The excludedvolume effect

Deviation from van't Hoff behavior increases with increasing colloid concentration for all the commonly available plasma substitutes, whether they are electrically charged or not, with the exception of polygeline (Webb 1.99.2.). In a polydisperse solution of macromolecules, the shape and size of the larger molecules may be such that they occupy a significant portion of the total volume of the solution. Thus the effective concentration of the smaller molecules is greater, since the volume available for molecular motion is reduced by the space taken up by the larger molecules.

This so-called excluded-volume effect is clearly not apparent for solutions of infinite dilution, since the solute will take up no space under these theoretical conditions.

Human albumin and modified fluid gelatin are the only two plasma substitutes containing charged colloid molecules; the remainder are neutral polymer mixtures. The non-linear relationship between COP and concentration is best explained by the excluded-volume effect for the neutral colloids, and this effect also makes an appreciable contribution for charged colloids. The molecular weight range of polygeline is well below that of the other neutral colloids; hence less space is taken up by these small molecules.

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