Pigment soil removal
If wash liquor alkalinity is maintained using a borate buffer, mutually repulsive charges are induced on soils and the
surfaces to which they adhere. This facilitates their removal and inhibits their redeposition.
For oxides and clays, common soil constituents, negative charges arise as a result of interaction between the oxide
surface and the potential-determining hydroxyl anions in the alkaline solution. This ionization process is helped by the
tendency of the generally larger cations to stay in the wash liquor and not adsorb at interfaces.
Inclusion of salts will normally mitigate the electrostatic forces (i.e. have a negative effect - ‘indifferent electrolytes’),
but some like STPP, silicates and borates work unusually in the opposite direction
and enhance the repulsive forces.
This is not solely a pH effect: it seems these ions, including the tetrahydroxy borate anion, adsorb onto the surface of
the soil. This enhances the negative surface charge, which is related to the measured zeta potential, leading to a greater
repulsive effect.
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Fig B9: deposition of iron oxide on cotton in washing tests; effects of salts and builders
Fig B10: iron oxide zeta potential; effect of salts and builders |
Oily and fatty soil removal
The alkalinity of wash liquors, conferred by borates, promotes the dislodging and emulsification of fatty/oily soils. This
is to be predicted, due to saponification of the fats which results in a substantial reduction of the interfacial tension
between the soil and its surrounding solution, at the usual wash pH.
pH alone, however, cannot explain the low interfacial tension values observed with borax, perborate or STPP.
While not able to reduce the oil-water interfacial tension to the low values accepted for efficient soil removal and
emulsification, the observed effect with borate is noticeable. This may involve the formation of an interfacially active
ester complex between the tetrahydroxy borate anion and trace amounts of diols - monoglycerides - present in the soil.
Carbonate and percarbonate do not exhibit this property.
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| Fig B11: olive oil/water interfacial tension; effect of salts |
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Detergency performance
In water hardness conditions often experienced (e.g., up to 300ppm CaCO3) it has been demonstrated that borax and sodium
perborate do work as better builder assists than sodium carbonate or percarbonate in laboratory soil removal tests, using
simple binary mixtures with the anionic surfactant sodium dodecylbenzenesulfonate (SDBS).
Similarly, with equal amounts of perborate and percarbonate substituting in an otherwise identical fully-formulated
zeolite-based detergent (IEC 456 - phosphate-free formula), the perborate was significantly better in dealing with
three soils, and equal on the fourth, as evidenced from Terg-o-tometer laboratory washing test results.
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| Fig B12 & B13: hard water detergency of borax vs. sodium carbonate; PBS1 vs. PCS |
Borates, builders, and the environment
Borates are present everywhere in the environment, and indeed boron is an essential plant micronutrient. Evidence that
boron is also necessary in mammalian nutrition is growing.
Additional borate can enter the environment through detergent usage, but this rapidly disperses and does not bio-accumulate,
so posing no long-term environmental threat.
In the case of (per)borate-polyhydroxycarboxylate builders, dissociation of the borate ester complexes occurs upon
discharge of the wash liquor into the waste water stream (promoted by dilution and a reduction in pH). This renders
the waste liquor much less able to sequester, transport or concentrate heavy metals in the environment.
The organic polycarboxylic component is likely to be biodegradable, thus ensuring the final destruction of any remaining
chelating capacity.
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