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| Fig E4: aqueous solution behaviour of borate-C12GA at 20oC |
N-alkylgluconamides
R-NHCO(CHOH)4CH2OH
(where R = alkyl, e.g. C12H25)
Members of this class of surfactant, an example of which is N-dodecylgluconamide (C12GA), is even less soluble in water
than the MEGA surfactants, and hence of little interest by themselves in detergent formulations. As the use of starch-derived
surfactants grows due to environmental and sustainability considerations, increasing their solubility is of great interest.
Surfactant insolubility, manifested by a tendency to form crystals is in this instance a function of strong hydrogen bonding
between the amide and hydroxy groups in the head group region.
Addition of sufficient borate produces a dramatic increase in solubility and the development of conventional low viscosity
solutions at 20oC.
At lower borate:surfactant ratios, the surfactant displays evidence of its inherent insolubility with the formation of gels
and crystal suspensions, although the gels appear not to be an equilibrium state.
Again, there is strong evidence that the solubility enhancement by borate is due to the latter's ability to form ester
complexes with the surfactant. This is believed to promote head group repulsion by the introduction of negative charge
into the head group region, opposing and disrupting the hydrogen bonding between the amide and hydroxy groups.
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Alkyl polyglucosides (APG)
This class of surfactant is intrinsically more water soluble than the polyhydroxy fatty acid amides. Members also interact
with tetrahydroxy borate anions in solution. APG surfactant micelles develop net negative surface charge as a result of
complexation with borates.
Monoglyceride surfactants
New synthesis methods for this type of natural surfactant have been developed. These rely upon the complexation between
the polyhydroxy groups in the surfactant precursor (e.g., glycerol) and borates or simple organoboron compounds to increase
yields of the target surfactant.
Boronic acids have been employed to act as solubilizing agents in non-polar solvents for the enzyme-catalyzed monoacylation
of glycerol with fatty acids. The boronic acid forms transient coordination compounds
with glucose - cyclic esters - which are soluble in organic solvents.
Using glycerol as the polyhydroxy substrate and long chain fatty acids as acyl donors, studies have shown that under lipase
catalysis monoglycerides are produced exclusively when phenylboronic acid is used as a solubilizing and protecting agent.
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Fig E5: freeze fracture transmission electron micrograph of borate - C12GA gel |
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