COLLOIDS AND COLLOIDAL SOLUTIONS 381 



B. Emulsions. 



Emulsions are systems of two liquids insoluble in each other ; they 

 consist of comparatively coarse liquid particles of one liquid in another 

 with which it does not mix. 



Emulsions are of two kinds : (a) a small quantity of a liquid in 

 suspension in a large amount of another liquid ; (ft) a large amount of 

 one liquid suspended in another liquid ; in this class the continuous 

 phase must consist of a solution of a colloid such as soap, protein, or 

 saponin. 



Formation of Emulsions. 



(a) An emulsion of oil in water is obtained if a fine stream of oil 

 be injected into water. 



* An emulsion is formed if an alcoholic solution of oil be poured into 

 water. 



* (b) Permanent emulsions are formed when colloids are present in a 

 solution and the solution is shaken up with another liquid The most 

 typical permanent emulsions are observed with fats and oils. 



The fats are neutral substances, but generally they contain a little 

 fatty acid, which gives them an acid reaction and causes the formation 

 of an emulsion when they are shaken up with alkali : 



In five test tubes are placed : 



(i) (2) (3) (4) (5) 



10 c.c. H 2 O 10 c.c. H 2 O 10 c.c. H 2 O 10 c.c. H 2 O 10 c.c. H 2 O 



2 c.c. neutral l I drop 8% NaOH 2 drops oleic acid i drop 8% NaOH i drop 8/ NaOH 



olive oil. 2 c.c. neutral 2 c.c. neutral 2 drops oleic acid 2 c.c. ordinary 



olive oil. olive oil. 2 c.c. neutral olive oil. 



olive oil. 



Each is shaken thoroughly. Only in (4) and (5) is a permanent 

 emulsion formed, separation occurring in (i), (2) and (3) after a short 

 time. (5) shows that ordinary fat contains free fatty acid. 



The same result can also be seen by dropping a little neutral olive 

 oil and a little ordinary olive oil on to the surface of some dilute sodium 

 carbonate solution in watch glasses. The neutral oil drop remains 

 clear, whilst the ordinary oil drop spreads out and gives a milky 

 emulsion. 



The formation of emulsions is due to the fact that a layer of soap, 

 formed by the combination of the free acid with the alkali, is made 

 round the fat particle. 



1 Neutral olive oil is prepared by dissolving it in ether, shaking up with dilute sodium 

 carbonate solution, washing free from alkali and finally distilling off the ether. 



