90 DISPERSE SYSTEMS 



about 2 per cent, per degree. With hyclrophilic colloids it varies 

 with the colloid in both degree and sign, e.g. gelatin is less viscous 

 while albumin is more viscous at 60° than at 10° C. 



Hydrogen ion Concentration. — At the isoelectric point (see 

 p. 92) physiological colloids have their lowest viscosity. Any 

 increase of H+ above this point leads to increased viscosity. 

 Decrease of H+ below the isoelectric point also increases viscosity, 

 but not so markedly. This increase of viscosity with altered pH 

 soon reaches a sharp upper limit. Any further alteration of pH 

 produces a decrease in viscosity due to a dehydration of the 

 colloidal micelle. 



Salts. — The effect of salts on the viscosity of a colloid at its 

 isoelectric point is very slight. It is found that the addition of 

 salts to an acid or to an alkaline protein tends to reduce the 

 viscosity to that found at the isoelectric point. The various 

 neutral salts differ in the intensity of their power to antagonise 

 acid or alkali in colloids, depending principally on their valency 

 (see Part II.). 



2. PROPERTIES OF COLLOIDS DEPENDING ON THEIR 



ELECTRIC CHARGE 



We have used terms in the discussion on viscosity above which 

 indicate that colloidal particles carry an electric charge. By 

 virtue of this charge the particles of the disperse phase will act like 

 ions and will migrate through the solution to any point of opposite 

 charge. This electrical migration is called cataphoresis (Figs. 19 

 and 20). If, on the other hand, the colloid cannot move, say it is 

 in gel form or associated with a membrane impermeable to it, then 

 the molecules of the water in which it is immersed will move 

 relative to it. (See Electrical Endosmose, p. 142). 



The charge on colloidal particles may be developed (a) electro- 

 statically, (b) by orientation of the molecules on the surface of the 

 colloid, or (c) by some adsorption effect. Most inert substances 

 when immersed in water collect a negative charge, while a very 

 few become positive. This is true whether we are dealing with 

 particles of colloidal size or with large masses, e.g. basins, beakers, 

 etc., and is generally considered as due to electrostatic causes, i.e. 

 the surface picks up electrons liberated by the energy of agita- 

 tion of the water molecules (ionisation, q.v.). Many colloids are 

 amphoteric, i.e. can give rise as occasion offers to either + or — 

 " ions." That is, they will have two ionisation constants — an 

 acid one and a basic one. Now if an ampholyte is immersed in 

 water and a strong acid added, the ionisation of the weak acid of 



