a liquid flow of the impact, but also part of the oil residual on the solid surface, as shown in Figure (b). Removal of the residual oil will require a more concentrated surfactant solution and/or stronger mechanical action.
(b) The contact angle of the oil contamination is less than 90°, and not all of it can be removed.
Removal of solid dirt
Unlike liquid dirt, solid dirt (granular) has only a small part of the surface contact and adhesion, and can not expand into a large area. The main force that causes solid particles to adhere to solid surfaces is van der Waals gravity, while electrostatic gravity is relatively weak, and it only accelerates the rate of adhesion of airborne dirt on solid surfaces, without increasing the strength of adhesion. The adhesion strength of particles increases in humid air compared to dry air and decreases considerably in water. In general, the adhesion strength increases with time.
The removal of solid dirt lies in the fact that both the dirt particles and the solid surface can be wetted by water. This reduces the adhesion of the particles and allows them to be washed away by the water flow. The realization of this wetting depends on the adsorption of the surfactant on the particle and solid surfaces.
Spreading is a very high condition for wetting, i.e., if a liquid can spread on the surface of a solid, it must be able to wet the solid.
On the other hand, adsorption of surfactant at the solid/liquid interface increases the surface potential of the solid. If the surface potentials of the solid and the dirt particles are of the same sign, electrostatic repulsive forces favor dirt removal and prevent redeposition. The ease of solid fouling removal is also related to the size of the fouling particles. Large particles by the impact of the water flow is large, easy to remove, while small particles by the impact of small, difficult to remove. Therefore, the removal of particles, mechanical action is particularly important.
The effect of adsorption on decontamination —- Adsorption profile related to decontamination
As mentioned before, adsorption of surfactants on solid surfaces and hydrophobic dirt surfaces is the basis for decontamination. If neither the fiber surface nor the dirt adsorbs surfactant, decontamination cannot be achieved. Another important factor is the mode of adsorption. The mode of adsorption that is beneficial for decontamination is the physical adsorption of hydrophilic groups toward water, i.e., the adsorption of surfactant molecules through the interaction of their hydrophobic groups with the hydrophobic groups within the fiber. Conversely, if the surfactant molecules are oriented towards water with hydrophobic groups, adhesion of the oil stain will be favored. The latter type of adsorption is therefore not expected.
