Chemistry Reference
In-Depth Information
more important than adsorption related effects such as wetting, emulsification, and
so on, in the overall cleaning process.
The degree of solubilization of oily soils depends on the chemical structure of
the surfactant, its concentration in the bath, and the temperature. At low surfactant
concentrations, solubilization occurs in small, roughly spherical micelles, and a
relatively small amount of oil can be solubilized. At surfactant concentrations
well above the cmc (10-100 times), larger micellar structures that have a greater
solubilizing capacity may be encountered, or some mechanism related to microe-
mulsion formation may take over.
In solutions of some ionic surfactants, the working concentration is seldom
much above the cmc, so that solubilization may not be a significant factor in oily
soil removal. For nonionic surfactants, the extent of solubilization depends on the
temperature of the cleaning solution relative to the cloud point of the surfactant,
since solubilization of oily materials increases significantly as the cloud point is
approached. This may account in part for the observation that soil removal by non-
ionic surfactants is often at a maximum at temperatures in the vicinity of the cloud
point. When insufficient surfactant is present to solubilize the oily soil, the remain-
der may be suspended by emulsification.
Since the detergent power of many surfactants cannot be directly related to their
efficacy as emulsifiers, there exists some question as to the importance of emulsi-
fication as a primary soil removal mechanism and redeposition control. Certainly,
for efficient solubilization to occur, the area of surfactant solution-soil interface
must be maximized, which implies a reduction in the solid substrate-oil interface.
A major criticism of the emulsification mechanism is that, since most detergent
class surfactants are not particularly good emulsifiers, emulsified soil droplets will
be very unstable, resulting in droplet coalescence and significant redeposition.
The rollback process, as well as any possible emulsification processes, will gene-
rally be aided by the addition of mechanical energy, although it is doubtful that
such added energy would be sufficient to significantly overcome the inherent insta-
bility of most detergent-emulsion systems.
10.7.8. Soil Redeposition
The term ''redeposition'' has already been used several times. Because most clean-
ing processes are ''batch'' processes, there will always exist the possibility that soils
removed from the substrate will be redeposited onto the surface as a result of a lack
of colloidal stability in the dispersed soils. For oily soils removed by solubilization,
the process is thermodynamically driven so that it is essentially a one-way street
and redeposition will be minimal. Solid soils, on the other hand, cannot be solubi-
lized and redeposition must be retarded by other kinetically controlled means.
Emulsified oily soils, where they occur, must be handled similarly.
As pointed out earlier, one major role of surfactants at solid interfaces is to
impart a degree of colloidal stability to finely divided particles in aqueous solu-
tions. The adsorption of ionic surfactants at the soil-water and substrate-water
interfaces produces an electrical double layer that retards the mutual approach of
