Chemistry Reference
In-Depth Information
nonionic surfactants, the mechanism is less clear-cut; however, steric repulsion
between adsorbed surfactant layers and solubilization is of primary importance.
10.7.5. Soil Types
In general, there are two types of soil encountered in detergency situations: liquid,
oily substances, and solid particulate material. Many stains on textiles such as
blood, wine, mustard, catsup, and the like involve proteins, carbohydrates, and rela-
tively high-molecular-weight pigmentlike materials that pose special problems in
terms of the interfacial interactions involved. The interactions of each class of
soil or stain with the solid substrate can be quite complex, and the mechanisms
of soil removal may be correspondingly complex.
Solid soils may consist of various mineral compositions, carbon (soot) having a
variety of surface characteristics, metal oxides and pigments, and other compounds.
Liquid soils may contain skin fats (sebum), fatty acids and alcohols, vegetable and
mineral oils, synthetic oils, and liquid components of creams and cosmetics. As
with the solid soils, the surface chemical characteristics of the liquid soils can
vary widely. It is not surprising, then, that the derivation of a comprehensive theory
for detergency that can characterize every situation poses a challenge. There
are some basic similarities between the two soil types, however, which allow for
some generalization, while each class will also have its special requirements for
efficient detergency.
The adhesion of both solid and liquid soils to solid substrates will, to a greater or
lesser extent, result from dispersion and van der Waals interactions. Adsorption due
to other polar forces such as acid-base interactions or hydrogen bonding is also
usually of minor importance except where highly polar soils and substrates are
involved. Adhesion by electrostatic interactions is generally less important for
liquid soil systems, but it can become important, and in fact, determining, in the
cases of some mineral and biological soils. When electrostatic forces are involved,
resulting soil stain can be very difficult to remove by normal cleaning processes.
Where soil adsorption predominantly a result of dispersion and van der Waals
interactions, nonpolar materials such as carbons and hydrocarbon oils can be espe-
cially difficult to remove from hydrophobic surfaces such as polyesters. More
hydrophilic soils such as clays, fatty acids, and other material, on the other hand,
can be more difficult to remove from hydrophilic surfaces such as cotton. Mecha-
nical forces can also inhibit cleaning action, especially in fibrous materials with
particulate soils, as a result of entrapment of the particles in the fibers. It is obvious,
then, that the cleaning process can be extremely complex, and optimum results may
be possible only for specifically defined systems. Like the universal solvent, the
universal detergent is, in all likelihood, beyond our technological reach.
10.7.6. Solid Soil Removal
The removal of solid, particulate soils from a substrate in an aqueous cleaning bath
involves the wetting of the substrate and soil by the cleaning bath followed by
adsorption of surfactant and/or other components at
the substrate-liquid and
