86 DISPERSE SYSTEMS 



In other words, a colloid is a suitable emulsifying agent only when 

 it holds a certain amount of water. That amount may vary 

 between an upper and a lower limit. If at any time the water in 

 the system oversteps either of the limits the emulsion will lose 

 its stability and will separate out. The emulsions hardest to 

 break are those where the emulsifying agent is a carbohydrate 

 like gum acacia, starch or dextrin. They hold their water of 

 hydration with avidity. Salts, acids, or alkalies in moderate 

 concentrations, alcohol, chloroform and ether have very little 

 action on them. Milk, an oil in protein emulsion, is very difficult 

 to break. Dilution has little effect and fat solvents do not readily 

 extract the fat. This is probably due to an adsorption effect in 

 which the carbohydrate plays a part as yet unknown. The 

 colloidal material comes to be concentrated on the surface between 

 the oil and the aqueous phase. These protecting films drawn 

 over the oil globules keep them from coalescing even when brought 

 close together and also form a membrane impermeable to fat 

 solvents. 



At this point the two theories seem to be on common ground. 

 Neither of them can, however, give a satisfactory explanation of 

 the stabilising effect of the presence of a small quantity of carbo- 

 hydrate on oil-protein or oil-soap emulsion. The carbohydrate 

 need not be a hydrophilic colloid itself. Cane sugar is an excellent 

 stabiliser. Why are tissues less easily poisoned by anaesthetics 

 and other poisons when glycogen is in the cells ? 



Similarly the colloid in a water-in-oil emulsion must be hydrated. 

 Using soap as his stabiliser, Pickering emulsified 99 per cent, oil 

 by volume in one volume of water. The resulting emulsion was 

 a stiff jelly which could be cut with a knife and the cube so prepared 

 would stand alone. These solid cubes when left standing in dry 

 air seem to liquefy. The reason for this is that the soap film loses 

 moisture by evaporation, cracks, and sets free the oil. The mass 

 does not become liquid because of the adsorption of water but 

 because of the loss of water. Several of the heavy lubricating 

 oils contain a considerable quantity of calcium soap. Now, 

 calcium soaps are very insoluble in water but form colloidal 

 solutions in oil, therefore, in these lubricants the water is emulsi- 

 fied into the oil and a thick grease is formed. Rosin acts similarly 

 to calcium soap and is used in the preparation of cheap brands 

 of ready-made paints as an instrument for the emulsification of 

 water in the linseed oil. As much as 80 per cent, water may be 

 absorbed in this way. 



