Friday, March 06, 2009

Lumpy Gravy

I'm getting a chance to work again with some water soluble polymers. For some reason, preparing aqueous solutions of these is always more difficult than dissolving any other polymer in a non-aqueous solvent. If you dump the powder in too fast, without adequate mixing, you end up with clumps that never dissolve - hence the title of this post.

(Obligatory Frank Zappa reference).

What's happening is that the water is quickly swelling and softening the surface of the powder. The grains then are easily able to stick together in a mass greatly slowing the uptake of further water and preventing complete dissolution. Certainly this could happen with solvents, but it never seems to. Only with water soluble polymers. (And this truly is what happens when you make lumpy gravy.)

An easy trick around this is to put some crushed ice in the water (being sure to incorporate the mass of the ice into the final concentration calculations). The cold water doesn't swell the polymer particles nearly as fast, thereby allowing for a complete dispersion of the powder. Once it is all dispersed, some heat melts the ice and the complete dissolution rapidly follows.

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