17 



the lowering of surface tension, although even the acid oleate NaOl, 

 HOI, causes a marked lowering of surface tension. 



Finally Hillyer investigated solutions of sodium palmitate and 

 stearate at 70° and 100°. The first traces of soap do not greatly lower 

 the surface tension of the water but as the concentration passes through 

 N/1000, the surface tension falls very rapidly at first and then above 

 N/80 more gradually up to N/20 solution. This anomaly in very 

 dilute solution was not further investigated, but was ascribed to 

 hydrolysis. Rosin soap, whether at 100° or in the cold, does not 

 differ markedly from sodium oleate, and is not very much affected 

 by temperature. Both, however, exhibit a slightly developed 

 inflexion in dUute solution Uke that of the stearate or palmitate. 



• Donnan and Potts {Kolloid Zeitsch, 1910, 7, 208) measured the 

 drop numbers of a nearly neutral paraffin oil against various 

 concentrations of the sodium salts of the saturated fatty acids from 

 the acetate up to the myiistate (C14). They found that the surface 

 tension was lowered roughly in proportion to the concentration of 

 the salts up to and including the heptoate (C,). Decmormal solutions 

 have a surface tension between 87 and 78 per cent, that of water. 

 The octoate which appears as the lowest soap, has a much more 

 marked effect, N/400 solution causing a lowering of 4 per cent. 

 A N/400 myristate has a surface tension only 58 per cent, of that of 

 water, and the curve connecting surface tension and concentration 

 is only sUghtly curved. Donnan's earlier work {Z. physikal Chem., 

 1899, 31, 427), in which various fatty acids were dissolved in oils 

 and their drop numbers determined against dilute alkali is explained 

 by those results owing to the formation of soap in these experiments. 

 This is invariably the case when commercial fatty oils come in contact 

 wtih alkaline solutions, since they all contain more or less of free 

 fatty acid, and it is only under these conditions that emulsification 

 occurs. 



Soap Solution — Benzene Interface. 



Harkins, Da vies, and Clark have further made exact measurements 

 of the surface tension of sodium oleate against benzene. It falls 

 extremely rapidly at first from the value for pure water, 35 • 03 dynes, 

 through 9-00 dynes for 0-005 N, and 2-22 dynes for 0-014 N to 

 1-78 dynes for 0-1 N soap. It may be noticed this behaviour differs 

 from both of those in the two interfaces already discussed. Briggs 

 {Jour. Phys. Chem., 1915, 19, 210) has done experiments in which 

 he measured instead the apparent amounts of sodium oleate removed 

 from the aqueous layer, using a somewhat unsatisfactory and very 

 incomplete method of analysis. He found a similar behaviour in 

 that the amount removed increased only slightly in higher concentra- 

 tions. Absolute results were not obtained. Briggs and Schmidt 

 {Ibid., 1915, 19, 479) found that the optimum amount of soap for 

 emulsification of the benzene is 1 per cent. ; that is, three times the 

 value given by Harkins, 0-01 N. Addition of 0-1 per cent, alkali 

 favoured emulsification, but larger amounts interfered. 



Shorter {Jour. Soc. Dyer-Colourists, 31, 1915, 64; ibid, 32 (1916) 

 92), Shorter and EUingworth {Proc. Roy. Soc, 1916, A, 92, 231), 



