Aqueous solubility of the components of detergent formulations at common wash temperatures is a prerequisite and as the
trend toward lower temperatures, more concentrated products and lower water usage continues, greater solubility under
these conditions may be required.
This is especially true for surfactants and bleaching agents: the addition of borate has been found to increase solubilities
markedly.
Surfactants with polyhydroxy functionality
Surfactants with amide linkages
N-alkanoyl-N-methylglucamides: the MEGA series surfactants (polyhydroxyfatty acid amides)
CH3(CH2)nCON(CH3)CH2(CHOH)4CH2OH
Non-ionic MEGA surfactants contain a polyhydroxyhydrocarbyl head group linked via an N-methyl amide group to an alkyl chain.
Homologues with alkyl chain lengths in the range of interest for detergent applications (n=10-16) have, in surfactant solution
phase behavior terminology, relatively high Krafft temperatures. In simple terms, these surfactants are rather insoluble in
water at low temperatures.
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Enlarge Image |
| Fig E1: laboratory demonstration of surfactant solubilization by borates |
For their successful application in liquid detergents, however, it is necessary to lower the Krafft temperature. The preparation of low viscosity, pumpable
isotropic (pre-mix) solutions of these surfactants is an important step in the formulation of liquid detergents. The high
Krafft temperature would ordinarily preclude this and result in the formation of an immobile two phase suspension of
surfactant crystals in dilute solution.
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Enlarge Image |
| Fig E2: complexation of the tetrahydroxy borate anion and the polyhydroxyhydrocarbyl surfactant head group |
The addition of borates to the system transforms the immobile suspension into the desired low viscosity isotropic surfactant
solution.
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Fig E3: schematic of a spherical surfactant micelle in cross section with complexed borate,
B- |
Borates achieve this by complexing with the hydroxyl groups. The resultant complex is more water soluble and tends to
aggregate in solution as charged micelles. Mono- or diester complexes form with the tetrahydroxy borate anion; the
concentration of the complexed species increasing with pH and borate concentration.
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