2 STRUCTURE OF GELS 65 



1% ScHWEiZER solution are completely solvated, i.e., completely 

 surrounded by molecules of the solvent, and free to move as in real 

 solutions. Their colloid character results merely from the fact that, the 

 molecular length of solute molecules in one dimension being almost 

 microscopic, they attract a large amount of solvent and thus increase 

 the viscosity. Staudinger denotes this state as "sol solution". From 

 a degree of polymerization of about loo onwards, however, a i% 

 ScHWEiZER solution can no longer completely solvate all the chain 

 molecules, and the solute molecules hamper each other's Brownian 

 movement. They are not completely dissolved but are in a state 

 intermediate between solid and liquid. At the highest degree of 

 polymerization detectable, this interaction of the giant chains with 

 2000 links is so intensified, that the fibre cellulose dissolves very 

 slowly. Solutions in which the chain molecules are hampered in their 

 Brownian movement for want of solvent were called "gel solutions" 

 by Staudinger (Staudinger and Sorkin, 1957b). There exists a 

 reliable method, based on the phenomena of capillary flow, by which 

 the concentration or particle size can be found at which the particles 

 in a colloid solution begin to disturb each other, viz., Hagen- 

 Poiseuille's law 



where q is the amount of Hquid flowing through a capillary of radius 

 r in a time t under the influence of a pressure gradient p/1. In this for- 

 mula the viscosity rj is independent of the pressure gradient p/1. 



This no longer applies when the colloid particles in the solution 

 influence each other's motion. In this case the viscosity depends on the 

 pressure gradient: r] = f(p/l), in the sense that the viscosity decreases 

 with increasing pressure gradient. This can be explained by the fact 

 that in these solutions the elastic properties of the solid substance are 

 not completely eliminated, since the parcicles, instead of being fully 

 dissolved, enter into some sort of relation with each other. With in- 

 creasing pressure gradient in the capillary these elastic forces are 

 progressively counteracted. For this reason, in colloid solutions with 

 long chain molecules the chains which are originally present in a ran- 

 dom and disorderly arrangement will be oriented parallel to the direc- 

 tion of flow, and thus the forces resisting the flow which are respon- 

 sible for the viscosity will be decreased. According to Table VIII, such 



