I recently acquired a sample of Sodium Alginate (unfortunately not food grade) and I plan to experiment with it soon. But here's what the Merck Index
says about sodium alginate.
Sodium Alginate (Alginic acid sodium salt):
A gelling polysaccaride extracted from giant brown seaweed [giant kelp, Macrocystis pyrifera (L.) Ag., Lessoniaceae] or from
horsetail kelp [Laminaria digitata (L.) Lamour, Laminariaceae] or from sugar kelp [Laminaria saccharina (L.) Lamour].
Cream colored powder. Sol in H2O forming a viscous, colloidal soln. Insol in alcohol and in hydro-alcoholic solns in which the alcohol content is
>30% w/w. Insol in chloroform, ether, in aq acid solns when pH<3.
Alginic acid (polymannuronic acid):
Mol wt about 240,000. A hydrophilic colloidal polysaccharide obtained from seaweeds which, in the form of mixed salts of Ca, Mg and other bases,
occurs as a structural component of the cell wall.
Alginic acid is a linear polymer of beta-(1-4)-D-Mannosyluronic acid and alpha-(1-4)-L-gulosyluronic acid residues, the relative proportions of which
vary with the botanical source and state of maturation of the plant.
Very slightly sol in H2O. Tasteless. Capable of absorbing 200-300 times its weight of water and salts to the extent of 60%. Resists hydrolysis. Sol
in alkaline solns. pH of a 3 in 100 suspension in water is between 2.0 and 3.4.
The calcium salt, (Sorbsan) forms gelatinous precipitate in water.
This page has the structure and even more information on sodium alginate:
http://www.fao.org/docrep/W6355E/w6355e0x.htm
I already forget where I saw this but the reason the calcium salt is insoluble is because the Calcium crosslinks the chains whereas sodium cannot.
(calcium being 2+ instead of sodium's 1+)
So I was thinking (and I plan to experiment with this); what if alginate was crosslinked with something 3+ (thus making 3 way crosslinks). The
polymer matrix would likely be much more stiff. I was thinking of trying to crosslink using NH3, B(OH)3, and some sort of Fe3+ salt.
Another thought, if the 2+ metal ion were larger/heavier than Ca, I wonder what sort of effect that would have on the properties of the polymer. (We
already know that the Mg salt is part of plant cell walls so it must be similar to the Ca salt) I was thinking of trying this with Sr(NO3)2, or maybe
some sort of Fe2+ salt... Cu2+?, Zn2+?
[Edited on 10-8-2007 by agent_entropy]
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