Gelling behavior

Two alternatives would seem to be open in the discussion of this subject. An exhaustive account of known gelling behavior would inevitably result in a technical bias. On the other hand, it should be possible to emphasize the principles, and to illustrate the limits of present understanding by chosen examples. The choice has been made reluctantly, because both types of discussion would be timely, but it would seem more sensible to marshal our ideas about the basis of gel formation before attempting to tabulate and organize the sum of factual knowledge. The present article is written from the viewpoint of the structural chemist who wishes to see how the overall properties of the gel are an outcome of molecular structure, and who would rather have a qualitative understanding in these terms than a physical or mathematical model which, even if capable of predictions with high precision, did not start from the molecular formula. ... 

Native starches due to their swelling and gelling behavior at body temperature were explored as a binder and a disintegrant in pharmaceutical formulation development. Out of all the starches, maize, potato, and now pea finds official status in many pharmacopoeias worldwide for their use... 

The effect of gel formation at the wall can be studied using the diffusion equation. Return to Figure 17-10. We implicitly assumed that the wall concentration was a variable that could increase without bound. In gelling systems once the wall concentration equals the gel concentration, Xg, the concentration at the wall becomes constant at x = Xg. As additional solute builds up at the wall, the gel concentration is unchanged, but the thickness of the gel layer increases thus, x becomes a constant set by the solute gelling behavior. This is illustrated in Figure 17-12. The value of Xg can vary from less than 1 wt % for polysaccharides to 50 vol % for polymer latex suspensions tBIattetal.. 19701. 

Fig. 4.40. Gelling behavior of alkyl celluloses (according to Balser, 1975). MC methyl cellulose, HG hydroxypropylmethyl cellulose with a hydroxypropyl content of about 6.5%. The numerical suffix is the viscosity (cps) of a 2% solution...

Kumar, S., Himmelstein, K.J. Modification of in situ gelling behavior of carbopol solution by hydroxypropyl methylcellulose. J. Pharm. Sci. 84(3), 344-348 (1995)... 

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