CHAPTER V. 



CONSISTENCY OF COLLOIDS. 



Internal Friction. 



THE various colloids show all possible transitions from fluids to 

 solid substances. A fluid may take on any shape, and the work 

 necessary to change its form, i.e., to overcome its internal friction, is 

 very slight. Solid substances, according to WILHELM OSTWALD, pos- 

 sess a form-energy also called elasticity; the energy necessary to 

 change their form, i.e., to overcome their internal friction, is very 

 great. If we picture to ourselves a number of colloids and gels we 

 pass from a true fluid, water for instance, through the albumoses 

 and albumin solutions to the semifluid gels (e.g., 1 per cent gelatin), 

 jellies and finally to the firm substances (e.g., horn). 



High internal friction, viscosity, is a typical property of hydrophile 

 colloids. Since colloids are diphasic systems, the internal friction 

 will depend, above all, upon the size of the free surface of the colloid, 

 i.e., upon the concentration. Changes in temperature are of great 

 importance. The absolute as well as the relative influence of concen- 

 tration is, indeed, characteristic for colloids. Even traces of colloids 

 (agar, gelatin) may increase the viscosity of water to an extraordi- 

 nary extent. We may obtain all degrees of internal friction with 

 agar and in fact a 5 per cent solution of agar is a solid body at room 

 temperature. 



Usually the viscosity increases with decrease in temperature, inas- 

 much as substances then approach the solid condition. Gelatiniza- 

 tion is analogous to the solidification of a molten fluid, where internal 

 friction rapidly rises within a small temperature range. 



J. FRIEDLANDER,* D. HOLDE* and V. ROTHMUND* proved that 

 artificial emulsions (gum water, castor oil, so-called solid fats) exhibit 

 a variation in their viscosity curves according to temperature and 

 concentration, similar to that shown by many natural hydrophile 

 colloids. T. B. ROBERTSON* found that emulsions of oil in water 

 became increasingly more viscous the higher the concentration of 

 the oil, until a critical point was reached when the viscosity decreased; 

 the water then became the dispersed phase. 



Internal friction is indeed a very complicated phenomenon. It 

 depends according to W. B. HARDY upon (1) the internal friction of 



64 



