Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 135

6.3 aPPlIcatIonS In cleanInG ProceSSeS
(SurFace and colloIdal aSPectS)
In almost all cleaning processes, unwanted materials (grease, dirt, color, bacteria)
need to be removed from surfaces or cloth (cotton, synthetics, wool).

6.3.1 d e T e r g e n c y
The detergent industry is a major field where surface and colloidal chemistry prin-
ciples have been applied extensively. In fact, some detergent manufacturers have
been involved in highly sophisticated research and development for many decades in
developing numerous patented processes.
Dirt adheres to fabric by way of different forces (such as van der Waals and elec-
trostatic). Some components of dirt are water soluble, and some are water insoluble.
Commercial detergents are designed specifically for these particular processes to
achieve various effects:

1. Water should be able to wet the fibers as completely as possible. This is
achieved by lowering the surface tension of the washing water, which thus
lowers the contact angle. The low value of surface tension also makes the
washing liquid to be able to penetrate the pores (if present) since, from the
Laplace equation, it can be seen that the pressure needed would be much
lower. For example, if the pore size of fabric (any modern type such as
microcotton and Gortex) is 0.3 μm, then it will require a certain pressure
(= Δ P = 2 γ/R) in order for water to penetrate the fibers. In the case of water
(γ = 72 mN/m) and using a contact angle of 105°, we obtain

Δ P = 2 (72 10 3 ) Cos (105)/0.3 10 6 = 1.4 bar (6.3)

2. The detergent then interacts with the dirt-soil complex to start the process
of the latter's removal from the fibers and dispersion into the washing water.
In order to be able to inhibit the soil once removed from readsorbing into
the clean fiber, polyphosphates or similar suitable inorganic salts should be
used. These salts also increase the pH (around 10) of the washing water. In
some cases, suitable polymeric antiredeposition substances (such as car-
boxymethyl cellulose) are preferred.

3. After the fabric is clean, special brighteners (fluorescent substances) can be
applied that give a bluish haze to the fabric. This enhances the whiteness
(by depressing the yellow tinge). Additionally, these also enhance the color
perception. Brighteners as used for cotton are different from those used for
synthetic fabrics.

Hence, the washing process is a series of well-designed steps that the industry has
enhanced with much information and state-of-the-art technology. Washing technol-
ogy has changed over the years as demands and preferences have changed. Washing
machines are now designed to operate in conjunction with the soap industry. The