Specialty surfactants
A number of surfactants with borate ester functionalities are known. These find application, for example, as emulsifiers and
antistats in cosmetics and hair care products. The surfactants have in common the cis-1,2- diol group, facilitating borate
ester formation.
Sodium perborate tetrahydrate
Unlike powder formulations, which are able to carry effective perborate-derived available oxygen into the wash liquor, the
aqueous solubility of sodium perborate tetrahydrate is inadequate (2.25 wt.% at 20oC) for practical bleaching through its
direct incorporation into ‘isotropic’ liquid detergents.
However, the addition of co-solutes is known to increase sodium perborate solubility substantially. Examples of suitable
co-solutes are: boric, phosphoric, sulfuric, tartaric and citric acids; other polyols and some mineral acids; acid salts
such as potassium saccharate, sodium and potassium gluconate; and sodium
dihydrogen phosphate.
Some of these co-solutes (tartaric acid, saccharate and gluconate) enhance perborate solubilization as a result of
complexation with B(OH)4-, which depends on the pH and stoichiometry, and the co-solute.
The remaining non-complexing acid co-solutes simply act to lower the pH of the perborate solution. As the pH is lowered,
solubilization occurs via complex ion formation between the borate species themselves.
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An interesting additional observation is that these perborate/co-solute solutions exhibit an increase in pH on dilution
(to 1%) of up to 2.5 units (e.g., from 6.5 to 9). This is discussed further in the section on pH effects [see pH effects section].
While it may be rendered adequately soluble, the available oxygen of aqueous sodium perborate tetrahydrate at its
natural pH, however, reduces over time, unless it is subject to a stabilizing agent [see
Stabilization section].
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| Fig E6: aqueous solubility of PBS4 with acids and acid salt co-solutes (dotted line: convergence of non-complexing co-solutes data) |
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