Chemistry Reference
In-Depth Information
the interfaces and prevents or at least hinders redeposition. Nonionic surfactants
perform the same task by the formation of a steric or entropic barrier to approach,
although the efficacy of such a mechanism is probably less than the electrostatic
repulsions in most aqueous systems.
10.7.9. Correlations of Surfactant Structure and Detergency
Any correlation between the detergent power of a surfactant and its chemical struc-
ture will be complicated by the existence of a wide variety of soil types that may
have vastly different interactions with a given surfactant type. It is important, there-
fore, to specify the exact nature of the system when trying to make any general
statements concerning such correlations.
For oily soils, where solubilization is the primary mechanism of soil isolation
and ultimate removal, it is reasonably safe to say that any structural characteristic
of the surfactant molecule that improves its solubilizing capacity as described in
Chapter 6 will tend to improve its performance in such systems. Equally, if soil
emulsification is important, surfactants with the proper HLB for emulsification
would be expected to have advantages over those lying outside the optimum
range for a given oil type. It has been reported that nonionic surfactants perform
well in the process of removing and preventing the redeposition of oily soils at
lower concentrations than anionic analogs, the reasoning being that their cmc is
reached at lower concentrations.
As indicated earlier, the orientation of the adsorbed surfactant molecule at the
solid-liquid interface will determine the physical result of the adsorption process.
In detergency, the surfactant must orient with the hydrophilic head group toward the
aqueous phase or the mechanisms for soil removal and prevention of redeposition
will not be operative. For that reason, the detergent activity of a given surfactant
in the system will also depend on the polar or ionic nature of the substrate. Both
anionic and nonionic surfactants, for example, may exhibit good detergent proper-
ties on relatively nonpolar surfaces such as polyesters and nylons. On cottons and
cellulose fibers, which are more hydrophilic, anionic surfactants will usually per-
form better than nonionic materials. It may reasonably be expected that the greater
hydrophilicity of such surfaces leads to substantial polar or hydrogen bonding inter-
actions with the POE units of the surfactant and forces its orientation to expose
more of the hydrophobic tail to the aqueous phase, or causes the surfactant mole-
cule to lie along the substrate surface (parallel) and thereby reduce the extent of
adsorption. Such orientation may increase or at least not sufficiently decrease the
interfacial energies at the soil-water and substrate-water interfaces and thereby
retard soil removal. As already mentioned, cationic materials are generally less use-
ful than the other types as detergents, but are especially so when the substrate has
significant anionic character, leading to a reversed molecular orientation and the
formation of a substantially hydrophobic surface.
Clearly, the extent and orientation of adsorption of surfactant molecules onto
a solid substrate are of primary importance to their action in the detergency pro-
cess. Therefore, any alterations in molecular structure that affect such adsorption
