12 



One solution may be cited to illustrate typical values of the 

 concentrations arrived at for the respective constituents • 6 N KOI 

 at 0° — 18°, is found to contain 0- 17 N potassium ion, not more"'than 

 0-01 of other crystalloidal matter, the remainder being entirely 

 colloid, probably largely included in the ionic micelle and comprising 

 0-16— 0-17 N aggregated oleate ion, with a total of 0-41— 0-43 N 

 aggregated neutral potassium oleate. 



In many other cases, however, the limits of concentration of the 

 constituents have not yet been so narrowly defined. 



V. — Physical Properties of Soap Solutions. 



Having discussed the general constitution of soap solutions, we 

 shall now discuss in turn a number of their chief physical properties. 



(a) Viscosity. (For references, see classified bibliography appended.) 



As in the case of so many coUoids the viscosity constitutes a 

 prominent characteristic responding in typical fashion to alteration 

 in experimental conditions. Whilst very readily measured and 

 reproducible, the data do not lend themselves to quantitative inter- 

 pretation and explanation. Nearly all our exact knowledge of this 

 subject is due to the numerous and careful measurements of F. Gold- 

 schmidt" ' and his collaborators, although Farrow carried out a series 

 of exact measurements at 70° in Donnan's laboratory. 



The viscosity increases with rise in concentration of the soap, 

 at first gradually, then enormously. For instance, potassium oleate 

 at 20° exhibits a viscosity of 1-19 for N/20, 1-87 for N/5, 8-02 for 

 0-4 N, and no less than 1573 for 0-6N, taking water as unity. 



The effect of temperature in the case of less viscous soap solutions 

 is practically that of the alteration of the fiuidity of water. 



Addition of hydroxide, chloride or carbonate at first lowers the 

 viscosity, which passes through a minimum, and thereafter rises 

 enormously. This was observed by Mayer, Schaeffer and Terroine,^* 

 Botazzi and Victorov-' and Leimdorfer, Farrow, -^ Goldschmidt and 

 Weissmann, and most carefuUy studied by Kurzmann." The more 

 concentrated the soap solutions, the more pronounced is the minimum, 

 and the less the salt required to produce it. 



The foUowing data present typical cases of such minima produced 

 by the addition of potassium hydroxide at 20° and at 90° to solution 

 of • 375 N, potassium laurate and • 6 N potassium oleate respectively, 

 the viscosity of water at 20° being taken as unity : — 



At 20° the viscosity of the laurate is decreased from 1-96 to 1-57, 

 that is, by 20 per cent. At 90° the decrease is from 0-604 to 0-522, 

 being 14 per cent. At 20° the values for the oleate are 1573 to 728, 

 by 54 per cent. At 90°, 3 • 80 to 3 • 01 by 21 per cent. These remarkable 

 changes are produced by the addition of 0-5 N KOH to the laurate, 

 but only - 02N KOH to the oleate. 



It is at once apparent that the lowering of the viscosity is dependent 

 upon the presence of ionic micelle. There must be at least three 

 primary effects at work in all these cases. The first, lowering is 



