16 



of surface tension 2-8 dynes per cm. at 20°, the surface tension falls 

 at first with extreme rapidity to 24-29 dynes when 0-002 N sodium 

 oleate has been added, but that further addition of sodium oleate 

 causes only a shght and gradual increase of the surface tension to 

 the value 27-20 dynes for decinormal solution. A 0-0001 N sodium 

 oleate has a surface tension as low as 60-46 dynes and, even when, 

 to diminish hydrolysis, 0-0002 N NaOH had been added, the surface 

 tension was still only 61-32 dynes. Similarly, a 0-008 N sodium 

 oleate solution of surface tension 25-30 dynes still exhibits a surface 

 tension of 31-79 dynes in presence of the comparatively large amount 

 of sodium hydroxide 0-008 N. Harkin's conclusion is that the 

 surface film consists largely of a single layer of sodium oleate molecules, 

 with possibly singly and multiply charged oleate ions (ionic micelle), 

 and hydroxyl and hydrogen ions, the film is thus entirely saturated 

 even at low concentration of solution. 



According to Hillyer the drop number of a decinormal rosin soap 

 is the same of that of sodium oleate. 



There is no recorded analysis of the composition of the substance 

 actually adsorbed. Perrin's work in 1918 in this connection is again 

 vitiated by the possibility that the observed effects were due to carbon 

 dioxide. 



It is worth while remembering that the power of foaming, like 

 emulsification, is not always or entirely due to low surface tension ; 

 for instance, saponin solutions whose surface tensions are only sUghtly 

 less than that of water, have a very great power of foaming on account 

 of the formation of a solid or highly viscous film of adsorbed material 

 in the surface. 



Soap Solution — Oil Interface. 



Here again the stalagometer or drop method has been almost 

 invariably employed. An excellent and luminous investigation was 

 published by Hillyer in 1903, and it was very unfortunate for this 

 subject that his researches were not continued since his elucidations 

 of the phenomena of surface tension and detergent action are. stiU 

 as convincing as ever. His papers are published in the Journal of the 

 American Chemical Society which, as the writer knows from personal 

 experience, has been inaccessible in many of the European Universities 

 in spite of its present enormous circulation in America. 



Hillyer foimd that in contradistinction to the behaviour of their 

 surface tensions against air, the surface tension of sodium oleate 

 solutions against paraffin oil (kerosene) continues to diminish rapidly 

 and steadily with increasing concentration up to decinormal. 

 Decinormal rosin soap Like decinormal sodium oleate exhibits a 

 surface tension which is only 5 or 6 per cent, of that of water. 

 Decinormal sodium hydroxide has almost as great a surface tension 

 against paraffin oil as pure water. Sodium oleate solution at 100° 

 exhibited no marked difference in the form of curve connecting surface 

 tension and concentration. 



HUlyer also investigated the effect of excess of alkaU and of acid, 

 the former had but little effect upon any concentration of sodium 

 oleate, whereas excess of oleic acid gradually and steadily removed 



